Sensitivity Experiments of an Eastward-Moving Southwest Vortex to Initial Perturbations

Abstract: Whether the initial conditious contain pronounced mesoscale signals is important to the simulation ofthe southwest vortex. An eastward-moving southwest vortex is simulated using the PSU/NCAR MM5. Amodest degree of success is achieved, but the most serious failure is that the formation and displacement ofthe simulated vortex in its early phase are about fourteen hours later than the observed vortex. Consideringthe relatively sparse data on the mesoscale vortex and in an attempt to understaud the cause of the forecastfailure, an adjoint model is used to examine the sensitivity of the southwest vortex to perturbations of initialconditions. The adjoint sensitivity indicates how small perturbations of model variables at the initial timein the model domain can influence the vortex. A large sensitivity for zonal wind is located under 400 hPa, alarge sensitivity for meridional wind is located under 500 hPa, a large sensitivity for temperature is locatedbetween 500 and 900 hPa, and almost all of the large sensitivity areas are located in the southwesternarea. Based on the adjoint sensitivity results, perturbations are added to initial conditions to improvethe simulation of the southwest vortex. The results show that the initial conditions with perturbatiouscan successfully simulate the formation and displacement of the vortex; the wind perturbations added tothe initial conditions appear to be a cyclone circulation under the middle level of the atmosphere in thesouthwestern area with an anticyclone circulation to its southwest; a water vapor perturbation added toinitial conditions can strengthen the vortex and the speed of its displacement.

Keywords:
• southwest vortex,
• mesoscale numerical simulation,
• adjoint model,
• sensitivity experiment