Analysis of the water vapor variability within a convective boundary layer over land using large eddy simulations and IHOP observations

This study presents a comprehensive analysis of the variability of water vapor in a growing convective boundary layer (CBL) over land.

A realistic Large Eddy Simulation (LES) is designed with the Meso-NH model and validated using the data collected during the IHOP_2002 (International H20 Project) field experiment. This unique dataset includes radiosondes, in situ aircraft measurements as well as water vapor mixing ratio measurements made with two airborne differential absorption lidars (one providing bidimensional vertical cross-sections of water vapor mixing ratio, whereas the other provided bidimensional horizontal cross-sections of water vapor in the CBL). The LES is able to reproduce the development of the CBL, including both mean and moment profiles of different variables, as assessed from comparisons with radiosonde and lidar data. The probability density functions of potential temperature, vertical velocity and water vapor mixing ratio derived from the LES are investigated. They show good agreement with aircraft in-situ measurements obtained in the middle of the CBL.

The water vapor fluctuations in the CBL are also investigated. The observed variability exhibits contributions from different scales. The influence of the mesoscale (larger than LES domain size) on the smaller scale variability is studied using LES and observations. The variability is found to be important and to be driven by the dynamics of the CBL. In particular, a negative skewness of the water vapor distribution is observed and simulated within the growing CBL. Both lidar and simulation show that is the result of descending dry downdrafts originating from above the CBL.