Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 347

incident upon the surface: temperature tendencies that would seemingly inhibit
convection by stabilizing the column. One resolution to this paradox is that
positive TOA forcing increases the export of energy from the convecting region,
resulting in anomalous convergence of moisture at the surface that makes the air
conditionally unstable and likely to ascend (Chou et al. 2005 ). This argument
makes certain assumptions that deserve further investigation. Most importantly, the
climate is assumed to respond to dust through a direct circulation. In the Tropics,
this circulation type dominates the transport of energy and moisture. Relatively
little attention has been devoted to how dust changes precipitation in the middle
latitudes. In general, extratropical cyclones have a complicated dependence upon
the time-averaged flow, and anticipating how they are modified by dust radiative
forcing is difficult. The precipitation response also depends upon the compensation
of the surface forcing by evaporation as opposed to ocean heat transport. The latter
has been calculated in only a few studies (Ramanathan et al. 2005 ; Bollasina et al.
2011 ).
While many studies emphasize the effect of dust forcing upon climatological
precipitation, there is recent interest regarding the effect of dust upon tropical
cyclones within the Atlantic, where dusty summers are associated with a reduc-
tion in cyclone activity. There has been extensive work untangling the radiative
and microphysical effects of dust upon tropical cyclone development from other
environmental factors like the low humidity that accompanies dusty air, along with
broader climate variations that modulate both cyclones and dust. Understanding
the observed relation remains a topic of active research and importance, given the
destructive power of cyclones and their potential to amplify in the warming climate.
Several studies have shown how climate anomalies created by dust forcing feed
back on the mobilization of additional soil particles. Surface wind speed responds
to forcing through either a perturbation to surface pressure or the sensible heat
flux that controls mixing within the boundary layer. Dust changes the strength
of mixing that delivers momentum to the surface each morning from jets that
form above the nocturnal boundary layer. In addition, a reduction of precipitation
by dust can diminish the extent of vegetation, leading to the mobilization of
additional particles and amplification of droughts like the Dust Bowl. What remains
in characterizing this positive feedback is to model quantitatively the impact
of the precipitation anomaly by dust upon vegetation. This in turn requires the
development of dynamical vegetation models with realistic sensitivity.
The largest contribution to the uncertainty of forcing and the attendant climate
response probably originates with the uncertain dust distribution. Estimates of the
global dust aerosol mass by current models range between 20 and 35 Tg in most
cases (Textor et al. 2006 ). To reduce the uncertainty of forcing and its impact
upon climate, we need a more confident estimate of the dust distribution. The
shortwave contribution to forcing is sensitive to the presence of absorbing minerals
like hematite, and routine measurements of mineral content will help to document
regional variations in the particle optical properties that are almost always neglected
by current ESMs. Forcing also varies among models due to environmental factors
like surface albedo. A more precise characterization of albedo remains a challenge