Aerosol Effects on Clear‐Sky Shortwave Heating in the Asian Monsoon Tropopause Layer. Issue 4 (18th February 2023)

• Record Type: Journal Article
• Authors: Gao, Jie; Huang, Yi; Peng, Yiran; Wright, Jonathon S.
• Is Part Of: Journal of geophysical research. Volume 128:Issue 4(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-18
• Subjects: ATAL -- radiative heating -- tropopause -- monsoon -- anticyclone -- UTLS; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2022JD036956
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: The Asian tropopause aerosol layer (ATAL) has emerged in recent decades with aerosol accumulation near tropopause above Asian Summer Monsoon region. Although ATAL effects on surface and top‐of‐atmosphere (TOA) 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 Modern‐Era Retrospective Analysis for Research and Applications, version 2 aerosol reanalysis are on the order of 15% 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 ∼10% or as large as ∼70%. Uncertainties in surface and TOA flux effects are also large, with values spanning one order of magnitude at the TOA. ATAL effects on radiative heating peak between 150 and 80 hPa (360–400 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. Plain Language Summary: Every summer, a layer of polluted air laden with aerosol particles collects above the convective storms of the Asian monsoon as part of a broad upper‐level circulation centered over the Tibetan Plateau. Researchers have developed a working understanding of how the dynamical environment shapes this Asian tropopause aerosol layer (ATAL). The motivating question for this work is: How might the aerosol layer reshape its environment? Aerosols can absorb and scatter sunlight, affecting both the amount of sunlight transmitted through the layer and the magnitude of solar heating within the layer. These effects depend on aerosol species and their vertical distribution within the layer, both of which are highly variable. In this paper, we translate variations and uncertainties in the amount, composition, and vertical distribution of aerosols near the Asian monsoon tropopause into variations and uncertainties in the absorption and scattering of solar radiation by the aerosol layer. We find that aerosols account for a substantial part (10%–70%) of heating by solar radiation near the tropopause in the region of ATAL. The vertical location and horizontal extent of the aerosol effects are distinct from those of other radiative effects. Key Points: The Asian tropopause aerosol layer produces a 10%–70% direct enhancement of clear‐sky shortwave heating above the summer monsoon Effects are largest where shortwave heating is weakest, with similar magnitudes to water vapor and ozone effects near the monsoon tropopause Discrepancies across recent aerosol analysis and forecast products cause large uncertainties in aerosol forcing of heating and fluxes