IBM T. J. Watson Research Center © 2009 IBM Corporation Developing Better Forecasts: An analysis of...

IBM T. J. Watson Research Center

© 2009 IBM Corporation

Developing Better Forecasts:Developing Better Forecasts:An analysis of two storms during summer 2009 An analysis of two storms during summer 2009

using the high resolution WRF model and using the high resolution WRF model and observations on July 7 and August 18observations on July 7 and August 18

Elena Novakovskaia, IBMBrandon Hertell, Consolidated Edison



IntroductionIntroduction• Focus is on detailed analysis of high-impact weather events for decision making, emergency response and preparedness, and planning for storm recovery actions

– utility companies;– private transportation;– emergency management by municipal and state government agencies;– and many more.

• High resolution localized numerical weather predictions provide information on the multi-faceted aspects of severe weather events and have advantages for allocation of resources with sufficient lead time

• Operational mesoscale prediction system, called “Deep Thunder”, has been developed at the IBM Thomas J. Watson Research Center. It is implemented and applied to business problems in a number of metropolitan areas in the US, including New York City.

• To assess the uncertainties in high-resolution mesoscale forecasts, we utilize ensemble-based probabilistic predictions suitable for risk analysis, and examine the most extreme and damaging scenarios.

• We analyze each event - whether the model forecast was correct or not - to find information that would enable the model to provide a better forecast in the future.



MethodologyMethodology

• Multiple forecast realizations in the ensemble are created using a variety of physical process schemes within the WRF-ARW model and the same initial and boundary conditions

IC/BCDecision Making based on Probabilistic Forecasting



Ensemble Run MatrixEnsemble Run Matrix

E1 Yonsei University Kessler Kain-Fritsch

E2 "" Lin et al. ""

E3 "" WSM 5-class ""

E4 "" Kessler Grell-Devenyi

E5 "" Lin et al. ""


E7 Mellor-Yamada-Janjic Kessler Kain-Fritsch

E8 "" Lin et al. ""


E10 "" Kessler Grell-Devenyi

E11 "" Lin et al. ""




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Simulation Model DomainSimulation Model Domain• WRF/ARW V3.1.1 model

• Domain: – Nested grid: 18/6/2 km (76x76x42) – Centered at 41°N, 73.5°W – 42 vertical levels

• Radiation: – Longwave – RRTM – Shortwave – Goddard

• Noah Land Surface Model

• Initial and Boundary conditions:– NAM (12 km)– SST RTG (0.5 deg)

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Soundings and Surface ObservationsSoundings and Surface Observations

Sites in NY, PA, VA and MD

• Observations from the closest NWS radiosonde sites within the model domain:

– OKX and ALB are within 6 km grid

– BUF, PIT, IAD and WAL – within 18 km grid

WeatherBug/AWS sites

• Surface observations from WeatherBug/AWS at 5 min sampling rate

• Data: winds, temperature, pressure, humidity, rainrate



Storm of July 7, 2009Case Study:



July 7, 2009July 7, 2009• It had been a stormy day in the NYC area

• At 7:45pm NWS Upton took down the Severe T-Storm Watch as all thunderstorm activity was dying out and moving away.

• At 9:30pm a broken line of thunderstorms entered and moved across Orange & Rockland County – strengthening rapidly as they moved east through approx 10:30pm.

Radar Image July 7, 2009 at 11:36pm EDT

• At 11:09pm NWS issued a Severe Thunderstorm Warning for Rockland and Westchester County

• Between 11:27pm – 11:53pm this storm moved through Westchester County.

• 11:34pm a tornado warning was issued for Westchester County based on the radar signature.



Microburst EventMicroburst Event

• That powerful and highly localized storm cell hit the city of Yonkers in Westchester County, NY. • Fallen trees caused power outages and road closings, local rescue teams rushed to help the recovery. • In the morning on July 8, 2009 Yonkers Third Deputy Fire Chief Ed Cucolo said: “We, the fire and police department are spread very very thin. It’s a mess.”

Photo by Adrienne Leptich, WFO NY, New York

• Extra manpower was called in by the Fire Department, including 16 bucket trucks from Consolidated Edison on the way in the city to help in the cleanup, Cucolo said. • Due to road closings and outages, summer schools were cancelled, emergency services, including distribution of ice, were provided to residents



• NWS determined that the damage caused by this storm in the Yonkers area of Westchester County was due to straight-line winds between 70-100mph.

• Magnitude of this storm at this time of night in this location caught many by surprise.• No “wow” factor in weather/atmospheric conditions that day which might have kept

awareness up that an event of this magnitude would occur that night. • Present factors: a strong upper level jet, wind shear, weak surface trough and weak

area of low pressure.

08/03z - Surface Analysis 08/00z - 300mb Analysis

Low pressure Low pressure

Surface TroughSurface Trough

Jetstream Wind Speed Jetstream Wind Speed over Westchester over Westchester July 7, 2009 – 8pmJuly 7, 2009 – 8pm

approx 85 kntapprox 85 knt

AnalysesAnalyses



Not impressive CAPE has decreased, LI = 0.

SoundingsSoundings

• Both soundings indicate strong upper level jet• Between 12z and 00z – increased wind shear, helicity and humidity at lower levels



INIT E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 E11 E12

Simulated Profiles for NWS site in Upton, NY

• Winds (in knots) up to 100 mb level at initial time (07/08/12z), each ensemble member at 07/08/00z, and observations at 07/08/00z

• Uncertainties in wind speed and direction at levels below 750 mb; most near-surface wind veering for MYI-K-GD (E10) and near 900-800 mb for MYJ-W-KF (E9)

OBS

08/00z



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INIT

07/12z

E1 E2 E3 E4 E5 E6

E7 E8 E9 E10 E11 E12

• On 07/08/00z, there are uncertainties in predicted moisture content not only at levels below 800 mb (950mb Td in [11.2, 16.1] ºC, but also up to 300 mb levels (450 mb Td in [-14.9, -0.9] ºC)

• For levels below 700 mb on 07/08/00z, the driest case (and most inconsistent with observations) is with MYJ-L-GD (E11)




Yonkers, NY on July 7, 2009Yonkers, NY on July 7, 2009

• For YSU-W-KF (E3) sharp wind speed change and more than average drop in temperature• YSU scheme in combination with GD has dry bias and higher surface pressure



Event Evolution from SimulationsEvent Evolution from Simulations

• Simulated localized convective event captured by E3 (YSU-WSM5-KF) with the highest wind speed after 8:00 pm EDT• Vertical velocity field (shaded) shows rising motion in the area of Yonkers, NY• Winds at 10 m height (streamlines) show inflow toward the center of convective cell• Rainrate - in blue contour lines at 1 mm/h intervals KOKX Nexrad – valid on 11:36pm

07/07/2009 9:30 pm EDT 9:40 pm EDT 9:50 pm EDTm/sec




• Stronger surface winds around 10:00 pm EDT (^) • Significant veering of the wind vector (O)• Almost 180 degrees change in wind direction (X) up to 920 mb

X X

X X

8:5

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^

2. (40. 936 ºN, 73.831 ºW)• BL winds simulated by E3• Locations in the area of Yonkers, NY

1. (40.940 ºN, 73.890 ºW)



Storm of August 18, 2009Case Study:



August 18, 2009August 18, 2009

• Winds in excess of 80 m.p.h. ripped through Central Park, Riverside Park, Thomas Jefferson Park, and Randall's Island, damaging at least one thousand trees.

• A strong line of thunderstorms developed in the late afternoon over central New York southwest through the Ohio River Valley which held together well as they moved east. By 10pm they were moving into the NYC area. One strong cell was in Essex County, NJ at 9:30pm and over Manhattan by 10:05pm.

KOKX radar – 10:05pm EDTPhoto by Tom Kaminski from WCBS news chopper-880



Straight Line WindsStraight Line Winds• NWS spotters and Weatherbug stations in Manhattan and the Bronx reported wind gusts in the 40-50mph range. Two offshore buoys in Long Island Sound off Queens and Port Chester reported 67mph gusts as the storm moved further east.• According to NWS spotters/reports over 100 tress were knocked down in Central Park. • This damage was also attributed to straight-line winds by the NWS.



AnalysesAnalyses• Obvious similarities between the two events:

– Time of year - summer– Time of day (although strength is abnormal for both since well after peak daytime heating

hours)– Both days were hot and humid

• On August 18 - no strong jet like July 7• There was a weak surface trough with an area of low pressure over Delmarva on the 19/00z

surface analysis

300mb Analysis: Valid 19/00z Surface Analysis: Valid 19/00z



SoundingsSoundings

• Soundings show atmospheric conditions more favorable for thunderstorm development– At 12z right near the surface – humid– No strong jet like July 7

• At 00z very humid environment near 850 mb level– Change in wind direction near the surface, also significant change in direction and

strengthening of winds at 500-200 mb levels



INIT E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 E11 E12 OBS

19/00z

Simulated Winds

• Winds (in knots) up to 100 mb level at initial time (08/18/12z), each ensemble member at 08/19/00z, and observations at 08/19/00z

• Ensemble consensus for wind speed and direction; lower level winds below 800 mb slightly stronger for E1,E2, E4,E5 (YSU-K, YSU-L)



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E1 E2 E3 E4 E5 E6

E7 E8 E9 E10 E11 E12

• On 08/19/00z, there are uncertainties in predicted moisture content at levels below 800 mb (950mb Td in [16.9, 18.7] ºC, and at levels up to 300 mb (400 mb Td in [-17.5, -15.7] ºC)

• For levels below 700 mb on 08/19/00z, the driest case (and most inconsistent with observations) is with MYJ-L-GD (E4)




Bronx, NY on August 18, 2009Bronx, NY on August 18, 2009

• For MYJ-K-KF (E7) sharp wind speed change and more than average drop in temperature• YSU scheme has dry bias and lower winds; for afternoon - also slightly higher surface

temperatures



Manhattan, NY on August 18, 2009Manhattan, NY on August 18, 2009

• Similar to Bronx, NY conclusions for MYJ-K-KF (E7)• More obvious dry bias and lower winds for YSU scheme; for afternoon - also slight positive bias

in surface temperatures



Event Evolution from SimulationsEvent Evolution from Simulations8:30 pm EDT 8:45 pm EDT 9:00 pm EDT

TDWR at Newark, NJ,valid for August 18, 2009

8:15 pm EDT8:00 pm EDT

• Simulated highly localized convective event captured by ensemble member #7 (MYJ-K-KF) with a wind speed peak shortly after 8:00 pm EDT• While simulated timing of the event is slightly earlier than observed, additional insight on event evolution is provided by the model• Sequence of snapshots at 15 min interval shows vertical velocity field (shaded), winds at 10 m height (streamlines), and rainrate (blue contour lines) at 1 mm/h intervals. Maximum rainrate at 8:30 pm – 2.2mm/15min, 8:45pm - 2.4mm/15min.



m/sec

Localized Convective CellLocalized Convective Cell8:30 pm EDT8:15 pm EDT

• Downdraft/updraft motion in the area of Manhattan and Bronx, NY (color shaded fields)• Winds at 10 m height (streamlines) show inflow toward the center of convective cell• Most intense rainfall in the center of convective cell (blue contours at 1 mm/h intervals)




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• Winds are much stronger than during the July 7,

2009 storm (^) • Most significant dynamics at levels below 980 mb (X)

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• Sites are in New York City • Significant veering of the wind vector, almost 180 degrees change in wind direction in 15 min (O)

1. (40.850 ºN, 73.870 ºW)

2. (40.880 ºN, 73.850 º)

^

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Event Similarity MatrixEvent Similarity Matrix

Present on July 7 Present on August 18 Captured by Model

Jet Stream Strengthening

X o X

Wind Shear/Veering

X(low level, sounding)

X(upper level, sounding)

X

Surface Front X X X

Low pressure area

X X X

Inflow/Outflow Model added value.

Thunderstorm and convective activity – on radar

Model added value. Thunderstorm and convective

activity – on radar

X

Vertical Velocity X

Rainfall X X X

Short durationlocalized event

X X X



ConclusionConclusion• Meteorological assessment of the event based on soundings and surface

observations shows a few similarities in atmospheric conditions between the two storms

• Ensemble forecasts were performed and validated against observations; the closest matching scenarios provided an additional insight on the events

• There are storm features at smaller scales that were revealed through simulations. While both events were very localized, simulations at 2 km resolution were able to capture additional details of these storms

• Events were different enough, so that analysis of the first event would not be sufficient to provide a better forecast for the second event. For the considered storms, the physical processes schemes of the most matching run were different (YSU-WSM5-KF and MYJ-K-KF). Using an ensemble with different schemes to simulate multiple scenarios for a high-impact weather event has an advantage of capturing the worst case scenario and estimating its probability.

• Our analysis indicates that a high-resolution mesoscale ensemble, which is based on a variety of physical schemes, is a valuable tool for developing better forecasts.

IBM T. J. Watson Research Center © 2009 IBM Corporation Developing Better Forecasts: An analysis of...

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