Impact of Tropical Easterly Impact of Tropical Easterly Waves during the North American Waves during the North American

Monsoon (NAM) using a Monsoon (NAM) using a Mesoscale ModelMesoscale Model

Jennifer L. AdamsJennifer L. AdamsCIMMS/University of OklahomaCIMMS/University of Oklahoma

Dr. David StensrudDr. David StensrudNOAA/National Severe Storms LaboratoryNOAA/National Severe Storms Laboratory

October 27, 2005October 27, 2005

What is the NAM?What is the NAM?

Distinct shift in mid-level winds accompanied by Distinct shift in mid-level winds accompanied by an increase in rainfallan increase in rainfall

Occurs over NW Mexico and SW United StatesOccurs over NW Mexico and SW United States

Onset usually in July and decays in SeptemberOnset usually in July and decays in September

Great deal of variabilityGreat deal of variability

NAM Moisture SourceNAM Moisture Source

Moisture source current consensusMoisture source current consensus– low-level moisture low-level moisture Gulf of California (GoC) Gulf of California (GoC) – mid-level moisture mid-level moisture Gulf of Mexico (GoM) Gulf of Mexico (GoM)

Transport of low-level moisture by gulf surges (one Transport of low-level moisture by gulf surges (one way)way)

Induced by passage of tropical easterly waves Induced by passage of tropical easterly waves (TEWs) over GoC and/or outflow boundaries/gust (TEWs) over GoC and/or outflow boundaries/gust frontsfronts

Gulf surgesGulf surges

Hales (1972) and Brenner Hales (1972) and Brenner (1974)(1974)

Cooler temps, increased Cooler temps, increased dewpoints, pressure rise, dewpoints, pressure rise, southerly windsoutherly wind

Increase in convectionIncrease in convection

Shallow vertical extentShallow vertical extent

Loss of definition upon entering Loss of definition upon entering desert SWdesert SW

Adams and Comrie (1997)

MotivationMotivation

NAM predictability very lowNAM predictability very low

TEWs influential to strength of NAMTEWs influential to strength of NAM

Inverse relationship between NAM and U.S. Inverse relationship between NAM and U.S. central plains rainfallcentral plains rainfall

GoalsGoals

Explore impact of TEWs on the NAMExplore impact of TEWs on the NAM– gulf surgesgulf surges– NAM region rainfallNAM region rainfall

Control run of MM5 compared to simulation Control run of MM5 compared to simulation where TEWs are removedwhere TEWs are removed

Model DescriptionModel Description

Pennsylvania State University/National Center for Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5)Atmospheric Research Mesoscale Model (MM5)

Model domain (350x180x23) at 25 km grid spacingModel domain (350x180x23) at 25 km grid spacing

Puerto Penasco

MM5 Parameterization SchemesMM5 Parameterization Schemes

Kain-Fritsch convective scheme (Kain and Fritsch 1990)Kain-Fritsch convective scheme (Kain and Fritsch 1990) MRF PBL scheme (Hong and Pan 1996)MRF PBL scheme (Hong and Pan 1996) Simple water and ice microphysics (Dudhia 1989)Simple water and ice microphysics (Dudhia 1989) Global terrain dataset – 10 minute resolution (25 USGS Global terrain dataset – 10 minute resolution (25 USGS

land use categories)land use categories) Rapid Radiative Transfer Model for radiationRapid Radiative Transfer Model for radiation 5-layer soil model (Dudhia 1996)5-layer soil model (Dudhia 1996)

Model initializationModel initialization: NCEP/NCAR reanalysis data: NCEP/NCAR reanalysis data– supply boundary conditions every 6-hsupply boundary conditions every 6-h– GoC SSTs set to constant 29.0GoC SSTs set to constant 29.0ºCºC

MethodologyMethodology

Four one-month periodsFour one-month periods– July 1990, July 1992, August 1988, August 1986July 1990, July 1992, August 1988, August 1986

ECMWF reanalysis data and CPC precipitation ECMWF reanalysis data and CPC precipitation analysisanalysis

Varying number of TEWs and rainfall amountsVarying number of TEWs and rainfall amounts

ECMWF Hövmoller Diagrams (850 mb)ECMWF Hövmoller Diagrams (850 mb)

July 1990

August 1986

July 1992

August 1988

MethodologyMethodology

Harmonic analysis to remove TEWs from Harmonic analysis to remove TEWs from boundary conditionsboundary conditions– Reed et al. (1977) – TEWs average wavelength 2500 Reed et al. (1977) – TEWs average wavelength 2500

km, propagation speed of 8 m/s, and average period of km, propagation speed of 8 m/s, and average period of 3.5 days3.5 days

TEWs with periods of approx. 3.5-7.5 days TEWs with periods of approx. 3.5-7.5 days identified – amplitudes replaced with value identified – amplitudes replaced with value of zero south ofof zero south of 30 30ºNºN– T, q, u, v, ght, and slpT, q, u, v, ght, and slp

HarmonicsHarmonicsHarmonicHarmonic Period (days)Period (days)

AA 3131

BB 15.515.5

CC 10.3310.33

DD 7.757.75

EE 6.206.20

FF 5.175.17

GG 4.434.43

HH 3.883.88

II 3.443.44

JJ 3.103.10

Harmonic Amplitudes (~40˚W)Harmonic Amplitudes (~40˚W)

TEW No-TEW

August 1988

Harmonic E

Harmonic E

MM5 HMM5 Hövmoller Diagrams (700 mb)övmoller Diagrams (700 mb)

TEW

No TEW

ResultsResults

18 surges over 4 18 surges over 4 months examinedmonths examined– 17 induced by 17 induced by

TEW/tropical stormTEW/tropical storm

Varying degrees of Varying degrees of strength and frequencystrength and frequency

MonthMonth # of surges# of surges

August 1986August 1986 66

August 1988August 1988 44

July 1990July 1990 55

July 1992July 1992 33

Surge CriteriaSurge Criteria

Used time-series data at Puerto Penasco, Mexico Used time-series data at Puerto Penasco, Mexico as a “first pass” to ID surge eventsas a “first pass” to ID surge events

Surges occur when:Surges occur when:– winds shift to southerlywinds shift to southerly– maximum daily dewpoint exceeding 65maximum daily dewpoint exceeding 65ºF for at least 2 ºF for at least 2

daysdays– peak wind speeds greater than 5 m/speak wind speeds greater than 5 m/s– decrease in daily max temp of greater than 5ºF from the decrease in daily max temp of greater than 5ºF from the

previous dayprevious day

August 1986 Time-seriesAugust 1986 Time-seriesAugust 1986 Wind Direction Time-series at Puerto Penasco,

Mexico

0.00

30.00

60.00

90.00

120.00

150.00

180.00

210.00

240.00

270.00

300.00

330.00

360.00

0 36 72 108144180216252288324360396432468504540576612648684720

Forecast hour

Wind direction (deg.)

TEW

August 1986 Time-seriesAugust 1986 Time-seriesAugust 1986 Temperature Time-series at Puerto Penasco,

Mexico

70.00

75.00

80.00

85.00

90.00

95.00

100.00

105.00

0 36 72 108144180216252288324360396432468504540576612648684720

Forecast hour

Temperature (F)

TEW

August 1986 ResultsAugust 1986 Results

6 surges in the control run6 surges in the control run– all show up in the time-series data at Puerto all show up in the time-series data at Puerto

PenascoPenasco

5 induced by TEWs and 1 initiated by a 5 induced by TEWs and 1 initiated by a tropical storm (Howard?)tropical storm (Howard?)

2 TEWs possibly contained in the model 2 TEWs possibly contained in the model initial conditionsinitial conditions

TEW passage (18Z Aug 26)TEW passage (18Z Aug 26)

TEW No TEW

Pre-surge (18Z Aug 26)Pre-surge (18Z Aug 26)

TEW No TEW

Surge onset (06Z Aug 27)Surge onset (06Z Aug 27)

TEW No TEW

Surge (12Z Aug 28)Surge (12Z Aug 28)

TEW No TEW

Post-surge (12Z Aug 29)Post-surge (12Z Aug 29)

TEW No TEW

Surge summarySurge summary

TEW passage 12 hours prior to surge onsetTEW passage 12 hours prior to surge onset

Entire GoC shifts to southerly windsEntire GoC shifts to southerly winds– 10 of 18 surges (most common)10 of 18 surges (most common)

Surge virtually absent from no-TEW Surge virtually absent from no-TEW simulationsimulation

TEWs and NAM rainfallTEWs and NAM rainfall

Absence of TEWs has impact on Absence of TEWs has impact on precipitation amounts over the NAM regionprecipitation amounts over the NAM region

Many areas receive more rainfall when Many areas receive more rainfall when TEWs are presentTEWs are present

Influences overall extent of NAM Influences overall extent of NAM precipitationprecipitation

August 1988 Rainfall Differences August 1988 Rainfall Differences (TEW-no TEW)(TEW-no TEW)

Central Plains Rainfall Differences Central Plains Rainfall Differences (TEW-no TEW)(TEW-no TEW)

August 1988

Meridional Moisture FluxMeridional Moisture Flux

August 1988 Meridional Moisture Flux Average at Dallas/Ft. Worth (s=0.995 to 0.675)

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

024 48 72 96

120144168192216240264288312336360384408432456480504528552576600624648672696720

Forecast hour

Meridional moisture flux (kg/kg*m/s)

TEW

no TEW

Rainfall Differences (00Z Aug 17-Rainfall Differences (00Z Aug 17-06Z Aug 23)06Z Aug 23)

August 1988

Rainfall Differences -- 12Z Aug Rainfall Differences -- 12Z Aug 19 - 00Z Aug 2519 - 00Z Aug 25

August 1986

Meridional Moisture FluxMeridional Moisture FluxAugust 1986 Meridional Moisture Flux Average at Dallas/Ft. Worth

(s=0.995 to 0.675)

-0.10

-0.05

0.00

0.05

0.10

0.15

0 24 48 72 96 120144168192216240264288312336360384408432456480504528552576600624648672696720

Forecast hour

Meridional moisture flux (kg/kg*m/s)

TEW

no TEW

Precipitable WaterPrecipitable WaterAugust 1986 Precipitable Water Time-series at Goodland, KS

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 42 84126168210252294336378420462504546588630672714

Forecast hour

Precipitable water (mm)

TEW

no TEW

Mid-latitude forcing -- 12Z Aug Mid-latitude forcing -- 12Z Aug 20, 198620, 1986

TEW No TEW

Adding TEWsAdding TEWs

July 1992 --> weak monsoon season July 1992 --> weak monsoon season July 1990 --> strong monsoon seasonJuly 1990 --> strong monsoon season

Removed waves from July 1992 boundary Removed waves from July 1992 boundary conditionsconditions

Inserted July 1990 TEWs into July 1992 Inserted July 1990 TEWs into July 1992 boundary conditionsboundary conditions

MM5 HMM5 Hövmoller Diagramsövmoller Diagrams

Hybrid

July 1992

12Z July 19 Hybrid run TEW passage12Z July 19 Hybrid run TEW passage

Surge (00Z July 20)Surge (00Z July 20)

Hybrid July 1992

Hybrid - July 1992 TEW RunHybrid - July 1992 TEW Run

ConclusionsConclusions

Harmonic analysis successfully removes TEWs Harmonic analysis successfully removes TEWs from the model boundary conditionsfrom the model boundary conditions

MM5 reproduces surges over the GoCMM5 reproduces surges over the GoC– full gulf, partial gulf, and SMOfull gulf, partial gulf, and SMO

NAM shows great deal of interannual variabilityNAM shows great deal of interannual variability

Surges impacted by absence of TEWsSurges impacted by absence of TEWs

ConclusionsConclusions

Reduction of surge events in the no-TEW run Reduction of surge events in the no-TEW run reduced rainfall amounts over the NAM regionreduced rainfall amounts over the NAM region

Absence of TEWs increases precipitation over the Absence of TEWs increases precipitation over the central United Statescentral United States– CAPECAPE– mid-latitude forcingmid-latitude forcing

Adding waves enhances NAMAdding waves enhances NAM– more distinct surge eventsmore distinct surge events– increase in rainfall over core monsoon regionincrease in rainfall over core monsoon region

Harmonic AnalysisHarmonic Analysis Since the model data used to create the boundary conditions are equally Since the model data used to create the boundary conditions are equally

spaced in time and contain no missing values, the model data can be spaced in time and contain no missing values, the model data can be represented exactly as a series of n points in time by summing a series of n/2 represented exactly as a series of n points in time by summing a series of n/2 harmonic functions….harmonic functions….

]}2sin[]2cos[{2/1 nktBnktAyy knkkt ππ++=∑= , (1)

where

)2cos(21∑==nttk nktynA π , (2)

)2 (sin21∑==nttk nktynB π . (3)