GEO - Clear Air Products
Miguel A. Martinez and Maria Putsay give an overview on the NWC-SAF clear air products and their applications.
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.