Tropical Cyclone Forecast Track and Intensity Sensitivities to Various Parameterizations using the WRF-ARW Model: Attempt at an Ensemble 2008 Cyclone Workshop Nick P. Bassill Advisor: Michael C. Morgan 2008 This work was supported by ONR Grant 144PN07

National Hurricane Center Discussion for Ernesto from 15 UTC 26 August 2006 … THE FORECAST TRACK IS MORE PROBLEMATIC AFTER 72 HR. THE LARGE-SCALE MODELS AGREE THAT THE MID-LEVEL RIDGE OVER THE GULF OF MEXICO WILL WEAKEN AS A SHORTWAVE TROUGH DIGS SOUTHEASTWARD THROUGH THE MISSISSIPPI VALLEY. HOWEVER ... THERE ARE DIFFERENCES IN HOW MUCH WEAKENING WILL OCCUR. THE ECMWF AND NOGAPS CALL FOR ERNESTO TO RECURVE OVER THE EASTERN GULF OF MEXICO ... WHILE THE UKMET SHOWS ENOUGH RIDGE TO KEEP ERNESTO MOVING WEST-NORTHWESTWARD …

WRF* Model Variables Grid Spacing Cumulus Parameters Horizontal: 30km, 45km, or 60km Vertical: 31 or 54 levels Cumulus Parameters (1) Kain-Fritsch (cu_physics=1) (2) Betts-Miller-Janjic (*=2) (3) Grell-Devenyi ensemble (*=3) Microphysics Parameters (1) Kessler (warm rain) (mp_physics=1) (2) Eta-Ferrier (time efficient) (*=5) (3) WSM6 (most complex of these) (*=6) *All simulations performed with the ARW core (version 2.1.2 - 2006 release)

There are two primary tracks North across Cuba and Florida West across the Yucatan Peninsula

Research Goals Attempt a daily “real-time” ensemble for the tropical North Atlantic Use an ensemble comprised of members differentiated only by choice of parameterizations (i.e. not initial/boundary conditions or dynamical core) Evaluate the ability of this type of ensemble to reproduce observed spreads seen in more “conventional” ensembles Specifically, we will focus on TC track, intensity, and genesis If this method shows promise, begin accumulating statistics on the above variables, with a goal of producing a “corrected consensus” of the ensemble members

Current Ensemble Information Uses 1200 UTC GFS 1.0º x 1.0º forecast for initial/boundary conditions Two domains: the outer and inner domains have a 90 km and 30 km horizontal grid spacing, respectively. Both have 31 vertical levels. The ARW core of WRFV3 is the dynamical core used The inner domain uses the outer domain to create its boundary conditions (one way nest) Currently, there are 10 ensemble members, with a run-length of 120 hours  MP CP BLP # 1 WSM3 Kain-Fritsch YSU # 2 Pleim # 3 Betts-Miller-Janjic # 4 # 5 Grell-3 # 6 Ferrier # 7 # 8 # 9 #10

Outline Discuss several examples of storm tracks - Tropical Storm Fay - Hurricane Hanna - Hurricane Ike Discuss a representative sample of intensity differences Discuss several examples of tropical cyclogenesis - Alma/Arthur - Hanna/Ike/Josephine Offer some preliminary conclusions/explanations

Example 1: Tropical Storm Fay ... WITH THE LARGE-SCALE MODELS FORECASTING A WEAKNESS IN THE RIDGE OVER THE EASTERN GULF OF MEXICO OR FLORIDA THAT SHOULD ALLOW FAY TO TURN TOWARD THE NORTH-NORTHWEST OR NORTH AFTER 36 HR. THERE IS A LARGE SPREAD IN THE MODEL GUIDANCE ON WHEN AND HOW SHARPLY FAY WILL TURN. THE ECMWF...CANADIAN ... AND NAM ARE FARTHEST TO THE EAST ... CALLING FOR FAY TO MOVE ACROSS CUBA TO THE SOUTHEASTERN COAST OF FLORIDA. THE GFDL IS A LITTLE FARTHER WEST. THE GFS ... HWRF ... FLORIDA STATE SUPERENSEMBLE ... AND THE CONSENSUS MODELS FORECAST FAY TO CROSS WEST CENTRAL CUBA AND MAKE LANDFALL ALONG THE SOUTHWEST COAST OF FLORIDA. THE UKMET AND GFDN ARE FARTHER WEST...FORECASTING FAY TO MOVE TOWARD THE FLORIDA PANHANDLE. THE NOGAPS IS THE FARTHEST WEST ... FORECASTING FAY TO MOVE TOWARD THE NORTHERN GULF COAST.

* # *http://euler.atmos.colostate.edu/~vigh/guidance/northatlantic/archive/ #http://cimss.ssec.wisc.edu/tropic2/archive/atlantic.php

Example 2: Hurricane Hanna TRACK MODELS INSIST THAT HANNA WILL START ITS MUCH-ANTICIPATED TURN TOWARD THE NORTHWEST SOON...AS A LARGE MID-LATITUDE TROUGH OVER THE WESTERN ATLANTIC LIFTS NORTHWARD AND THE SUBTROPICAL RIDGE BUILDS BACK IN. SUCH A TRACK WOULD TAKE THE CYCLONE NORTHWESTWARD THROUGH THE CENTRAL AND NORTHWESTERN BAHAMAS DURING THE NEXT TWO DAYS. HANNA IS THEN EXPECTED TO ACCELERATE AS IT MAKES ITS APPROACH TO THE SOUTHEASTERN UNITED STATES. HOWEVER ... UNTIL HANNA BEGINS A MORE DEFINITIVE MOTION ... IT WILL BE DIFFICULT TO NARROW THE POTENTIAL IMPACT AREA. IN ADDITION ... THE ANTICIPATED ANGLE OF APPROACH TO THE SOUTHEAST UNITED STATES COAST MEANS THAN ONLY A SMALL DEVIATION LEFT OR RIGHT OF THE TRACK WILL HAVE LARGE IMPLICATIONS IN BOTH TIME AND LOCATION OF LANDFALL.

* # *http://euler.atmos.colostate.edu/~vigh/guidance/northatlantic/archive/ #http://www.wunderground.com/hurricane/at200808.asp

Example 3: Hurricane Ike THE RIDGE IS FORECAST TO SHIFT EASTWARD DURING THE NEXT COUPLE OF DAYS ALLOWING IKE TO MAKE A TURN WESTWARD. IN 3 TO 4 DAYS ... A SHORTWAVE TROUGH MOVING OVER THE EASTERN UNITED STATES IS EXPECTED TO CREATE A WEAKNESS TO THE NORTH OF IKE RESULTING A TURN TOWARD THE WEST-NORTHWEST OR NORTHWEST. THIS SHORTWAVE TROUGH WILL BE A CRITICAL PLAYER IN IKE'S EVENTUAL TRACK AT THE EXTENDED RANGES. DYNAMICAL MODELS ARE SPLIT INTO TWO CAMPS AT THIS TIME ... WITH SOME MODELS SHOWING THE SHORTWAVE PASSING SUFFICIENTLY FAR TO THE NORTH TO KEEP IKE MOVING WEST-NORTHWESTWARD ... WHILE OTHER MODELS SHOW IKE TURNING NORTHWARD INTO THE WEAKNESS.

* # *http://euler.atmos.colostate.edu/~vigh/guidance/northatlantic/archive/ #http://cimss.ssec.wisc.edu/tropic2/archive/atlantic.php

Kain-Fritsch Members

Betts-Miller-Janjic Members

Grell-Devenyi Members

A Comparison of Maximum Intensities CP: BLP: WSM3 KF YSU Pleim BMJ Grell-? Ferrier 6th 950.1 942.5 970.5 956.8 995.1 946.9 940.5 971.0 956.7 992.5 7th 952.4 940.4 972.4 958.8 989.5 954.3 942.8 974.2 963.6 989.4 9th 947.2 933.0 969.4 952.3 949.9 948.7 937.5 977.2 961.8 955.4 10th 950.7 935.7 967.0 954.1 950.2 950.3 936.6 973.9 960.1 959.7 Avg: 937.9 969.8 955.5 939.4 974.1 The above table shows the minimum sea level pressure attained by Hurricane Ike in the Gulf of Mexico at any point during the 120 hour duration of each simulation, over four different days of simulations

A Comparison of Maximum Intensities CP: BLP: WSM3 KF YSU Pleim BMJ Grell-? Ferrier 6th 950.1 942.5 970.5 956.8 995.1 946.9 940.5 971.0 956.7 992.5 7th 952.4 940.4 972.4 958.8 989.5 954.3 942.8 974.2 963.6 989.4 9th 947.2 933.0 969.4 952.3 949.9 948.7 937.5 977.2 961.8 955.4 10th 950.7 935.7 967.0 954.1 950.2 950.3 936.6 973.9 960.1 959.7 Avg: 937.9 969.8 955.5 939.4 974.1 The above table shows the minimum sea level pressure attained by Hurricane Ike in the Gulf of Mexico at any point during the 120 hour duration of each simulation, over four different days of simulations

A Comparison of Maximum Intensities CP: BLP: WSM3 KF YSU Pleim BMJ Grell-? Ferrier 6th 950.1 942.5 970.5 956.8 995.1 946.9 940.5 971.0 956.7 992.5 7th 952.4 940.4 972.4 958.8 989.5 954.3 942.8 974.2 963.6 989.4 9th 947.2 933.0 969.4 952.3 949.9 948.7 937.5 977.2 961.8 955.4 10th 950.7 935.7 967.0 954.1 950.2 950.3 936.6 973.9 960.1 959.7 Avg: 937.9 969.8 955.5 939.4 974.1 The above table shows the minimum sea level pressure attained by Hurricane Ike in the Gulf of Mexico at any point during the 120 hour duration of each simulation, over four different days of simulations

Some Observations Choice of CP seems to determine storm track much more than either choice of MP or BLP Conversely, choice of BLP seems to impact storm intensity to a similar degree as choice of CP For these parameterizations, choice of MP seems to affect track and intensity surprisingly little Storms using the same CP can have much different intensities, but this does not appear to influence the storm track significantly

TC Cyclogenesis: Alma/Arthur From Jeff Masters’ Wunderground Blog* from 26 May 2008: Possible development in the Western Caribbean or Eastern Pacific late this week For the past 3-6 days, our most reliable global computer weather forecast models have been predicting the development of a low pressure system near or over Central America by Friday of this week. Given the persistence in the models in developing this low, we need to be alert to the possibility of a tropical depression forming in either the Western Caribbean or Eastern Pacific, on either side of Costa Rica and Nicaragua. It is uncertain which ocean basin such a storm might form in, and whether or not there will be a tropical wave around to help kick off development. It may be that the low pressure region will stay anchored over land south of the Yucatan Peninsula, preventing any development. This is the solution preferred by the ECMWF model in its last few runs. However, the GFS, NOGAPS, and Canadian models all predict a tropical depression might form in the Western Caribbean near Mexico's Yucatan Peninsula. In contrast, the UKMET shows development in the Eastern Pacific, on the Pacific side of Central America. Climatologically, May tropical storms are much more common in the Eastern Pacific than the Western Caribbean, so we should not discount the UKMET solution, even though it is an outlier. All five models predict that the Central American low pressure area will move northward towards the Gulf of Mexico, and wind shear may fall enough to allow a tropical depression to form should the low's center emerge over water. I'll be posting daily updates on the situation this week. - Jeff Masters *http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=947&tstamp=200805

Inner Grid 84 Hour Forecast Initialized 1800 UTC 26 May

Inner Grid 120 Hour Forecast Initialized 1200 UTC 28 August

Preexisting Hurricane Gustav

Future Hurricane Hanna

Future Hurricane Ike

Future Tropical Storm Josephine

The “Unnamed Storms”

Outer Grid 120 Hour Forecast Initialized 1200 UTC 28 August

Conclusions & Future Work Initial results suggest this method of ensemble generation can effectively reproduce spreads more typical of a “traditional” ensemble This is true for track, intensity, and TC cyclogenesis Ideally, the ensemble size could be increased to include additional CPs, MPs, BLPs, or additional categories of parameterizations Given enough data, hopefully statistics could be created that would enhance the usefulness of this ensemble Additional variables could be studied, such as radius of maximum wind or precipitation amounts More detailed comparisons between the two types of ensembles need to be performed

http://aurora.aos.wisc.edu/tropical/WRF.shtml