USING THE METEOSAT-9 IMAGES TO THE DETECTION OF DEEP CONVECTIVE SYSTEMS IN BRAZIL

The purpose of this article is to present a simple method of identification of deep convective clouds using water vapor (WV) and thermal infrared (IR) brightness temperature differences from the multispectral images of Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) sensor. The use of this method is part of an international effort to calibrate the radiances of SEVIRI sensor for microphysical properties of deep convective systems. This approach was applied to the image from 08 September 2009 for the demonstration of its efficacy analysis. The results show that the difference values larger than –2o C for BT Differences (WV6.2 μm – WV7.3 μm) and + 50o C (IR3.9 μm –IR10.8 μm) were associated with areas of intense precipitation. The method can be easily implemented and effectively utilized in operational basis to monitor deep convective cloud clusters over Brazil.

, while uncertainties in calibration of the SEVIRI 10.8 μm channel are below 0.25 K at 300 K (Schmetz et al., 2002).Differencing the 3.9 μm channel with the 10.8 μm channel allow the determination of liquid water clouds and high cirrus clouds.This is due to the 10.8 μm detecting all clouds close to blackbody temperature.Because of emissivity values of clouds in the 10.8 channel are greater than 0.9.
And it detects a warmer cloud top temperature whereas 3.9 μm channel detects much less than a blackbody for water clouds.However, this type of analysis with BTD method is not able to compute too high overshooting tops with high precision, in part because their height reduces the total amount of moisture above them (Setvák et al., 2008).
An alternative approach is the use of 6.2 μm BT -7.  (1993) and from a few other authors since then (e.g., Schmetz et al., 2002;Setvák et al., 2008).The BTD (WV6.2-WV7.3)approach described above can be applied for detection of the overshooting tops.3 ).This processing level corresponds to image data corrected for radiometric and geometric effects, geolocated using a standard projection, finally calibrated.
To compute the radiance for each channel scaling parameters (cal_slope and cal_offset) have to be identified.The scaling parameters are contained into the header file named "prologue" of Level 1.5 SEVIRI images (HRIT format).Radiance values can be calculated by means of the following formula (EUMETSAT, 2008): where DC (i,ch) and L (i,ch) are the digital count and radiance of pixel i and channel ch, respectively.For SEVIRI thermal channels (4-11), brightness temperature, expressed in 10 - 3 Wm -2 sr -1 [cm -1 ] -1 , can be calculated by simply inverting the Planck function at the channel wavelength, that is: where λ 0 is the central wavelength of the channel expressed in μm and c 1 and c 2 channel varying constants listed in the EUMETSAT documents (EUMETSAT, 2007a).

The 08 September 2009 storm
The case study for which results are presented here is a frontal system over southeastern region of Brazil on 08 September

Summary and conclusion
In this paper, we described our weather monitoring, particularly in case of complex and intense events.In this direction its use for nowcasting is to be considered in a short while.

BARBOSA
Using multispectral SEVIRI radiances at the top of deep convective storm as a powerful tool for data has been available free to the academic and scientific communities since January 2004.The MSG SEVIRI is positioned at 0° longitude and 0° latitude, approximately 36 thousand km above the Gulf of Guinea.This sensor operates with 11 spectral channels that provide measurements with a resolution of 3×3 km 2 at the subsatellite point every 15 minutes and a High Resolution Visible (HRV) channel whose measurements have a resolution of 1×1 km 2 (EUMETSAT, 2008).The primary mission of the second-generation Meteosat satellites is the continuous observation of the Earth's full disk with a multi-spectral imager.The repeat cycle of 15 minutes for full-disk imaging provides multi-spectral observations of rapidly changing phenomena such as deep convection.The imaging is performed by utilizing the combination of satellite spin and scan mirror rotation, a process known as stepping.The images are taken from south to north and east to west. Figure 1 provides an example the view of the sample distance on ground scanned by SEVIRI.Data is then processed and wavelet compressed, then uplinked via the EUMETCast servicea new C-band satellite reception facility to collect data from SEVIRI to the commercial telecommunication geostationary satellites from which it can be disseminated to meteorological communities.

Figure 2 .
Figure 2. SEVIRI channel normalized weighting functions at 60°.Source EUMETSAT.: 3 μm BT method to obtain more precise information concerning the overshooting tops in comparison to the WV -IR method when the overshooting tops do not exhibit any BT minimum.The physical mechanisms associated with the overshooting tops are well documented.These were studied on the pioneering work of Fritz and Laszlo,

The
storms over the Brazil since 2006.The severity criteria of the U.S. National Weather Service for a convective storm can be classified as severe if it presents on the following characteristics: a) tornado, b) wind gusts ≥ 50 knots (~ 25 m s-1) and c) hailstones with diameter ≥ ¾ inch (~ 2 cm)(http://www.weather.gov/glossary/index.php?l etter=t).It is also well documented(Schmetz et al., 2002;Barbosa and Ertürk 2009;Ertürk and Barbosa 2009) that cloud top brightness temperature (BT) directly related to the cloud top level environment temperaturevalid only for opaque clouds (i.e., cumulonimbus clouds) and state of thermal equilibrium between the cloud and its environment.In particular, critical to the success of any attempt to spot the satellite-based storm cell is the BT isotherm of~ 240 -230 K.The atmosphere must already be conditionally unstable and the large-scale dynamics must be supportive of vertical cloud development.Understanding the characteristics of convective storms that impact the weather conditions in Brazil is of importance to help forecasters to improve their capability as regards to the forecast of strong convective events.Since 2006 there were many cases when deep convective storms developed in Brazil, the 08 September 2009 storm was a strong convective event.Although this study is a preliminary one, and the number of cases is limited to only one event, the next step will be the acquisition of more events and the implementation of a database of the aforementioned information as support to the comprehension of such events and to the forecasting activity.The paper is organized as follows.Section 2 describes the MSG satellite data acquisition, decoding, and analysis.Section 3 details a case study of a deep convective storm over the south-eastern Brazil., the cloud-top SEVIRI data from 08 September 2008 with a temporal resolution of 15 minutes were retrieved from the EUMETCast service through the reception station at the Federal University of Alagoas (UFAL).EUMETCast is a content delivery network used by EUMETSAT for transporting SEVIRI data (MSG-2 satellite) received at Darmstadt (Germany) to the end users.Raw count data received by this service are referred to as level 1.5 data (EUMETSAT, 2008), that is, image data ready to use with calibration and geo-location information appended.The level 1.5 data have a 10 bit digitization and provide the basis for all further processing and for the derivation of meteorological products.They are processed and uplinked to NSS-806 in wavelet compressed the high rate information transmission (HRIT) format.From there the images can be received with a standard dish receiving system in the EUMETCast C-band.At UFAL they are archived in compressed form on external drives linked to the UFAL network, and accessible through ordinary PCs.The PC system has a built in DVB-S card that is connected to the dish and besides the EUMETCast Key Unit (EKU), which hold the key for encrypting the received data.Each file is compressed by means of a wavelet algorithm.Furthermore the PC system is connected to the UFAL LAN to have the ability to serve the end user with the MSG full disk that is composed by 8 segment files, each one consisting of 464 lines (i.e., HRIT format).This data consists of geographical arrays of 3712 × 3712 pixels.Each pixel contains 10 bit data that represents the radiance value, expressed in 10-3 Wm-2sr-1[cm-1]-1, codified in digital count (DC) form.MSG SEVIRI data have been received at UFAL since 2007 (Fig.

Figure 3 .
Figure 3. Overview of the broadcasting ground reception and processing system at the University of Alagoas (UFAL) in Brazil.

Figure 4 .
Figure 4. METEOSAT 9 SEVIRI color-enhanced IR 10.8 image dated on 08 September 2008 at 13:00 UTC.(a) For the MSG full disk.(b) For the geographic domain used in the paper.
2009 at 13:00 UTC.At this time the cloud storm was already in the mature stage.This anomalous event was characterized by very unstable weather, in particular over the eastern State of São Paulo.The general situation, shown in the synoptic chart (Fig. 5), suggests that a strong pressure gradient produced high winds bringing the cold air from South to South-eastern Brazil, producing upper-level cyclonic vortices.According to reports from meteorological stations, the average velocity of winds in parts of São Paulo city on 08 September was 70km/h.The geographic area under consideration is approximately centred over Brazil (Fig. 5).It covers from 25N to 35°S and from 5 to 73°W.
-2) received through the EUMETCast service using RGB composite for characterizing cloudy (and potentially precipitating) pixel areas relative to a severe convective event, take on 8 September 2009, over the South-eastern Brazil.In this respect, the software tools developed at LAPIS, based on open source codes for geolocation and geographical information systems, written for the transformation of the 1.5 SEVIRI radiances into the geo-physical values (i.e., the solar reflectance in the solar bands and brightness temperature in the thermal bands) were employed.In conclusion, the study shows that difference values larger than -2º C for BT Differences (WV6.2 μm -WV7.3 μm) and + 50º C (IR3.9 μm -IR10.8μm) were found to correspond well with deep convective storms.The study opens up an avenue for successive validation of the MSG data for

Table 1 .
Spectral bands of the SEVIRI instrument, commonly used for monitoring of convective storms.