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Introduction

This page describes two studies of convection with exhibiting different characteristics; one over N. America associated with the formation of severe tornadoes, the other over central Africa. The N.America case has strong large scale forcing whereas the central African case is driven by the diurnal cycle. The role of convection in these cases is quite different.

Both cases use a forecast from the same date/time (initial conditions).

In these case studies, you will carry out a control forecast, followed by any number of suggested sensitivity experiments.

Info

These cases were used in the 2014 OpenIFS user workshop at the University of Stockholm.
A Linux 'virtual machine' with a complete copy of the Stockholm workshop exercises is available on request from openifs-support@ecmwf.int.
For more details and copies of handouts, please see the workshop page.

US Tornado convection case (Arkansas)

On the 27 April 7pm local time (00UTC 28 April), tornadoes hit towns north and west of Little Rock, Arkansas killing approx 17 people  (see http://edition.cnn.com/2014/04/28/us/severe-weather/index.html?hpt=hp_c2). On the evening on the 28 April fatal tornadoes occurred over Mississippi ( see: http://www.bbc.co.uk/news/world-us-canada-27199071).

This case study will look at the role of convection and the large scale in these events.

More information can also be found on the ECMWF Severe Event Catalogue 201404 - Convection - Arkansas U.S.

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Tornado damage

African diurnal deep convection (Central Africa)

Over tropical land masses, incoming radiation strongly heats the surface leading to the development of deep convection and precipitation. Observations show that convective activity and precipitation peak in the late afternoon or early evening. Until very recently, numerical weather prediction models struggled to reproduce this diurnal cycle, often predicting convective activity to peak too early in the day. In this case study, aspects of the convective parameterization scheme can be altered to see how the intensity and the diurnal cycle of convection responds.

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Info

Note that if using OpenIFS version 38r1 some additional code is needed for the diurnal correction of convection case study.

Please retrieve the modified source code in the file 'src38r1-conv.tgz' from the same directory as the initial conditions as described below.

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The initial conditions are available at a range of different resolutions and start dates for a 30hr forecast. The experiment ids are created at ECMWF and used for identifying the model forecasts on the ECMWF archive system (for those with access).

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titleAvailable start dates
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We recommend that the initial files from the 27th April are used for a 30hr forecast for these exercises. The initial files from the 22nd are provided for interest in examining a longer forecast lead-time for the N.American case.

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ResolutionExpt idStart datesFilenameFile size
T159L62g4a52014
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ResolutionExpt idStart datesFilenameFile size
T159L62g4a52014/04/22 at 00Zt159l62_g4a5_2014042200.tgz19Mb
  2014/04/27 at 00Zt159l62_g4a5_2014042700.tgz19Mb
T255L62g4a42014/04/22 at 00Zt255l62_g4a4_2014042200.tgz51Mb
  2014/04/27 at 00Zt255l62_g4a4_2014042700.tgz51Mb
T255L91g4552014/0427 at 00Zt255l91_g455_2014042700.tgz76Mb
T511L62gflf20140422 at 00Zt511l62_gflf_2014042200.tgz190Mb
  20140427 at 00Zt511l62_gflf_2014042700.tgz190Mb
T799L62gflg20140422 at 00Zt799l62_gflg_2014042200.tgz441Mb
  20140427 at 00Zt799l62_gflg_2014042700.tgz441Mb
T1279L62gflh20140422 at 00Zt1279l62_gflh_2014042200.tgz1.1Gb
  20140427 at 00Zt1279l62_gflh_2014042700.tgz1.1Gb
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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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titleExample using T159T255
% mkdir -p runs/convection/t255
% cd runs
% ftp ftp.ecmwf.int
ftp> cd case_studies/lotharconvection_USA_stormAfrica
ftp> binary
ftp> get 1999122412t255l62_T159g4a4_fqar2014042700.tgz
ftp> quit
% tar zxf 1999122412t255l62_T159g4a4_fqar2014042700.tgz
% cd 2014042700
% ls
1999122412_T159.tgz  ICMCLfqarINITICMCLg4a4INIT  ICMGGfqarINITICMGGg4a4INIT  ICMGGfqarINIUAICMGGg4a4INIUA  ICMSHfqarINITICMSHg4a4INIT  ecmwf
% ls ecmwf
NODE.001_01.model.1  ifs.start.model.1  namelistfc

Note that the initial conditions unpack into a directory named by the date/time of the forecast start.

The 'ecmwf' directory contains the files produced at ECMWF when this experiment was run:

  • namelistfc : copy this file to 'fort.4' to run the experiment (modify as required)
  • NODE.001_01.model.1 : this is the model output file as run at ECMWF. If your run fails, it may be useful to compare with this file.

Perform control forecast and analysis

The first step is to run the control forecast. Both cases can be studied with a single 30 hour forecast.

Use the initial files dated 27th April 2014 (filenames include 2014042700) starting at 00Z. Some additional initial files are provided for 22nd April and can be used for the N.America tornado case to investigate the impact of lead time on the forecast.

See below for tasks and key questions to address for the control forecast before moving on to the sensitivity experiments..

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We suggest starting with horizontal resolution of T255 for these exercises. Higher resolutions can be used for comparison.

 

Case study: N.America deep convection

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titleKey questions and tasks using the control forecast

  1. Understand the weather situation resulting in tornadoes.
  2. Evaluate the control forecast and compare it to the ECMWF reanalysis and observations (perhaps plot hourly precipitation rates).
  3. What is the area of threat according to the control forecast?
    Area of threat = is the area where severe weather can expected. This can be identified by considering plotting parameters such as CAPE (convective available potential energy), CIN , (convective inhibition) and 850-hPa equivalent potential temperature. 
  4. How does the convective adjustment process takes place and and what is the role of large scale forcing (why and where it happens)?
    Perhaps plot soundings (tephigrams) (9 pt area average) in threat area before and after the 'tornado event' to see convective adjustment.

 

Case study: African deep convection

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