Nowcasting SAF products
Cloudtype
The cloud type (CT), developed within the SAF NWC context, mainly aims to support nowcasting applications. The main objective of this product is to provide a detailed cloud analysis.
It may be used as input to an objective meso-scale analysis (which in turn may feed a simple nowcasting scheme), as an intermediate product input to other products, or as a final image product for display at a forecaster 's desk. The CT product is essential for the generation of the cloud top temperature and height product and for the identification of precipitation clouds. Finally, it is also essential for the computation of radiative fluxes over sea or land, which are SAF Ocean and Sea Ice products.
Convective rainfall rate
The objective of the CRR product is to estimate the precipitation rate associated to convective clouds. The final output is a numerical calibrated product (in mm/hr) divided into classes in an image format. This product provides to forecasters complementary information to other SAF NWC products related to rain and convection monitoring as Precipitating clouds and Cloud type.
Total precipitable water
Total Precipitable Water (TPW) is the amount of liquid water, in mm, if all the atmospheric water vapour in the column were condensed. High values of TPW in clear air often become antecedent conditions prior to the development of heavy precipitation and flash floods. When high TPW 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. Within the SAF NWC context, the main goal is to provide TPW data in clear air pixel by pixel in image format for Nowcasting purposes.
Precipitating clouds
The objective of the PC product is to support detailed precipitation analysis for nowcasting purposes. The focus is on the delineation of non-precipitating and precipitating clouds for light and heavy precipitation, rather than quantifying the precipitation rate. Particular attention will be given to the identification of areas of light frontal precipitation.
The product provides probability results, i.e. probabilities of precipitation intensities in pre-defined intensity intervals. From the probabilities a categorical estimate of precipitation intensity may be derived. It is not intended to provide information on the type of precipitation.
Stability analysis imagery
The Stability Analysis Imagery (SAI) was developed by the NWC SAF. The central aim of the SAI is to provide estimations of the atmospheric instability in cloud-free areas. Among all potential indices the Lifted Index (LI) has been implemented and codified and presented in this case for central Europe on the 19th June 2006.
The lifted index of SAI is only done for clear sky conditions, therefor for SAI the first step is to compute the Cloud Mask product (CMa). This CMa allows the identification of cloud free and cloud contaminated areas. The SAI product itself uses the corrected normalized IR SEVIRI radiance values of the following channels WV6.2, WV7.3, IR8.7, IR9.7, IR10.8, IR12.0 and IR13.4μm), and provides as output the normalized lifted index.
GII
Global instability index (GII) is an airmass parameter indicating the stability of the clear atmosphere. The GII product should serve as a nowcasting tool to identify the potential of convection and possibly of severe storms in still preconvective conditions. The applied retrieval method makes use of six MSG SEVIRI thermal bands, and together with the a priori information of forecast profiles, the scheme infers an updated atmospheric profile for each MSG pixel, from which instability indices can be computed. Several instability indices are used in this case and presented. The images are presented in 1 hour sequence.
K-Index
The K-index is a widespreaded method amongst meteorologists to make a stability analysis of the atmosphere. In the above link the K-index as computed by the GII algorithm is presented in 1 hourly interval. To find out more on how the K-index is computed you can look at the following
animation. Since the K-index makes use of the Temperature and dewpoint Temperature at 850 Hpa. the retrieval of the K-index over the Alps can be somehow problematic.
Lifted Index
A second index that is computed from GII is the Lifted Index. In 1 hour interval the Lifted Index is presented for the 19th June 2006 over Central Europa. If you want to learn more on Lifted Index and how it is derived click "
here".
Precipitable Water
One final product to be presented is the precipitable water. For a Meteorologist this product can be of extreme value when doing a nowcast. It represents the total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section.
INCA
The products that are presented below are derived from the INCA model which was developed at the Austrian Meteorological Service. "INCA" is the abbreviation for “Integrated Nowcasting through Comprehensive Analysis”.
The model is in fact the combination from different data sources such as observation, remote sensing and NWP data from the LAM model Aladin, on a 1 kilometer resolution grid making so optimal use of the advantages of all data sources. Parameters like temperature, precipitation, cloudiness and wind are parameters are analysed all 15 minutes and can in fact be nowcasted by several methods for the next 2 - 6 hours.
The results of this analysis and nowcasting method are operationally used for a public warning system. In the frame of this case study we will look at the data INCA has to offer in respect to convection.
Rain Rate
CAPE
CAPE is an often used index to characterise how (un)stable the atmosphere is. One great advantage to other indices like K-index or Lifted Index is that it is not calculated from just single levels but by integrating over a large part of the atmosphere. To find out more on how CAPE is computed you can look at the following
animation.
Lifted Index
A second index that can be calculated by the INCA method is the Lifted Index. In 15 minute interval the Lifted Index is presented for the 19th June 2006 over Austria. If you want to learn more on Lifted Index and how it is derived click
here.
Showalter Index
The Showalter Index is like CAPE, Lifted Index and K-index the fourth stability parameter introduced in this case study. Its been widely accepted among meteorologist in Europe and is also used here at ZAMG when doing Satrep Analyses. If you want to learn more on Showalter Index and how it is derived click
here.
MOCON (Moisture Convergence)
Radar
Large hail is regularly observed in association with intense thunderstorms (as in this case study) and is often the cause of severe damage on e.g. crops, roofs and cars. Hail is a local phenomenon, in both time and space, so that it can not be easily detected using satellite imagery or with surface observations (since the density is to coarse). Due to its widespatial coverage and relatively fine spatial and temporal resolution, weather radar appears as a valuable tool for the real-time detection of hail (Holleman, 2002).
For both convective regions the radar is presented and described for the 19th June 2006.
Radar Composite: time sequence
Summary of the investigations in this chapter
Four different methods for nowcasting were presented and discussed in this chapter. The NWCSAF using MSG, GII, using MSG and ECWMF. INCA which uses a mixture of NWP (Aladin), Remote Sensing (MSG), surface observations (TAWES) and radar data. Finally the radar data of the 19th June was presented with marks on where hail was expected due to high reflectivity.
NWCSAF
A range of five products were presented that are most affiliated to convection. In the Cloudtype the cells where correctly identified as cold and highreaching with a thin cirrus shield flanking them. This Cloudtype often serves as the base to further side products like Convective Ranfall Rate (CRR) and Precipitating Clouds (PC). With the CRR is is very easy to recognise the seperate cells. A rain rate is computed and presented in different classes. As the convection reached its climax in the afternoon a rain rate of 5 mm/hr was computed. Two other products that basically give the same information are the before mentioned Precipitating Clouds (PC) and the total precipitable water (TPW). In the cloudfree also products exist such as the SAI. With SAI the Lifted Index is computed by an algorithm which takes the different brightness temperatures of MSG thermal channels. It would last untill the morning hours before the SAI showed negative results and the indication the atmosphere was unstable. And when it finally did retrieve negative results it did in abundance as the whole of Central Europe was covered by a negative LI making an exact diagnosis on where the new convection would start impossible.
GII
From GII three products that describe instability where presented. They were the K-indes, the Lifted Index and the Total Precipitable Water. From K.index and the Lifted Index we learned that the values that were calculated by the GII algorithm were correct as a direct comparison with values from radiosoundings was possible. Already at an early stage there were high values for KI and negative values for LI computed so that at an early stage one could see where the next convection would likely start. In the animation three different timesteps were chosen and the stability indices gave a good indication on where the next development would start. The TPW also proved to give correct values. Striking is that the values computed by GII are about 10mm/hr higher than the TPW computed by the NWCSAF.
INCA
Five products from INCA were presented that are related to convection. Three of them are stability indices and one was Moisture Convergence. Interesting enough all three of the stability parameters already gave signs that it would be a convective day very early on the 19th June. Three areas really popped out as being extremely unstable that were Upper Austria, the northeastern part of Lower Austria and Styria, close to the border with Slovenia. In the later however no convection occured. From the radiosounding which was presented in the GII we learned that over Budapest there was a strong capped inversion that might have prevented convection to trigger. Such a situation is most likely also the cause that nothing occurred over Southern Styria. More to the north however orographic lifting along the Alps triggered the first cells over Austria on the 19th. Also over Lower Austria already at 11UTC the first cells emerged. In Upper Austria the situation was unstable but it would last till the late afternoon when cells from Switzerland moved in bringing thunderstorms to that area.
The Moisture Convergence is extremely usefull as it realy pinpoints on the exact location were maximum convective forcing occured. Along the Alps were most of the convection started on the 19th several of these areas were observed. The MOCON inc combination with stability parameter like CAPE makes the nowcasting of convection much better.