The highest precipitation rates in midlatitude cyclones are often associated with the narrow cold-frontal rainband. In this paper the formation of precipitation in this type of rainband is analyzed with the aid of a diagnostic cloud model and field measurements.
Model results indicate that the high precipitation rates in narrow cold-frontal rainbands are associated with graupel. The graupel forms when ice particles, which originate in the stratiform cloud ahead of the rainband, grow rapidly by riming after entering the strong updraft (and hence a region of high liquid water content) associated with the rainband. When this source of input ice particles is not present the precipitation is somewhat weaker, but still mainly associated with graupel. In this case, the graupel forms as small, frozen drops grow by riming. The drops form in the updraft region due to the autoconversion of cloud water and they are frozen by collisions with small ice crystals. A series of sensitivity studies dealing with various model inputs and parameters are discussed. The model results agree reasonably well with field measurements.