Hydrological sensitivity to greenhouse gases and aerosols in CESM
Maria M. Kvalevåg, Bjørn H. Samset and Gunnar Myhre
Center for International Climate and Environmental Research – Oslo (CICERO), Norway
Keywords: Precipitation response, climate change, radiative forcing, CESM,
CO2, CH4, aerosols, sulphate, BC, soot, convection, atmospheric absorption
We present a set of climate model experiments using the NCAR Community
Earth System Model (CESM1.03) to investigate the relationship between
precipitation changes and surface temperature change for several forcing
mechanisms. The model simulations include changes in climate forcers since
preindustrial times causing either warming or cooling, in order to study the
energy budget at different levels (surface, atmosphere and top of
atmosphere), temperature changes and precipitation change. On a short
timescale the precipitation changes are linked to atmospheric stability and
reduced convection caused by the presence of a forcing mechanism in the
atmosphere. On a longer timescale it is the adjusted surface temperatures
that drive the changes.
Method
In order to study the hydrological sensitivity on different time scales we
divide the precipitation changes into a fast and a slow response term similar to
Andrews et al. 2010:
(a)
(b)
Figure 1: Relationship between a) atmospheric absorption and fast
precipitation changes and b) radiative forcing at top of atmosphere and
slow precipitation changes. Results from CESM (+) and Andrews et al
2010 (O). Correlation coefficients are given for CESM results only.
(a)
(b)
Observed mechanisms include:
Radiative forcing  atmospheric heating  fast precipitation response
Radiative forcing  surface temperature change  slow precipitation response
Cases
CO2
Description
CO2 concentration increased 286 ppm  379 ppm
CO2x2
CO2 concentration doubled 286 ppm  572 ppm
CH4
CH4 concentration increased 805 ppm  1774 ppm
CH4x2
CH4 concentration increased 805 ppm  2745 ppm
CH4 concentration increased 805 ppm  5650 ppm
SO4
SO4 concentration increased from preindustrial level
to present levels
SO4x5 SO4 concentration increased from preindustrial level to
5 times preindustrial levels
BC
Black carbon concentrations increased from preindustrial level
to present levels
BCx10 Black carbon concentration increased from preindustrial level
to 10 times preindustrial levels
BC@xxx Total black carbon burden change inserted at 850mb, 750mb,
650mb or 550mb respectively
Case
∆Ptot
∆Pfast
∆Pslow
RF
RFs
RFatm
∆Ts
(%)
(%)
(%)
(W/m2)
(W/m2)
(W/m2)
(K)
CO2
2,08
-1,04
3,12
1,35
0,51
0,84
1,16
CO2x2
5,31
-2,28
7,59
3,55
1,59
1,96
2,92
CH4
1,28
-0,29
1,57
0,41
0,06
0,35
0,56
CH4x2
2,09
-0,39
2,45
0,90
0,58
0,32
0,82
CH4x5
3,90
-0,62
4,53
1,84
1,28
0,56
1,65
Solar
7,34
-0,82
8,16
3,54
3,18
0,36
2,83
SO4
-0,69
-0,12
-0,57
-0,12
-0,37
0,24
-0,26
SO4x5
-3,82
0,05
-3,87
-1,43
-1,99
0,56
-1,39
BC
-0,19
-0,33
0,14
-0,04
-0,36
0,31
-0,03
BCx10
-1,46
-2,31
0,86
0,48
-2,60
3,08
0,35
Table 2: Precipitation response (total, fast and slow), radiative forcing (TOA, surface and in the
atmosphere), and surface temperature change due to climate forcing mechanisms described in Table 1.
In conclusion, we observe:
• a strong positive correlation between radiative forcing at the top of the
atmosphere and slow precipitation changes, attributable to a long-term
heating of both surface and atmosphere. (Fig. 1b)
(c)
(d)
• a strong negative correlation between radiative forcing in the
atmosphere and fast precipitation changes, attributable to increased
atmospheric stability on the short term, as well as lower relative humidity
and a semi-indirect cloud effect. (Fig. 1a)
• convection is increased by greenhouse gases and solar forcing but
decreased by aerosols (both SO4 and BC). (Fig. 2b-d)
CH4x5
Table 1: Overview of NCAR CESM1.0 climate simulations. All cases are performed with CAM4/CLM4
including either fixed SST or slab ocean and run for 30 years. Baseline year is 1850.
Results contd.
Results
Introduction
• surface temperature change per change in precipitation (i.e. the
hydrological sensitivity) is found to be different for climate forcing
mechanisms with different absorption efficiencies. (Fig. 2a)
Figure 2: a) Surface temperature change (K) vs fast precipitation changes
(%). The dotted lines indicate climate forcers with different absorption
efficiencies. b-d) Convection rate vs precipitation change (%). (b) shows
the total response, (c) fast response, (d) slow response
• BC-aerosols (soot) in the atmosphere absorb short wave radiation and
contribute to atmospheric heating and increased stability, and thus inhibit
cloud formation and precipitation. The forcing due to BC aerosols is altitude
dependent and is very sensitive to the presence of clouds above or below
the aerosol layer (not shown here).
References
Andrews et al., Geophysical Research Letters, vol. 37, L14701, 2010
Kvalevåg, MM., Samset, BH. and Myhre, G., in prep.