EULINOX

(European Lightning Nitrogen Oxides Project)

Work Programme

Experiment plan (Postscript)

Annual Report 1998 (for participants only)


Proposal

1 Objectives

2 Work Content

2.1 Project Concept

2.2 The Field Experiment
2.2.1 General Design


2.2.2 Localisation and characterisation of lightning flashes
2.2.3 Radar observations
2.2.4 Airborne chemical measurements
2.2.5 Satellite observations
2.2.6 Supporting activities

2.3 Parameterisation of lightning and NOx production

2.3.1 Parameterisation of lightning
2.3.2 Parameterisations of NOx production

2.4 Numerical modelling

2.4.1 Mesoscale modelling
2.4.2 European-scale modelling

2.5 NOx inventory and environmental implications

References


1 OBJECTIVES

General objective

The objective of this project is to refine the knowledge on the upper tropospheric budget and concentration of nitrogen oxides (NOx) over Europe and in particular to understand and quantify the contributions from thunderstorm lightning-induced sources of NOx to the composition of the atmosphere over Europe.

Issues

Ozone in the upper troposphere is an important greenhouse gas relevant to the earth's climate and essential for the photochemistry and oxidising capacity of the atmosphere (CEC 1993). Reactive nitrogen oxides NOx (NOx=NO+NO2) are central to photochemistry. The local ozone production rate depends strongly and nonlinearly on the local NOx concentration (Ehhalt and Rohrer 1995). The main inputs of NOx to the middle and upper troposphere are from deep to moderate convection or frontal activity over boundary layer source regions and from lightning production associated with thunderstorms, transport from the stratosphere, and in-situ emissions of NOx from aircraft (Levy et al. 1996). The global lightning induced NOx (LNOX) source represents the largest uncertainty in the global NOx budget, with estimates varying between 1 and 220 TgN/yr (Liaw et al. 1990). More recent model analysis and field experiments come up with more narrow ranges of 2 to 5 TgN/yr (Levy et al. 1996, Ridley et al. 1996). An overview of the estimates for the various sources is given in Table 1.

Source

Best Estimate (TgN/yr)

Uncertainty Range (TgN/yr)

Lightning

5

2 - 20

Aircraft

0.8

0.59 - 0.95

Biomass Burning

7.9

3 - 15

Industry and Surface Traffic

22

13 - 31

Soil Microbial Production

7

4 - 12

Stratospheric Production

0.64

0.4 - 1

Table 1: Global NOx sources in Tg N/year (Lee et al., 1997)

The amount and distribution of the LNOX source must be known in particular for assessments of the impact of aircraft NOx emissions on the state of the atmosphere (Beck et al. 1992, WMO 1995, Brasseur et al. 1996). The following concentration estimates have been computed assuming about 5 TgN/yr of LNOX source (Table 2).

The importance of LNOX over Europe is uncertain. Whereas the global LNOX estimate can be constrained by comparison to measured NOx concentrations, modelled sink terms, and measured deposition of nitrate (Levy et al. 1996), the regional distribution cannot be assessed because the existing NOx measurements in the free troposphere are very sparse, especially over Europe (Emmons et al. 1997, Ehhalt et al. 1992, Luke et al. 1992). Systematic measurements are needed because NOx concentrations are highly variable, even in the upper troposphere, due to the relatively short lifetime of NOx (order days to weeks), and localised sources and sinks.

Reference

(1)

(2)

(3)

(4)

Lightning

15-20%

16%

<10%

10-25%

Fossil Surface Sources

15-20%

56%

30-60%

-

Aircraft

25-30%

16-24%

25-40%

-

Stratospheric Production

-

5%

<5%

-

Table 2: Calculated relative contributions (%) of the individual NOx sources to the total atmospheric burden of NOx as a mean over (1) 40N-60N and 8-11 km height (Lamarque et al. 1996), (2) 30N-60N and 175-325 hPa (Köhler et al. 1997), (3) 40N-50N, 0E-25E, about 10 km altitude (Ehhalt et al. 1992), (4) 40N-60N, 190 - 500 hPa (Levy at al. 1996).

Though most LNOX is produced in the tropics over land, lightning has been observed to occur at all latitudes up to the polar circles (Goodman and Christian 1993, Mackerras and Darveniza 1994). About 25% of the estimated global production takes place in the northern mid-latitudes (30N-60N) (Price et al. 1996). Lightning observations from space (OTD satellite data) and some models (Levy et al. 1996) suggest considerable LNOX sources in particular over southern Europe. Because of small lifetime of NOx and rather large transport time from the tropics to mid-latitudes, it appears likely that the NOx concentration at mid-latitudes is only little affected by tropical LNOX sources.

In fact, the lightning source over Europe and their effects might have been underestimated. A recent analysis of 3 years of lightning observations over South Germany reveal an annual mean flash rate of 2.8/(km2 yr) which is 1/3 of that observed over Florida (Finke and Hauf 1996, Orville 1994). Based on the observed flash rate over Germany and LNOX production rates per flash from the literature one computes that the LNOX source rate over Europe may be 3 times larger or 3 times smaller than aviation NOx sources in the upper troposphere over mid Europe depending on whether one uses the largest or smallest of the previously published LNOX sources per flash (Liaw et al. 1990).

Though surface emissions and upward transport by convection are expected to dominate the NOx concentration in the troposphere over Europe (Ehhalt et al. 1992), some models reveal strong importance of LNOX source in the upper troposphere even at northern mid-latitudes (Larmarque et al. 1996). All models find very strong sensitivity of the ozone impact to the details of the LNOX source model (Beck et al. 1992), because of the large efficiency of NOx in catalysing ozone production in the upper troposphere. Hov and Flatoy (1997) find that the concentration fields of NOx, ozone, OH, and non-methane hydrocarbons differ significantly when a fixed lightning source is applied compared to a source variable in time and space as a function of cloud convection. Different versions of LNOX representation may cause 100% changes in NOx, OH, and NMHC and 10% change in O3. Hence, there is strong need to reduce the presently large uncertainty in NOx concentrations and lightning contributions over Europe.

State of the art

Models predict NOx concentration increases within thunderstorms due to LNOX of the order 1 ppbv, i.e., 5 to 20 times the background NOx concentrations in the free troposphere over western Europe. Up to now only few measurements have been performed relating NOx concentrations to thunderstorm activities (Dickerson et al. 1987). Airborne in-situ NOx measurements showed peak concentrations of up to 4 ppbv over Kansas (Luke et al. 1992), up to 1.3 and 2.0 ppbv NO within the anvil of 2 thunderstorms over New Mexico (Ridley et al. 1996), about 0.7 ppbv NO after 24 hours in the outflow of a thunderstorm over Spain (Huntrieser et al. 1996), and more than 2 ppbv in a thunderstorm anvil over Germany. The latter case could be attributed to observed lightning activity (under investigation).

Global and regional chemical transport models have been developed (Flatoy and Hov 1996, Wauben et al. 1997) and are available to quantify the contributions from various NOx sources to the NOx

concentration of the free troposphere and the impact these sources have on the ozone chemistry and the chemical composition. The existing chemical transport models still use highly simplified LNOX source models. Numerical Weather Prediction (NWP) models can deliver detailed data for the convective activity, vertical transport, cloud cover and precipitation, including the vertical distribution (Flatoy and Hov 1996). However, up to now neither the cloud models nor the LNOX source models got sufficiently validated. Field data that can be used to evaluate the calculation of these processes are scarce.

Hence, it is very important to develop and test realistic cloud convection models and LNOX source models. The data collected within this project gives an excellent opportunity to characterise the dynamics of the convective cloud systems observed and quantify the importance of the meteorological parameters and processes involved. Even if LNOX production over Europe turns out to be of minor importance, the contributions from convection to upper tropospheric NOx sources and the NOx concentration over Europe remain to be quantified.

Mesoscale cloud models can simulate dynamical and microphysical processes in thunderstorms in great detail. It is possible to study individual storm cells or storm systems like squall lines. The microphysical evolution of precipitation particles as well as updraft or downdraft systems can be represented. The three-dimensional information on the flashes can be used to follow the transport and the dispersion of the lightning produced NOx. Thereby, a comparison with in situ aircraft measurement is feasible.

Ground based measurements of the electromagnetic radiation (VHF-UHF) have successfully been used for remotely sensing the three-dimensional structure of flashes (Richard et al. 1986, Rhodes et al. 1994). Such details have only been studied on a regional scale. Data from two-dimensional location systems became more and more available during the past few years. Lightning location systems (LLS) have been installed in many European countries. Recently such data have been used for establishing regional thunderstorm statistics (Finke and Hauf 1996).

Radar observations of the three-dimensional wind field of thunderstorms in combination with lightning localisation have also been performed (Ray et al. 1987). But airborne NOx measurements were not available for these investigations. By polarimetric radar observations it has not only been possible to discriminate between different types of precipitation particles (e.g. Brandes et al. 1995) but also to observe the build-up and breakdown of the electrical field in thunderstorms due to the presence of electrically aligned particles (Krehbiel et al. 1996).

In order to develop realistic LNOX source models, one has to understand the details of lightning and its dependence on cloud dynamics. The idea that lightning could be a significant source of atmospheric fixed nitrogen was first proposed by J. von Liebig (1827). Much has been learned on the relationships between lightning occurrence, flash structure, cloud dynamics and microphysics in recent years based on measurements with lightning location systems (Holle and Lopez 1993, Richard et al. 1986, Finke and Hauf 1996), Doppler and polarisation radars (Carey and Rutledge 1996, Höller et al. 1994), satellite observations (Goodman and Christian 1993, Kumar et al. 1995), and comparisons to cloud models at various scales (Levy et al. 1996). However, the knowledge is far from complete and not yet adapted to the parameters available in CTM for ozone assessments. Hence, there is a need and there are new possibilities to develop suitable and testable relationships between observable and computable quantities for better LNOX model parameterisations in CTMs.

Questions to be answered

According to the objectives discussed in the previous Chapter the following major questions will be addressed in the EULINOX project:

Compliance with the Work Programme

Contribution to 1.2.1.2. Tropospheric Physics and Chemistry Research Task 3: relative influence of anthropogenic emissions and natural ones, quantification of the emissions of nitrogen species.

2 WORK CONTENT

2.1 Project Concept

The general objective of the work proposed is to use new experimental evidence for a re-examination (comparison with aircraft measurements) of the lightning NOx production and thus to provide an improved knowledge of the distribution of NOx sources on the European scale (see Figure 1 for illustration). The objective will be addressed by field experiments and modelling studies both at regional scale (in Southern Germany) and at the scales of Western Europe (Ireland to Poland, Italy to Norway).

Figure 1 Schematic of general proposal idea: improved distribution of NOx sources on European scale

This goal will be approached by the following steps:

The regional scale measurements are based on the experience from a recently performed national experiment LINOX, from the previous ORLANDO 92 experiment and from the ongoing American upper tropospheric experiments within STERAO, see Chapter 10. The European scale experiment makes use of experience obtained within various European activities such as the CEC funded projects AERONOX, POLINAT, AEROCHEM and others, and the European Co-ordinated activity on active radar systems COST 75.

The experiments includes a series of existing and well-tested experimental tools:

  1. airborne in-situ measurements of NOx concentration fields together with O3 and CO2 (as an air mass tracer) and meteorological parameters (temperature, wind, humidity) either on long-distance flights at mid-tropospheric levels or in the anvils and vicinity of individual thunderstorms, together with
  2. ground based trace species measurements using an instrumented vehicle,
  3. measurements of the lightning activity with 2D (at European sale) and 3D (regional scale) lightning detection systems,
  4. radar observations with radar networks (European scale) or 2 special Doppler- and Doppler-polarisation radars measuring the 3D wind field as well as the distribution of different hydrometeor species (at regional scale), and
  5. satellite observations.

The measurement plan is to obtain radar, lightning, and satellite data for a period of at least 3 months in one summer period. Aircraft measurements will be performed with a research jet aircraft at altitudes up to the tropopause during 10 flights of about 4 hours each during one month with thunderstorm peak activity of the same summer period. The aircraft measurements will be performed regionally near the radar stations to measure the NOx outflow from individual thunderstorms if present, or will be performed along long-distance flights within Europe to measure large-scale NOx concentration fields in connection with various synoptic scale convective events.

The experimental data will be analysed to identify first on regional scales and for individual thunderstorms

  1. the relation between the 2D and 3D lightning structure from the different measuring systems,
  2. the dynamics and hydrometeor distribution within thunderstorms and from radar measurements.

On the European scale, data will be used as obtained with lightning detection and radar networks and from satellite observations. Data from the European scale observations will be used likewise to extrapolate from the more detailed cloud scale results to the less resolved European (and possibly global) scale observables and computed parameters.

According to the experimental scales discussed above the models are subdivided into:

  1. models at cloud scale, which allow to study the dynamics and microphysics of an individual thunderstorm in such detail that the transport of LNOx originating from a well defined single flash can be followed. Thereby, for the regional experiment, a comparison of the aircraft measurements with the modelling results will be possible by testing different LNOx parameterisations for the flash components and their distribution with height,
  2. European, hemispheric and global scale models enable a comparison of the LNOx distribution according to the large scale parameterisations and the airborne measurements. Conventional parameterisations as already implemented in the models, as well as new formulation derived from the EULINOX measurements will be tested. The more detailed cloud scale results (e.g. the flash type or the height-dependence of the LNOx sources) will be checked for their ability to be extended to the larger scale,
  3. trajectory analysis to identify the history of air masses and possibly various sources contributing to measured NOx concentration values,
  1. after having established an optimum LNOx parameterisation these results will be used to evaluate the data from the total experimental period (summer 98; not restricted to days with aircraft data available) with respect to the total LNOx production over Western Europe,
  2. the LNOx will be compared to NOx produced from aircraft and ground sources,
  3. the impact of the LNOx on the ozone concentration as relevant for climate assessment studies will be addressed.

According to the objectives described above the project is organised around four Work Packages (WPs) which, in turn, are subdivided into different activities (A). Table 3 provides an overview of the project's structure including an indication of the partner's involvement in the different Activities. A detailed scientific and technical description is given in the following Paragraphs.

Work Packages and Activities

WP 1 Field experiment
A 1.1 Regional experiment
A 1.2 European-scale observations
A 1.3 Data processing and set-up of an experimental data base
WP 2 Development of new parameterisations
A 2.1 Lightning parameterisation
A 2.2 Parameterisation of NOx production
WP 3 Numerical modelling of case studies, test of parameterisations
A 3.1 Mesoscale modelling, MM5
A 3.2 Mesoscale modelling, CTM
A 3.3 European large scale modelling
A 3.4 Trajectory analysis
WP 4 NOx inventory and environmental implications
A 4.1 Total lightning NOx production, NOx inventory
A 4.2 LNOx compared to aircraft and ground sources
A 4.3 LNOx impact on O3

Table 3 Breakdown of the project into Work Packages (WP) and Activities (A).

2.2 The Field Experiment

2.2.1 General Design

The objective of the field experiment is twofold:

An improved understanding of the processes on the small scale is necessary for a better representation of the effects on the larger scale.

The observations cover the general dynamical development, the microphysical development, the occurrence of lightning flashes in time and space, the inflow and outflow air masses and the content of NOx from measurements at different levels inside and around thunderstorms (for illustration see Figure 2). This data set together with qualified assumptions on the NOx production by individual lightning flashes should allow to quantify the NOx production in thunderstorms.

The EULINOX field experiment will take place during the summer 1998, with an intense measuring phase (IMP) during July 98. The general design of the experiment and the principal measuring systems are illustrated in Figure 3. The polarization Doppler radar (POL), the aircraft facilities, an operation centre, the central VHF interferometer station and a site for launching radiosondes will be available at the main experimental site. The additional Doppler radar (DOP) is located about 40 km from POL. This configuration ensures an optimum Doppler analysis area covering the field site for the aircraft operations.

Figure 2 General design of the field experiment showing the principle measuring systems.

Several years of radar observations as well as extended regional statistics from climatological station have shown that the mean thunderstorms occurrence ranges between 20 and 35 days each year (mainly May through August). Thus 5 to 10 days can be expected for the intense measuring phase (IMP) in July 98. The area coincides with the one selected for an earlier experiment on convective clouds performed in summer 92. The experimental organization will be complemented by the transportable interferometer facilities of partner B which has been extensively used in previous experiments.

Figure 3 Map of the experimental area and flight patterns for the regional experiment.

The objective of the European-scale investigations is to extend the local investigations to larger scales in order to assess the total NOx production from a convective system and, thereby, its global relevance. For approaching these goals the aircraft chemical measurements will be performed on Western European air routes suitable for covering conditions upwind and downwind of the convective system. Figure 4 illustrates possible flight patterns along and across a line of thunderstorms embedded in a cold front over Europe.

Figure 4 Possible flight tracks for the European scale long range NOx measuring flights to be performed at different altitudes. A Meteosat IR (grey scale) is shown (left panel) superimposed by a Swiss-German radar composite (colour scale). Different LLSs used throughout Europe and Doppler radar sites are also indicated (right panel).

About 5 different flights are planned on the European scale. A thunderstorm line across Western Europe (Spain/France) can be identified as a suitable weather condition. Take off will be during daytime hours, as soon as an active thunderstorm line is visible in the satellite images (over Spain/ France) and the movement of this line can be followed over Western Europe for the next 1 or 2 days in the weather forecast charts.

Flight levels between 6 and 10 km (FL 180-330) will be chosen depending on the cloud top height. The flight pattern will be constructed along ordinary air routes towards Spain/ France/ Scandinavia in order to measure the distribution of NOx in the thunderstorm line region as well as to the front and to the rear sides of the line. Vertical profiles from cloud top down to the boundary layer will be measured in these regions. Depending on the synoptic development, the time evolution of the line can be followed over the Western part of Europe during several flights (possibly on successive days).

2.2.2 Localisation and characterisation of lightning flashes

3D Lightning Localisation

A lightning flash, either intra-cloud (IC) or cloud-to-ground (CG), consists of branched discharge channels which propagate mainly inside the cloud. The lightning discharge is initiated by a bi-directional leader which consists of neutral, positive, and negative thermalised discharges propagating in opposite direction in the atmospheric electric field. The physics of positive and negative leaders are similar but the differences in the mean field value in a positive and negative streamer is responsible for a different electrical behaviour (about 4.7 kV/cm for the positive and 7.5 kV/cm for the negative streamer at sea level standard pressure and temperature). The positive leader involves slower processes and the current is commonly an order of magnitude lower (Bondiou and Gallimberti 1994).

A new component in the experiment is the use of a 3D VHF interferometer (ITF3D) for 3D location of lightning flashes. The system measures the VHF radiation emitted from negative discharges in a lightning flash. The ITF3D system provides continuous detection and location of lightning discharges. The VHF receiving frequency is tuned to the protected aviation navigation band 100-118 MHz; the bandwidth is 1 MHz. The equipment consists of two independent remote stations, located at a distance of about 40 km. Data reduction is done off-line. The accuracy is about 500 m in all co-ordinates within a radius of about 30 km. 2D locations are also available within a radius of about 100 km. The time resolution is 23 s, and it is possible to describe a single flash with up to 4000 different locations. The system has been applied, e.g., to determine the relationship between the cloud electrical potential and CG flashes (Mazur et al. 1995).

2D Lightning Localisation

For 3 months in summer, we collect the data from the existing lightning location systems (LLS) in Western Europe. Most of the operating cloud to ground (CG) LLS are either LPATS (Lightning Position and Tracking System) or LLP (Lightning Location and Protection, Inc.) networks. The main characteristics of these systems are:

The system is measuring the electrical component of the radiation emitted by the discharge. The location is based on time-of-arrival of the signals for at least 3 stations. The systems output is time, 2D-location and peak current amplitude of single return strokes. The type of discharge (CG, IC) is inferred from the signal shape and polarity. In the case of a CG flash this roughly corresponds to the location of the flash's impact on the ground. Data from such a system have been analysed by Finke and Hauf (1996). The local experiment will make use of lightning data collected from a 2D LPATS network.

The system is a magnetic direction finder. Time and 2D-location for return strokes are recorded. Additionally, current amplitude for the first return stroke and number of return strokes constituting the flash are provided. The flash location is determined by the intersection of the source directions measured at two stations. Only 2 stations are needed for the flash location. The position accuracy, however, decreases strongly for narrow angles.

The overall detection efficiency for both systems is about 70%. Position accuracy is generally better than 1-2 km and time resolution is 7 ms.

A SAFIR network consists of at least three independent stations equipped with VHF interferometers. 2D horizontal locations of both, CG and IC flashes, are returned.. As for LLP, location is determined by the intersection of the source directions measured by two stations. The time resolution is 100 s and the accuracy is claimed by operators to be 1-2 km in a range of about 150 km.

The Netherlands, France (2 networks) and Belgium operate a SAFIR. Some of the LPATS and LLP-systems are currently updated (e.g. finished in Austria) to the IMPACT system which utilises a combination of the direction finder and time of arrival techniques. IMPACT exhibits a better accuracy than the stand alone LLP or LPATS.

Conventional 2D LLS are currently installed in Spain (LLP), France (LLP, SAFIR), Italy (LPATS), Austria (IMPACT), Switzerland (LPATS), Germany (LPATS), Netherlands (LPATS, SAFIR), Belgium (SAFIR), Norway (LPATS), Sweden (LPATS), UK (VFL mapping system). Thus the area of Western Europe is completely covered by the 'national' networks. However there is no lightning composite available until now. The LLS are operated by different organisations or companies - national weather services or various agencies for security of electrical power networks.

In EULINOX the LLS data will be used for:

  1. Comparison of the data from a conventional 2D intra-cloud LLS with the 3D location system (ITF3D) for an extended observation period. Then comparison of the data from a conventional 2D cloud-to-ground LLS with 2D and 3D intra-cloud LLS can be done. This will give the opportunity to assess the reliability and the quality of the LPATS data and enable the generalisation of the findings with the 3D interferometer to larger areas.
  2. Comparison of 3D and 2D lightning data with radar observations in order to study the interrelations between lightning characteristic and cloud structure.
  3. Merging of the data from the different networks into a lightning composite picture. The aim is to calculate the complete production of NOx by lightning for thunderstorm episodes in Western Europe.

2.2.3 Radar observations

Polarimetric radar measurements provide estimates of different kinds of hydrometeors. From the radar reflectivity Z, the differential reflectivity ZDR and the linear depolarisation ratio LDR it is possible to classify the hydrometeors in a thunderstorm according to their thermodynamical phase, tumbling behaviour or shape into rain, graupel, snow or hail particles. Moreover, the mass concentrations in each category can be assessed from by using empirical relations between the radar parameters and the microphysical fields. These fields are basically identical to those used as prognostic variables in mesoscale cloud models. Thereby, a comparison of measurements and model calculations will be possible in terms of cloud microphysics. The microphysical classification will also enable the comparison of the lightning observations with the cloud microphysical state of the storm.

In addition, Doppler measurements with the same radar (POL) will be performed showing interesting storm developments in a range up to about 100 km from the radar. The same volume will be covered by another Doppler radar (DOP) thus ensuring dual-Doppler analysis. The main deliverables of the Doppler scans will be the 3D fields of radar reflectivity and Doppler (radial) velocity from both radars. Thereby, a dual-Doppler analysis is possible providing the 3D wind field which will be used to study the LNOx transport through the storm system (by numerical modelling) and to compare the resulting concentrations with the aircraft measurements.

It is intended to construct an European radar composite for EULINOX. Several European operational weather radar networks are established today:

Depending on positive negotiations with various national weather services, data from the European radar networks are collected into one data set. It is intended to use all available composites with a space resolution of at least 4 km and a time resolution of half an hour for interesting periods of some days. For NORDRAD e.g. the geographical extent, duration and accumulated values of precipitation will be available but not the vertical extent of cloud systems. In Germany, beside the composite, further information is provided by the echo top product including the maximum reflectivities and their heights. Contacts to all European weather radars are established via COST 75.

2.2.4 Airborne chemical measurements

The objective of these measurements is to characterise the NOx concentration increase due to lightning in the anvil outflow region of electrified mid-latitude thunderclouds.

Airborne measurements will be performed using a jet research aircraft (ceiling 13.7 km, range 3700 km or 5 hours) instrumented with in-situ probes for NO, NO2, CO2, and O3. The CO2, and partly O3 and the meteorological parameters will be used as tracer of air-mass origin. The measuring system has been successfully applied in various recent experimental campaigns. In addition to the trace gas measurements the basic meteorological parameters will be also recorded during the aircraft flights using well tested sensors for temperature, wind, and humidity.

Characteristics of the airborne instruments to be used for EULINOX on board the jet are given in Table 4. All instruments have been extensively used in airborne operations.

Species

Technique

Time Resolution

Detection Limit

NO Chemiluminescence

1s

30 pptv (1s); 5 pptv (60 s)
NO2 Photolytic NO2- Converter + Chemil.

1s

40 pptv (1s); 10 pptv (60s)
O3 UV-Photometry

1s

2 ppbv (1s)
CO2 IR-absorption

1s

1 ppm

Table 4 Characteristic data of the aircraft-borne instruments

The aircraft will be directed from the radar-equipped operation centre to penetrate through the anvil of isolated thunderclouds which are also observed by the additional remote sensing systems of EULINOX. Moreover, the distribution of the trace gases of interest will be measured in the investigation area before and after a storm event as well as in the storm environment. Preliminary flights of this kind have been performed in 1996.

Two basic flight strategies can be distinguished:

About 5 different flights are planned on the European scale. The flights will be directed towards regions with convection along lines from Germany to either Spain, Italy, Poland, Norway or England (Figure 4).

Storm anvils will be penetrated in several levels (see Figure 5).

Figure 5 Schematic of aircraft operations for thunderstorm penetrations. Upper panel show vertical section through typical thunderstorm, the lower panel illustrates a horizontal projection of the flight pattern.

The basic evaluation of the data collected during the EULINOX measuring flights include control of the instrument performance and data acquisition during flight, correction of the instrument signals for the time shift of the individual probe delay and time constant, and the production of combined sets of meteorological and trace gas data. After validation of the data by the aircraft measuring group further data exploitation and interpretation will be performed in co-operation with the EULINOX partners.

It is noted that one cannot, in all cases, uniquely discriminate between NOx from lightning and NOx from surface sources transported upwards within the thunderstorms. Therefore, the aircraft measurements have to be analysed in correlation with model and trajectory analysis. Also, the surface measurements with the vehicle described next will help to discriminate between surface and lightning sources.

2.2.5 Satellite observations

METEOSAT is the European meteorological satellite in geostationary orbit. METEOSAT covers the whole region of interest (Europe) every 30 min. Information from 3 spectral channels (visible, infra-red, water vapour) is available, at least during daytime. The data are available from EUMETSAT in Darmstadt.

METEOSAT data are used to identify convective events, their motions and development, and the cloud top temperature as a measure for cloud altitude. The infra-red data will be used for monitoring purposes during the experimental phase as well as for deriving quantitative information on cloud top temperatures. These will be used for comparison (establishing correlations) with other observational data like radar or lightning events. The data will also be transformed to the grid of the NWP model of Partner C and used to evaluate the convective cloud parameterisation included in the model.

The polar orbiting NOAA satellites carry the AVHRR (Advanced Very High Resolution Radiometer) with 5 spectral channels and a resolution of order kilometres. The data from this sensor are received from 5 overpasses daily by partner A.

The AVHRR data are analysed by means of the 'AVHRR processing scheme over cloud land and ocean' (APOLLO) which identifies cloud filled pixels, and which can be used to relate cloud reflectance to optical properties (optical thickness) and to cloud physical entities like liquid or ice water paths. The data will be used for inter-comparison with METEOSAT, radar, lightning data and model results.

NASA (Marshall Space Flight Center) operates a space-based lightning sensor (OTD - Optical Transient Detector). The OTD instrument detects and locates the lightning discharges that occur within its field-of-view, marks the time of occurrence of the lightning, and measures the radiant energy. The spatial resolution of the instrument is 10 km and the temporal resolution is 2 ms. The OTD detects both IC and CG discharges during day and night conditions with a high detection efficiency. The OTD circles the earth once every 100 minutes at an altitude of 700 km. Using its 128 x 128 pixel photo-diode array and wide field-of-view lens, the OTD instrument is capable of viewing a total area of 1300 km x 1300 km. Given the field-of-view and the orbital trajectory, the OTD can monitor individual storms and storm systems for about 4 minutes, a period long enough to obtain a measure of the lightning flashing rate in these storms. A complete orbit takes 100 minutes. The time interval between subsequent passes for a given location is between 8-14 hours. Data from the OTD are available from NASA for scientific research.

The OTD data will be used for:

The GOME (Global Ozone Monitoring Experiment) instrument on board of ERS-2 is a spectrophotometer that can measure multiple trace gases. NO2 column data can be derived from the GOME spectra and is useful for the detection of high NO2 events. Partner D will analyse the GOME NO2 data in order to detect possible signatures of NOx production by lightning, in particular for the convective systems sampled during EULINOX. Attention will be paid to the retrieval as the presence of clouds can lead to miss-interpretation.

2.2.6 Supporting activities

A brief summary of activities supporting the experiment is listed below:

For planning and analysis of the measurement campaigns trajectory calculations will be performed with data from the ECMWF (European Centre for Medium range Weather Forecasts) model and a Limited Area Model.

In summary, the main physical parameters provided by the different measuring systems are listed in Table 5.

Jet Aircraft time series
Volume Concentrations of NO, NO2, O3 pressure, temperature, relative humidity
wind u, v, w
position
Video
Polarisation Radar POL PPI and RHI scans
radar reflectivity at arbitrary polarisation angles (generally horizontal and vertical) ZHH, ZVV, ZHV, ZVH
Differential phases DP, KDP
Correlation HV
Differential reflectivity ZDR
Linear depolarisation ratio LDR
Doppler velocity vD, spectral width ws
derived properties
water and ice contents for rain, graupel, hail, and snow
VAD wind profiles
Doppler-Radar DOP PPI and RHI scans
radar reflectivity Z, Doppler velocity vD, spectral width ws
Operational Radars radar reflectivity Z
VHF-Interferometer 3D (close range) and 2D (far range) location of channels of flashes
Lightning (LPATS) 2D location of cloud to ground lightning flashes
Radiosoundings vertical profiles of pressure, temperature, humidity
Satellite IR, vis, WV images
O3, NO2
Table 5 The field experiment data catalogue

2.3 Parameterisation of lightning and NOx production

The objective of this work package is to set-up parameterisations of lightning and NOx production. The work will be based on analysis of both, previous observations from precursor field studies (see Chapter 10) and on the field experiments to be performed in WP1.

In the following Activities, data will be collected and analysed in order to provide the required LNOX source information. Activity A2.1 provides information on the distribution of lightning as a function of other observables. Activity A2.2 will provide the LNOX source strength as a function of lightning and other observables.

2.3.1 Parameterisation of lightning

It has been shown by VHF location systems of different type that the length of the leader branches inside the cloud is much larger than the typical length of the return stroke phase of a CG (Proctor et al. 1988, Richard et al.1986, Krehbiel et al. 1996). The 3D locations observed during the EULINOX experiment will be used to identify the actual length of each type of component. It is intended to establish statistics of the time and height dependence of these components for the different types of storms observed during the regional experiment. For example, the observations will be used to determine the IC/CG flash rates, a number which varies between 1 and 20, and which is very badly known (Price and Rind 1992, Carey and Rutledge 1996).

This comparison will be made in order to set up a parameterisation of 3D IC and CG from 2D high resolution long range LLS. The three-dimensional structure of the different flash components as obtained from ITF3D will be compared to the two-dimensional representation from LPATS. This parameterisation will be used to make local scale analysis of storms when 3D IC locations are not available.

Recent radar and lightning observations over Colorado (Carey and Rutledge 1996) suggest strong correlations between IC flash rates and graupel and between CG flash rates and hail concentrations in cloud updrafts. Some circular polarised radar data have provided clear indications of the build-up of electric fields inside storms due to ice crystals which get aligned with the electrostatic fields and the sudden collapse of the field at the time of lightning (Krehbiel et al. 1996). We plan similar investigations using our radar and lightning detection data for mid-Europe.

Moreover, the structure and development of storms deduced from radar will be compared to lightning activity in terms of flash production. This analysis will be done as an attempt to evaluate the lightning activity from the microphysics and dynamics of clouds.

Type and stage of development of storms (as derived from radar measurements and lightning detection) will also be compared to general atmospheric information like the vertical thermodynamical profile of the atmosphere prior to convection. The convectively available potential energy (CAPE) or the bulk Richardson number might be a useful parameters in this context, as they are important for updraft speed or storm type, respectively. This kind of relation is especially useful as these parameters are available from most dynamical models.

Satellite data will be compared to 3D description of storm and lightning structure. Some models parameterise the vertical and horizontal lightning activity as a function of convective activity and cloud top altitudes H. Flash frequencies have been found to increase with H4.9 over land and with H1.7 over oceans. Similar correlations were found for updraft velocities, probably because of different lapse rates in the rather dry atmosphere over lands compared to the more humid atmosphere over the oceans (Price and Rind 1992). Previous investigations used satellite cloud climatologies or model data to determine the cloud height. Here, we will apply METEOSAT and radar data to determine the correlation between lightning, cloud top height, and updraft velocity.

Some of the parameters mentioned previously are available from NWP or mesoscale models. Local shear and stability should be mentioned here. Especially CAPE is expected to correlate well with lightning frequency as it determines the vertical velocities in the updrafts. These, in turn, trigger the microphysical processes and therefore the charging mechanisms. Mesoscale models will be used to model the non-hydrostatic dynamics and the microphysical properties of clouds and relate observed lightning fields to the detailed physics.

Whereas most existing NWP models parameterise lightning as a function of cloud top height, some recent analysis show that lightning, and in particular the IC/CG ratio, is controlled by the altitude difference between cloud top and the 0°C-level within the clouds (depth of the supercooled part of the cloud). Information on this altitude difference can be made available form results of NWP analysis.

2.3.2 Parameterisations of NOx production

The parameterisations of NOx production by lightning discharges is a difficult task (Liaw et al. 1990). Evaluation of this production can be based upon laboratory measurements (Levine et al. 1981), physical modelling (Goldenbaum and Dickerson, 1993, Chameides et al. 1987) and field experiments (Noxon 1976, Frantzblau and Popp 1989, Ridley et al. 1996).

In this activity we will set-up models which relate the LNOx source to the flash rate. We shall use the available results of laboratory measurements as input for this parameterisation. Existing long leader discharge models like those developed by Bondiou and Gallimberti (1994) for the positive polarity and Bacchiega et al (1994) for the negative polarity will be used to extend the laboratory results to the case of an actual lightning flash. We shall investigate the case of (i) the corona phase of a discharge, (ii) the leader phases of the discharge, and (iii) the arc phases of the stroke processes.

The 3D locations observed by VHF observations will be used to identify the actual length of each type of component and to evaluate the production of NOx using the parameterisation set up as described above.

The LNOX is primarily NO, which achieves quickly a photochemical equilibrium with NO2 by reactions with O3 and photolysis of NO2. The NOx can be treated as a passive tracer within the thunderclouds, because conversion to NOy is slow and scavenging and rainout of NOx is small.

The LNOx source strength (in mass of NO per unit time) can be parameterised, e.g., as


(Price et al. 1996), where fCG is the cloud-to-ground flash frequency (flashes/sec), fIC is the intra-cloud flash frequency, ECG is the mean energy per CG flash (J) and EIC the mean energy per IC flash, P is the production rate in terms of molecules NO per unit energy in J, and C is the mass of an NO molecule (14 g / 6.02·1023). The total energy E in a lightning flash (stroke) can be determined as V Q, where V is the breakdown potential and Q the charge deposited in one flash. The charge deposited is the time-integral of the current pulse I(t), which is a function of the potential V, the length of the flash, the electric conductivity of the lightning channel and its radius.

At a global scale, the source strength is often modelled more simply from G = f·PF·C where PF is the production rate in terms of molecules NO per flash (Liaw et al. 1990).

For models, one needs the volume specific source strength (mass of N per unit time and unit volume) as a function of altitude and horizontal co-ordinate and time. However, little is know yet on the effective vertical release height of LNOX. 3D lightning data are needed to locate the sources, and 3D mesoscale model simulations are needed to determine the effective release altitude.

2.4 Numerical modelling

The new parameterisations which have been established by the work described in the previous Chapter have to be tested now in numerical models in order to assess their capability to describe the actual NOx production. Even though a total NOx production could be calculated from the assumptions on lightning frequency, flash length or type etc., it is not clear a priori if these assumptions can explain the observed concentrations. Without considering transport processes in the storms this assessment cannot be made. Besides the pure transport processes, the models also have to consider additional NOx sources like contributions coming from the ground or air traffic, or chemical reactions, if necessary.

According to the different scales investigated experimentally, the models also have to handle the cloud (meso) scale as well as the larger European scale.

2.4.1 Mesoscale modelling

Mesoscale models can simulate thunderstorms in great detail. It is possible to investigate the main updraft and downdraft systems and to look at the microphysical processes responsible for the growth of precipitation particles. Lightning activity and the associated NOx amounts can be introduced. Following its release, NOx can be transported and diffused through the storm, reach the anvil and finally the location corresponding to the aircraft penetration. The key issue is the comparison of simulated and measured NOx values at the location and for the times of the measurements. Sensitivity studies will be performed to demonstrate the key factors influencing NOx production and NOx distribution inside the cloud and its environment. This concerns (i) the lightning location, (ii) the evaluation of parameters needed for the lightning model, (iii) the LNOX source model, (iv) the transport calculation, e.g., the determination of the velocity field (main up- and downdrafts) with radar and/or model.

For this purpose the MM5 model (Anthes and Warner 1978) will be used in a cloud resolving mode. It is a non-hydrostatic model which is well established and which is also able to handle different kind of hydrometeors. As the model will be applied on a relatively short time scale (1-2 hours) chemical reactions are not included here and NOx is looked upon as a passive tracer. The boundary layer concentration of NOx will be adjusted according to the ground emission inventory and the results obtained from the measuring vehicle.

Furthermore, a mesoscale-regional chemical transport model (CTM) including a tropospheric chemistry module with aqueous phase and scavenging effects will be used to investigate the potential transformation and transport of chemical species inside a cloud system. The model will use the forecast mode of MM5 as dynamical and meteorological input over a 36 hours period on a mesoscale range (i.e. approximately 30 km of resolution).

Calculation will be carried out in order to interpret the observed chemical fields. Assessments will be conducted to fit the model results with observations, this will be based on the definition of the lightning NOx sources for all 3-D flashes mapped by the ITF3D. A diagnostic analysis by trajectory simulation using winds from MM5 will also be performed.

2.4.2 European-scale modelling

A 3D numerical limited area model for chemistry and transport (MCT) coupled to a numerical weather prediction (NWP) model will be employed by Partner C. Analysis provided every 6 h from the ECMWF model are used as initial and boundary conditions. The model uses 10 unequally spaced vertical layers. The models can easily be adapted to different domains and grid resolutions depending on the scale of the phenomenon studied. The model contains a lightning parameterisation based on the convective activity computed with the NWP and a comprehensive description of photooxidant gas phase chemistry. For surface emissions, the most recent EMEP emissions inventories are utilised.

A large scale Chemistry-Transport Model (CTM) has been used in the past by Partner D to study the budgets of NOx and ozone in the troposphere. The CTM used is an off-line chemistry-transport model whose meteorological input is taken from the ECMWF weather forecast model. A regional version of the CTM, embedded in the global version (for boundary conditions), and centred upon Europe, will be applied to the EULINOX measurement episodes. The advantage of the regional version of the CTM is that the horizontal and vertical resolutions can be much refined (e.g. 0.5 degree). The CTMK has been used in the past to study the budgets of NOx and ozone in the troposphere.

The chemistry module in the CTM contains the basic mechanisms pertinent to ozone formation and destruction in the free troposphere and lower stratosphere. Presently, NOx surface emissions compiled by IGAC/GEIA and aircraft emissions of NOx from ANCAT are implemented in the model. The model contains a parameterisation of the production of NOx by lightning following Price and Rind (1992). Wet deposition of HNO3 is is modelled using precipitation data and an effective scavenging coefficient.

Partner D's trajectory model has proven to be very useful for the interpretation of in-situ measurements of the atmospheric composition in the neighbourhood of large convective systems and will as such be used during the planning and interpretation of the EULINOX campaigns. As input it uses the three-dimensional wind field from a weather forecast model.

In a first activity the convective cloud/precipitation parameterisation employed in the numerical weather prediction (NWP) model is tested against satellite data or other measured data, under different conditions. Processes like convective initiation from orographic forcing, large scale convergence, or local instability can probably have impact on the model results. This is likely to be a critical topic. If the modelled convection does not resemble the observed one, it will be difficult to find a general parameterisation for NOx emissions. Major factors to be studied include fractional cloud cover, aggregate phase (water/ice), convective intensity (vertical exchange), and rainout efficiency. The model resolution is also critical for the above mentioned effects.

For model purposes the most advantageous is a parameterisation that computes the lightning NOx emissions from data available from the driving NWP model. If the measured data can be used to construct relationships between modelled convection and NOx emissions from lightning, i.e. to develop a dynamic NOx lightning emission inventory, it will mean that reasonable estimates of the emissions can be produced also when no measurements are available, which is the normal situation. This is of especial importance when future impact of anthropogenic emissions are considered and since climate models with time will include simplified ozone chemistry and a NOx lightning source based on computed convection will then be necessary.

After comparison of the current CTM model to the EULINOX measurements, the parameterisation of lightning in the model will be tuned to the measurements performed in EULINOX making use of the simulations with the fine grid models, such as MM5, and data from the lightning detection network, in such a way that the NOx production for the measurement episodes comes in agreement with the observations.

2.5 NOx inventory and environmental implications

After having tested or verified the different model predictions against the observational material (radar, lightning, aircraft observations) for individual case studies an attempt will be made to arrive at total NOx production rate assessed for the complete observational period (summer 1998 or appropriate period where data are available). This will be the final result of the project as announced in the Project Concept description (Chapter 2.1). It will also enable to place the results obtained from the present study in the broader context of global NOx production as discussed in the recent literature.

Based on the model results, computations will be performed with the regional and global scale models to identify the importance of LNOX sources relative to aircraft and ground sources. The sources from aircraft and ground based sources will be taken from existing data bases such as described in Lee et al. (1997).

On the regional scale data on the actual air traffic are also available from the air traffic control radars. Such information can be used not only for the interpretation of the aircraft measurements but also for assessing the NOx input from air traffic for a period covered by the mesoscale cloud simulations (few hours).

The regional and global scale chemical transport models will be used to assess the impact of NOx sources on the ozone concentration in the upper troposphere, as relevant for climate assessment studies. In this part the existing photochemical models and emission data bases are used, as described before. As a result, we will determine the importance the LNOX source compared to other NOx sources with respect to the impact on photochemical ozone production.

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List of Abbreviations

CG Cloud to Ground

CTM Chemical Transport Model

ECMWF European Centre for Medium Range Weather Forecast

HDR Hail Signal (Hail Differential Reflectivity)

IC Intra Cloud

IMP Intense Measuring Phase

ITF3D Three-dimensional Interferometer

IR Infra-Red

KDP Specific Differential Phase

LDR Linear Depolarisation Ratio

LLS Lightning Location System

LNOX Lightning induced NOx

NOx Nitrogen Oxides (NO and NO2)

NWP Numerical Weather Prediction

VHF Very High Frequency

VIS Visible

WV Water Vapour

ZDR Differential Reflectivity

2D, 3D Two-dimensional, Three-dimensional

Tg Teragram = 1012 g

TgN Teragram of Fixed Nitrates in Mass Units of Nitrogen