Exploring the Details of Convective Downdrafts
Researchers use radar data from the ARM GOAmazon field experiment to develop observational constraints on properties of convective downdrafts.
Deep convective clouds can occur as isolated cells or as strongly organized convective systems that cover large areas known as “mesoscale convective systems”. Understanding and simulating how clouds organize in mesoscale convective systems is important for accurate model simulations of precipitation. Downdrafts, air that moves rapidly downward within a storm system, are a critical aspect of convective organization. Convective downdrafts involve complex interactions between dynamics, thermodynamics, and microphysics across scales. They form cold pools, evaporatively cooled areas of downdraft air that spread horizontally and can initiate convection at their leading edge. In this study, data from the ARM GOAmazon2014/15 campaign is used to examine downdraft and cold pool characteristics for both organized mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convective clouds in order to gain insight into how to better simulate these features in earth system models.
The results show that both organized and unorganized convection show similar mean downdraft magnitudes and probabilities with height. Downdrafts originate throughout the lowest few kilometers of the convective systems. In order to develop constraints for model parameterizations of downdrafts, the researchers calculate bounds on mixing coefficients and also find statistically robust relationships between precipitation from convective systems and changes in surface equivalent potential temperature. The robustness of these statistics over land and ocean, and to averaging in space at scales appropriate to the typical resolution of an earth system model grid cell suggests the potential for use of these statistics as model diagnostic tools and observational constraints for downdraft parameterizations.
Downdrafts and cold pool characteristics for strong mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convection are analyzed using multi-instrument data from the DOE Atmospheric Radiation Measurement (ARM) GoAmazon2014/5 campaign. Increases in column water vapor (CWV) are observed leading convection, with higher CWV preceding MCSs than for isolated cells. For both MCSs and isolated cells, increases in wind speed, decreases in surface moisture and temperature, and increases in relative humidity occur coincidentally with system passages. Composites of vertical velocity data and radar reflectivity from a radar wind profiler show that the downdrafts associated with the sharpest decreases in surface equivalent potential temperature have a probability of occurrence that increases with decreasing height below the freezing level. Both MCSs and unorganized convection show similar mean downdraft magnitudes and probabilities with height. Mixing computations suggest that, on average, air originating at heights greater than 3 km must undergo substantial mixing, particularly in the case of isolated cells, to match the observed cold pool equivalent potential temperature implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature exhibits a strong relationship because the most negative values are associated with a high probability of precipitation. The more physically motivated conditional average of decreases in surface equivalent potential temperature on precipitation shows that decreases in equivalent potential temperature level off with increasing precipitation rate, bounded by the maximum difference between surface equivalent potential and its minimum in the profile aloft.
University of California, Los Angeles
This research was supported in part by the Office of Biological and Environmental Research of the U.S. Department of Energy grant DE-SC0011074, National Science Foundation grant AGS-1505198, National Oceanic and Atmospheric Administration grant NA14OAR4310274, and a Dissertation Year from the University of California, Los Angeles Fellowship (KS).
Schiro, K. A. and Neelin, J. D. “Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection.” Atmos. Chem. Phys. 18(3),1997-2010 (2018). DOI:10.5194/acp-18-1997-2018