Are low-latitude cloud characteristics changing with Hadley cell expansion?

Voiceover

Presenter(s)

Logan Longacre

Abstract or Description

Whether due to natural or anthropogenic forcing, recent Hadley cell (HC) expansion from observations and reanalysis is on the order of 0.1° latitude decade^-1. The most obvious impact of a widening HC includes the poleward migration of the subtropical dry zones although the response of clouds and precipitation across the global tropics remains less well understood. Of particular interest are the impacts on various cloud characteristics and low-latitude precipitation. This study will examine changes in tropical and subtropical cloud types and quantify differences in their frequency, geometric thickness, and precipitation rates, among other relevant variables. We speculate that the depth of clouds, particularly for deep convection, will become deeper or shallower in some areas associated with the tropical expansion. On a broader scale, we intend to determine where the horizontal distribution of clouds changes the most and document whether these patterns represent known climatological features (e.g., ENSO, ITCZ, and/or monsoon anomalies) and how they are related to interannual variability or long-term trends for HC width.

This study will use data from CloudSat and MERRA-2. The HC edge will be defined from monthly reanalysis data using the zero line at 500 hPa for the meridional mass streamfunction (Ψ₅₀₀) and reversal of near-surface zonal winds (U_sfc) based on previous work. We also use a gridded climatology of monthly cloud characteristics from CloudSat to examine how frequency, thickness, cloud water content, and precipitation vary by cloud type for HC width extremes. We hypothesize that the expansion of the tropics will amount to changes in the spatial distribution of cloud types and the depth of those clouds, which will impact quantities such as precipitation within the ITCZ and subsiding branches of the HC. Future work will examine changes in diabatic heating associated with anomalous cloud occurrences to better understand the potential mechanisms and dynamic links for HC widening.

Mentor

Justin Stachnik