Characterizing the Atmospheric Mixed Layer During the North American Monsoon at Walnut Gulch Experimental Watershed

Abstract

The Haar wavelet covariance transform is applied to ground-based LIDAR backscatter profiles to provide high resolution local time series of planetary boundary layer (PBL) height. This method is used to produce a PBL height dataset at the USDA ARS Walnut Gulch Experimental Watershed (WGEW) in Tombstone, AZ over the 2017 summer North American monsoon season. The WGEW site has a long term record of hydrological and meteorological measurements in an arid environment with complex terrain and variable vegetation, which provides an interesting case to study of temporal and spatial variations of the convective PBL in such an environment, and their relationship to the development of summer air mass thunderstorms. PBL height is correlated with heat and moisture fluxes in the area, as well as precipitation rates and soil moisture, to help characterize the effects of recirculation of moisture over the course of the monsoon season on rainfall in the region. We find that the boundary layer tends toward lower heights of and later daily development over the course of the season, dropping around of 540 m on average by September. An analysis of dry-down times following active monsoon periods reveals a change of average PBL h of 220 m/day for the 3 days following significant rainfall, slowing to 80 m/day for the remaining 2 days before the lower atmosphere and surface dry out. Results are compared with a single slab boundary layer model based on surface sensible heat flux, and a strong linear correlation is found, especially earlier in the season, with an R2 of 0.992 for the month of June and an average R2 of 0.95 for the entire summer.