Climate Change 2001:
Working Group I: The Scientific Basis
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5.4.4 Model Validation of Indirect Effects

Validation of the simulations from global models is an essential component of estimating and reducing the uncertainties in the indirect forcing. Comparisons of observations and modelled concentrations of chemical species have been discussed in Section 5.4.1.3 while comparisons of modelled and satellite-derived aerosol optical depths were discussed in Section 5.4.1.4. Here, comparisons with observations of several other model products important for indirect forcing are examined. Unfortunately, there is only a very small set of observations of the physical, chemical, and radiative properties of clouds from in situ methods available. Thus, validations with these types of datasets are left to limited temporal and spatial scales and to comparing relationships among various quantities. Lohmann et al. (2001), for example, compared prognostic simulations with observations of the relationships between particulate sulphate and total particle mass, between particle number concentration and sulphate mass, and between Nd and sulphate mass. The relationships between sulphate mass and total particle number concentrations was larger than observations in the case of internal mixtures but was smaller than observations in the case of external mixtures. Ghan et al. (2001a) found that the results of their determination of Nd using their mechanistic parametrization were comparable to the results using the empirical parametrization of Boucher and Lohmann (1995). Such tests are important for large-scale model parametrizations because comparisons of absolute concentrations on the scale of the model grid size are difficult.

Satellites offer observations over large temporal and spatial scales; however, for the derived parameters of interest, they are much less accurate than in situ observations. Han et al. (1994) retrieved an reff for liquid-water clouds from ISCCP satellite data that showed a significant land/sea contrast. Smaller droplets were found over the continents, and there was a systematic difference between the two Hemispheres with larger droplets in the Southern Hemisphere clouds. The reff calculated by different models and the observations from Han et al. are shown in Table 5.12. Since the reff tends to increase with increasing height above cloud base and the satellite observations of reff are weighted for cloud top, the satellite observations will tend to overestimate the overall reff compared with that determined from in situ studies. In situ data sets against which to make absolute comparisons are few in number (e.g. Boers and Kummel, 1998). However, for now model evaluations are better done using the contrasts in reff between the land and ocean and between the Southern Hemisphere and the Northern Hemisphere. Most of the models listed in Table 5.12, with the exception of Roelofs et al. (1998), approximate the difference between reff over the Southern Hemisphere ocean vs the Northern Hemisphere ocean. Over land, the Southern Hemisphere vs Northern Hemisphere difference from Roelofs et al. (1998) is closest to the observed difference. For reff over Southern Hemisphere land vs Southern Hemisphere ocean, several models are relatively close to the difference from the observations (Boucher and Lohmann, 1995; Chuang et al., 1997; Roelofs et al., 1998; Lohmann et al., 1999b,c). Some models compare with observations better than others, but there is no model that is able to reproduce all the observed differences. The reff calculated with the same parametrization but using different GCM meteorologies are quite different (compare Jones and Slingo (1996) vs Boucher and Lohmann (1995)). As noted above, Jones and Slingo determined the "cloud top" reff by assuming a LWC profile that increased with height from cloud base to cloud top. Such a profile is more similar to observed profiles and might be expected to produce a better comparison with the observations. While the Jones and Slingo (1996) model does reasonably well in terms of hemispheric contrasts, their results indicate a land-ocean contrast in the opposite direction to that from the other models and the observations. We note that many factors may affect the results of this type of comparison. For example, Roelofs et al. (1998) estimate the sensitivity of the reff calculations to uncertainties in the sulphate concentration field, cloud cover and cloud liquid-water content to be of the order of a few micrometres. Moreover, the satellite determination of reff is probably no more accurate than a few micrometres (Han et al., 1994).

Table 5.12: Cloud droplet effective radius of warm clouds (in µm).
All results for 45° S to 45°N
Ocean Southern Hemisphere
Ocean Northern Hemisphere
Land Southern Hemisphere
Land Northern Hemisphere
Total
Han et al. (1994)
11.9
11.1
9.0
7.4
10.7
Boucher and Lohmann (1995)
8.9 to 10.1
8.3 to 9.3
5.4 to 8.7
4.9 to 8.0
 
Jones and Slingo (1996)
9.6 to 10.8
9.0 to 10.4
10.2 to 11.8
9.9 to 10.8
9.5 to 11.1
Roelofs et al. (1998)
12.2
10.3
8.8
6.9
10.4
Chuang et al. (1997)
11.6 to 12.0
10.7 to 11.4
8.8 to 9.1
8.6 to 9.0
10.7 to 11.2
Lohmann et al. (1999b,c)
10.7
10.2
8.3
4.9
 
Rotstayn (1999)
11.2
10.9
9.8
9.5
10.7
Ghan et al. (2001a,b)        
11.0 to 11.7

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