Mesoscale data assimilation of Myanmar cyclone Nargis part II: Assimilation of GPS-Derived precipitable water vapor

Four-dimensional variational (4d-var) data assimilation (da) experiments using global positioning system (gps)derived precipitable water vapor(pwv)were conducted for the tropical cyclone(tc) nargis in 2008.in order to analyze the initial field at 1200 utc 30 april 2008,12,24,36,and 48 h sequential da experiments with 3 h assmilationwindows were performed. the initial fields madeby these da experiments were applied to subsequent forecast experiments using a nonhydrostatic model (nhm) with a horizontal resolution of 10 km. nhm predictions using initial fields produced by da experiments that used only ordinary observational data (without gpspwv) exhibited alargevariation of predicted maximum tc intensity (958 to 983 h pa) for each experiment. inthese experiments,a longer assimilation period did not necessarily resultin better prediction. the da of gps pwv yieldeda smaller variation of predicted maximum tc intensity (964 to 974 hpa),and a longer assimilation period tended to bring deeper depression of tc central pressure. overall,tc intensities determinedbyda experiments with gps data were closer to the best track produced by the regional specialized meteorological centre (rsmc) new delhi than the da experiments without gps data. the 48 h da without gps pwv resulted in the weakest prediction of tc development with the deepest tc central pressure of 983 h pa,while 48 h da with gps pwv successfully predicted rapid tc development with the deepest pressure of 967 h pa. one cause of the in complete development of nargisin the 48 h da experiment without gps pwv was insufficient observations in the bay of bengal,especially in the first 12 h. underestimation of precipitation was conspicuous in the first 12 h of the da. implementation of gps pwv in to the da contributed to increasing the precipitation and changed the fields of pressure and wind in the bay. in the first several hours,modifications of the fields of pressure and wind around the andaman islands were conspicuous. these affected areas extended with time and createda morefavorable environment for tc development. © 2011,meteorological society of japan.