Net Cloud Thinning, Low-Level Cloud Diminishment, and Hadley Circulation Weakening of Precipitating Clouds with Tropical West Pacific SST Using MISR and Other Satellite and Reanalysis Data

Terence L Kubar,Jonathan H Jiang

doi:10.3390/rs11101250

Abstract

Daily gridded Multi-Angle Imaging Spectroradiometer (MISR) satellite data are used in conjunction with CERES, TRMM, and ERA-Interim reanalysis data to investigate horizontal and vertical high cloud structure, top-of-atmosphere (TOA) net cloud forcing and albedo, and dynamics relationships against local SST and precipitation as a function of the mean Tropical West Pacific (TWP; 120°E to 155°W; 30°S–30°N) SST. As the TWP warms, the SST mode (~29.5 °C) is constant, but the area of the mode grows, indicating increased kurtosis of SSTs and decreased SST gradients overall. This is associated with weaker low-level convergence and mid-tropospheric ascent (ω500) over the highest SSTs as the TWP warms, but also a broader area of weak ascent away from the deepest convection, albeit stronger when compared to when the mean TWP is cooler. These associated dynamics changes are collocated with less anvil and thick cloud cover over the highest SSTs and similar thin cold cloud fraction when the TWP is warmer, but broadly more anvil and cirrus clouds over lower local SSTs (SST < 27 °C). For all TWP SST quintiles, anvil cloud fraction, defined as clouds with tops > 9 km and TOA albedos between 0.3–0.6, is closely associated with rain rate, making it an excellent proxy for precipitation; but for a given heavier rain rate, cirrus clouds are more abundant with increasing domain-mean TWP SST. Clouds locally over SSTs between 29–30 °C have a much less negative net cloud forcing, up to 25 W m−2 greater, when the TWP is warm versus cool. When the local rain rate increases, while the net cloud fraction with tops < 9 km decreases, mid-level clouds (4 km < Ztop < 9 km) modestly increase. In contrast, combined low-level and mid-level clouds decrease as the domain-wide SST increases (−10% deg−1). More cirrus clouds for heavily precipitating systems exert a stronger positive TOA effect when the TWP is warmer, and anvil clouds over a higher TWP SST are less reflective and have a weaker cooling effect. For all precipitating systems, total high cloud cover increases modestly with higher TWP SST quintiles, and anvil + cirrus clouds are more expansive, suggesting more detrainment when TWP SSTs are higher. Total-domain anvil cloud fraction scales mostly with domain-mean ω500, but cirrus clouds mostly increase with domain-mean SST, invoking an explanation other than circulation. The overall thinning and greater top-heaviness of clouds over the TWP with warming are possible TWP positive feedbacks not previously identified.

Highlights

The nature of tropical convection—including its vertical distribution, horizontal extent, and optical properties, and associated top-of-atmosphere (TOA) radiative effects—is associated with Sea-surface temperature (SST), SST gradients, and regional and large-scale circulation and dynamics
Using primary satellite data from Multi-Angle Imaging Spectroradiometer (MISR), with complementary data from CERES, Tropical Rainfall Measurement Mission (TRMM), and ERA-Interim reanalysis data, this study has focused on how changes of the domain-mean SST of the tropical western Pacific (TWP) influence the redistribution of horizontal SST within the domain and local cloud relationships with local SST and rain rate
Going beyond previous investigations which have quantified total Tropical West Pacific (TWP) high cloud relationships with domain-mean TWP SST, some of which have identified an Iris Effect whereby high cloud amount decreases as the TWP warms, this study has explored sub-TWP relationships of clouds, rain rates, and SST

Summary

Introduction

The nature of tropical convection—including its vertical distribution, horizontal extent, and optical properties, and associated top-of-atmosphere (TOA) radiative effects—is associated with SST, SST gradients, and regional and large-scale circulation and dynamics. SST gradients were posited by Lindzen and Nigam [4] to determine airflow and low-level convergence and convection; moisture boundary layer convergence was later confirmed to be a cause of convection by Back and Bretherton [5]. A top-heavy profile, such as over the western tropical Pacific, in which ascent peaks near 400 hPa, in conjunction with higher upper-tropospheric humidity, may help sustain more cirrus clouds than in regions with bottom-heavy ω-profiles [6], e.g., the eastern Pacific. The vertical and horizontal cloud structure differences over the western tropical Pacific contribute to clouds having a significantly smaller net TOA cooling effect versus cloud profiles in the eastern Pacific [2]

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Conclusion