Abstract
Air–sea fluxes are greatly enhanced by the winds and vertical exchanges generated by mesoscale convective systems (MCSs). In contrast to global numerical weather prediction models, space-borne scatterometers are able to resolve the small-scale wind variability in and near MCSs at the ocean surface. Downbursts of heavy rain in MCSs produce strong gusts and large divergence and vorticity in surface winds. In this paper, 12.5 km wind fields from the ASCAT-A and ASCAT-B tandem mission, collocated with short time series of Meteosat Second Generation 3 km rain fields, are used to quantify correlations between wind divergence and rain in the Inter-Tropical Convergence Zone (ITCZ) of the Atlantic Ocean. We show that when there is extreme rain, there is extreme convergence/divergence in the vicinity. Probability distributions for wind divergence and rain rates were found to be heavy-tailed: exponential tails for wind divergence (P∼e−αδ with slopes that flatten with increasing rain rate), and power-law tails for rain rates (P∼(R*)−β with a slower and approximately equal decay for the extremes of convergence and divergence). Co-occurring points are tabulated in two-by-two contingency tables from which cross-correlations are calculated in terms of the odds and odds ratio for each time lag in the collocation. The odds ratio for extreme convergence and extreme divergence both have a well-defined peak. The divergence time lag is close to zero, while it is 30 min for the convergence peak, implying that extreme rain generally appears after (lags) extreme convergence. The temporal scale of moist convection is thus determined by the slower updraft process, as expected. A structural analysis was carried out that demonstrates consistency with the known structure of MCSs. This work demonstrates that (tandem) ASCAT winds are well suited for air–sea exchange studies in moist convection.
Highlights
The interaction of the atmosphere with the ocean is strongest in the tropics, where deep convection and rain-induced downbursts drive intense vertical exchanges of momentum, heat, and moisture
These moist convective processes organize upscale through aggregation of individual cloud systems into a mesoscale convective system (MCS)—a large contiguous area of convective and stratiform rain spanning about 100 km or more in at least one horizontal direction that can persist for several hours [1]
MCSs and other forms of organized convection are absent from contemporary global numerical weather prediction (NWP) and global climate models
Summary
The interaction of the atmosphere with the ocean is strongest in the tropics, where deep convection and rain-induced downbursts drive intense vertical exchanges of momentum, heat, and moisture. These moist convective processes organize upscale through aggregation of individual cloud systems (thunderstorms) into a mesoscale convective system (MCS)—a large contiguous area of convective and stratiform rain spanning about 100 km or more in at least one horizontal direction that can persist for several hours [1]. MCSs and other forms of organized convection are absent from contemporary global numerical weather prediction (NWP) and global climate models. Cloud formation (GCMs) due to insufficient numerical resolution and inadequate parameterizations [2,3]. The global observing system does not allow the 4D tracking of these smallscale processes in NWP data assimilation [4].
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