Lidar Observations of Updrafts and Water Vapor Anomalies at ARM SGP on Days with Cumulus Convection

Poster PDF

Description

Cumulus convection is initiated by boundary layer updrafts penetrating to their condensation level. Representing this seemingly simple process in climate models remains a challenge due, in part, to uncertainties in the size and shape distribution of updrafts and their associated moisture transport. To better inform the parameterization of these processes we examine a multi-year observational data set featuring co-located Doppler and Raman lidars in order to develop statistics and a physical understanding of subcloud circulations and water vapor fluxes. Observations are from the ARM southern great plains (SGP) site on days with cumuli of varying depths. In this poster we specifically examine: (1) The mean and varying profiles of water vapor mixing ratio and vertical velocity during cumulus convection. (2) The mean profile of water vapor flux (i.e., the covariance of vertical velocity and water vapor mixing ratio) up to cloud base, and how this flux varies with cloud fraction. (3) The size and shape distribution of water vapor and vertical velocity anomalies associated with individual cumulus clouds. Based on these analyses, we show that the water vapor flux profile increases approximately linearly with height from the surface through the upper convective boundary layer, then decreases to near zero at the CBL top (Zi). Peak water vapor fluxes are ~500 W m-2 and occur near 0.7Zi. Cloudy periods have much larger fluxes than clear periods due to the presence of stronger and more prevalent updrafts. We also show that updrafts are typically narrower than their associated water vapor anomaly, which is consistent with the dynamics of a central updraft flanked by counter rotating vortices that laterally redistribute moist updraft air. This process appears contributes to a broader joint probability distribution function (JPDF) of vertical velocity and water vapor than might otherwise be expected. For example, there are many observations of anomalously moist downdrafts. Other results include the tendency for cloudy updrafts to be stronger