Working Group I: The Scientific Basis



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5.4.1.3 Comparison of modelled and observed aerosol concentrations

Like previous model intercomparisons, the IPCC comparison showed large differences (factor of 2) in model predictions of the vertical distribution of aerosols. The model simulations of surface sulphate concentrations (Figure 5.8) indicate that much of the difference in sulphate radiative forcing reported in the literature is most likely to be associated with either variations in the vertical distribution or with the response of sulphate aerosols to variations in relative humidity (Penner et al., 1998b).

The IPCC comparison showed that the capability of models to simulate other aerosol components is inferior to their capability to simulate sulphate aerosol. For example, sea salt in the North and South Pacific shows poorer agreement, with an average absoute error of 8 mgm-3 (Figure 5.9) than the corresponding sulphate comparison which is less than 1 mgm-3 (Figure 5.8) (see Table 5.9 also).

Figure 5.8: Observed and model-predicted annual average concentrations of non-sea salt sulphate (in µgm ^-3 ) at a series of stations in the North and South Atlantic. The models are listed in Table 5.8. Data were provided by D. Savoie and J. Prospero (University of Miami). Stations refer to: Heimaey, Iceland (HEI); Mace Head, Ireland (MAH); Bermuda (BER); Izania (IZO); Miami, Florida (RMA); Ragged Point, Barbados (BAR); Cape Point, South Africa (CPT); King George Island (KGI); and Palmer Station, Antarctica (PAL).

Figure 5.9: Observed and model-predicted annual average concentrations of sea salt (as Na) (in µgm-3) at a series of stations in the North and South Pacific. The models are listed in Table 5.8. Data were provided by D. Savoie and J. Prospero (University of Miami). Stations refer to: Cheju, Korea (CHE); Hedo, Okinawa, Japan (HOK); Midway Island (MID); Oahu, Hawaii (OHU); Fanning Island (FAN); American Samoa(ASM); Norfolk Island (NOR); Cape Grim, Tasmania (CGR); and Wellington/Baring Head, New Zealand (WEL).

For dust the model-observation comparison showed a better agreement with surface observations in the Northern than in the Southern Hemisphere. For example, the average absolute error in the Northern Pacific was 179%, while it was 268% in the Southern Pacific. In the Southern Hemisphere, almost all models predict concentrations higher than the observations at all stations poleward of 22°S. Thus, it appears that dust mobilisation estimates may be too high, particularly those for Australia and South America. The paucity of dust from these regions relative to other arid dust source areas has been noted previously (Prospero et al., 1989; Tegen and Fung, 1994; Rea, 1994), and may reflect the relative tectonic stability, low weathering rates, duration of land-surface exposure, and low human impacts in this area.

Table 5.9a: Comparison of models and observations of aerosol species at selected surface locations ( µg/m³)^a,b.

Model	Sulphate		Black carbon		Organic carbon		Dust		Sea salt
Model	Average bias (µg/m³)	Average absolute error (µg/m³)	Average bias (µg/m³)	Average absolute error (µg/m³)	Average bias (µg/m³)	Average absolute error (µg/m³)	Average bias (µg/m³)	Average absolute error (µg/m³)	Average bias (µg/m³)	Average absolute error (µg/m³)

GISS	0.15	0.33	0.16	0.61	0.69	1.52	5.37	5.37	3.90	11.94
GSFC	-0.10	0.28	0.71	1.00	0.71	1.57	-0.5	1.98	-3.02	9.23
Hadley	-0.54	0.55	0.74	1.18			-2.47	3.48
CCM/Grantour	-0.31	0.40	-0.18	0.50	-0.84	1.20	1.77	2.99	5.26	14.48
ECHAM	0.09	0.42	0.74	1.07	1.52	2.09
Stochem	0.34	0.40
ULAQ	0.18	0.34	-0.30	0.48	-0.47	1.43	1.82	3.69	0.81	12.57
Mozart	0.05	0.39	-0.34	0.51
ECHAM/Grantour	0.26	0.28	0.07	0.55	-0.57	1.40	5.2	5.27	2.07	10.55
TM3	0.27	0.47
PNNL	-0.04	0.28	0.16	0.64	0.79	1.50	-2.48	2.64	-13.46	13.74
Average of all models	0.03	0.38	0.20	0.73	0.26	1.53	2.73	3.86	-0.74	12.09

^a Aerosol sulphate, dust and sea salt were compared to observations at a selection of marine locations. The observations for organic carbon and black carbon were those compiled by Liousse et al. (1996) and Cooke et al. (1999). ^b The average bias and the average absolute error is the average differences between each model result and the observations over all stations.

Table 5.9b: Comparison of models and observations of aerosol species at selected surface locations (%)^a,b.

Model	Sulphate		Black carbon		Organic carbon		Dust		Sea salt
Model	Average bias (%)	Average absolute error (%)	Average bias (%)	Average absolute error (%)	Average bias (%)	Average absolute error (%)	Average bias (%)	Average absolute error (%)	Average bias (%)	Average absolute error (%)

GISS	26	31	85	127	91	121	121	121	37	40
GSFC	7	15	189	219	109	134	39	42	21	30
Hadley	-11	16	140	220
CCM/Grantour	1	15	43	111	13	85	78	80	63	68
ECHAM	32	35	253	276	276	285
Stochem	30	34
ULAQ	10	17	-10	84	23	100	21	35	81	88
Mozart	28	31	164	211
ECHAM/Grantour	31	31	204	230	88	135	70	70	29	33
TM3	43	46
PNNL	17	21	75	133	189	220			-12	16
Average of all models	19	26	127	179	112	154	66	70	36	46

^a Aerosol sulphate, dust and sea salt were compared to observations at a selection of marine locations. The observations for organic carbon and black carbon were those compiled by Liousse et al. (1996) and Cooke et al. (1999). ^b The average bias and the average absolute error is the average percentage differences between each model result and the observations over all stations.

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