HIRLAM
In chapter 2 we have given you the set of basic and derived parameters from the ECMWF model. This chapter will go a bit more in depth in the topic of convection by presenting you a range of convective parameters from the HIRLAM model. In addition we also try to make the link in this direction by demonstrating you the potential of the various products from the NWCSAF and from MPEF. For latter we will show you the Global Instability Index product that uses a first gues of ECMWF and updates it with MSG information to retrieve a sounding and a stability analysis. Finally we this chapter will give some vertical profiles made over Mallorca and compare them with the forecasted soundings.
HIRLAM: Precipitation forecast
The model run from HIRLAM of 3rd of October 0000 UTC is the basis for the calculation of the total precipitation that was forecasted during the events of this case over the Iberian Peninsula and the Balearic Islands. These forecast images are shown in combination with the enhanced IR10.8 satellite image to make a comparison between model and real time imagery.
HIRLAM: Convective Parameters
In the above two image there are 8 panels plotted that a forecaster in Spain has operational to see from the model where there is potential for convection. In this part of this chapter we will zoom in to this product and look at these different parameters from the HIRLAM model and describe the details we see in relation to this severe weather over the Balearic Islands. We will start with shortly identifying which these parameters are:
- PW, or precipitable water, is, in this interpretation of AEMET, the amount of liquid water, in mm, if all the atmospheric water vapour in a column from the surface to 300 hPa. were condensed. High values of PW in clear air often become antecedent conditions prior to the development of heavy precipitation and flash floods. When high PW values areas present a lifting mechanism and warm advection in low levels, heavy precipitation often occurs. These data can provide to forecasters an important tool for very short range forecasting.
- The opposite to CAPE is CIN which we also see plotted in the right image in the top right panel. CIN, which stands for convective inhibition is a numerical measure in meteorology that indicates the amount of energy that will prevent an air parcel from rising from a given level to the level of free convection. Sometimes is is also referred to as CAP or CAP inversion. There are many ways of compute CIN, AEMET's diagnostic tool takes an average parcel represented by first 100 mb of the atmosphere to compute it.
- The winx in the bottom left is derived from the WINDEX or Wind Index which is based on observations and numerical models and gives an estimation of max wind gust that can be generated by convective processes. It is considered that when a gust front moves perpendicular and towards high values of WINDEX contours, downburst activity has a chance to take place.
- CIZ6 is an indirect measure of vertical wind shear computed from the difference between the lowest 6 km mean wind and the lowest 500m mean wind. It is equivalent to twice the square root of the BRN shear, which is more widely use in forecast offices for the same purposes. The advantage of Ciz6 is that it offers to the forecaster values in m/s directly, (instead of m2/s2 given by BRN shear) which are more intuitive for the assesment task. According to most climatologies performed in the US, this parameter have a very good skill for discriminating organized convection: values between 12 and 24 m/s have to be taken into account for this pourpose. Values higher than 24 m/s tend to be detrimental for convective development, and values below 12 m/s tend to be not enough for well organized convection.
- Top left the Lifted Index is plotted. Negative values indicate potential buoyancy for a low level average parcel.
- Top right CAPE is plotted. CAPE which stands for (Convective Available Potential Energy) is a measure of the amount of energy available for convection.
- To the bottom left we see SRH which stands for Storm Relative Helicity. In this case SRH refers to storm relative helicity in the first 3km. It helps to estimate the capacity of a thunderstorm to produce rotating updrafts in a given environment. The computation is dependant on the direction and speed of the storm, so an approach has to be taken into account before computing it. In the case of AEMET diagnostic tool, the classical approach is adopted, that is to consider that the storm will move 30º to the right of the environmental mean flow and at a speed which is a 75% of that of the mean flow. A forecaster added value to this paramenter is straightforward by just changing the classical approach with the radar observed storm speed and direction, which would give much more realistic SRH values.
- Bottom right ACON is a combination of several convective parameters where different thresholds are set:
Deep convection | LI<0; CAPE>600J Kg-1; CIN<300 J Kg-1 |
---|---|
Structurised deep convection | CAPE>700J Kg-1; CIZ>9 m s-1; RH (70 - 50 kPa)<60% |
Supercell | CAPE>700J Kg-1; CIZ>9 m s-1; RH (70 - 50 kPa)<60%; SRH>150 m2 s-2 |
If you look closer you also see in some of the figures some lines and windbarbs plotted. This is the direct output from the model that provide guidance about the dynamics in the area of interest.