Turbulence and density currents below heavily precipitating marine stratocumulus clouds.

 

Submitter:

Ghate, Virendra Prakash — Argonne National Laboratory
Cadeddu, Maria Paola — Argonne National Laboratory

Area of research:

Cloud Processes

Journal Reference:

Ghate V, M Cadeddu, and R Wood. 2020. "Drizzle, Turbulence, and Density Currents Below Post Cold Frontal Open Cellular Marine Stratocumulus Clouds." Journal of Geophysical Research: Atmospheres, 125(19), 10.1029/2019JD031586.

Science

Downdrafts associated with heavy drizzle in stratocumulus clouds can bring colder air within the cloud layer to the surface. We have combined data from multiple instruments to characterize the coupling between rain rates, downdrafts. and changes in surface meteorological fields.

Impact

Our results show that the downdrafts get stronger with increasing rain rates, and are associated with a decrease in surface air temperature, and an increase in surface air pressure and density. The results highlight the drizzle-turbulence-surface coupling in these systems, an important aspect controlling the lifetime of these cloud systems and necessitating their accurate representation in Earth System Models.

Summary

Post cold-frontal open cellular marine stratocumulus clouds are routinely observed at the Atmospheric Radiation Measurement (ARM)’s Eastern North Atlantic (ENA) site. Here we have combined data from multiple instruments to characterize drizzle, turbulence, and precipitation-induced density currents within them. On average, the drizzle shafts were ~18 km wide with cloud base rain rate of 7.78 mm day-1, and cloud base drizzle modal diameter of 393 micrometers. The downdrafts in the lowest 500 m within the drizzle cells strengthened with increasing cloud base rain rate, while the updrafts within drizzle cells did not exhibit this behavior. On average, the surface air density, pressure, and water vapor mixing ratio were higher by 1.88 g m-3, 8.60 Pa, and 0.24 g kg-1, respectively, from their background value during the drizzle shaft. On the other hand, the surface air temperature was lower by 0.43 K during the drizzle shaft than its background value. The coincident measurements highlight the drizzle-turbulence-surface coupling in these cloud systems.