Comparisons of observations with model analyses showed overestimated specific humidity values in ECMWF's boundary layer (Schäfler et al., 2010, 2011). Most likely these errors are associated to insufficiently parameterized boundary layer processes. The current observational system lacks high-resolution observations of moisture as ground observations are sparse of the oceans and satellite observations have insufficient vertical resolution, especially close to the ground.
As diabatic processes related to the lifting of moist air masses can strongly influence the dynamics of the extratropics an accurate representation of the boundary layer humidity can be crucial. Schäfler and Harnisch (2015) showed that initial condition errors in the low-level moisture supply of a WCB deteriorate the forecast quality. In some unstable flow situations, e.g. baroclinic cyclones, the small-scale errors can grow rapidly and impact the large-scale flow and propagate downstream.
The observation of horizontal moisture transport also provides a link to "atmospheric rivers" (filaments of strong WV transport). These particularly strong exports of tropical moisture can have substantial impact by causing intense rain in the mid-latitudes (e.g. Cordeira et al. 2013)
The collocated deployment of wind and water vapour lidar instruments has shown to be a valuable instrument combination to observe both horizontal (Schäfler et al. 2010) and vertical transport of humidity (Kiemle et al. 2011).
|Moisture Structure in the Boundary layer|
|Planned Observations||HALO and Falcon (dropsondes, DIAL, wind lidar, radar)|
|Schematic flight plan:|