Three-dimensional Extensions of the  Applications of Dynamic Retrieval  Dynamics of precipitating convection

Dynamics of frontal circulations

Another example of this retrieval approach is retrieval of pressure and virtual temperature perturbations from within a surface cold front with intense but shallow convection at its leading edge (Parsons et al. 1987). This study also provided further verification of the validity of the technique with values of E_r averaged over the radar volume as low as 0.16 and relatively favorable comparisons between the derived pressures and surface pressure observations, between the retrieved virtual temperature and rawinsondes measurements taken ahead of and behind the front, and between derived fields and a simple numerical simulation of the flow. Their retrievals found the strong upward motions to be co-located with negative buoyancy (Fig. 13.6a) and virtual temperature deficits (Fig. 13.6b):

 

Figure 13.6: Average cross-frontal characteristics obtained from thermodynamic retrieval for a surface cold front observed in central California by multiple Doppler radar. a) Buoyancy, b) virtual temperature and cloud water loading, and c) pressure deviations relative to a sounding taken just ahead of the front. (from Parsons et al. 1987).

There was also no indication of convective instability in the sounding. As it turned out the vertical pressure gradient (Fig. 13.6c) was positive and therefore its contribution to the vertical acceleration was positive as well. Therefore, contrary to popular belief, positive buoyancy is not a necessary condition for strong updrafts. Since these shallow circulations also produced a tornado it is also evident that the potential for positive buoyancy in the ambient environment is also not a necessary condition for the onset for these types of tornadoes that occur along shallow air mass boundaries.

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