Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

Journal Article · 11 February 2018 · Atmospheric Chemistry and Physics (Online)

DOI: https://doi.org/10.5194/acp-18-1997-2018 · OSTI ID:1502121

Schiro, Kathleen A. ^[1]; Neelin, J. David  ^[2]

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1. Univ. of California, Los Angeles, CA (United States); Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA, USA
2. Univ. of California, Los Angeles, CA (United States)

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 (θ_e) 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 3km must undergo substantial mixing, particularly in the case of isolated cells, to match the observed cold pool θ_e, implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature (Δθ_e) exhibits a strong relationship because the most negative Δθ_e values are associated with a high probability of precipitation. The more physically motivated conditional average of Δθ_e on precipitation shows that decreases in θ_e level off with increasing precipitation rate, bounded by the maximum difference between surface θ_e and its minimum in the profile aloft. In conclusion, robustness of these statistics observed across scales and regions suggests their potential use as model diagnostic tools for the improvement of downdraft parameterizations in climate models.

• Research Organization: Univ. of California, Los Angeles, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
• Grant/Contract Number: SC0011074
• OSTI ID: 1502121
• Journal Information: Atmospheric Chemistry and Physics (Online), Journal Name: Atmospheric Chemistry and Physics (Online) Journal Issue: 3 Vol. 18; ISSN 1680-7324
• Publisher: European Geosciences UnionCopyright Statement
• Country of Publication: United States
• Language: English