Sophie Douet-Bastin

Sophie Douet - Bastin (EOL), worked on two detailed case studies (29 May and 12 June 2002 ) from the International H2O Project (IHOP). IHOP_2002 is a field experiment that took place in the Southern Great Plains of the United States of America during May-June 2002 (Weckwerth et al., 2004). It was dedicated to the study of the four-dimensional water vapor distribution in the atmosphere in order to improve the understanding and prediction of convection and associated rainfall. There were four coordinated and overlapping research components during IHOP 2002: instrumentation research (IR), Atmospheric Boundary Layer (ABL), Convection Initiation (CI) and quantitative precipitation forecast (QPF).

29 May case was classified as an ABL case with strong east-west gradients of water vapor within the ABL. My objective in this study is to understand the role of the soil forcing (vegetation, soil moisture.) in the boundary layer heterogeneity at different time and spatial scales. I'm using both observations and a mesoscale model (MM5) to conduct the study. The observations show that the soil moisture can have an important impact at local scale, enhancing locally and significantly the water vapor in the ABL. The model is unable to reproduce the small scale heterogeneities but it does a great job in reproducing the mesoscale variability of water vapor. From the model (by performing a series of experiments to test the sensitivity of the ABL water vapor mixing ratio heterogeneities to the modification of the soil forcing), it was possible to determine the areas where the soil forcing variability was responsible for the atmospheric variability and the areas where the advection of different air masses was the reason of such variability. My close collaborators in this study are Fei Chen (NCAR/RAL) and Philippe Drobinski (CNRS/SA, France ). The other involved scientists (collecting and providing data.) are Cyrille Flamant and Jacques Pelon (CNRS/SA, France ), Christoph Kiemle (DLR, Germany ), Kevin Manning (NCAR/MMM). I presented this case study at different conference and workshop and was the occasion to meet Peggy LeMone.

12 June 2002 is a CI case. My objective in this study is to investigate the relative importance of the multi-scale processes involved in the initiation of convection. Several pre-convective features are observed: a dryline, an outflow boundary that intersect with the dryline to form a triple point, a mesocyclone, gravity waves and horizontal convective rolls. From observations, it seems that the mesoscale convergence at the dryline and at the triple point does not produce enough instability and wind shear to initiate convection. The horizontal convective rolls that are visible east of the dryline are likely the process that triggers convection. Sophie tries to simulate this case using the WRF model to further document the detailed kinematic and thermodynamical structure of the environment close to the dryline and understand why convection does not initiate at the triple point but a few kilometers to the southeast. The simulation of this case is a struggle that will hopefully have an happy end. I performed a high number of simulations testing different configurations. The use of data assimilation (in particular measurements from dense surface networks) is likely to be necessary to simulate this case by producing a more detailed initial state for the model. My close collaborator in this study is Tammy Weckwerth but this case was also a good opportunity to meet and discuss with lots of NCAR scientists: Morris Weisman, Jimy Dudhia, Wei Wang, Stan Trier, Andrew Crook.

I also met Rita Roberts (NCAR/RAL) and Frederic Fabry (McGill university, Canada ) in the framework of the REFRACTT experiment. Refractivity fields should be a useful tool to better understand convection initiation, so I'm staying in touch with them to use these products in models or as useful observations.

Funding Sources

This research is supported by the National Science Foundation.