airmass

Ordinary Cells -- Air Mass Convection

Introduction
Ordinary convective cells are exemplified by air mass thunderstorms. These cells often short-lived and less organized than the dynamically induced multicellular systems and supercells. Ordinary convection is typically not associated with a front or a synoptic-scale forcing mechanism (Branick, 1999). Instead warm, humid air in the summer months allows for destabilization of the environment leading to positive buoyancy (rising air). Convective maximums of this type typically occur in mid and late afternoons in the Southeastern U.S., but can occur at any location experiencing a warm, moist air mass. Subsequently, as soon as the insolation is halted by the setting of the sun, the environment begins to stabilize, reducing positive buoyancy and, hence, thunderstorm activity.
Air mass thunderstorms are usually described as 'popcorn convection' or 'pulse storms' because of their random, bubbled appearance in radar reflectivity images. Because shear values are usually weak or nonexistent in air mass thunderstorms, buoyancy processes dominate cell generation and evolution.

Ordinary Cell over Texas

NSSL

Buoyancy Processes and the Parcel Theory
   Buoyancy processes describe how air rises and falls and may be illustrated by the vertical momentum equation. The vertical momentum equation states that a change in the vertical velocity of a parcel is equal to the vertical pressure gradient plus buoyancy term (a forcing description). Lapse rates and moisture stratification are important in determining the potential for convective development in weak dynamic, buoyancy dominated convective scenarios. Specifically, water vapor and potential temperature perturbations are important in the production of positive buoyancy. Potential temperature is used to define whether a parcel is warmer than its surroundings, in which case it will rise. Water vapor adds buoyancy to a parcel and, thus, aids upward motion. Through the above discussion, it is easy to understand that an environment with high dew points and temperatures is ideal for the generation of air mass convection.
   Negative buoyancy contributions include cloud water and precipitation. Negative buoyancy is best described by the addition of 'weight' to the parcel, causing it to sink. Negative buoyancy is an important concept in the generation of downdrafts and there associated severe wind phenomena.
   A thermodynamic diagram is an important forecasting tool for estimating the potential for buoyancy. The diagram aids forecasters in determining the potential strengths of both convective updrafts and downdrafts. Analysis of sounding data and thermodynamic diagrams is out of the scope of this module; the reader should review Bluestein and Jain (1985), Evans (1998), Weisman and Przybylinski (1998) for further discussion on these forecasting techniques. An internet-based tutorial on thermodynamic diagrams is found here.
   One of the most useful values that can be derived from a thermodynamic diagram is convective available potential energy (CAPE). CAPE (measured in J/kg) is a value that defines the integration of the difference between the lifting temperature and ambient temperature from the level of free convection (LFC) to the equilibrium level (EL). CAPE is important for determining the potential instability of the environment. Values of 1000-2000 J/kg are associated with typical air mass convection. Values greater than 2000 J/kg are associated with strong to severe convection (Kreighton et al. 1996). CAPE that is concentrated in the lower and mid-levels produces stronger updrafts (storms) because acceleration of the parcel is maximized, entrainment of stable air is minimized, and the affects of precipitation loading are minimized (Kreighton et al. 1996).   A more significant term for determining the potential for damaging wind events is downdraft available potential energy (DAPE).

   An environment that is characteristic of deep convective clouds (airmass convection) includes:
        * abundunt low-level moisture
        * little shear in the horizontal and vertical
        * a low level of free convection (LFC)
        * a moderate amount of CAPE (1000+ J/kg)
        * little convective inhibition (capping)
        * a high Equlibrium Level (EL)
        * a forcing mechanism (e.g. sea breeze front, outflow boundary)

A description of the lifecycle of an ordinary cell (Kreighton et al. 1996):
1) Cumulus Stage: Air is lifted, due to buoyancy, to the lifting condensation level (LCL) where it begins to form a cloud. If the uplifted air is warmer than the surrounding environment it will continue to rise (at the LFC), forming an ever-larger cloud. Click here to see a schematic of this stage.

2) Mature Stage: Ascending air eventually reaches the EL where the parcel temperature is equal to the surrounding environment's temperature. Little or no increase in the storms vertical extent occurs after the EL is reached. During this stage, new air is continually fed into the storm from the boundary layer: adding more moisture and heat to the developing cloud. The air continues to spread out below the tropopause forming an anvil. All the while moisture is condensing in the updraft. Eventually the condensed moisture becomes to 'heavy' to be kept aloft by the updraft and begins to fall to the earth as precipitation. The downward moving precipitation drags air earthward leading to negative buoyancy and the production of a downdraft. The drag induced by the precipitation is termed precipitation loading and is the most significant contributor to the downdraft strength. Additionally, entrainment of drier air (from the mid-levels, 3-5 km AGL) into the downdraft increases evaporation and, thus, cooling. Because the downdraft is cooler than the surrounding air sinks, accelerating the downdraft. Eventually the downdraft reaches the earth's surface where it spreads out in all directions producing a gust front. Heavy rain, gusty winds, lightning, and possible hail typify the mature stage of an ordinary convective cell. Click here to see a schematic of this stage

3) Dissipating stage: Precipitation loading reduces the positive buoyancy associated with the updraft. In addition, the spreading downdraft at the earth's surface cuts off the heat and moisture supply to the updraft. Because upward motion begins to wean, the downdraft becomes dominant. Precipitation continues to fall until all available moisture has been depleted. Eventually all that is left of the thunderstorm is a debris shaft and spreading anvil. Click here to see a schematic of this stage

Ordinary Convection Evolution in the Vertical

The gust front described in stage 2 and 3 may continue to propagate away from the storm for tens of kilometers (termed an outflow boundary). The outflow boundary is similar to a miniature cold front and may aid in the initiation of convection at its head (produces additional low-level convergence). A higher probability of new convection may be expected at the conjunction of two outflow boundaries.

Ordinary Cell Severe Weather

NWS

Additional resources:
Airmass thunderstoms radar tutorial (by Texas A&M) is here.
Single cell tutorial (by COVIS) is here.

Next Page - Multicellular

The Thunderstorm

Multicellular Systems

Supercells