The impact of convergence zones on the Initiation of Deep Convection

1. The impact of convergence zones on the Initiation of Deep Convection B. Adler*, N. Kalthoff, Ch. Barthlott, U. Corsmeier, S. Khodayar, Ch. Kottmeier, K. Träumner, S. Mobbs, S. Crewell, V. Smith * Institut für Meteorologie und Klimaforschung, IMK Karlsruhe Institute of Technology, KIT

2. 1230 UTC 1500 UTC 1430 UTC • 1115 UTC: first cumulus clouds over Black Forest • 1230 UTC: cumulus clouds over Vosges; two weak north-south oriented cloud bands east of Black Forest crest • 1500 UTC: deep convection southeast of Freudenstadt with rain rate of roughly • 21-35 mm h-1 • Question: Why did deep convection develop and why only over the Black Forest area? • 1400 UTC: both cloud bands intensified • 1430 UTC: isolated deep convective cells with rain rates of roughly 5-8 mm h-1 1. Evolution of deep convection IOP 8b (15 July 2007)

3. 2. Synoptic conditions hPa h-1 • COPS area was in the transition zone between an eastern Atlantic trough and an eastern European ridge • Surface low in the west (south-westerly wind) and surface high in the east (easterly wind), convergence in between • No significant upper level lifting in the COPS area 1200 UTC 1200 UTC 1800 UTC

4. Heselbach, 1100 UTC Achern, 1100 UTC UTC CAPE (J kg-1) CIN (J kg-1) LFC (m asl) CCL (m asl) FZK 1100 408 147 3229 2797 Burnhaupt 1100 483 253 3676 2438 Achern 1059 90 327 3577 2579 Heselbach 1129 361 78 3260 3022 2. Convective conditions • Achern (Rhine valley): low CAPE, high CIN • Heselbach (Black Forest): moderate CAPE, lower CIN • Conditions not too favourable for deep convection • If trigger mechanism present, convection most likely in Black Forest area

5. western slope eastern slope 3.1 PBL processes relevant for triggering of convection • 0700 to 0800 UTC: Onset of up-slope winds resulting in convergence over the mountain crest

6. 3.2 Evolution of a convergence zone between the surface low and high

7. 3.2 Evolution of a convergence zone between the surface low and high • Meistratzheim • change from SE to SW wind finished at 1000 UTC in lowest 2 km

8. 3.2 Evolution of a convergence zone between the surface low and high • Achern • change from SE to NW wind finished at 1030 UTC

9. 3.2 Evolution of a convergence zone between the surface low and high • Hornisgrinde: • 1100 UTC change from SE to W wind • convergence zone SUPERIMPOSES on the stationary slope-wind generated one • updrafts of 2 m s-1 extend up to 3100 m asl penetrate capping inversion and nearly reach LFC • 15 min after the convergence zone passed the Black Forest crest first clouds formed

10. 3.2 Evolution of a convergence zone between the surface low and high • Barongartenhütte • change from SE to NW wind finished at 1330 UTC

11. 3.2 Evolution of a convergence zone between the surface low and high • Heselbach • change in wind direction from SE to SW in lowest 2 km finished at 1400 UTC • base of deep convection at 2.5 km agl (3 km asl) • at the same time increase of humidity

12. 3.2 Evolution of a convergence zone between the surface low and high • Igelsberg • change from SE to SW wind finished at 1400 UTC

13. 3.2 Evolution of a convergence zone between the surface low and high • near Barongartenhütte vertical updraftsup to 3.5 m s-1 (DO 128) • over Murg valley updrafts up to 5 m s-1 (BA 146; flight level: 1900 m asl) DO 128: 1217 – 1328 UTC, flight level: 1500 m asl

14. 15 July 2007 deep convection developed over the Black Forest area Conditions were not too favourable for convection, because high CIN and moderate CAPE no significant upper air lifting If a trigger mechanism was present, the Black Forest area would be slightly favourable to the evolution of deep convection The reason for development of convection was an optimal superposition of different convergence zones over the Black Forest resulting in vertical updrafts of about 2 m s-1 which penetrated the lids and triggered convection East of the Black Forest crest the convergence zone intensified, strong updrafts of up to 5 m s-1 below convective cells were measured Summary

15. Thanks for your attention!

16. 0748-0900 UTC: Dornier 128 flight at level 1500 m asl SW wind in the whole COPS domain • along Black Forest crest vertical velocity up to 0.5 m s-1 • 1217-1328 UTC: SW wind in most parts