Atmospheric Ocean System

Geographical Pattern Scaling, model description

The main assumption of the approach used in GPS is the linear climate response to greenhouse forcings, while the response to aerosol (sulphate) forcings is non-linear. In this approach geographical patterns of reponse to greenhouse gases and to sulphate aerosols are combined by adding the appropriate weights to the patterns. The different geographical patterns are obtained as follows:

with:
T_cell(m) = temperature of grid cell in year t relative to 1990 for GCM run m
∆ T_GCM,cell(m) = temperature change of grid cell from GCM run m
∆ mean (T_GCM,global) = global mean surface temperature change from GCM run
∆ mean (T_UDCM,global) = global mean surface temperature change from UDCM in year t

This equation is applied eight times to take the linear effect of GHG and the non-linear effect of sulphate aerosols into account:

• The first run of UDCM is based on the climate forcing after 1990 by greenhouse gases only. The corresponding temperature change is scaled with a GHG-only pattern from the selected GCM, as explained in the above equation. The Hadley GCM run with greenhouse gases only (HADCM2) is used as default in IMAGE 2.4. 
• The second run of UDCM uses the climate forcing by sulphate only (other greenhouse gases held constant after 1990). The corresponding temperature change is scaled with a sulphate-only pattern taken from the UIUC-GCM (University of Illinois at Urbana-Champaign). The results of the first and second iteration are added (no weights) to obtain the combined pattern of radiative forcings by greenhouse gases and sulphate aerosols. 
• So far, only linear responses are taken into account. Runs 3-8 are made to account for the non-linear climate response to radiative forcing by sulphate aerosols. These runs are based on the relative change in radiative forcing since 1990 in six UIUC regions (see  UIUC regions vs IMAGE2.4 regions ).  

The weighting for the regional patterns are calculated as follows:     

• First, the regional sulphate burden in 1990 is calculated using regression equations presented by Schlesinger et al. (2000) relating the sulphate aerosol burden (mg m^-2) and annual SO₂ emissions (g m^-2) for the six regions (i).
• The ratio g_i of the sulphate burden in region i to the global sulphate aerosol burden is calculated for the year 1990 (no scenario dependency).
• The future ratios of regional to global sulphate burden are calculated for each scenario with the same regression equations. This gives time-dependent ratios f_i,j.
• The weighting factor d_i,j is calculated as the difference: d_i,j = g_i - f_i,j
• For years after 1990 the weighting factor d_i,j is used to multiply the regional forcings calculated from the emissions of SO₂ in UDCM.
• The temperature changes from runs 3-8 are scaled with six UIUC-GCM patterns constructed with increased forcings in the six UIUC regions.
• These six scaled temperature patterns are added to the combined pattern of the first two runs for forcing due solely to greenhouse gases or sulphate. In this way the combination of the first two runs is corrected for the regional differences in forcing by future SO₂ emissions.       

The result is a pattern in which the non-linear effect of sulphate is taken into account. This pattern is added to the 1990 observed climate (New et al., 1999) to obtain a total temperature pattern for the 0.5 by 0.5 degree IMAGE grid cells every five years.

• to  Geographical Pattern Scaling, UIUC regions vs IMAGE2.4 regions
• to  external references
• to  MNP references