Updraft and Downdraft Events in the Atmospheric Boundary Layer Over the Equatorial Pacific Ocean

Gary K. Greenhut Office of Weather Research and Modification, Environmental Research Laboratories/N0AA, Boulder, CO 80303

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Siri Jodha Singh Khalsa Cooperative Institute for Research in Environmental Sciences, University of Colorado/N0AA, Boulder 80309

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Abstract

Conditional sampling has been applied to aircraft turbulence measurements in order to study updrafts and downdrafts over the central equatorial Pacific Ocean. Average event size, number density and proportion of time series occupied are obtained for the drafts, along with conditional averages of horizontal momentum, moisture and virtual temperature and the draft contributions to the total fluxes. It is found that updrafts are usually cool/moist and warm/moist while downdrafts are most often warm/dry. Convective mass flux parameterizations of the sensible and latent heat flux are tested. Results consistent with previous workers are obtained when the flux production is mainly by updrafts. However, when downdrafts dominate, they must be taken into account explicitly in order for the parameterization to be accurate.

Abstract

Conditional sampling has been applied to aircraft turbulence measurements in order to study updrafts and downdrafts over the central equatorial Pacific Ocean. Average event size, number density and proportion of time series occupied are obtained for the drafts, along with conditional averages of horizontal momentum, moisture and virtual temperature and the draft contributions to the total fluxes. It is found that updrafts are usually cool/moist and warm/moist while downdrafts are most often warm/dry. Convective mass flux parameterizations of the sensible and latent heat flux are tested. Results consistent with previous workers are obtained when the flux production is mainly by updrafts. However, when downdrafts dominate, they must be taken into account explicitly in order for the parameterization to be accurate.

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