Targeted dropwindsondes in complex terrain

The dropwindsonde (or dropsonde) is a frequently utilized tool in geophysical research and its use over ocean and flat terrain is a reliable and well-established practice. Its use in complex terrain, however, is complicated by signal acquisition challenges that can be directly related to the ground target location, local relief, and line of sight to flight tracks relevant to the observation sought. This note describes a straight-forward technique to calculate the theoretical altitude above ground to which a ground-targeted dropsonde will provide data for a given airborne platform. It is found that this height H_Cq can be calculated from expected airborne platform horizontal velocity U_ag, mean dropwindsonde vertical velocity W_s, the relevant barrier maximum H_B, and the horizontal distance from the target area to the barrier maximum D_B. Here, H_Cq is found to be weakly dependent on release altitude through W_s. An example from the Terrain-induced Rotor Experiment (T-REX) is used to show that for modern aircraft platforms and dropwindsondes signal loss can occur 1-2 km above ground if mitigation is not pursued. Practical mitigation techniques are described for those complex terrain cases where signal propagation problems would create a significant negative scientific impact.