The Application of Sea Level Pressure and Vorticity Fields derived from the University of Washington Planetary Boundary Layer Model in the NOAA Ocean Prediction Center

Abstract

The SeaWinds scatterometer onboard the NASA QuikSCAT satellite has been providing forecasters in the Ocean Prediction Center (OPC) with near-real time (NRT) ocean vector winds over large ocean areas since 1999. The OPC forecasters routinely use QuikSCAT winds in their analysis and forecast process to position frontal features, centers of high and low pressure and to determine the category and location of short term wind warning areas. QuikSCAT has also given forecasters the ability to detect hurricane force conditions within extratropical cyclones. Since QuikSCAT has been fully integrated in to OPC operations, OPC forecasters' assessment of the surface wind field over the open oceans is more accurate than ever before. As part of the warning and forecast process, OPC forecasters prepare a manual sea level pressure (SLP) analysis four times daily for both the North Atlantic and North Pacific. These OPC SLP analyses are disseminated directly to ships at sea and are heavily relied upon by the marine community for safe and economic operations. These analyses are also a key element in the forecast process as accurate initial conditions are essential to the production of precise forecasts. Although QuikSCAT's impact on the analysis and forecast process has been significant to the short-term wind warning process, this positive impact has not carried over to the analysis of the sea level pressure field over the open oceans. In an effort to improve their SLP analyses, OPC began to run the University of Washington Planetary Boundary Layer (UWPBL) model to derive SLP, surface vorticity and surface wind speed fields using the NRT QuikSCAT winds from NOAA/NESDIS as input. The UWPBL model derived SLP, surface vorticity and surface wind speed fields were made available to OPC forecasters within their N-AWIPS workstations so that they could overlay these products with other observational fields and model guidance. The SLP fields from the UWPBL model were examined daily over a three-month period. The model was found to produce dynamically consistent SLP fields the majority of the time. A comparison of the SLP fields derived from the UWPBL model with the OPC manual surface analyses and the Global Forecast System Model (GFS) surface pressure fields revealed that in most cases the central pressure of the cyclones were not analyzed to be deep enough by either the OPC manual analyses or the GFS model output. There were occasional instances, however where the UPWBL model produced central pressures that were unrealistically low and/or high. This problem was determined to be related to stratification issues and to the method of assimilation of available ship and buoy observations into the model to seed the pressure gradient field. This paper will present several case studies illustrating the application of UWPBL derived sea level pressure and vorticity by OPC forecasters. Comparisons of OPC manual analyses, numerical model analyses and the UWPBL fields will be shown. The UWPBL model using QuikSCAT winds as input provides very high quality sea level pressure fields associated with intense ocean storms. In particular, the retrieved sea level pressures contain strong pressure gradients in areas of very high winds. This strength of the pressure retrieval system has made it very useful to OPC forecasters in daily operations and as a training tool