4.2 An analysis of the pre-storm environment of intense convective systems in West Africa

Monday, 17 August 2009: 4:15 PM
The Canyons (Sheraton Salt Lake City Hotel)
Stephen D. Nicholls, Rutgers University, New Brunswick, NJ; and K. I. Mohr

Previous studies offer only general descriptions of the thermodynamical and dynamical environment of strong, well-organized convective systems in West Africa. In this study, we extract and analyze the pre-storm environments of a large sample of intense and non-intense convective systems that occurred in 2003. These convective systems were identified by contiguous multi-pixel 85 GHz brightness temperature depressions in the 1B11 TRMM product. Convective systems with a minimum brightness temperature less than 135K (lowest 10%) were classified as “intense”. The intense convective systems were sub-divided into those near high terrain (terrain-intense) and those that were not (far-intense). The terrain-intense classification was awarded to 0.25 deg or within 6 hours of the 500 m terrain contour

We matched cases either to in-situ soundings at 8 sites around the region or to reconstructed soundings from ECMWF operational analysis. Intense cases were associated with significantly warmer surface equivalent potential temperature (theta-e), a deeper and more unstable convectively unstable layer, and a higher and faster African easterly jet (AEJ). Low-level shear was twice as large for intense cases than for non-intense cases with far-intense cases having the highest low-level shear. Most of the intense cases were within 2 deg of the AEJ axis, accounting for the increased low-layer shear. Although there was no significant difference between the boundary layer mean theta-e between intense and non-intense cases, the boundary layer was several degrees warmer and deeper for cases far from high terrain than near it.

From U- and V-wind Hovmöller plots, southerly 925-hPa V-winds were well correlated to the development of intense convection in areas west of 10E due to advection of high theta-e air northward behind the monsoon trough. Cases east of 10E were correlated with low-level trajectories of high theta-e air from the Central African rainforests. On each map of 9-day mean 925-hPa theta-e, an axis of intense cases appeared, forming orthogonal to the strongest 925-hPa theta-e gradient and wind direction. Over 98% of cases occurred where the 925 hPa theta-e was greater than 334K.

We assessed the influence of African easterly wave (AEW) on the pre-storm environment using 4 criteria: 1-AEW Trough Length, ½-AEW Trough Length, 200 km, and 500 km from case to trough. Under the most generous criterion (500 km), 41% of all cases and 20% of intense cases could be directly associated with an AEW.

From the MODIS Deep Blue aerosol optical depth product, we inferred the relationship between case location and dust density. All cases decreased with increasing aerosol optical depth, with 85% of cases occurring in areas with little or no dust. Intense cases that did occur in moderately dusty environments were more likely to have higher low-level shear and boundary layer mean theta-e. The development of intense convective systems is most likely in environments with little dust and an active monsoon in which the 925-hPa theta-e is greater than 334K and a strong regional 925-hPa theta-e gradient develops near the jet axis.

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