Shelby Frisch___, and Paquita Zuidema_ _ NOAA/Environmental
Technology Laboratory and Colorado State University _ NOAA/Environmental
Technology Laboratory and University of Colorado
There are two components of
the vertical flux of liquid water in stratus clouds, one component is due to
the mean fall velocity of the cloud droplets, also referred to as gravitational
settling, while the other is due to any turbulent motion which can redistribute
the cloud droplets. Previous work has shown these two terms can be comparable
in magnitude, even for non-drizzling cloudy portions (Nicholls, 1984). Nicholls
(1984) also found that calculations of the implied cloud-top entrainment flux
were sensitive to the liquid water flux term, because the liquid water flux
offsets the apparent upward moisture flux. A correct treatment of the total
water flux and entrainment flux, both in modeling and data analysis, must
therefore also include the droplet settling term. Since millimeter cloud radars
can detect these droplets, cloud microphysical retrievals can be used to
estimate the stratus cloud droplet liquid water flux. Earlier retrievals using
cloud radars have been used to retrieve the effective radius from the
reflectivity measurements. By using additional information about cloud droplet
fall velocities in the Stokes range, we can estimate the liquid water flux in
the non-drizzling portion of stratus clouds. Furthermore, by taking the
divergence of the gravitational settling term, we can calculate the associated
latent heating and cooling rates. These can be compared to the radiative
heating rates calculated
from similarly-retrieved
liquid water contents and effective radii, as one measure of the relative
impact of gravitational settling upon the total diabatic heating. The knowledge
of both diabatic heating terms also provides useful constraints on the modeling
of mixed-layer clouds. In this initial investigation we focus upon a
nondrizzling stratus day observed during the stratocumulus leg of the East
Pacific Investigation of Climate (EPIC), held during October 2001 in the
southeastern Pacific region. This two-week ship-based stratocumulus study,
discussed further in Bretherton et al. (2003), included in its goals an
increased understanding of the heat and water fluxes for this region, and the
measurement of the vertical structure of the atmospheric boundary layer.
Bretherton, C. S., T. Uttal,
C. Fairall, S. E. Yuter, R. A. Weller, D. Baumgardner, K. Comstock, and R.
Wood, 2003: The EPIC stratocumulus study. Bull. Amer. Meteor. Soc. submitted;
available through http://www.atmos.washington.edu/ breth/.
Nicholls, S., 1984: The
dynamics of stratocumuls: aircraft observations
and comparisons with a
mixed-layer model. Quart. J. Roy. Meteor. Soc., 110, 783–820.