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Volume 18, issue 3
Atmos. Chem. Phys., 18, 1997-2010, 2018
https://doi.org/10.5194/acp-18-1997-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5)...

Atmos. Chem. Phys., 18, 1997-2010, 2018
https://doi.org/10.5194/acp-18-1997-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Feb 2018

Research article | 12 Feb 2018

Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

Kathleen A. Schiro and J. David Neelin Kathleen A. Schiro and J. David Neelin
  • Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA, USA

Abstract. 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 (θe) 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 θe, implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature (Δθe) exhibits a strong relationship because the most negative Δθe values are associated with a high probability of precipitation. The more physically motivated conditional average of Δθe on precipitation shows that decreases in θe level off with increasing precipitation rate, bounded by the maximum difference between surface θe and its minimum in the profile aloft. 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.

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Downdraft processes in climate models are poorly constrained, largely due to a lack of robust, statistical relationships describing them. Using multi-instrument data from the GoAmazon2014/5 campaign, downdraft properties of mesoscale-organized and unorganized systems are compared and such statistics are presented. Both vertical velocity retrievals and thermodynamic arguments are consistent in suggesting a spectrum of downdraft mass origin levels throughout the lowest few kilometers.
Downdraft processes in climate models are poorly constrained, largely due to a lack of robust,...
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