Abstract. Black carbon (BC) and dust impart significant effects on the South Asian
monsoon (SAM), which is responsible for ∼80 % of the
region's annual precipitation. This study implements a variable-resolution
(VR) version of the Community Earth System Model (CESM) to quantify two
radiative effects of absorbing BC and dust on the SAM. Specifically, this
study focuses on the snow darkening effect (SDE), as well as how these
aerosols interact with incoming and outgoing radiation to facilitate an
atmospheric response (i.e., aerosol–radiation interactions, ARIs). By running
sensitivity experiments, the individual effects of SDE and ARI are
quantified, and a theoretical framework is applied to assess these aerosols'
impacts on the SAM. It is found that ARIs of absorbing aerosols warm the
atmospheric column in a belt coincident with the May–June averaged location
of the subtropical jet, bringing forth anomalous upper-tropospheric
(lower-tropospheric) anticyclogenesis (cyclogenesis) and divergence
(convergence). This anomalous arrangement in the mass fields brings forth
enhanced rising vertical motion across South Asia and a stronger westerly
low-level jet, the latter of which furnishes the Indian subcontinent with
enhanced Arabian Gulf moisture. Precipitation increases of 2 mm d−1 or
more (a 60 % increase in June) result across much of northern India from
May through August, with larger anomalies (+5 to +10 mm d−1) in the
western Indian mountains and southern Tibetan Plateau (TP) mountain ranges due to orographic
and anabatic enhancement. Across the Tibetan Plateau foothills, SDE by BC
aerosols drives large precipitation anomalies of > 6 mm d−1
(a 21 %–26 % increase in May and June), comparable to ARI of absorbing
aerosols from April through August. Runoff changes accompany BC SDE-induced
snow changes across Tibet, while runoff changes across India result
predominantly from dust ARI. Finally, there are large differences in the
simulated SDE between the VR and traditional 1∘ simulations, the latter
of which simulates a much stronger SDE and more effectively modifies the
regional circulation.
Supplementary material to "Quantifying snow-darkening and atmospheric radiative effects of black carbon and
dust on the South-Asian Monsoon and hydrological cycle: Experiments using
variable resolution CESM
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