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Column Introduction This page describes two studies of convection exhibiting different convective casescharacteristics; 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 are studied starting the 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 sensitivities experiments 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.  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. Panel borderColor white bgColor white titleBGColor white Image RemovedImage Added African diurnal deep convection (Central Africa) To be doneOver 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. Column Panel bgColor white titleBGColor #dcf0f7 borderStyle dotted title On this page... Table of Contents maxLevel 3 indent 10px 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.

Initial conditions

Initial conditions

To be done.

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. 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.

Download instructions

% mkdir -p runs/convection/t255
% cd runs
% ftp ftp.ecmwf.int
ftp> cd case_studies/convection_USA_Africa
ftp> binary
ftp> get t255l62_g4a4_2014042700.tgz
ftp> quit
% tar zxf t255l62_g4a4_2014042700.tgz
% cd 2014042700
% ls
  ICMCLg4a4INIT  ICMGGg4a4INIT  ICMGGg4a4INIUA  ICMSHg4a4INIT  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

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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.

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

Section
Column Image Added Column Image Added

Image Removed

1. Perhaps plot soundings (tephigrams) (9 pt area average) in threat area before and after the 'tornado event' to see convective adjustment.
   

Note
icon false title Key questions and tasks using the control forecast
Understand the weather situation resulting in tornadoes. Evaluate the control forecast and compare it to the ECMWF reanalysis and observations (perhaps plot hourly precipitation rates). What is the area of threat according to the control forecast? TODO: explain Area of threat is the area where severe weather can expected. This can be identified by plotting parameters such as CAPE (convective available potential energy), CIN (convective inhibition) and 850-hPa equivalent potential temperature.  How does the convective adjustment process takes place and and what is the role of large scale forcing (why and where it happens)?
Column

 

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.

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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.

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• How important is the correct diurnal cycle of precipitation and radiation for 2m temperature and dewpoint forecast?

Further reading

Journal publications

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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 especially grateful to : Peter Bechtold for suggesting these case studies, assisting in their preparation and for his support during the OpenIFS workshop. We also give a special thanks to: Glenn Carver, Filip Vana , and 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.

 We also thank the participants of the OpenIFS user workshop in Stockholm for their participation and enthusiasm.

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