Lagrangian Diagnostics of Tropical Deep Convection and Its Effect upon Upper-Tropospheric Humidity

Brian J Soden,Ákos Horváth

doi:10.1175/2007jcli1786.1

Brian J Soden, Ákos Horváth

Open Access

https://doi.org/10.1175/2007jcli1786.1

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Abstract

AbstractThis study combines geostationary water vapor imagery with optical cloud property retrievals and microwave sea surface observations in order to investigate, in a Lagrangian framework, (i) the importance of cirrus anvil sublimation on tropical upper-tropospheric humidity and (ii) the sea surface temperature dependence of deep convective development. Although an Eulerian analysis shows a strong spatial correlation of ∼0.8 between monthly mean cirrus ice water path and upper-tropospheric humidity, the Lagrangian analysis indicates no causal link between these quantities. The maximum upper-tropospheric humidity occurs ∼5 h after peak convection, closely synchronized with the maximum cirrus ice water path, and lagging behind it by no more than 1.0 h. Considering that the characteristic e-folding decay time of cirrus ice water is determined to be ∼4 h, this short time lag does not allow for significant sublimative moistening. Furthermore, a tendency analysis reveals that cirrus decay and growth, in terms of both cloud cover and integrated ice content, is accompanied by the drying and moistening of the upper troposphere, respectively, a result opposite that expected if cirrus ice were a primary water vapor source. In addition, it is found that an ∼2°C rise in sea surface temperature results in a measurable increase in the frequency, spatial extent, and water content of deep convective cores. The larger storms over warmer oceans are also associated with slightly larger anvils than their counterparts over colder oceans; however, anvil area per unit cumulus area, that is, cirrus detrainment efficiency, decreases as SST increases.

Highlights

Water vapor is arguably the most important greenhouse gas; a thorough understanding of its global distribution is essential for making credible climate change projections
Note that cirrus anvils (CAs) ice water path (IWP) and optical thicknesses over land are at the higher end of the distribution and have a narrower range compared to oceanic values. (The same is true for deep convection (DC) ice content as well.) This finding is consistent with Zipser et al (2006), who observed the strongest storms occurring preferentially over land
When upper-tropospheric humidity (UTH) is plotted against ice water path (Fig. 2a), the correlation is significantly lower over land (Rland ϭ 0.56) than over ocean (Rsea ϭ 0.75); when it is plotted against cloud optical thickness (Fig. 2b), the discrepancy between land and ocean correlations disappears

Summary

Introduction

Water vapor is arguably the most important greenhouse gas; a thorough understanding of its global distribution is essential for making credible climate change projections. The dependence of the upper-tropospheric vapor budget on tropical deep convection (DC) has been recognized at least since the study of Gray et al (1975). Soden and Fu (1995) have found a strong positive relationship between the frequency of tropical deep convection and upper-tropospheric humidity (UTH) over a wide range. The existence of such apparently strong spatial relationships has led some investigators to conclude that the evaporation–sublimation of cloud condensate, of cirrus ice, plays a significant role in moistening the tropical upper troposphere (Soden 1998; Su et al 2006). Other diagnostic studies have argued that the amount of cirrus ice is too small for substantial sublimative moistening (Luo and Rossow 2004; John and Soden 2006); the upper-tropospheric vapor budget is, instead, more likely to be influenced by dynamical mechanisms responsible for the formation and maintenance of cirrus anvils (CAs; Soden 2004)

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