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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 storms to hit Europe in December 1999 and Lothar severely affected northern France, Switzerland and Germany (for more details see: Ulbrich et al., 2001, Weather, 56, 70-80).

These 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 model accurately the high wind speeds observed, particularly over the mountainous regions of Europe.

A number of initial conditions are provided along with suggested exercises.

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Note that ERA-Interim has a resolution of T255 and the operational resolution at that time was T319. Initial data has been spectrally interpolated to the model resolutions.

The ERA-Interim analysis is an improvement over the original analysis which did not have as many observations. The scientific content of the IFS operational model at that time was significantly different to the more modern OpenIFS. A rough proxy for the how the forecast at the time performed would be to run OpenIFS at T255, the resolution of the initial data.

 

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As OpenIFS is a spectral model, the 'T' number refers to the triangular truncation is spectral space. Equivalent grid-pt resolutions are:
T159 is approximately 125km resolution, T255 - 80km, T511 - 40km, T799 - 25km, T1279 - 16km.

 

 

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ResolutionExpt idStart dateAnalysisFilenameFile size
T159L60fqar1999/12/24/12zERA-InterimT159_1999122412_fqar.tgz22Mb
T255L60fqak1999/12/24/12zERA-InterimT255_1999122412_fqak.tgz54Mb
T511L60fqaj1999/12/24/12zERA-InterimT511_1999122412_fqaj.tgz205Mb
T1023L60fs2y1999/12/24/12zERA-InterimT1023_1999122412_fs2y.tgz780Mb
T1279L60fqaf1999/12/24/12zERA-InterimT1279_1999122412_fqaf.tgz1.2Gb
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To unpack files with .tgz, either use:

tar zxf T159_1999122412_fqar.tgz

or if your tar command does not support compression:

mv T159_1999122412_fqar.tgz T159_1999122412_fqar.tar.gz
gunzip T159_1999122412_fqar.tar.gz
tar xf T159_1999122412_fqar.tar

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As ERA-Interim is an improved analysis, forecasts from these starting initial conditions will not reproduce the actual operational forecast of the storm. For that, the model should be run with operational dataBecause of changes to the forecasting system, this is impossible to do 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. 

The IFS is highly tuned to give the best forecast over a range of initial conditions. However, it is instructive to try some sensitivity experiments to understand the role of various physical and dynamical processes.

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

  • What's the difference and why between forecasts started with the operational analysis of the time and the ERA-Interim analysis?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?

  • Reduce the timestep of the model - does this 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:

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

to:

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Line 108: !  Revised gwd parameter values
Line 109: GKDRAG = 0.075_JPRB   !  half GWD coefficient: 0.15_JPRB

 

  • Increase the precipitation auto conversion rate

    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)

 

Further reading

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

This article in a recent ECMWF Newsletter has a description of student projects at the University of Stockholm using the Lothar storm case study.

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