Observation of convection initiation processes with a suite of state‐of‐the‐art research instruments during COPS IOP 8b

Abstract

In the afternoon of 15 July 2007, a thunderstorm was initiated within a line of cumulus clouds which formed parallel to the crest of the Black Forest mountains during the Intensive Observation Period (IOP) 8b of the Convective and Orographicallyinduced Precipitation Study (COPS). This paper extends the analysis of processes that led to convection initiation (CI), i.e. the transition from shallow to deep convection, on this day with the data from several COPS instruments that have not been considered in previous studies. In particular, the boundary-layer structure, lids and the water-vapour field in the pre-convective environment of the event are discussed. For this purpose, we investigated measurements of water-vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and gridded data of weather stations as well as GPS integrated-water-vapour data and satellite imagery. Thermally driven circulation systems formed over both the Black Forest and the Vosges mountain ranges which resulted in local convergence zones. These superimposed with the large-scale convergence in the Black Forest area. In the presence of sufficient moisture and updraught, clouds formed close to the mountain crests. The related latent-heat release allowed larger thermals to be produced, which may have had a positive feedback on stabilizing these convergence zones as a whole. We believe that differences in the moisture field explain why convection remained shallow and sparse over the Vosges mountains because these differences were responsible for differences in convective inhibition (CIN). The stationary location of the convergence zone over the southern Black Forest was probably decisive for CI because it constantly transported sensible and latent heat into the area in which CI took place.

The Convective and Orographically‐induced Precipitation Study (COPS): the scientific strategy, the field phase, and research highlights

“…The synergy of these observations is extensively applied for process studies, e.g. Kiemle et al (2011) and Behrendt et al (2011).…”

“…The frequency of low CIN was higher in the Black Forest than in the Rhine valley, which facilitates convection initiation over the mountain sites (e.g. Kalthoff et al, 2011;Behrendt et al, 2011).…”

“…The strong spatial correlation of the convergence with subsequent cloud formation strongly suggests that lifting along this convergence line was the key trigger mechanism of the observed convective cell on that day. A detailed observational study was performed by Behrendt et al (2011) and further model studies of this fundamental case are presented by Barthlott et al (2010.…”

Initiation of deep convection at marginal instability in an ensemble of mesoscale models: a case‐study from COPS

“…From the measurements, it is not obvious whether the northern and southern convergence lines were connected. For information about the pre-convective atmosphere and the moisture distribution with emphasis on data exploitation of lidars, we refer to Behrendt et al (2011).…”

Influence of the wind profile on the initiation of convection in mountainous terrain

“…The region is well known for convective events during summertime, and several studies have shown the effects of orography on convection (e.g. Hannesen, 1998;Gysi, 1998;Dotzek, 2001;Barthlott et al, 2006;Meissner et al, 2007;Behrendt et al, 2011;Corsmeier et al, 2011;Weckwerth et al, 2011).…”