96 Modelling theclimate
for showers and thunderstorms) are affected by higher surface tem-
peratures. All these processes are already well included in weather
forecasting models, and water vapour feedback has been very thoroughly
studied.^13 The most important of the others are cloud radiation feedback
and ocean-circulation feedback. How are these incorporated into the
models?
For modelling purposes, clouds divide into two types –layerclouds
present on scales larger than the grid size andconvectiveclouds generally
on smaller scales than a grid box. For the introduction of layer clouds,
early weather-forecasting and climate models employed comparatively
simple schemes. A typical scheme would generate cloud at specified
levels whenever the relative humidity exceeded a critical value, chosen
for broad agreement between model-generated cloud cover and that ob-
served from climatological records.More recent models parameterise the
processes of condensation, freezing, precipitation and cloud formation
much more completely. They also take into account detailed cloud prop-
erties (e.g. water droplets or ice crystalsand droplet number and size)
that enable their radiative properties (e.g. their reflectivity and transmis-
sivity) to be specified sufficiently well for the influence of clouds on the
atmosphere’s overall energy budget to be properly described. The most
sophisticated models also include allowance for the effect of aerosols
on cloud properties – denoted the indirect aerosol effect in Figure 3.8.
The effects of convective clouds are incorporated as part of the model’s
scheme for the parametrisation of convection.
The amount and sign (positive or negative) of the average cloud-
radiation feedback in a particular climate model is dependent on many
aspects of the model’s formulation as well as on the particular scheme
used for the description of cloud formation. Different climate models,
therefore, can show average cloud-radiation feedback that can be either
positive or negative; further, the feedback can show substantial regional
variation. For instance, models differ in their treatment of low cloud such
that in some models the amount of low cloud increases with increased
greenhouse gases, with other models it decreases. Uncertainty regarding
cloud-radiation feedback is the main reason for the wide uncertainty
range in what is calledclimate sensitivity(see Chapter 6) or the change
in global average surface temperature due a doubling of carbon dioxide
concentration.
The second important feedback is that due to the effects of the ocean
circulation. Compared with a global atmospheric model for weather fore-
casting, the most important elaboration of a climate model is the inclu-
sion of the effects of the ocean. Early climate models only included the
ocean very crudely; they represented it by a simple slab some fifty or
one hundred metres deep, the approximate depth of the ‘mixed layer’ of