Aerosol Effects on Clear-Sky Shortwave Heating in the Asian Monsoon
Tropopause Layer
Abstract
The Asian Tropopause Aerosol Layer (ATAL) has emerged in recent decades
to play a prominent role in the upper troposphere and lower stratosphere
above the Asian monsoon. Although ATAL effects on surface and
top-of-atmosphere radiation budgets are well established, the magnitude
and variability of ATAL effects on radiative transfer within the
tropopause layer remain poorly constrained. Here, we investigate the
impacts of various aerosol types and layer structures on clear-sky
shortwave radiative heating in the Asian monsoon tropopause layer using
reanalysis products and offline radiative transfer simulations. ATAL
effects on shortwave radiative heating based on the MERRA-2 aerosol
reanalysis are on the order of 10% of mean clear-sky radiative heating
within the tropopause layer, although discrepancies among recent
reanalysis and forecast products suggest that this ratio could be as
small as ~5% or as large as ~25%.
Uncertainties in surface and top-of-atmosphere flux effects are also
large, with values spanning one order of magnitude at the
top-of-atmosphere. ATAL effects on radiative heating peak between 150
hPa and 80 hPa (360 K–400 K potential temperature) along the southern
flank of the anticyclone. Clear-sky and all-sky shortwave heating are at
local minima in this vertical range, which is situated between the
positive influences of monsoon-enhanced water vapor and the negative
influence of the ‘ozone valley’ in the monsoon lower stratosphere. ATAL
effects also extend further toward the west, where diabatic vertical
velocities remain upward despite descent in pressure coordinates.