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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: Wernli et al., 2002, QJRMS;  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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  • 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

Wernli et al., 2002, QJRMS

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

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