A Numerical Investigation of Slabular Convection and Moist Absolute Instability in Hurricane Isabel

Jeremy Ross, Richard James, George Bryan, Charles Hosler, and

J. M. Fritsch

The Pennsylvania State University

University Park, PA  16802

Many investigators have presented evidence demonstrating that, under certain conditions, the atmosphere, despite being conditionally unstable, overturns in a manner more resembling mesoscale slabs (or sheets) of ascending air overrunning slabs of descending air rather than as an ensemble of individual convective towers (e.g., Thorpe et al. 1982, Rasmussen and Rutledge 1993, Liu and Moncrieff 1996, and Bryan and Fritsch 2000). These slabs are typically several hundred km wide and exhibit an overrunning-underrunning zone that extends horizontally from a few tens to several hundred km (e.g., Houze 1977, Roux et al. 1984, Chong et al. 1987, Trier and Parsons 1993).  Recently, Kain and Fritsch (1998), Bryan and Fritsch (2000) and Rogers and Fritsch (2001) have used observations and numerical model simulations to demonstrate that, in some situations, slab overturning creates a moist absolutely unstable layer that extends vertically through hundreds of hPa and spans horizontal distances of several hundred km.  In these zones, the typical cellular (thunderstorm) mode of overturning is replaced by a sheet of relatively uniform but powerful ascent that extends, unbroken, along the length of the slab.  Observed by radar, slab convective overturning often appears as a mesoscale solid swath of high reflectivity with the inflow side of the swath paralleling a front or outflow boundary.  Bryan and Fritsch (2000) found that the low-level mesoscale forced ascent in these regions exceeds the vertical motions that could arise from buoyancy.  Of interest here is whether or not slab convection occurs in tropical storms or hurricanes.  In particular, does slab convection form in the mesoscale regions of strong ascent associated with spiral bands and the eye wall?  And, if so, do moist absolutely unstable layers form in the rapidly ascending lower layer of the slabs?  

In order to address these issues, high-resolution numerical experiments were conducted in which Hurricane Isabel was simulated with a multi-nested three-dimensional model.  The outermost domain spanned approximately 10⁴ km; the innermost domain spanned 600 km and was run with a horizontal grid spacing of Dx = 250 meters.   Results from the innermost domain clearly revealed the formation of slab-type convection and moist absolutely unstable layers.  In particular, slabs formed along the inflow edge of the spiral bands and the eye wall.  Typically, these zones exhibited the strongest mesoscale ascent.  Examination of the vertical motion and buoyancy in these regions indicates that, as with the mid-latitude systems, the forced mesoscale ascent was comparable to or exceeded the vertical motions that could be produced by buoyancy.  Also similar to the mid-latitude systems, the moist absolutely unstable layers gradually transformed into saturated moist neutral ascent by the time they reached mid-levels.    Investigations are continuing to determine the dominant processes whereby the atmosphere transforms from a sub-saturated conditionally unstable state, to moist absolutely unstable, to moist neutral.  Model thermodynamic structure is compared to hurricane dropsondes.

References:

Bryan, G. H. and J. M. Fritsch, 2000: Moist absolute instability:  The sixth static stability state.  Bull. Amer. Meteor. Soc., 81, 1207-1230.

Chong, M., P. Amayenc, G. Scialom and J. Testud, 1987:   A tropical squall line observed during the COPT 81 Experiment in West Africa.   Part I: Kinematic structure inferred from dual-doppler radar data.   Mon. Wea. Rev., 115, 670-694.

Houze, R. A., 1977:   Structure and dynamics of a tropical squall-line system.   Mon. Wea. Rev., 105, 1540-1567.

Kain, J. S., and J. M. Fritsch, 1998:  Multi-scale convective overturning in mesoscale convective systems:  Reconciling observations, simulations, and theory.  Mon. Wea. Rev., 126, 2254- 2273.

Liu, C. and M. W. Moncrieff, 1996:   Mass and momentum transports by organized convection:  Effects of shear and buoyancy.   J. Atmos. Sci., 53, 964-979.

Rasmussen, E. N. and S. A. Rutledge, 1993:  Evolution of quasi-two-dimensional squall lines. Part I: Kinematic and reflectivity structure.  J. Atmos. Sci., 50, 2584-2606.

Rogers, R. and J. M. Fritsch, 2001:   The mechanism of mid-level warm-core vortex penetration to the surface.   Mon. Wea. Rev., 129,  605–637.

Roux, F., J. Testud, M. Payen and B. Pinty, 1984:  West African squall-line thermodynamic structure retrieved from dual-Doppler radar observations.  J. Atmos Sci., 41, 3104-3121

Thorpe, A. J., M. J. Miller and M. W. Moncrieff, 1982:  Two-dimensional convection in non-constant shear:  A model of mid-latitude squall lines.  Quart. J. Roy. Meteor. Soc., 108, 739-762.

Trier, S. B. and E. B. Parsons, 1993:   Evolution of environmental conditions preceding the development of a nocturnal mesoscale convective complex.   Mon. Wea. Rev., 121, 1078-1098.