‘SEVIRI Physical Retrieval’ products of NWC SAF

• Chapter I: Introduction
•  
• Introduction

Introduction

The purpose of this tutorial is to help the reader understand and use the SEVIRI Physical Retrieval (SPhR) product.

SPhR's purpose is to provide information on convective environmental parameters, particularly on moisture content and atmospheric instability (Martinez, Romero and Li, 2012a and 2012b). These parameters are crucial in studying the potential for deep convection, and in predicting the development of convective clouds. Moisture, instability and a lifting (trigger) mechanism are needed for the formation of deep convection. The definition of conditional instability used in this tutorial can be found in Glickmann, 2000; or in Emanuel, 1994. The concepts of stable and unstable conditions used later in the text refer to this kind of instability, which can be studied in several ways, such as with mean temperature lapse-rates or by studying the temperature exchange between air parcels and their environment when the parcels are adiabatically displaced. The reader should note that there are several other definitions of instability related to deep convection (and likewise several other methods of investigation) but that discussion is rather beyond the scope of this tutorial. For more on the subject, refer to the textbooks of Bluestein, 1992 and 1993, Holton, 2004 and Emanuel, 1994.

SPhR outputs cannot predict whether convective storms will occur. Rather, they help to estimate the potential for later deep convection. Once the outbreak of convection occurs, the parameters provided by SPhR in clear air surrounding the convective system may help forecasters predict the evolution of convection. Correct interpretation of convective environmental parameters requires a skilled local forecaster.

The SPhR product is retrieved from SEVIRI data, which is an instrument onboard the Meteosat Second Generation (MSG) geostationary satellite. Some of its main features are as follows:

• The temporal resolution is excellent: 15 minutes. Around Europe and the Atlantic it can reach 5 minutes when using rapid scan service (RSS) data. Good temporal resolution is essential for monitoring and forecasting convection.
• The spatial resolution is comparable or better than that of most Numerical Weather Prediction (NWP) data. The nominal spatial resolution of the SPhR product is 9 x 9 km by default. However, it is configurable, and the best spatial resolution can be narrowed down to 3 x 3 km.
• Satellite-based moisture and temperature profile retrieval methods usually require additional information, whereas SPhR uses numerical moisture and temperature profiles as 'background' information. SPhR can only slightly improve the NWP humidity profiles, as it is based on a limited number of spectrally broad IR channels. However, this minor improvement might still be useful; SPhR can improve the shape of some mesoscale features, such as the exact location of a moisture boundary or a local moisture gradient.
• The product is calculated from infrared (IR) data. SEVIRI does not carry any microwave channels, i.e. retrieval is possible only for cloud-free areas.
• Undetected clouds result in overly high values of humidity and instability.

In summary, SPhR significantly improves the temporal resolution of humidity parameters compared to the corresponding numerically forecasted parameters in cloud-free areas, and it may improve the forecast in cases where satellite observations are not in agreement with numerical prediction.

 

Short description of the images presented in this product tutorial

The images were processed at the Hungarian Meteorological Service using 15-min SEVIRI data. The SPhR products were calculated by the 2012 version of SAFNWC/MSG program package using the default configuration. 3-hourly half degree (latitude/longitude) spatial resolution ECMWF data was used as 'background' information.

The images were constructed by the Hungarian Advanced Weather Workstation (HAWK) visualization system (Kertësz, 2000; Rajnai et al., 2005). 10 minutes were added to the satellite nominal time to get a 'European time', a time when SEVIRI starts to scan the northern part of the full disk containing Europe. The images are converted to a stereographic map projection. The SPhR outputs are visualized as proposed by the product developers: in cloudy areas the infrared (IR10.8) image is presented in shades of gray to roughly represent cloud top height, while in cloud-free areas the convective environmental parameters are presented in colors. The color scales and ranges applied were those used by the developers. SPhR outputs are presented together with RGB or IR10.8 images to better assess the weather situation, especially the formation of clouds. The EUMETSAT RGB's used here are standard ones: Dust, 24 Hour Microphysics, Day Microphysics and HRV Cloud RGBs (Kerkmann et al., 2004).

Some examples use radiosonde measurements taken from the database of the University of Wyoming
(http://weather.uwyo.edu/upperair/sounding.html).
The surface analyses are from the database of the German Weather Service (http://www2.wetter3.de/fax.html).