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Lothar storm

Introduction

The Lothar storm that swept across Europe during 24-27 December 1999 is provided as a sample case study to use with OpenIFS. This storm was one of several severe storms to hit Europe in December 1999 and Lothar affected northern France, Switzerland and Germany (for more details see: Ulbrich et al., 2001, Weather, 56, 70-80).

The storms were characterized by record-breaking windspeed observations and rapid development across Europe. There was also a band of extremely high baroclinity near the cyclone track over the N. Atlantic and partly into Europe associated with Lothar.

The ECMWF forecasting system of the time did not accurately capture the storm's intensity though the strong jet stream was predicted some 9 days earlier. The storm initiated from a small disturbance in the Atlantic. More recently, Wedi et al, 2012, ECMWF Newsletter, have shown that very high resolution (T7999; ~2.5km) is necessary to accurately model the high wind speeds observed, particularly over the mountainous regions of Europe.

A number of initial conditions are provided along with suggested exercises. Feedback on this case study (and others) is welcomed.

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Initial conditions

Case study initial conditions for the Lothar storm are provided on the OpenIFS ftp site. The Lothar depression developed initially on 24th December off the North American east coast at 35N.

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ResolutionExp idStart datesAnalysisFilenameFile size
T255g8oz1999/12, 14th-25th, 12zERA-InterimT255_199912_14-25_g8oz.tar.bz2660Mb
T511g8su1999/12, 14th-25th, 12zERA-InterimT511_199912_14-25_g8su.tar.bz22.3Gb
T1279g8t31999/12, 14th-25th, 12zERA-InterimT1279_199912_14-19_g8t3.tar.bz2
T1279_199912_20-25_g8t3.tar.bz2
6Gb
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Info

These files use the 'bzip2' command rather than 'gzip', to achieve a better compression.

Uncompressing may take a long time depending on your system.

To uncompress:

bzip2 -d T255_199912_14-25_g8oz.tar.bz2
tar xf T255_199912_14-25_g8oz.tar

 

Download instructions

Code Block
titleExample using T159
% mkdir -p runs/lothar/t159
% cd runs
% ftp ftp.ecmwf.int
ftp> cd case_studies/lothar_storm
ftp> binary
ftp> get 1999122412_T159_fqar.tgz
ftp> quit
% tar zxf 1999122412_T159_fqar.tgz
% ls
1999122412_T159.tgz  ICMCLfqarINIT  ICMGGfqarINIT  ICMGGfqarINIUA  ICMSHfqarINIT  ecmwf
% ls ecmwf
NODE.001_01  namelistfc

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  • namelistfc : copy this file to 'fort.4' to run the experiment (modify as required)
  • NODE.001_01 : this is the model output file as run at ECMWF. If your run fails, it may be useful to compare with this file.

Suggested sensitivity experiments

As ERA-Interim is an improved analysis, forecasts from these starting initial conditions will not reproduce the operational forecast of the storm as it was in 1999. Because of changes to the forecasting system, this is impossible to reproduce with OpenIFS. A proxy is to run the model at the same resolution as the ERA-Interim data (T255) as this is close to the resolution of the operational model of the time. 

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  • What's the impact of the different 'lead times' on the forecast of the storm (i.e. starting from different dates)?

  • What's the impact of resolution on the forecast of the storm: both for it's development and impact over areas worse hit in Europe?

  • Does reducing the model timestep improve or worsen the forecast?

  • Reduce the gravity wave drag - how does this affect the forecast in the upper and lower levels?

    Expand
    titleHow to change the code (click here to expand)

     Edit the source code to half the gravity wave drag coefficient

    File: ifs/phys_ec/sugwd.F90, change:

    Code Block
    Line 108: !  Revised gwd parameter values
    Line 109: GKDRAG =0.15_JPRB

    to:

    Code Block
    Line 108: !  Revised gwd parameter values
    Line 109: GKDRAG = 0.075_JPRB   !  half GWD coefficient: 0.15_JPRB
  • Increase the precipitation auto conversion rate - what impact does this have?

    Expand
    titleHow to change the code (click here to expand…)

    Edit the source code to increase the auto conversion rate by 20%

    File: ifs/phys_ec/sucldp.F90, change:

    Code Block
    line 123: RKCONV=1._JPRB/6000._JPRB   ! 1/autoconversion time scale (s)

    to:

    Code Block
    line 123: ! RKCONV=1._JPRB/6000._JPRB   ! 1/autoconversion time scale (s)
    line 124: RKCONV=1.2_JPRB/6000._JPRB    ! default scaled by 20%: 1/autoconversion time scale (s)
  • Change the surface transfer coefficient in the turbulence scheme

    Expand
    titleHow to change the model code (click here to expand)

    Reduce the coefficient by 20%.

    Alter surf/module/surfexcdriver_ctl_mod.F90 from :

    Code Block
    line 671:  DO JL=KIDIA,KFDIA
    line 672:    IF (JTILE == IFRMAX(JL)) THEN 
    line 673:      PKHLEV(JL)=ZKHLEV(JL)
    line 674:    ENDIF
    line 675:  ENDDO

    to:

    Code Block
    line 671:  DO JL=KIDIA,KFDIA
    line 672:    IF (JTILE == IFRMAX(JL)) THEN 
    line 673:      PKHLEV(JL)=ZKHLEV(JL)
    line 674:    ENDIF
    line 675:    !  reduce surface transfer coeff by 20% in turbulence scheme
    line 676:    ZCFMTI(JL,JTILE)=0.8_JPRB*ZCFMTI(JL,JTILE)
    line 677:  ENDDO
  • Reduce the asymptotic mixing length scale (K) - how does this affect surface & near-surface fields?

    Expand
    titleHow to change the model code (click here to expand)

    For this change, two files need to be edited:

    Code Block
    titleifs/phys_ec/suvdf.F90
     line 53: RLAM   = 75.0_jprb  !! 150._JPRB: reduce to 75m 

    and:

    Code Block
    titleifs/phys_ec/vdfexcu.F90
     ZKLEN     = 75.0_jprb  !! 150.0_JPRB     ! asymptotic K length scale troposphere - Reduce to 75m
  • For these last 4 cases where the model's parametrizations have been altered, which make the biggest difference and why? Does any of the changes improve the forecast in any way?

  • If you were providing forecasts for wind and precipitation to the general public based on these experiments, what could you say with certainty and what is less certain? How would this change over different countries?

 

 

Further reading

Ulbrich et al., 2001, Weather, 56, 70-80

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Cyclone Lothar and Martin, Wikipedia article, retrieved 17/12/14.

 

Acknowledgement

We gratefully acknowledge Dr Anton Beljaars (ECMWF) suggestions and code changes for the parametrization changes in the list of sensitivity experiments.

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