Controls on cloud base mass flux and cloudiness of fair-weather cumuli in a marine environment

 

Authors

Bruce A. Albrecht — University of Miami
Pavlos Kollias — Stony Brook University
Virendra Prakash Ghate — Argonne National Laboratory

Category

Dynamics/Vertical Motion

Description

The parameterization of fair-weather cumuli in numerical models is frequently based on cumulus mass flux approaches. Two critical components of these fluxes are the cloud base mass flux and the vertical distribution of the cumulus mass flux. Direct evaluation of existing mass flux parameterizations has been limited primarily to the use of Large-Eddy Simulations (LES). In this study we apply the evaluation approach used with LES to clouds characterized by a profiling Doppler cloud radar operating as part of the Atmospheric Radiation Program (ARM) Climate Research Facility at Nauru Island (0.5°S, 167°E). The radar measurements provide statistics on the in-cloud vertical air motions in non-precipitating fair-weather cumuli observed from 1999–2000 during a prolonged period of suppressed conditions at the island. These (nighttime only) observations are used to estimate cumulus mass fluxes extending from cloud base to cloud top for 24 consecutive months. Application of a mixed-layer model to the subcloud layer using monthly averaged near surface temperature and moisture measurements and cloud base height estimates from a ceilometer indicates that the subcloud layer is in radiative-convective equilibrium. The associated surface virtual temperature fluxes from these steady conditions provide monthly estimates of the convective velocity scale w*. This velocity scale is used with the boundary layer thermodynamics and static stability at the top of the sub-cloud layer for evaluating different cloud base mass flux parameterizations. The vertical profiles of the observed mass flux are compared with those from LES. The monthly averaged fractional cloudiness used in this study varies from about 10–20% on a seasonal time scale. The factors affecting this observed variability in the cloudiness are examined in light of the Nauru observations.