Water vapor turbulence profiles in stationary continental convective boundary layers

Poster PDF

Description

The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility’s Raman lidar at the Southern Great Plains (SGP) site in north-central Oklahoma has collected water vapor profile data more than 90% of the time since October 2004. Three hundred cases were identified where the convective boundary layer was stationary and well-mixed for a 2-hour period, and the variance and skewness profiles were derived from the 10-second, 75-meter resolution water vapor data. These cases span the entire calendar year and demonstrate that the water vapor variance at the mixed layer top (Zi) changes seasonally and is mainly related to the gradient of the water vapor across the entrainment zone. However, the water vapor variance at Zi shows only weak correlations (r < 0.3) with sensible heat flux, Deardorff convective velocity scale, and turbulence kinetic energy measured at the surface. The mean water vapor skewness is asymmetric around Zi, with the mean water vapor skewness being most negative at 0.85 Zi, zero at approximately 0.95 Zi, and positive above Zi; the negative skewness below Zi is attributed to the narrow dry downdrafts penetrating into the mixed layer while the positive skewness above Zi is due to the moisture plumes that ascend above the mean mixed layer top. The spread in the water vapor skewness is smallest between 0.95 Zi and Zi. The water vapor skewness at altitudes between 0.6 Zi and 1.2 Zi is correlated with the magnitude of the water vapor variance at Zi, with increasingly negative values of skewness observed deeper in the boundary layer as the variance at Zi increases, suggesting that in cases with larger variance at Zi there is deeper penetration of the warm, dry free tropospheric air into the boundary layer.