A case study of the variational assimilation of GPS zenith delay observations into a mesoscale model

Results from a case study of the four-dimensional variational assimilation of total zenith delay (TZD) observations from a dense global positioning sy stem (GPS) network into the Pennsylvania State University-National Center f or Atmospheric Research Fifth-Generation Mesoscale Model are reported. TZD is made up of the rescaled pressure and precipitable water at the site of t he GPS receiver. Profiler-wind and radio acoustic sounding system (RASS) vi rtual temperature observations are also included in the assimilation experi ments. Four experiments are performed. The study targets the 12-h period fr om 0000 to 1200 UTC 6 December 1997, characterized by the passage of a fron tal system that produced intense rainfall over southern California. Forecas ts prior to data assimilation underestimate the observed 6- and 12-h accumu lated rainfall for most of the domain. The (sole) assimilation of TZD obser vations is found to have a small but beneficial impact on the short-range p recipitation forecast. Measured against the control forecast, area-mean imp rovements of up to 33.15% and 25.08% are found in the 6- and 12-h accumulat ed rainfall in Los Angeles County. The inclusion of profiler-wind observati ons is found to have a significant impact on the model precipitation, with improvements in the 6- and 12-h accumulated precipitation as high as 88.26% and 32.53%, respectively. However, these increments are noticeably reduced when the TZD data are excluded from the assimilation experiments. Further improvements are achieved when the TZD and profiler-wind data are assimilat ed along with the RASS virtual temperature data. Increases of up to 93.21% and 50.58% are found in the 6- and 12-h accumulated precipitation, respecti vely. Because the virtual temperature also contains information on the thre e-dimensional moisture field, these findings point to the potential benefit that may result from the future assimilation of GPS slant-path delay data.