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SEMI-INTELLIGENT USE OF ATMOSPHERIC MODELS

Michael Eckert

HYDROMETEOROLOGICAL PREDICTION CENTER

CAMP SPRINGS, MD

E-MAIL ADDRESS: meckert@ncep.noaa.gov

Presented by: Pete Manousos

COMET Hydromet 00-2

Friday, 25 February 2000

Understanding the performance of an operational model is critical to being able to forecast the sensible weather

All models have strengths and weaknesses.All models have strengths and weaknesses. All have trouble handling smaller scale features. All have trouble handling smaller scale features. All have problems with convection.All have problems with convection. All do a decent job in handling the short range All do a decent job in handling the short range

(0-36 hr) forecast of synoptic scale features. (0-36 hr) forecast of synoptic scale features.

Why models have forecast problems Initialization and quality control smooth data fields, but some of the Initialization and quality control smooth data fields, but some of the

lost detail may be important. lost detail may be important. May have poor first guessMay have poor first guess Lack of data over the oceans and Mexico. Lack of data over the oceans and Mexico. Atmospheric processes are non-linear; small changes in initial Atmospheric processes are non-linear; small changes in initial

conditions can lead to large forecast variations (this is the basis for conditions can lead to large forecast variations (this is the basis for ensemble forecasting).ensemble forecasting).

Model physics are approximationsModel physics are approximations

for lower resolution models, convection is parameterizedfor lower resolution models, convection is parameterized for higher resolution models the micro-physical for higher resolution models the micro-physical

processes are parameterized processes are parameterized

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INTELLIGENT USE OF THE MODEL REQUIRES THAT THE FORECASTER

COMPARE THE INITIAL 00HR COMPARE THE INITIAL 00HR FORECAST WITH DATAFORECAST WITH DATA

BE FAMILIAR WITH CHARACTERTIC BE FAMILIAR WITH CHARACTERTIC MODEL ERRORS AND BIASES. MODEL ERRORS AND BIASES.

HAVE A ROUGH UNDERSTANDING HAVE A ROUGH UNDERSTANDING OF HOW APPROXIMATIONS OF THE OF HOW APPROXIMATIONS OF THE PHYSICS MAY NEGATIVELY IMPACT PHYSICS MAY NEGATIVELY IMPACT A FORECAST. A FORECAST.

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The performance characteristics of the Eta have changed dramatically during the past year. QPF forecasts during the past winter deteriorated QPF forecasts during the past winter deteriorated

when compared to the AVN or NGM. when compared to the AVN or NGM. ETA surface and 500 mb forecasts have also been ETA surface and 500 mb forecasts have also been

worse compared to the other models. worse compared to the other models. April 1999 ETA 500 h and 250 mb forecasts usually April 1999 ETA 500 h and 250 mb forecasts usually

verified worse than the AVN.verified worse than the AVN.

DESPITE ITS RECENT PROBLEMS, THE ETA IS STILL USUALLY BETTER THAN THE AVN OR NGM FORECASTING PRECIPITATION OVER COMPLEX TERRAIN DURING WINTER IN A ZONAL PATTERN.

12-36 H NGM V.T. 12Z 3 JAN 9712-36 H ETA V.T. 12Z 3 JAN 97 ANALYSIS V.T. 12Z 3 JAN 97

Note that the Eta max in California is a little too far west. It also often under predicts precipitation over the Siskiyou Mountains of northern California.

The T126 version of the AVN does not have sufficient resolution to adequately depict the rain shadow east of the Cascades. A forecaster needs to know

the climatology of precipitation during certain flow regimes.

Prism climatological precipitation data. Note the distinct rain shadow to the east of the Cascades

12-36 AVN V.T. 12Z 23 FEB 1999

PRECIPITATION ANALYSIS AVN V.T. 12Z 23 FEB 1999

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Eta Model Physics

Eta model calculates grid-scale precipitation Eta model calculates grid-scale precipitation using a simplified explicit cloud water schemeusing a simplified explicit cloud water scheme includes super-cooled water, simplified snow includes super-cooled water, simplified snow

processes and the advection of cloud water and processes and the advection of cloud water and cloud ice cloud ice

but does not include horizontal advection of snow but does not include horizontal advection of snow and rain. and rain. In fast flow snow can advect 50 to 100 km downwind In fast flow snow can advect 50 to 100 km downwind

of its source region (Rauber, 1992)of its source region (Rauber, 1992)

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EXPLICIT CLOUD PREDICTION SCHEME (large scale) Cloud condensation is allowed to occur when the RH Cloud condensation is allowed to occur when the RH

reaches a critical valuereaches a critical value Cloud evaporation is allowed to take place only when the Cloud evaporation is allowed to take place only when the

RH falls below the critical valueRH falls below the critical value 70% over land, 80% over water70% over land, 80% over water the difference in the critical value between land and water can the difference in the critical value between land and water can

produce discontinuities along the coastproduce discontinuities along the coast this may be one of the reasons the Eta over predicts cold season this may be one of the reasons the Eta over predicts cold season

precipitation along the Gulf and Atlantic Coasts. precipitation along the Gulf and Atlantic Coasts.

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More about the BMJ scheme

then develops sounding based on a reference profile.then develops sounding based on a reference profile. The important thing to remember about the profile is it will not The important thing to remember about the profile is it will not

allow the sounding to become completely saturated.allow the sounding to become completely saturated. The saturation pressure deficit is found for three levels (cloud The saturation pressure deficit is found for three levels (cloud

base, freezing level and cloud top) and the final reference base, freezing level and cloud top) and the final reference profile based on the type of cloud efficiency profile based on the type of cloud efficiency

Saturation pressure deficits are then found for other points Saturation pressure deficits are then found for other points along the profile along the profile

• when the saturation pressure deficits are exceeded, the moisture when the saturation pressure deficits are exceeded, the moisture produces rain. produces rain.

• however, the amount of rainfall must be in balance with the however, the amount of rainfall must be in balance with the latent heating. latent heating.

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The BMJ scheme Was developed for tropical systemsWas developed for tropical systems

does not handle elevated convection welldoes not handle elevated convection well the convection may not extend through a deep enough layerthe convection may not extend through a deep enough layer

does not develop realistic downdrafts/outflow does not develop realistic downdrafts/outflow boundariesboundaries therefore, during summer it sometimes predicts the therefore, during summer it sometimes predicts the

convective development too far northconvective development too far north the saturation pressure deficits in the scheme are the saturation pressure deficits in the scheme are

different over land and water different over land and water

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THE ETA OFTEN FORECASTS TOO MUCH RAINFALL NEAR THE GULF AND SOUTHEAST COASTS BECAUSE OF THE PROBLEMS WITH THE WAY THE ETA HANDLES THE LAND-SEA INTERFACE

12-36 HR PRECIPITATION FORECAST V. T. 12Z 1 APR

24 HR PRECIPITATION ANALYSIS V. T. 12Z 1 APR

FOR ANY MODEL, ALWAYS BEWARE OF THE 1ST GUESS

00 HR ETA 500 MB HEIGHT AND VORTICITY V.T. 00Z 3 SEP 1998

00 HR ETA SURFACE AND 1000-500 MB THICKNESS V.T. 00Z 3 SEP 1998

SURFACE ANALYSIS V.T. 00Z 3 SEP 1998

TROPICAL STORM EARL WAS LOCATED JUST SOUTHWEST OF THE FLORIDA PENINSULA. THE 1ST GUESS WILL SOMETIMES OVERRIDE DATA WHEN INTENSE SMALL SCALE FEATURES ARE PRESENT.

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WHAT HAPPENED?

THE MRF 1ST GUESS FIELD TRIED TO DRAW TO THE DATA BUT THE FIRST GUESS FIELD OVERWHELMED IT. IF NCEP HAS A COMPUTER

FAILURE, THE ETA DATA ASSIMILATION MAY BE CANCELLED AND THE ETA MAY RUN ON AN EARLIER GLOBAL MODEL 1ST GUESS

A POOR INITIAL ANALYSIS CAN PRODUCE HUGE FORECAST ERRORS

ETA 48 HR V.T. 00Z SEP 5 ETA 48 HR V.T. 00Z SEP 5

ETA 00 HR V.T. 00Z SEP 5

EARL

EARL

ETA 00 HR V.T. 00Z SEP 5

EARL

EARL

IN THE PAST, ETA MODEL HAS HAD PROBLEMS PREDICTING THE STABILITY. PROBLEM IS OFTEN TIED TO THE 1ST GUESS

WHEN SOIL MOISTURE IS HIGH, THE ETA DEWPOINTS ARE TOO HIGH AND LOW-LEVEL TEMPERATURES ARE TOO LOW.

FORECAST OBSERVED

HIGH SOIL MOISTURE CASE

THE ETA FORECAST CAPE=1177, LI=-4

OBSERVED CAPE=5, LI=2

THIS SOMETIMES CAUSES THE MODEL TO BE TOO UNSTABLE

WHEN HIGH SOIL MOISTURE IS PRESENT, OR WHEN THE MODEL FIRST GUESS THINKS THE SOIL MOISTURE IS HIGH,

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20

2428 SURFACE

TEMPERATURE

DEWPOINT TEMPERATURE

SURFACE PRESSURE

20

24

16

12

10241020

1016

1012

20/0018/00 19/1819/1219/0619/0018/1818/1218/06

THEN, THE MODEL FORECASTS SURFACE DEWPOINTS TOO HIGH AND SURFACE TEMPS TOO LOW.

OBSERVED

ETA FORECAST

THE MODEL UNDERPREDICTS THE BOUNDARY LAYER WINDS. HOWEVER, MODEL FORECASTS OF 850 MB WINDS ARE OFTEN TOO

STRONG

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WHEN LOW SOIL MOISURE IS PRESENT DURING SUMMER OVER THE HIGH PLAINS, ESPECIALLY WEST TX, THE

FORECAST CAPE IS TOO LOW

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WHEN SOIL MOISTURE IS LOW IN SUMMER IN THE PLAINS, THE SURFACE DEWPOINT IS TOO LOW AND THE TEMPERATURE IS TOO HIGH

OBSERVED

ETA FORECAST

SURFACE TEMPERATURE

DEWPOINT

TEMPERATURE

SURFACE PRESSURE

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202428

36

12

16

20

1016

21/0020/18020/1220/0620/0019/1819/1219/0619/00

1008

MAY 1998

OKLAHOMA CITY

ETA SURFACE WINDS WERE TOO WESTERLY, WAS THERE TOO MUCH DOWN-SLOPE?

THE DEWPOINTS IN THE PLAINS AND SOUTHWEST WERE TOO LOW THIS SUMMER

Forecast -Vs- Observed Best CapeSpring 96

Line x=y

Line x=y

Forecast precipitation 1 - less than .25”

2 - more than .25”

Note the large spread. The model stability forecasts are worst when precipitation is forecast

MORE ON ETA PERFORMANCE

TOO WET IN FLORIDATOO WET IN FLORIDA SOMETIMES OVERDEVELOPS LOW-LEVEL JETSOMETIMES OVERDEVELOPS LOW-LEVEL JET HAS BEEN TOO FAST BRINGING SHORTWAVES HAS BEEN TOO FAST BRINGING SHORTWAVES

THROUGH THE ROCKIES INTO THE PLAINS.THROUGH THE ROCKIES INTO THE PLAINS. HAS BEEN TOO FAR SOUTH WITH CLOSED LOWS HAS BEEN TOO FAR SOUTH WITH CLOSED LOWS

COMING EASTWARD INTO THE PLAINSCOMING EASTWARD INTO THE PLAINS OVERFORECASTS THE STRENGTH OF OVERFORECASTS THE STRENGTH OF

ANTICYCLONESANTICYCLONES HAS PROBLEMS INITIALIZING TROPICAL HAS PROBLEMS INITIALIZING TROPICAL

SYSTEMSSYSTEMS

ETA AND STORM TRACKS DURING 1999 TENDED TO BE TOO FAR SOUTH WITH LOWS AS THEY REFORMED EAST OF ROCKIES. THIS ERROR USUALLY CONTINUED UNTIL THE LOW MOVED EAST OF THE MISSISSIPPI RIVER.

BY CONTRAST, THE AVN IS SOMETIMES TOO

FAR NORTH TENDS TO SOMETIMES TRACK LOWS TOO FAR NORTH AND WEST WITH LOWSALONG THE EAST COAST.

ESPECIALLY DURING MAJOR CYCLOGENESIS WHEN A COASTAL TROUGH IS PRESENT

COMMON ETA ERROR ALONG EAST COAST

48 H ETA 500H V.T. 12Z 23 APR 98 48 H NGM 500H V.T. 12Z 23 APR 98

WHEN A CLOSED UPPER LOW APPROACHES THE COAST THE ETA SOMETIMES HAS PROBLEMS FORECASTING THE LOCATION OF THE SURFACE LOW. NOTE WHERE THE UPPER LOW IS CENTERED AND WHERE THE STRONGEST UPPER-LEVEL DIVERGENCE IS IMPLIED.

Based on the 500 h and vorticity pattern, where would you predict the surface low?

NOTE THAT THE ETA SURFACE LOW IS A LITTLE WEST OF ITS 500 MB CENTER. THE NGM HAS A

MUCH BETTER FIT TO THE 500 MB PATTERN.

48 H ETA SURFACE V.T. 12Z 23 APR 98 48 H NGM SURFACE V.T. 12Z 23 APR 98

THE STRONG EASTERLY COMPONENT TO THE WINDS NORTH OF THE ETA MODEL LOW ALLOWS IT TO WRAP MOISTURE AND PRECIPITATION TOO FAR WEST

THE LOW VERIFIES A LITTLE NORTH AND EAST OF THE NGM. REMEMBER, THE NGM IS TYPICALLY TOO SLOW WITH LOWS ALONG THE COAST.

ETA SURFACE LOW

L

VERIFYING SURFACE V.T. 12Z 23 APR 98 VERIFYING 500H V.T. 12Z 23 APR 98

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When the NGM and AVN sheared 500 troughs approaching the east coast in 1999, the Eta often amplified the trough and over-deepened the surface low. An example:

48 HR ETA 500 48 HR ETA SFC

48 HR NGM SFC48 HR NGM 500

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The Eta predicted a major east coast snowstorm. The NGM and AVN predicted light snow at best

36-48 hr ETA precipitation 36-48 hr NGM precipitation

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HOW THE MODEL VERIFIED. NO

MAJOR SNOWSTORM DEVELOPED.

48 HR ETA 500

48 HR NGM 500 VERIFYING SFC

VERIFYING 500 MB

LOWS TO THE LEE OF THE ROCKIES THE AVN AND NGM USUALLY PREDICT THE AVN AND NGM USUALLY PREDICT

THEM TO FORM TOO FAR NORTHTHEM TO FORM TOO FAR NORTH THE ETA IS SOMETIMES A LITTLE TOO FAR THE ETA IS SOMETIMES A LITTLE TOO FAR

SOUTHSOUTH USE THE 300 MB UPPER LEVEL JET. THE USE THE 300 MB UPPER LEVEL JET. THE

SURFACE LOW IS USUALLY FOUND IN THE SURFACE LOW IS USUALLY FOUND IN THE LEFT EXIT REGION OF THE JET, USUALLY LEFT EXIT REGION OF THE JET, USUALLY JUST TO THE NORTHJUST TO THE NORTH

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28 ETA model runs were evaluated during the period from 00Z March 30-12 Z April 13. During the entire period the mean 500h

pattern was similar to the one shown below.

THE ETA SHOWED A CONSISENT CHARACTERISTIC ERROR DURING THE PERIOD. THE NEXT FEW SLIDES WILL DESCRIBE THE ERROR

A RIDGE AND POSITIVE ANOMALY NEAR 160W, BELOW NORMAL HEIGHTS OVER ALASKA AND A TROUGH NEAR OF JUST INLAND FROM THE WEST COAST WITH BELOW NORMAL HEIGHTS EXTENDING EASTWARD INTO THE SOUTHWESTERN U.S.

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As the upper trough digs into the west the ETA did not dig the shortwaves strongly enough once the trough reached the ca coast. Note how much lower the heights are across

NV and CA.

48 hr ETA valid 00Z 1 April 00 hr Eta valid 00Z 1 April

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The Eta underplays the second short-wave diving into the mean trough and

overplays the first one.

48 hr ETA 500 h and vorticity v.t. 12Z 4 April 00 hr ETA 500 h and vorticity v.t. 12Z 4 April

THE ETA PREDICTED THE UPPER LOW ASSOCIATED WITH THE FIRST SHORTWAVE TOO FAR SOUTH AND EAST IN THE PLAINS. INSTEAD THE INITIAL SHORTWAVE LIFTED MORE TO THE NORTH BEFORE BEING FORCED EASTWARD. THIS HAPPENED SEVERAL TIMES DURING THE STUDY.

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The Eta was generally too fast and far southeast with the 500h low over the Plains with 120 meter errors over MO and IA. This can have a very serious impact on frontal speed and on the position of the

low level convergence and resulting convection.

546558

48 hr Eta v.t. 12Z 10 Apr00 hr Eta v.t. 12Z 10

Apr

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The Eta surface low and associated fronts can also be affected. The slower eastward movement of the ridge axis may allowed for the flow along the east to be more northwesterly which allowed the surface

boundary to sink farther to the south

48 hr Eta v.t. 12Z 10 Apr 00 hr Eta v.t. 12Z 10 Apr

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Why models have problems with arctic airmasses Terrain is averagedTerrain is averaged Initialization process sometimes robs shallow Initialization process sometimes robs shallow

airmass of its coldnessairmass of its coldness Models have problems handling the strength of the Models have problems handling the strength of the

inversioninversion The sigma coordinate system, the Eta coordinate The sigma coordinate system, the Eta coordinate

system does bettersystem does better The leading edge of the ETA LI gradient is often The leading edge of the ETA LI gradient is often

the best indicator of the frontal positionthe best indicator of the frontal position

THE NGM AND AVN/MRF HAVE SERIOUS PROBLEMS WITH ARCTIC AIRMASSES.

36 HR NGM V.T. 00Z APR 09, 1995

36 HR AVN V.T. 00Z APR 09, 1995

AVN ANALYSIS V.T. 00Z APR 09, 1995

L

TEMPERATURES ACROSS KANSAS WERE IN THE LOW TO MID 50s WITH STRONG NORTH WINDS. SOUTH OF THE FRONT

TEMPERATURES WERE IN THE UPPER 70s TO LOW 90s.

PRIOR TO THE 1998-99 WINTER SEASON ,ETA USUALLY HANDLED ARCTIC AIR MASSES BETTER.

AVN/MRF Still Often Has Problems Handling Upslope Events

Around 75% of the precipitation predicted by the AVN during this event was grid scale, rather than convective, precipitation. In these cases, the model QPF is often too far to the northwest. The maximum rainfall falls farther to the south along the surface front.

12-36 hr AVN QPF V.T. 12Z 27 APR 98 VERIFYING 24H PRECIPITATION V.T. 12Z 27 APR 98

4”3”

4”3”

5”

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One of the typical errors of the AVN is to predict convective systems too far north

12-36 HR QPF V.T. 1200Z 29 JAN 1999

12-36 HR QPF V.T. 1200Z 30 JAN 1999

ANALYSIS V.T. 1200Z 29 JAN 1999

ANALYSIS V.T. 1200Z 30 JAN 1999

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The AVN/MRF may be right on the synoptic scale

features but cannot handle outflow boundaries, etc. 36 HR QPF V.T. 1200Z 29 JAN 1999

36 HR QPF V.T. 1200Z 29 JAN 1999

ANALYSIS V.T. 1200Z 29 JAN 1999

A SLOW MOVING 500 CLOSE LOW AND QUASISTATIONARY FRONT WERE WELL FORECAST BY THE MODEL. HOWEVER, THE EFFECTIVE BOUNDARY REMAINED

SOUTH OF THE MODEL FORECAST

The AVN broke continuity on this forecast being much faster than previous runs or models from other centers

When systems are digging into the west with no kicker evident upstream, it is usually smart to follow the lead of the slowest model. In this case the AVN was much to fast with the upper low in the west.

If the 500 forecast is poor, the surface forecast will also be corrupted. Note that surface low in the

plains is too deep and far north on the forecast.

When the AVN is in error to the lee of the mountains. This is the typical error.