Introduction

Saturation adjustment schemes are usually designed to bring the relative humidity back to exactly 100% when supersaturation occurs. In doing so, the enthalpy of condensation or deposition is released, the temperature is increased just the right amount for 100% humidity, and the air becomes laden with condensate in the form of cloud droplets at temperatures warmer than 273.15 K. At temperatures colder than freezing, in order to adjust the relative humidity to 100% with respect to ice, a mixture of cloud droplets and ice crystals may be found, and finally at temperatures colder than 233.15 K, only ice crystals are generally produced. For the case of a mixture of cloud droplets and ice crystals, the adjustment is made such that the saturation mixing ratio of each phase, liquid and ice, is weighted in the calculation of relative humidity (Tao et al.1989). Some of the earliest adjustment schemes were described by McDonald (1963), for example, to simulate fog formation. The adjustment process can be prescribed for a single step as in Rutledge and Hobbs (1983; 1984), or an iteration process such as that in Bryan and Fritsch (2002), using potential-temperature, vapor, and mixing ratios. In Tripoli and Cotton (1981), an ice-liquid potential temperature and vapor are used to diagnose quickly the cloud-water mixing ratio required to bring a parcel to 100% humidity with an appropriate associated temperature increase (condensation) or temperature decrease (evaporation).