Numerical Simulations of the Evolution of a Cold Front and Its Precipitation

Abstract

Short-range (36-h) simulations of a surface cold front that occurred over the Great Plains of the United States during the SESAME-AYE III period are made using a significantly modified version of a hydrostatic, sigma-coordinate, quasi-Lagrangian, primitive-equation gridpoint model. The necessity of using blended data from FGGE Level IIIb and IIb for input is demonstrated. The evolution of the environment associated with the cold front was well predicted by the model when compared with analyses and observations of the cold front location, horizontal temperature gradient, wind field, dry line, temperature inversion, and precipitation. The current model results caught several observed features that were not present in the other mesoscale model results. Also, comparisons between moist and dry versions of the current model showed a stronger frontal upward motion and a faster moving front showed in the moist version of the model experiment than in the dry version, as expected. The evolution of several local maxima of frontal intensity, defined in terms of temperature gradient, were related to the combination of local maxima of convergence and deformation effects in different locations along the cold front. The genesis of negative relative vorticity along the inverted cold front over the lee of the United States Rockies was found to be related to the effect of solenoidal forcing. The negative solenoidal forcing here was formed by the orographic pressure gradient perpendicular to the strong temperature gradient. The movement of the local maximum of mesoseale lower-level convergence along the cold front moving from the southern portion of the front to north drove the major frontal intensity from south to north in terms of frontogenesis and cyclogenesis. The model precipitation agreed well with observation in terms of timing and location. The evolution of maximum precipitation followed the evolution of the maximum low-level convergence ahead of the cold front, where ample moisture combined with potential instability. The generation of the temperature inversion in the vicinity of the frontal surface over the southern portion of the cold front prohibited convection there, and localized the frontal precipitation along the northern portion of cold front and east of southern temperature inversion. The temperature inversion was generated by differential temperature advection through the frontal circulation within the PBL, and modified by the vertical stretching effect that stabilized the lower layer behind the cold front and destabilized ahead of the cold front.