Regime Dependence of Cloud Water Variability Observed at the ARM Sites

Ahlgrimm, M., European Centre for Medium-Range Weather Forecasts

General Circulation and Single Column Models/Parameterizations

Cloud Life Cycle

Ahlgrimm M and RM Forbes. 2016. "Regime dependence of cloud condensate variability observed at the Atmospheric Radiation Measurement Sites." Quarterly Journal of the Royal Meteorological Society, 142(697), 10.1002/qj.2783.


The figure shows time-height cross sections of observed cloud water amount at the Eastern North Atlantic ARM site. A typical day with shallow cumulus convection is shown in the top panel, while the bottom panel shows a typical day with stratocumulus clouds. It is evident that the cloud water amount is more variable on the cumulus day. The new parameterization tries to captures these differences in a systematic way.


The figure shows time-height cross sections of observed cloud water amount at the Eastern North Atlantic ARM site. A typical day with shallow cumulus convection is shown in the top panel, while the bottom panel shows a typical day with stratocumulus clouds. It is evident that the cloud water amount is more variable on the cumulus day. The new parameterization tries to captures these differences in a systematic way.

Science

Observations from ARM sites across the world have improved our understanding of how the amount of water in clouds varies at small scales. Based on these observations, a new way of describing how cloud water varies with cloud regime has been developed for use in global weather and climate models.

Impact

Physical processes, such as the interaction of cloud with sunlight or the rate at which cloud produces rain, depend on the amount of cloud water available locally at scales smaller than can be represented by current global model grid resolutions. The new parameterization will allow models to calculate more accurate process rates by providing a better representation of how the amount of cloud water varies within the cloud at these subgrid scales.

Summary

A number of different retrieval products for cloud condensate are assessed for five different geographical regions for multiple years and seasons. The retrieval reliability varies with cloud type, but for cloud categories largely unaffected by precipitation, a comparison across sites and longer time periods is possible. These observations confirm previously documented variability behaviour as a function of cloud fraction, but also reveal a systematic regime dependence that is not captured by existing parameterizations. Condensate variability measured as a fractional standard deviation (FSD) in warm boundary-layer clouds is greater in the tropics than in the mid- and high-latitudes for scenes with comparable cloud type and fraction, with the observed FSD varying from 1.2 in the tropics to 0.4 in the Arctic. A parameterization of the FSD of cloud liquid condensate based on the grid box mean total water amount and cloud fraction is formulated and shown to better capture the observed range of FSD values across the different geographical sites and seasons.