|
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.
The initial conditions are available at a range of different resolutions and start dates for a 10 day forecast. The experiment ids are created at ECMWF and used for identifying the model forecasts on the ECMWF archive system (for those with access).
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 how the forecast at the time performed would be to run OpenIFS at T255, the resolution of the initial data.
As OpenIFS is a spectral model, the 'T' number refers to the triangular truncation in spectral space. Equivalent grid resolutions are: The number of vertical levels is given after the letter 'L' e.g. L62 means 62 vertical levels. Please note that higher resolutions progressively require more processors and computer memory to run. |
|
|
% 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 |
The 'ecmwf' directory contains the files produced at ECMWF when this experiment was run:
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.
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.
Reduce the gravity wave drag - how does this affect the forecast in the upper and lower levels?
Edit the source code to half the gravity wave drag coefficient File: ifs/phys_ec/sugwd.F90, change:
to:
|
Increase the precipitation auto conversion rate - what impact does this have?
Edit the source code to increase the auto conversion rate by 20% File: ifs/phys_ec/sucldp.F90, change:
to:
|
Change the surface transfer coefficient in the turbulence scheme
Reduce the coefficient by 20%. Alter surf/module/surfexcdriver_ctl_mod.F90 from :
to:
|
Reduce the asymptotic mixing length scale (K) - how does this affect surface & near-surface fields?
For this change, two files need to be edited:
and:
|
Ulbrich et al., 2001, Weather, 56, 70-80
Wikipedia article:
Cyclone Lothar and Martin, Wikipedia article, retrieved 17/12/14.
This article in a recent ECMWF Newsletter has a description of student projects at the University of Stockholm using the Lothar storm case study.
A. Hannachi, J. Kjellsson, M. Tjernström, G. Carver, 2012, Teaching with the OpenIFS at Stockholm University, ECMWF Newsletter No. 134, Winter 2012/13.
The forecasting system at ECMWF makes use of "ensembles" of forecasts to account for errors in the initial state. In reality, the forecast depends on the initial state in a much more complex way than just the model resolution or starting date. At ECMWF many initial states are created for the same starting time by use of "singular vectors" and "ensemble data assimilation" techniques which change the vertical structure of the initial perturbations.
As further reading and an extension of this case study, research how these methods work.
We gratefully acknowledge: Dr Anton Beljaars (ECMWF) for suggestions and code changes for the parametrization changes in the list of sensitivity experiments; Prof Erland Kallen for reviewing & comments on the text.
<script type="text/javascript" src="https://software.ecmwf.int/issues/s/en_UKet2vtj/787/12/1.2.5/_/download/batch/com.atlassian.jira.collector.plugin.jira-issue-collector-plugin:issuecollector/com.atlassian.jira.collector.plugin.jira-issue-collector-plugin:issuecollector.js?collectorId=5fd84ec6"></script> |