State University of New York at Albany NY 12205
The response and impact of clouds remains one of the largest outstanding questions in GCMs. Clouds are not homogeneous, though they are treated as such in the models. When averaged over areas typically used as numerical grid elements by GCMs, observations suggest that there are some clouds at all relative humidities. Fractional cloud cover at 100% relative humidity is rarely 100%, and totally clear skies rarely occur, even for low relative humidities (see Figure 23.1). Relative humidity is the best single indicator of cloud coverage. However, if there is a relationship between cloud coverage and relative humidity, our current models and observations are inadequate to reveal exactly what that relationship is. It does appear that cloud coverage decreases exponentially as humidity falls below 100%.
Climate change predictions are extremely sensitive to modeled cloud cover estimates. There are numerous methods for predicting cloud cover from relative humidity, yet none of them are firmly grounded in observation. The intent of this research is to study the relationship between cloud cover and relative humidity using available observations.
In Walcek's research, U. S. Air Force 3DNEPH cloud data is correlated with relative humidity fields produced by assimilating radiosonde observations using a mesoscale meteorology model. This produces a scatter diagram of cloud cover versus relative humidity, the gross features of which are summarized in Figure 1. Clearly, in contrast to current GCM methodologies, clouds exist over a wide range of relative humidities, rather than disappearing below some arbitrarily defined threshold, typically 60-80%, depending on height in the atmosphere.
The purpose of current models of climate change is to attempt to discern how small changes in temperature and humidity will affect cloud cover and climate. But current models are unable to respond subtly. In going from 85% to 80% relative humidity, some models go from 100% cloud cover to 0%.
Not withstanding the current uncertainties inherent in observations of both cloud cover and relative humidity, the best fit to the data appears to be an exponential decrease in cloud cover with falling relative humidity.
Of all atmospheric layers, the lower boundary layer is by far the most sensitive to changes in relative humidity. For example, a 1% change in relative humidity is associated with a 4-6% change in cloud cover in the lower boundary layer (Figure 23.2). This means that if relative humidity increased from 82% to 84%, cloud cover would increase 15%, which according to Slingo (1990), would be enough to offset the temperature rise that could result from a doubling of CO2.
Reference
Slingo, A., 1990: Sensitivity of the earth radiation budget to changes in low cloud amount. Nature, 343, 49- 51.
Walcek, C. J., 1994: Cloud cover and its relationship to relative humidity during a springtime mid-latitude cyclone. Monthly Weather Review, 122, 1021-1035.
