Air can rise or fall at any level in the troposphere. In addition, the air may be rising at some levels of the troposphere and sinking at others. The type of clouds viewed depends on which level of the troposphere air is rising. Air that rises only in the PBL and not in the middle and upper levels of the troposphere will produce fair weather cumulus or low level stratocumulus. Air rising in the middle levels of the troposphere will produce altocumulus or stratocumulus. Air rising in the upper levels of the troposphere will produce cirrus. In the case of a thunderstorm, air is convectively rising throughout the troposphere. This will produce cumulonimbus. There are two types of convection that forecasters are concerned with. These two types are surface based deep convection and elevated convection. Surface based convection initially begins as parcels rise from the PBL due to surface heating from the sun. If the troposphere is unstable in the middle and upper levels, the surface based convection will build into the troposphere producing thunderstorms. Surface based convection occurs in the warm sector of a mid-latitude cyclone and in an air mass thunderstorm situation. The second type of convection is elevated convection. This is convection that begins above the PBL. Elevated convection occurs when the PBL is stable. The PBL may be stable due to low level cold air advection, a stable lapse rate or very dry air. The lifting mechanisms for elevated convection include: isentropic lifting (lifting of warm/moist air over cooler PBL air), positive vorticity advection, and jet streak divergence. In an elevated convection situation, the cloud base will often be higher up than that associated with surface based convection. Surface based convection tends to occur in the warm season while elevated convection tends to occur in the cool season. When surface based convection is in vertical alignment with upper level forcing mechanisms such as vorticity and jet streaks, storms have a higher probability of being severe. Elevated convection is common on the cool side of a warm front boundary and on the cold side of a cold front boundary or when an upper level low or upper level forcing mechanism causes air to rise in the middle and upper levels of the troposphere. Many snow and wintry precipitation events are the result of elevated convection. Elevated convection tends to have a lighter precipitation rate than surface based convection since surface based convection is associated with a warm / moist PBL and there is less moisture to lift in the middle and upper levels of the troposphere in an elevated convection situation. Dry air in the low levels of the troposphere can evaporate elevated convection as it falls (results in virga). Elevated convection can have characteristics of both cumulus and stratiform precipitation while surface based convection is more associated with cumulus type convection only. The next time it rains or precipitates, ask yourself, is this surface based or elevated convection? |