ABSTRACT
Precipitation includes all water particles, whether liquid or solid, that fall from clouds and reach the ground. Precipitation includes both liquid (drizzle and rain), freezing (freezing drizzle and freezing rain) and frozen (snow, ice crystals, and hail) water.[1] For precipitation to occur, air must be cooled sufficiently to cause condensation and droplet growth. The mechanism that causes precipitation is adiabatic-expansion cooling as air is lifted in the atmosphere. When cooling is sufficient, vapor condenses on nuclei that are generally small particles of dust or salt, and combustion products that are always present in the atmosphere to form either ice crystals and supercooled liquid cloud droplets, or only liquid cloud droplets. Clouds that extend above the 0C level are referred to as cold clouds and those that do not extend above the 0C level are called warm clouds. Ice particles grow to sufficient mass to fall as precipitation in cold clouds by three processes; vapor condensation, collisions with supercooled droplets, and aggregation with other ice particles. In warm clouds, droplets grow large enough to fall as precipitation through the coalescence process where larger particles (which fall faster than small particles) collide and coalesce. As shown in Fig. 1, air is generally lifted by four means: 1) frontal convergence (cyclonic convergence); 2) orographic lifting; 3) thermal convection; or 4) tropical cyclones (hurricanes).[2-4]
FRONTAL CONVERGENCE
Precipitation caused by frontal convergence occurs when the general atmospheric circulation brings air masses of different temperatures and moisture from high-pressure regions (cold, relatively heavy air) to low-pressure regions (warm, relatively light air) which forces the air to rise, producing adiabatic cooling. Areas of high pressure at the surface are associated with converging air on the west side of high altitude troughs and areas of low pressure at the surface are associated with diverging air on the east side of the troughs. These cyclonic systems are usually larger than 500 km across and in the mid-latitudes, the air is lifted at the frontal surface as shown in Fig. 1A. Non-frontal
convergence generally occurs in the tropics within a mass of warm, moist air.
