YELLOW SPOT IN THE CONVECTIVE STORMS RGB IMAGE CAUSED BY A PILEUS CLOUD

Transcription

1 YELLOW SPOT IN THE CONVECTIVE STORMS RGB IMAGE CAUSED BY A PILEUS CLOUD André Simon, Mária Putsay, Ildikó Szenyán and Ákos Horváth Hungarian Meteorological Service, Kitaibel Pál u. 1, H-1024 Budapest, Hungary Abstract Pileus clouds were visually observed and time-lapse was taken on a severe convective system over Hungary. On the same time a yellow spot appeared in the convective storms RGB images. The paper discusses this yellowish spot and its relation to the pileus clouds. Such roundish, relatively small size (2-4 pixels in diameter) yellow spot on the top of a convective cloud is usually interpreted as a (very likely) indicator of strong updraft. IR3.9-HRV blended images were created from the 5-minute METEOSAT-8 imagery, and analyzed together with the simultaneous photographs (and with IR10.8 images). The analysis showed that the yellow spot (the increased IR3.9 brightness temperature spot) was probably not related to particles originating within the thunderstorm updraft. The presence of the pileus cloud over the thunderstorm top could be one of the possible explanations why a spot with increased IR3.9 BT value can appear on the satellite images. INTRODUCTION On 27 May 2010 thunderstorm cells developed north from Lake Balaton, Hungary. The most intense thunderstorm existed for more than 2 hours. Radar observations showed high radar reflectivity (over 55 dbz) and a Weak Echo Region (WER) at the southern flank of this cell. The maximum of the radar measured echotops was 13 km. Hail was observed at town Pápa and in the village Magyaralmás, probably related to this thunderstorm. A wall cloud was observed at Lake Velence and in Budapest at later stages of the cell. Figure 1: Photograph of the thunderstorm taken at Siófok on 27 May 2010 at 15:27 UTC (left) and the corresponding METEOSAT-8 convective storms RGB image from 15:28 UTC (right). The small white square marks the position of Siófok.

2 Several pileus clouds formed at the tops of the most intense thunderstorms (Fig. 1). The most intense formation phase of the pileus clouds was around 15:20-29 UTC. Photographs and time-lapse of the cloud development were taken at Siófok, Lake Balaton (marked by white square in Fig. 1). Studying the satellite imagery it was found that a yellowish spot appeared in the convective storms RGB around the same time (see Fig.1), when the top of the thunderstorm reached its maximum and the pileus was observed. The decay of the spot was fast. Such roundish, relatively small size (2-4 pixels in diameter) yellow spot on the top of a convective cloud is usually interpreted as an (very likely) indicator of a strong updraft (Kerkmann et al. 2006). The paper discusses that in this case the yellowish spot was related to the pileus clouds, or not. WHAT WE KNOW ABOUT PILEUS CLOUDS Pileus cloud may form when a moist layer is locally lifted due to a rising cloud below (definition from Glossary of Meteorology, edited by Glickman, (2000)). Pileus might also form on a top of a stationary wave as mentioned by Schaefer and Day (1998), similarly to mountain waves and lee clouds. Updraft would represent the mountain crest. The rapidly rising convective cell causing the formation of a pileus cloud above it might shoot through the pileus layer in a later phase. If the pileus cloud forms at high altitude just below the tropopause the water vapor deposits directly to ice crystals. As there is not much moisture at high levels so the high-level pileus will consist of very small size ice crystals. Garrett et al. (2006) reported ice crystals of size of only 2-4 micrometers in pileus clouds. THE ORIGIN OF YELLOWISH SPOTS IN THE CONVECTIVE STORMS RGB IMAGERY In this RGB image type the interpretation of the colors is as follows: high-level ice cloud tops are reddish (opaque ice cloud with large particles), yellowish (opaque ice cloud with small particles), or rose (semitransparent ice clouds). However, the yellowness depends not only on the particle size, but also on the cloud top temperature. In some special cases the pixel can be yellow because of the very low cloud top temperature and not because of the small particle size. To avoid this uncertainty we studied rather the IR3.9 channel brightness temperature (BT) values. This is the channel containing daytime - the information on cloud top microphysics. Figure 2: Simultaneous convective storms RGB (upper left), IR3.9 (upper right) and IR10.8 (lower left) channel brightness temperature and retrieved cloud top particle effective radius (lower right) from 27 May :23 UTC. Meteosat-8 SEVIRI imagery.

3 During daytime the radiation measured in the IR3.9 channel is a mixture of the reflected sunlight and the emitted thermal radiation. The first term is usually much bigger. The IR3.9 reflectivity depends both on the phase and the particle size. Ice clouds have low cloud top reflectivity in the IR3.9 channel. However, ice clouds with small ice crystals have relatively high reflectivity, up to 10%. In Fig. 2 we see the convective storms RGB together with the IR3.9, IR10.8 BTs and the retrieved effected radius (Reff) image at 15:23 UTC. The Reff, the IR3.9 and the convective storms RGB images show the spot at the same location. In the IR10.8 image there is also a spot with slightly lower BTs, but this spot has different structure. The Reff values were retrieved with the MSG_RGB visualization tool of Lensky and Rosenfeld (2006). Note that in Reff image the cloud system has a slightly different shape, because the effective radius can be calculated only if some conditions are fulfilled. It was not calculated for some pixels at the southern part of the studied convective system. Additionally, it contains the Reff values for water clouds on the south-western edge of the convective system as well. The Reff image (lower right panel) shows that the particle size is much lower in the studied patch, than in its surrounding. In the IR3.9 image (upper right panel) we can see a dark red patch at the same location. Here the BT is increased locally relative to the other parts of the convective cloud top. It is increased because of the bigger reflectivity caused by smaller ice particles. On the basis of the retrieved Reff and the IR3.9, we can say that the yellowish spot appeared due to small size of the particles at the cloud top, not because of the low temperature. Small ice particles on the cloud top may be due to cells with strong updrafts. In strong updrafts the small water particles formed at the cloud base reach the cloud top very quickly, without much time for interacting and growing (Kerkmann et al., 2006; Lensky and Rosenfeld, 2006). the presence of over-anvil cirrus cloud. For example, over-anvil plumes consist of small particles (Levizzani and Setvak, 1996), but their forms are different. To clarify whether the small ice crystals in our case are due to the pileus or they origin within the strong updraft, we need to know at which level are these particles observed. Are they on the top of the convective clouds, or in an over-anvil cloud? DATA AND METHODOLOGY We studied the appearance and disappearance of the increased IR3.9 BT patch on the top of this convective cloud by analyzing simultaneous photographs and 5-minute METEOSAT-8 imagery. Several single channels and RGBs were analyzed, however, the main emphasis was on the IR3.9 and on HRV imagery. If the small cloud top ice crystals are due to strong updraft, then they most likely originate from overshooting top(s). In HRV images one can often see the development of the overshooting tops. The High Resolution Visible (HRV) images provide information on the cloud top topography due to the shadows cast by higher-elevated parts of the cloud. A further advantage of the HRV channel is its three times bigger horizontal resolution. We had to accurately identify the relative location of the increased IR3.9 brightness temperature and the overshooting top on HRV. To solve this, we created blended images from IR3.9 and HRV channels. The method of visualization of two channels as a blended image was suggested by Setvak (2009). He uses this visualization method for the IR10.8 and the HRV channels to study the cloud top features on METEOSAT SEVIRI imagery this is the so-called sandwich product. He blends two channels in the following way: HRV image as background layer and the color enhanced IR10.8 brightness temperature data as the upper layer (partly transparent). To increase the contrast he suggests to use the multiply blended option (e.g. with the Image Magics software).

4 Following his concept, we blended the IR3.9 brightness temperature with the HRV image. We can see an example in Fig. 3. Here we visualized separately the IR10.8, the IR3.9 brightness temperature and the HRV images and also the blended images created from them. In the blended images one can see the cloud top topography and on the top the brightness temperature distribution in colors. For comparison we visualized the convective storms RGB as well. Figure 3. METEOSAT-8 IR10.8 (upper left) and IR3.9 (upper middle) brightness temperature, convective storms RGB (upper right), HRV-IR10.8 blended (lower left) and HRV-IR3.9 blended (lower middle) and HRV (lower right) images from 27 May 2010 at 15:23 UTC. RESULTS Fig.4a-f show simultaneous photographs, IR3.9-HRV and IR10.8-HRV blended satellite images between 15:13 and 15:38 UTC in 5 minute time steps. (We added 3 minutes to the nominal time of the METEOSAT-8 rapid scan images to get the scanning time over the Lake Balaton). Note that the timelapse was taken from the surface, from a distance of km to south-southeast. The photos show the south-southeastern side of the system. Figure4a. Simultaneous photograph (left), IR3.9-HRV blended (in the middle) and IR10.8-HRV blended (right) images, all taken at 15:13 UTC. In the 15:13 UTC IR3.9-HRV blended image (Fig. 4a) we can see a slightly increased, blurred IR3.9 BT spot on the cloud top. The IR10.8-HRV blended image shows homogeneous BT for this area,

5 except a slightly colder pixel corresponding to an overshooting top. In the photograph we can see a thin, but extended cirrus cloud layer over the middle of the system. Figure4b. Simultaneous photograph (left), IR3.9-HRV blended (in the middle) and IR10.8-HRV blended (right) images, all taken at 15:18 UTC. In the 15:18 UTC photograph (Fig. 4b) we can see a pileus cloud (white arrow) left from the elevated dome (black arrow). In the IR3.9-HRV blended image the reddish patch (indicating the increased IR3.9 BT) is slightly darker and the contrast between the patch and the environment is higher than 5 minutes earlier. The IR10.8-HRV blended image does not show this pattern. Figure4c. Simultaneous photograph (left), IR3.9-HRV blended (in the middle) and IR10.8-HRV blended (right) images, all taken at 15:23 UTC. In the 15:23 UTC photograph (Fig. 4c) a well developed pileus can be observed over the elevated dome (see the arrows). In the IR3.9-HRV blended image the reddish patch on the yellowish cloud top is even better expressed, it is darker and bigger. The highest IR3.9 BTs in the patch were found in this slot. This might be caused by thickening of the pileus cloud. In the IR10.8-HRV blended image there is also some spot with slightly lower BTs, but this spot has a different structure. Figure4d. Simultaneous photograph (left), IR3.9-HRV blended (in the middle) and IR10.8-HRV blended (right) images all taken at 15:28 UTC. The 15:28 UTC IR3.9-HRV blended image (Fig. 4d) is very interesting. The dark red (higher IR3.9 BT) spot is divided into two parts (white arrows) by the two overshooting tops (yellow arrow) seen between them in the yellowish area (lower IR3.9 BT). It means that the effective particle size on the overshooting tops and in their surroundings is bigger. The patch with the smaller particle size was pushed aside in two directions with the appearance of the overshooting tops. This could be the moment when the overshooting tops shot through the pileus clouds. While we see two patches in the IR3.9-HRV blended image we have only one patch in the IR10.8-HRV blended image, somewhere in between, closer to the overshooting tops. It is cold because of the adiabatic cooling of the

6 overshooting top. In the photograph we see the pileus cloud over the south-southeastern part of the thunderstorm. Its horizontal extent increased compared to previous images. Figure4e. Simultaneous photograph (left), IR3.9-HRV blended (in the middle) and IR10.8-HRV blended (right) images all taken at 15:33 UTC. At 15:33 UTC the two dark spots in the IR3.9-HRV blended image are already less dark (Fig. 4e) this could be explained by the start of the mixture of the pileus cloud with the environmental air or with the air originating from the thunderstorm cloud. We still have only one cold patch in the IR10.8-HRV blended image, somewhere in between, close to the overshooting top. In the photograph we see that the pileus layer spreads and it probably also gets thinner. Figure4f. Simultaneous photograph (left), IR3.9-HRV blended (in the middle) and IR10.8-HRV blended (right) images all taken at 15:38 UTC. In the 15:38 UTC photograph (Fig. 4f) it is not possible to see the pileus cloud any more. In the IR3.9- HRV blended image almost the whole ice cloud top is yellowish without any signal of small ice particles. We note that a warm spot shape developed close to this cloud top area minutes later. CONCLUSION The photographs and the above presented HRV-IR3.9 blended image indicate that in this case the increased IR3.9 BT values were very likely due to appearance of pileus clouds. In 15:28 UTC the blended image (Fig. 4d) we can see that the particle size around the overshooting top was big. The appearance of the overshooting top decreased the IR3.9 BT. If the yellow spot would have been caused by small particles just coming out of an updraft channel; it would have appeared simultaneously with the overshooting top. The presence of the pileus cloud could be one of the possible explanations why a spot with increased IR3.9 BT value can appear over a convective storm. ACKNOWLEDGEMENTS Part of this research was supported by the Hungarian National Program for Research and Development (NKFP, project number 3/0022/2005), the Operational Program of Environment and Energy (KEOP-630/2F/ ), the DMCSEE (Drought Management Centre for South East Europe) project of South East Europe Transnational Cooperation Program and the Hungarian Scientific Research Fund (OTKA68277). The authors are grateful to Martin Setvák (Check Hydrometeorological Institute) for helpful discussions. The authors wish to thank EUMETSAT for the beneficent training workshops on MSG applications on convection and the lecturers at these workshops for their valuable instructions.

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