Introduction
This page describes two studies of different convective cases; one over N. America associated with formation of severe tornadoes, the other over central Africa.
Both cases are studied starting the forecast from the same date/time (initial conditions).
The N.America case has strong large scale forcing whereas the central African case is driven by the diurnal cycle.
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. (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 (http://www.bbc.co.uk/news/world-us-canada-27199071).
More information can also be found on the ECMWF Severe Event Catalogue 201404 - Convection - Arkansas U.S.
African diurnal deep convection (Central Africa)
To be done.
Initial conditions
To be done.
Key questions to address with the control forecast
TODO: Add maps.
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.
Case study: African deep convection
TODO: note area of interest (show WV image?)
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.
- What's the impact of the different 'lead times' on the forecast of the convection (i.e. starting from different dates)? (N.America only)
- What's the impact of resolution on the forecast of the convection? (both)
- Does reducing the model timestep improve or worsen the forecast? (both)
Impact of the improved diurnal cycle of convection.
In this sensitivity experiment, look at the timing of convective and precipitation events by changing how the model parametrizes the diurnal cycle.Increase the precipitation auto conversion rate - what impact does this have? (both)
Impact of the convective time scale adjustment
An optimization factor in the parametrization is used for tuning the diurnal cycle. This can be altered by changing a value in the model namelist.
Sensitivity to entrainment rate:
Namelist block NAMCUMF, parameter ENTRORG
ENTRORG= 1.75E-3 default setting
ENTRORG= 5.8E-4 reduced by factor 3 (mostly shallow convection regime)
ENTRORG= 5.25E-3 multiplied by factor 3 (mostly deep convection regime)
Look at the cloud top height, precipitation and eventually changes in temperature and moisture
fields with respect to the reference. Note also this is having less impact with the diurnal cycle
activated.
Additional questions
- How important is the correct diurnal cycle of precipitation and radiation for 2m temperature and dewpoint forecast?
Further reading
More information about the N.American tornadoes can be found on the ECMWF Severe Event Catalogue - 201404 - Convection - Arkansas U.S
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