P67  WRF Simulations of Hurricane Irene During its Interaction with the US East Coast

Klausmann, Alfred M., Exponent

During the period when Hurricane Irene was moving northward along the US east coast the storm was encountering increasing wind shear and cooler sea surface temperatures and was slowly weakening as it tracked from the Carolinas to New England.  Despite this, Irene caused widespread and significant impacts along the east coast, from severe flooding from the Mid-Atlantic States into eastern New York and western New England to widespread wind damage and power outages across a large portion of southern and central New England.  The objective of this study is to conduct number of retrospective WRF simulations using the WRF-ARW model in an effort to reconstruct the storms surface wind field and rainfall during the period from August 27-29, 2011.  The initial WRF simulation was conducted with Four Dimensional Data Assimilation using a 12 km and 4 km grid with 40 vertical levels. The Kain-Fritsch cumulus parameterization scheme was used on the 12 km domain while convection was explicitly simulated on the 4 km grid. The WSM5 microphysics scheme, the YSU PBL scheme, and the NOAH land surface model were implemented on both domains. The National CenterŐs for Environmental Prediction (NCEP) 1 degree final analysis (FNL) data was used for the initial and lateral boundary conditions along with the Real Time Gridded 1/12 degree sea surface temperature data. In the initial simulation the storm was initialized using just the FNL data. Tropical cyclone bogussing was not used in the initial simulation since the storm had a large circulation envelope and a poorly defined inner core structure while it was along the east coast during the modeling period.  Analysis nudging was performed on the 12 km domain using the FNL gridded analysis data. Several additional simulations were conducted to examine different model configurations.

The initial WRF simulation was compared against the Hurricane Research Divisions HWIND analysis as well as with observations at selected land based surface stations and buoys. For comparisons with direct observations, time series plots were constructed from the observations and compared against time series from the WRF simulation at the observation locations and the root mean square errors for each station were computed. The initial results show that the WRF track of Irene showed a southwestward error by about 30 km compared to the best track data, but overall, WRF handled the storm track well. The WRF 10-meter wind field compared reasonably well with the HWIND analysis but did show somewhat higher wind speeds covering larger spatial areas than indicated by the HWIND analysis particularly over the ocean east of the center. The peak winds from the WRF simulation were compatible with maximum winds from the best track data. Time series plots of WRF and observed data showed an over prediction of the 10-meter winds over several stations near and along the storm track but much closer agreement with observations at stations well east of storm center. This may be the result of the slight southwestward error in the model storm track. Computed RMSE values ranged from about 4 m/s over the western most stations which were located along and near the storm track to about 2 m/s at stations east of the center.  The total storm rainfall pattern was compared against the Advanced Hydrologic Prediction System (AHPS) multi-sensor precipitation analysis. The results show that the spatial pattern of total storm rainfall along the east coast was well simulated by WRF. However WRF overestimated the rainfall amounts particularly in the northern mid-Atlantic region and into eastern New York where it showed too much areal coverage of 8-10 inch rainfall amounts when compared to the AHPS multi-sensor analysis. The multi-sensor analysis showed a broad swath of 6-8 inch amounts and pockets of rainfall totaling 8 inches or more. Additional results will be presented showing the effects of using different model configurations on the WRF simulation of Hurricane Irene.