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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-684
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
28 Aug 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Tropical Continental Downdraft Characteristics: Mesoscale Systems versus Unorganized Convection
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 mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convection are analyzed using multi-instrument data from the GOAmazon campaign. For both MCSs and isolated cells, there are increases in column water vapor (CWV) observed in the two hours leading the convection and an increase in wind speed, decrease in surface moisture and temperature, and increase in relative humidity coincident with system passage. 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 that increases towards lower levels below the freezing level. Both MCSs and unorganized convection show similar mean downdraft magnitudes and probabilities with height. This is consistent with thermodynamic arguments: if θe were approximately conserved following descent, it would imply that a large fraction of the air reaching the surface originates at altitudes in the lowest 2 km, with probability of lower θe dropping exponentially. 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 θe, likewise implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature (Δθe) exhibits a strong relationship because the largest Δθe values are associated with high probability of precipitation. The more physically motivated conditional average of Δθe on precipitation levels off with increasing precipitation rate, bounded by the maximum difference between surface θe and its minimum in the profile aloft. Precipitation values greater than about 10 mm h−1 are associated with high probability of Δθe decreases. 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.

Citation: Schiro, K. A. and Neelin, J. D.: Tropical Continental Downdraft Characteristics: Mesoscale Systems versus Unorganized Convection, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-684, in review, 2017.
Kathleen A. Schiro and J. David Neelin
Kathleen A. Schiro and J. David Neelin
Kathleen A. Schiro and J. David Neelin

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Short summary
Downdraft parameterizations in climate models are poorly constrained, largely due to a lack of robust, statistical relationships describing these processes. This study utilizes a unique set of multi-instrument data from the Green Ocean Amazon campaign (2014–2015, Manacapuru, BR) to study downdraft and cold pool characteristics of mesoscale-organized and unorganized convection. Radar vertical velocity retrievals explore downdraft magnitudes and origin height.
Downdraft parameterizations in climate models are poorly constrained, largely due to a lack of...
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