Observation of a weak, warm and dry downdraft in the vicinity of a convective storm system, inhibiting new convective initiation

Downdrafts are an essential part of any convective system. In scientific literature, downdrafts forced by hydrometeor evaporation, water loading, and by dynamically-forced pressure perturbations are most discussed. Here we present observations of a weak but broad downdraft outside the storm's updraft, which does not appear to be caused by any of the aforementioned processes. We argue that it develops in response to a buoyancy-induced perturbation of the pressure gradient, similar to the process responsible for upper-level downdrafts that Yuter and Houze (1995) have found in Florida thunderstorms. This downdraft had an inhibiting effect on the development of new convective cells in the vicinity of the storm system that caused it.

We present observations of a convective storm system in the northern Black Forest in southwest Germany made on 12 July 2006 as part of the field campaign PRINCE (PRediction, Identification and trackiNg of Convective cElls). Airborne measurements reveal a local maximum in temperature (1.5 K warmer) and a minimum in mixing ratio (4 g kg-1 drier) under the storm's anvil cloud. Given that the moisture monotonously decreased with increasing altitude, this is a sign for subsidence. The subsidence is confirmed by data from radiosondes, released by mobile teams in the vicinity of the storm. The aircraft measurements indicate that the subsiding air had a lower density than that at the same altitude further away from the storm system, which rules out negative thermal buoyancy as a forcing mechanism. Moreover, the weakly-sheared atmosphere renders dynamically-induced pressure contributions negligible as well.

Doppler lidar measurements on a nearby low mountain top indicate that the warm and dry downdraft air spread out above the boundary layer. In the remaining 6 hours of insolation after dissipation of the storm system and its anvil clouds, a circular zone with a radius of approximately 40 km around its previous location remains void of any new convective storm development. This is in contrast to locations outside this zone, where new storms developed. Radiosonde data suggests that the boundary layer air remains effectively capped near the original storm system, due to both a reduction of low-level moisture compared to pre-convective conditions, and higher temperatures directly above the boundary layer.

References:

Yuter, S.E. and R. A. Houze Jr.: Three-dimensional kinematic and microphysical evolution of Florida cumulonimbus. Part II: frequency distributions of vertical velocity, reflectivity and differential reflectivity, Monthly Weather Review, 123, 1941—1963