Impact of land surface heterogeneities on atmospheric water vapor distribution during the IHOP_2002 29 May 2002 case

The International H2O Project (IHOP_2002) took place in the South Great Plains of the U.S. during May-June 2002. One of its goals was the improvement of the understanding of the water vapor distribution in the atmospheric boundary layer (ABL). To achieve this goal, a dense network of instruments has been deployed during this campaign. In particular, on 29 may 2002, the airborne differential absorption lidar LEANDRE 2 allowed to evidence a strong heterogeneity of the ABL depth and its water vapor content, with well marked east-west gradients in the investigated area. The day of 29 May is characterized by a weak synoptic forcing and by the existence of soil humidity gradients due to the occurrence of rainfalls on the day before over one part of the domain.

A control simulation using the non-hydrostatic MM5 model and initialized by the NCEP EDAS operational fields, was performed to identify the sources of water vapor variability. This simulation shows that the soil moisture enhances the impact of land surface heterogeneities on the atmospheric dynamics. In regions of strong soil humidity, thermal circulations are generated and an important correlation exists between the surface heterogeneities (vegetation, land use) at different scales and the atmospheric water vapor variability, while in dryer areas, the role of the surface forcing is more secondary.

A second simulation was run, similar to the control simulation except that a high-resolution land-surface data assimilation system (HRLDAS) was used to initialize the soil temperature and moisture. This study was conducted to examine the sensitivity of the simulation to the surface forcing and to compare the performances of these two simulations to reproduce the observed thermodynamical fields.