Sugar, Gravel, Fish, and Flowers: Dependence of Mesoscale Patterns of Trade-Wind Clouds on Environmental Conditions.

Sandrine Bony,Bjorn Stevens,Hauke Schulz,Jessica Vial

doi:10.1029/2019gl085988

Abstract

Trade‐wind clouds exhibit a large diversity of spatial organizations at the mesoscale. Over the tropical western Atlantic, a recent study has visually identified four prominent mesoscale patterns of shallow convection, referred to as flowers, fish, gravel, and sugar. We show that these four patterns can be identified objectively from satellite observations by analyzing the spatial distribution of infrared brightness temperatures. By applying this analysis to 19 years of data, we examine relationships between cloud patterns and large‐scale environmental conditions. This investigation reveals that on daily and interannual timescales, the near‐surface wind speed and the strength of the lower‐tropospheric stability discriminate the occurrence of the different organization patterns. These results, combined with the tight relationship between cloud patterns, low‐level cloud amount, and cloud‐radiative effects, suggest that the mesoscale organization of shallow clouds might change under global warming. The role of shallow convective organization in determining low‐cloud feedback should thus be investigated.

Highlights

Shallow cumuli are ubiquitous over the world ocean, and their sensitivity to a change in environmental conditions has the potential to greatly influence Earth's radiation balance and climate sensitivity
Stevens et al (2019) showed, based on a visual and subjective classification, that the tropical western Atlantic during boreal winter is associated with four prominent mesoscale patterns of shallow convection
The present study shows that these patterns can be objectively identified based on the size and degree of clustering of segmented cloud objects as identified from infrared brightness temperatures

Summary

Introduction

Shallow cumuli are ubiquitous over the world ocean, and their sensitivity to a change in environmental conditions has the potential to greatly influence Earth's radiation balance and climate sensitivity. It remains an open question as to whether the mesoscale organization of clouds, which is left out of most parameterizations and the many large-eddy simulations over small domains, influences how shallow convective clouds respond to warming To help answer this question, here we first explore whether the observed organization of shallow convection can be linked to variability in large-scale environmental conditions and whether the different patterns of organization imprint themselves differently on the radiation budget. This region is associated with sea surface temperatures (SSTs) of 26 to 28 ◦ C, a moderate large-scale subsidence in the free troposphere (about 25 to 30 hPa d−1) and a predominance of shallow clouds (Stevens et al, 2016) In this region, the prominent patterns of organization do not correspond to the classical and well-characterized open and closed patterns of mesoscale cellular convection found over colder oceans (McCoy et al, 2017; Wood & Hartmann, 2006).

Classification of Mesoscale Organization Patterns

Dependence of Patterns on the Large-Scale Environment

Summary and Discussion