Introduction

This page describes two studies of convection with 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.

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.

These case studies 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.

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 convection 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.

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

Initial conditions

Case study initial conditions for this case study are provided on the OpenIFS ftp site.

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).

All of the initial data provided are derived from ECMWF operational analyses (not ERA-Interim). These operational analyses use a horizontal resolution of T1279 and 137 vertical levels. Lower resolutions are spectrally fitted and interpolated to produce balanced initial conditions.

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

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.

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
T1023L60fs2y1999/12/24/12zT1023_1999122412_fs2y.tgz780Mb
T1279L60fqaf1999/12/24/12zT1279_1999122412_fqaf.tgz1.2Gb

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

Download instructions

% mkdir -p runs/convection/t255
% 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:

Run the control forecast

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.

Case study: N.America deep convection

On 27 April 2014 7pm local time (00UTC 28 April), tornadoes hit towns north and west of Little Rock, Arkansas.


  1. Understand the weather situation resulting in tornadoes
  2. Evaluate the control forecast and compare it to the ECMWF reanalysis and observations
  3. What is the area of threat according to the control forecast? Area of threat = the area where severe weather can expected. This can be identified by considering parameters such as CAPE, CIN, 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)?

 

Case study: African deep convection

Deep convection develops over central Africa as shown on the water vapour satellite image below, with the accompanying ECMWF forecast shown as a false satellite image. Note that local time for the location in central Africa shown on the map is UTC+2hrs.

 

  1. Understand the weather situation over Africa.
  2. What is the role of large scale in this case (compare with N.America case).
  3. Look at the diurnal variation of key parameters (2m temperature, surface heat fluxes, precipitation, outgoing-longwave-radiation) for location 0N,25E.
  4. Compare differences in convection profiles between Central Africa and (i) open ocean, and (ii) Amazonia.

Sensitivity experiments

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.

Not all of the suggested experiments are applicable to both cases, indicated in brackets.

Additional questions

Further reading

  1. P. Bechtold et al, 2014, Representing Equilibrium and Nonequilibrium Convection in Large-Scale Models. J. Atmos. Sci., 71, 734–753. http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-13-0163.1

  2. See section on convection in description of Atmospheric Physics: http://www.ecmwf.int/en/research/modelling-and-prediction/atmospheric-physics
  3. More information about the N.American tornadoes can be found on the ECMWF Severe Event Catalogue - 201404 - Convection - Arkansas U.S

  4. ECMWF training course lecture notes on : Introduction to Moist Processes (PDF), Convection lecture 1 (PDF), Convection lecture 2 (PDF), Convection lecture 3 (PDF)

  5. IFS documentation, Part IV, Physical Processes - Chapter 6: convection (PDF).

  6. ECMWF Newsletter, summer 2014, number 140. Article on OpenIFS user workshop 2014 (Stockholm), page 2 (PDF)

 

Comments

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.

Acknowledgements

We are grateful to: Peter Bechtold, Filip Vana, Sandor Kertesz in preparing the material for the OpenIFS user workshop in Stockholm 2014, from which most of the material on this page is derived. We also thank the forecast department for their material on the ECMWF Severe Event Catalogue that was used in preparing these cases.

 

 

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