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.