18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001: 2:00 PM
On adequate resolution for the simulation of deep moist convection: theory and preliminary simulations
George H. Bryan, Penn State Univ., University Park, PA; and J. M. Fritsch
Poster PDF (182.8 kB)
Deep moist convection organizes in many different forms: from isolated cumulonimbus cells, to squall lines, to mesoscale convective complexes. Even individual categories of mesoscale organization can contain vast differences in structure. For instance, some squall lines are best described as a collection of individual cumulonimbus cells, while other squall lines overturn as slabs with > 100 km long updrafts. It is natural to ask why convection organizes in such different manners.

The depth and magnitude of vertical wind shear has been studied by many authors as an important factor in the organization of convection. Furthermore, the relationship between environmental wind shear and convective available potential energy (CAPE) has been identified as a strong determinant of convective structure in supercells and squall lines.

Other parameters that may influence convective organization, but have been explored to a much lesser extent, are the thermodynamic properties of the local environment. This study focuses on the vertical distribution of environmental static stability and its impact on the internal structure of mesoscale convective systems (e.g., depth and horizontal extent of the stratiform region, strength of the mesohigh, cellular versus slabular convective overturning, etc.). Analytic soundings were developed for use in three-dimensional cloud-resolution simulations over a mesoscale (400 km X 100 km) domain. The soundings have the same amount of total CAPE, but the vertical distribution of the CAPE is altered by changing the stability just above the level of free convection (LFC) and by changing the height of the tropopause. The absolute humidity profiles and the wind profiles were kept fixed in the various experiments in order to isolate the effects of the stability differences.

Results show that a spectrum of convective organization can be obtained by varying the static stability profile just above the LFC, even though total CAPE for the experiments remains the same. The organizational differences are related to the structure of moist absolutely unstable layers (MAULs) that form in a 100-300 mb deep layer just above the LFC. For situations with similar MAUL depth, the portion of the total CAPE that exists in the MAUL is considerably larger for soundings that have steep lapse rates above the LFC. Since the stability in the MAUL affects the ability of parcels to accelerate upwards (away from the LFC), the nature of turbulent mixing is different -- i.e., the structure of the convection is different. These results suggest that new parameters need to be developed to assist operational forecasters in anticipating the organization of convection. We propose that the amount of CAPE in low levels (i.e., from roughly 100-300 mb above the LFC) may be a useful parameter for operational forecasters.

Supplementary URL: http://www.ems.psu.edu/~bryan/mpc2001/