Factors Controlling Boundary Layer Cloud Fraction and Mass Flux at the ARM Nauru Site

Bruce Albrecht University of Miami
Virendra Ghate Argonne National Laboratory
Pavlos Kollias Stony Brook University

Category: Cloud Properties

Working Group: Cloud Life Cycle

The climatology of nighttime boundary layer clouds and surface turbulent fluxes estimated at the ARM Nauru site from January 1999–2001 has been studied within the context of a simple atmospheric mixed- layer representation of the sub-cloud layer applied to monthly averages. The period for this analysis is chosen since the nighttime winds observed at Nauru are very steady from the east during the entire observing period and very little precipitation was observed. It is shown that the mixed-layer formulation provides an excellent framework for representing the close coupling among sea surface temperature, wind speeds, atmospheric mixed-layer properties, and the sub-cloud layer radiative cooling. The monthly averaged low-cloud fractional cloudiness estimated from the ceilometer varies from 0.09–0.21 during this period. The fractional cloudiness is negatively correlated (-0.66) with the convective velocity scale (the Deardorff velocity) estimated from the surface fluxes and the sub-cloud layer depth. There is a positive correlation (0.60) between the monthly fractional cloudiness and the lower tropospheric stability (difference between the 700 mb and 1000 mb potential temperature) estimated using the soundings from the site; but there is a stronger negative correlation (-0.86) between the fractional cloudiness and the SST (upstream from Nauru). The cloud base cumulus mass fluxes for this time period have been calculated using the MMCR Doppler observations. The relationships between these fluxes and the fractional cloudiness, the convective velocity scale, and static stability at the top of the sub-cloud layer are being explored.

This poster will be displayed at ASR Science Team Meeting.