Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 331

Table 13.1 Forcing and response of surface air temperature T S , precipitation P, and evaporation
E (global and JJAS average) as a function of the shortwave absorptivity of the dust particles.
The forcing is calculated using the dust distribution calculated by Miller et al. ( 2006 ). Particle
optical properties are taken either from the compilation of Sinyuk et al. ( 2003 ) or Patterson et al.
( 1977 ), where the latter assumes greater shortwave absorption. As a third sensitivity experiment,
even greater absorption is prescribed by reducing the single scatter albedo $ 0 by a factor of 0.9
compared to the case derived from Patterson et al. ( 1977 )

Patterson et al. ( 1977 )

Sinyuk et al. ( 2003 ) Patterson et al. ( 1977 ) 0:9 $ 0

Forcing (W m 2 )
F T (TOA) 0:39 0:39 1:35

F S (Surface) 1:13 1:81 2:68

Surface temperature response (K)
ıT S 0:28 0:07 0:30

Precipitation (and evaporation) response (mm day 1 )
ıP . D ıE/ 0:033 0:026 0:014

humidity, respectively. Thus, if relative humidity is constant (so that C P ıT S C Lıq S

can be written as ˛C P ıT S ), then ( 13.2 ) implies

˛ıT S D ıT E : (13.3)

The parameter ˛ is related to the surface relative humidity, and can be generalized
to include the effects of feedbacks by upper tropospheric water vapor and the lapse
rate, for example. Both of the conditions leading to ( 13.1 )and( 13.2 ) are generally
satisfied within regions of tropical convection or the mid-latitude storm tracks,
so that adjustment of the surface air temperature and humidity are related to the
temperature anomaly at the emitting level, where anomalous OLR compensates the
TOA forcing.
Tab le 13.1 shows that on a global scale, the anomalous surface air temperature
calculated by an ESM increases with TOA forcing, even though forcing at the
surface decreases. This is consistent with ( 13.1 )and( 13.2 ). As the prescribed par-
ticle absorption of solar radiation increases, the TOA forcing increases. OLR must
increase in compensation, raising the emitting temperature and the surface value
despite increased dimming of the surface. Similar behavior has been demonstrated
in a single-column model (Cess et al. 1985 ).
Figure 13.5 shows the regional response of surface air temperature and moist
static energy to the forcing shown in Fig. 13.1 . (This particular model has a relatively
large climate sensitivity of 4.2 K for a doubling of CO 2 that magnifies the climate
response by about 50 % compared to the current CMIP5 model version: Schmidt
et al. 2014 .) An increase in TOA forcing is associated with a larger value of
moist static energy at the surface, as expected from the combination of ( 13.1 )and
( 13.2 ). (h S is divided by C p in Fig. 13.5 to have units of temperature.) Despite
the increase of the global average surface air temperature with TOA forcing as
shortwave absorption by the dust particles increases, the surface air cools in some