Transport of Mass and Water Vapor in Cumulus Topped Boundary Layer: A Case-Study from Arm Darwin Facility

Abstract:

Shallow cumulus clouds are intimately tied to the turbulence in the boundary layer and transport momentum and enthalpy upwards from the surface. These clouds have a significant impact on the Earth’s radiation budget as they reflect more incoming solar radiation back to space compared to the underlying surface. They form when water vapor is transported upwards from the surface above the lifting condensation level at which point the water vapor condenses to form cloud droplets. These clouds typically have a life-time of less than an hour after which they evaporate, with the active cumuli venting the boundary layer moisture into the free troposphere.

We use data collected during a 24-hour period at the Atmospheric Radiation Measurement (ARM) observing facility at Darwin, Australia to study the turbulent transport of mass and water vapor associated with shallow cumulus clouds. The instruments at the site include a vertically pointing Doppler cloud radar, Doppler Lidar, ceilometer among others. Three balloon borne radiosondes were also launched during the study period. Data from the cloud radar and Doppler Lidar were combined to retrieve the vertical velocity structure of the entire boundary layer at a high resolution (2 sec; 30 m). Additionally, high resolution (10 s; 37 m) retrievals of water vapor mixing ratio were also performed using the data collected by the collocated Raman Lidar.

We will use the high resolution observations of vertical velocity and water vapor to characterize the second order turbulent transport terms of water vapor and vertical velocity. These turbulent transport terms will then be used together with a parcel model to calculate entrainment rates of individual cloud elements. The contrast in the entrainment rates of forced, active and passive cumuli will be presented together with the moisture and dynamic structure of surrounding environmental air.