NoWCasting SAF - Event Week 2013

To provide the forecasters with tools for severe weather events monitoring with a suitable spatial and temporal resolution is very important. The NWCSAF project develops several products to make easier forecasters their nowcasting tasks.

The NWCSAF/MSG software package contains two products devoted to precipitation. Precipitating Clouds (PC) product assigns a probability of precipitation occurrence on each SEVIRI pixel. Convective Rainfall Rate (CRR) product estimates rain rates on convective, and stratiform associated to convection, events. The algorithms, on which those products are based, have currently two different approaches.

PC product takes advantage of surface temperatures and of those SEVIRI spectral features which have the highest correlation with precipitation, to construct a Precipitation Index (PI). According to this PI, which has been calibrated against rain gauges, a likelihood of precipitation occurrence is assigned to each SEVIRI pixel. CRR algorithm assigns rain rates to each pixel according to some SEVIRI channel information and some calibration functions, which have been calibrated using radar data. The influences of environmental and orographic effects on the precipitation distribution are taken into account through some corrections that use NWP data. Besides, two new products are included in the NWCSAF/MSG v2013 software package. These products also estimate the probability of precipitation and convective rainfall rates using a common approach, which takes advantage of cloud microphysical properties.

In a second talk, Natasa Strelec-Mahovic will present applications of the NWCSAF Precipitation Products.

Since 2010, the PGE13 SEVIRI Physical Retrieval (SPhR) is available as an operational product of the NWCSAF/MSG software package. The algorithm is based on the physical retrieval algorithm for GOES developed by Dr. Jun Li (CIMSS at Wisconsin University) but using RTTOV as the radiative transfer model and the coefficients adapted for SEVIRI. The PGE13 has been optimized for the purpose of operational implementation; near real time images could be seen on the NWC SAF reference system (http://www.nwcsaf.org).

The improvements on version 2013 of PGE13 are presented first. The main improvement on the 2013 version is that it allows the use as the background NWP input of ECMWF GRIB files on hybrid levels; the 2012 version only allows the use as NWP input of GRIB files on fixed pressure levels.

This step is a small milestone in the exploitation of this kind of MSG L2 products. Now, the spatial and temporal resolution could be used together with vertical resolution provided by the hybrid levels GRIB files to get information of 3D structures. The reason it that the use of hybrid GRIB files as input to the algorithm avoids the stepwise aspect on the vertical cross sections created by the linear interpolation between too separated levels in the fixed pressure levels PGE13 version. Other point is that the local execution of PGE13 allows the generation of optional files with fields like 3D equivalent potential temperature arrays. Thus, forecasters can fully exploit the 3D structure allowing the detection of instability vertical gradients on medium levels.

All these changes allow forecasters the use of the PGE13 for a better monitoring of key ingredients in regions where likely triggering of convective storm could develop and also advise of the regions where the NWP disagree with the PGE13 physical retrieved profiles. The use of PGE13 in several case studies will be presented.

The NWCSAF Clear Air product consists of total and layer water vapour contents and instability indices. This product is retrieved from SEVIRI data for cloud-free pixels. As it can be calculated in 15-minute time step, even in 5-minute time step it is usable to monitor convection ability.

Case studies will be presentation and analyzed: cases, when the NWP forecasted and the satellite retrieved convective environmental parameters are close to each other and when not. We will show selected cases when the NWCSAF Clear Air product has added value compared to the NWP forecast. The effect of the undetected clouds is also analysed.

As optional output the whole retrieved temperature and humidity profiles can be study. We show a case when the retrieved profiles were also analyzed. Statistical results will be presented on using the satellite retrieved Clear Air environmental parameters to separate stable from unstable environments and the environment where forming severe storm is not likely from the environment where it is possible.

The “NWC SAF/HRW High Resolution Winds” provides a detailed calculation of Atmospheric Motion Vectors locally and in near real time. It calculates the AMVs considering cloud and humidity patterns in up to seven different MSG/SEVIRI channels (HRVIS, VIS06, VIS08, WV062, WV073, IR108, IR120), together with NWP data and the “NWC SAF Cloud products (Cloud type, Cloud top temperature and height)”.

It considers up to two tracer scales (Basic and detailed), two tracer determination methods (Gradient and tracer characteristics), two tracking methods (Euclidean distance or cross correlation), and two Height level assignment methods (Brightness temperature interpolation method and Cross correlation contribution method). As other AMV products worldwide it also includes a quality control flagging based on the Quality Indicator Method developed at EUMETSAT.

NWC SAF/HRW product can be useful in Nowcasting applications in synergy with other data available to the forecaster: monitoring of low level convergence, divergence at the top of developed systems, or other cases of small scale circulation or wind singularities. It can also be used as an input in analysis and forecasting applications like NWP models. The development and main characteristics of the product are explained with examples, useful for the potential users to know what the product can offer. The validation results related to the latest version on the product (HRW v4.0) are also explained.

The role of the Atmospheric Motion Vectors (AMV) in the modern data assimilation systems is evident nowadays. The AMVs – along with satellite observations – have an important key for our operational ALADIN data assimilation system as well. In the framework of the EUMETSAT SAF High Resolution Winds (HRW) products had been developed by AEMET for Nowcasting and Very Short Range Forecasting purposes. The HRW products provide quasi real time data of AMVs from MSG HRVIS and IR10.8 channels every hour in a day. A collaboration between AEMET and OMSZ have been focused to compare the benefit of the operationally used AMVs (received through the EumetCAST) with the newly processed HRW winds. Recent work was an extensive evaluation and monitoring of the assimilation and forecast system. The impact of the AMV winds was examined using different diagnostic techniques, verifications tools and case studies. The first results are promising regarding the use of the HRW AMV. The statistical scores on the full domain showed neutral or slightly positive impact in most of the parameters and the experiment based on HRW AMV data provided better estimate of extreme precipitation events.

RDT-CW (Rapid Development Thunderstorm Convection Warning) product is an object-oriented diagnostic for convective clouds or cells. RDT-CW is mainly based on satellite data. RDT-CW software tracks clouds, identifies those that are convective (discrimination), and provides some descriptive attributes for their dynamics. Year after year many attributes have been added to the convection object. These improvements offer end-users the possibility to focus on specific parameters according to their center of interest: dynamic of the system (cooling rate, motion vector) lightning activity, rainfall, main cloud phase of the cell, etc. These attributes may come from various sources such as NWP data, other PGEs (Product Generation Element), lightning network. In the v2013 release, an overshooting top detection inside RDT cell is proposed to users. This new attribute allows to focus on potentially hazardous areas.

RDT cell is proposed to users. This new attribute allows to focus on potentially hazardous areas. In the future releases the description of convective cells will be enriched, giving a high priority to the use of other SAFNWC products. A nowcast of RDT up to one hour will be implemented.

Several SAFNWC processing chains are implemented at Meteo-France over various geographical areas: those covered by MSG of course, but also those covered by other geostationary satellites, like overseas territory where Meteo-France centers are still implemented. Forecasters in these centers are the main users to benefit from RDT as an additional product. Productions over Europe and Africa benefit to Aeronautical forecasters from Meteo-France’s National Forecast center, to French Army, and also to ACMAD, Niamey.

A target visualization tool with nowcasting capabilities will be presented, which allows taking advantage from RDT end-product. Presentation of RDT for various situations and over various areas will also be undertaken, and the latest development implemented in v2013 release will be highlighted.