Assimilation of MODIS winds at ECMWF

ECMWFArctic Imaging Workshop, Copenhagen, 20/21 August 2008

Assimilation of MODIS winds at ECMWF

Niels Bormann, Jean-Noël Thépaut, and Graeme Kelly*

(ECMWF)

(* now at UK Met.Office)

(with contributions from Dave Santek and Jeff Key, CIMSS)


Outline

1. Introduction: Polar winds from MODIS

2. Forecast impact of MODIS winds

3. Forecast example of MODIS winds impact

4. AVHRR winds

5. Summary


Outline




4. AVHRR winds

5. Summary


1) Polar winds from MODIS

• Wind derivation developed at CIMSS (Cooperative Institute for Meteorological Satellite Studies), Madison • Based on feature-tracking in subsequent MODIS swaths (similar to “cloud track winds” from geostationary satellites)

• Height assignment to cloud top: IR brightness temperature/WV-intercept method


1) Polar winds from MODIS (cnt’d)

• Primary source of wind observations over the polar regions• Assimilated operationally at ECMWF since 14 January 2003.

Sample of IR winds

Sample of WV winds


Outline




4. AVHRR winds

5. Summary


2) Forecast impact:Data assimilation system

The observations are used to correct errors in the short forecast from the previous analysis time.

Every 12 hours we assimilate 4 – 8,000,000 observations to correct the 100,000,000 variables that define the model’s virtual atmosphere.

This is done by a careful 4-dimensional analysis in space and time of the available observations; this operation takes as much computer power as the 10-day forecast.


assimilation window

t0 tnti

yo

yo

yo

yo

previous forecast

xb corrected forecast (analysis)

xa observations

2) Forecast impact:Schematic of 4d variational data assimilation (4DVAR)


2) Forecast impact of MODIS winds

Assimilation experiments:- 3DVAR, T159/159 (~125 km res.):

- 5 March—3 April 2001 (30 forecasts)- 4DVAR, T511/159 (~40 km/125 km model/analysis res.):

- 13 July—29 August 2002 (48 forecasts) - 5 March—3 April 2001 (30 forecasts)

CTL: No MODIS winds; otherwise full operational set of observations.MODIS: MODIS winds added to CTL.

MODIS winds used: IR, WV cloudy and clear, Terra data only Over land: IR and WV winds above 400 hPa. Over sea: IR winds above 700 hPa, WV above 550 hPa.

(see also Bormann and Thépaut, 2004, MWR)


2) Forecast impact (3DVAR)

Anomaly correlation for 500 hPa geopotential, each experiment verified against its own analysis (25 cases).

Northern Hemisphere Arctic (N of 65N)


2) Forecast impact (3DVAR, cnt’d)

Anomaly correlation for 500 hPa geopotential, each experiment verified against its own analysis (25 cases).

Southern Hemisphere Antarctic (S of 65S)


2) Forecast impact (4DVAR)

Anomaly correlation for 500 hPa geopotential, each experiment verified against own analysis (58 cases).


2) Forecast impact (4DVAR, cnt’d)

Anomaly correlation for 500 hPa geopotential, each experiment verified against own analysis (58 cases).


Outline




4. AVHRR winds

5. Summary


3) Forecast Example

Analysis MODIS

CTL

5 day forecast of 500 hPa geopotential, initialised 15 March 2001, 12 UTC,

3DVAR experiment


3) Forecast Example: Snowfall

“Analysis” (corresp. 0-12 h forecast) MODIS

CTL

96-108 h forecast of 12h snowfall [mm water equivalent], initialised

15 March 2001, 12 UTC


3) Forecast Example: Difference tracking

Forecast of 500 hPa geopotential [gpdm] initialised 15 March 2001:

blue: MODIS forecastblack: CTL forecast

red/green: Difference MODIS-CTL (positive/ negative)


Outline




4. AVHRR winds

5. Summary


4) AVHRR AMVs

CIMSS-derived polar AMVs from AVHRR from NOAA-15, -16, -17, -18. No WV channel on AVHRR, so IR winds and height assignment only.

Assimilation experiments:

12-hour 4DVAR

Resolution: T511L60 (~40 km, model), T159 (~125 km, analysis)

1 January 2007 – 14 February 2007 (45 forecasts)

Control: Conventional observations + NOAA-18 AMSU-A

AVHRR: As Control, but plus AVHRR winds

AMVs used over land above 400 hPa, over sea/ice above 700 hPa.

MODIS: As Control, but plus MODIS winds

IR AMV usage as for AVHRR;

WV AMVs used over land above 400 hPa, over sea/ice above 550 hPa.


4) AVHRR AMVs: Coverage Number of used winds (all levels), 1 Jan – 14 Feb 2007:

AVHRR N.Pole AVHRR S.Pole

MODIS N.Pole(IR & WV)

MODIS S.Pole(IR & WV)


U-component:

V-component:

Std.dev [m/s] Bias [m/s]

Std.dev [m/s] Bias [m/s]

• Statistics for used AMVs over Antarctica for AVHRR and MODIS (IR & WV).

• AVHRR winds show larger departures and worse biases against the FG than MODIS winds.

Obs-FG MODISObs-FG AVHRRObs-AN MODISObs-AN AVHRR

4) AVHRR AMVs


Normalised

differences in RMS of

48-hour forecast

errors for the 500 hPa

geopotential

(negative = polar

winds “good”)

AVHRR – Control

MODIS – Control

4) AVHRR AMVs


Outline




4. AVHRR winds

5. Summary


5) Summary

MODIS winds provide important upper air wind observations over polar regions.

Significant positive forecast impact when first introduced, also outside high-latitudes.

Used by all main NWP centres.

ECMWF plans to re-evaluate MODIS winds impact to provide input to Arctic imager initiatives.

WV winds provide 2/3 of the number of MODIS winds; lack of WV channel on AVHRR means fewer AVHRR winds with poorer quality.

AVHRR winds will provide only limited polar wind coverage after the lifetime of the MODIS instruments.


Arrival time

4 Z extraction time for early delivery

Timeliness of MODIS AMVs

Direct broadcast MODIS winds

NESDIS MODIS winds

Nu

mb

er

of o

bse

rva

tion

sAccumulated number of data for 21-3 Z early delivery data window vs arrival time

Data window for early delivery analysis

Documents

Assimilation of MODIS winds at ECMWF