Extended Abstract (1.4M)P1.4
The Structure and Dynamics of Moist Absolutely Unstable Layers in Simulated Squall Lines
George H. Bryan, Penn State Univ., University Park, PA; and J. M. Fritsch
Moist absolute instability is a thermodynamic state wherein a saturated layer of air exhibits a lapse rate greater than moist adiabatic. Moist absolutely unstable layers (MAULs) can form as a result of lifting an initially sub-saturated, conditionally unstable environment. It is traditionally argued that MAULs quickly overturn (i.e., in a matter of minutes), since all parcels displaced vertically within the layer will accelerate in the direction in which they are displaced. Nevertheless, observations and numerical simulations suggest that MAULs may be able to persist for considerably longer periods of time if the rate at which they are being created by lifting exceeds the rate at which they are being dissipated by turbulent mixing.
Simulations of squall lines are used to examine the structure and dynamics of MAULs. The highest resolution simulations utilized 125 m grid spacing. Results confirm that MAULs form in the inflow region of the squall lines where a deep layer of strong, dynamically-forced, mesoscale ascent saturates the conditionally unstable environment. Horizontally, regions of moist absolute instability extend continuously in the along-line direction, and extend 20-50 km behind the gust front in the cross-line direction. Vertically, the deepest uninterrupted layers of moist absolute instability are 200-250 mb deep, and are located above the gust front. Rearward of the gust front, continuous layers of moist instability become generally shallower as turbulent mixing processes eliminate the instability. However, the largest total depth of the absolutely unstable state (i.e., the sum of the depths of all MAULs in a vertical column) is > 400 mb and is located in the transition zone between the convective region and the stratiform region.
The numerical simulations confirm that MAULs can be created and maintained in squall lines. In particular, it is shown that MAULs can last much longer than has been previously thought. From a system-relative viewpoint, a layer of deep, continuous moist absolute instability is always present - e.g., as the system propagates, air is continuously saturated at the leading edge of the system. From a viewpoint fixed in space, soundings show that MAULs last for 15 to 45 minutes, depending mainly on the propagation speed of the squall line. Presumably, for a quasi-stationary system, the unstable state can persist locally for much longer time periods. From the viewpoint of a parcel entering the squall line from the pre-squall-line boundary layer, unstable conditions can be experienced for greater than 20 minutes.
An analysis of the dynamics governing the creation and removal of MAULs, as well as the relationship between MAULs and the various regions of squall lines (e.g., convective, transition, and stratiform regions) will also be presented.
Poster Session 1, Mesoscale Convective Systems and MCVs
Monday, 12 August 2002, 3:00 PM-4:30 PM
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