T-re Plots Generated from MSG Data in Severe Storms Forecasting Testing in Central Europe

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1 WDS'11 Proceedings of Contributed Papers, Part III, 88 92, ISBN MATFYZPRESS T-re Plots Generated from MSG Data in Severe Storms Forecasting Testing in Central Europe M. Pokorný and M. Žák Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. The T-re plots derived from the satellite retrieved data represent vertical distribution of effective particle size, which is closely related to the updraft strength inside convective storms developing within the target area. Considering the shape of these plots and the size of the particles, the special severe storm forecasting technique was tested in the USA and other countries. This technique was used to forecast and predict dangerous phenomena occurring in severe storms. There are tests done for the Central Europe also. This paper presents the preliminary results of testing the T-re plots concept during selected severe storms episodes in the Czech Republic. The discussion of the possible benefits for the Czech weather service is also given. Introduction The use of T-re profiles (T represents cloud top temperature, re means particles effective radius) in the severe storms and their dangerous accompanying attendant phenomena forecasting (such as hails or tornadoes) was introduced by Daniel Rosenfeld (Institute of Earth Sciences, The Hebrew University, Jerusalem, Israel) and Itamar Lensky (The Department of Geography and Environmental Studies, The Hebrew University, Jerusalem, Israel) already in 1998 [4]. According to their assumptions, the MSG data can be effectively used to track cloud particles effective radius evolution depending on convective cloud top temperature, these theories were used in various projects. T-re plots from different satellite data (mainly GOES) were tested and verified in several countries, namely Texas, California, Brazil, Israel and southern Africa, where specifically MSG data was used to observe the evolution of T-re relationship [2]. Their method can therefore provide another nowcasting tool aimed at dangerous weather event forecasting in Central Europe and can improve forecasting success of these weather phenomena. Basic T-re plots theory CAPE (convective available potential energy) and wind shear in the lowest 6 km of the atmosphere are frequently used for prediction of severe storm formation, however updraft in clouds plays the crucial role according to Rosenfeld as well [7]. He considers that the speed of updrafts is the basic presumption for severe storms and their accompanied dangerous attendant phenomena. High updraft speed in clouds supports hail formation, help forming tornadoes etc. Hence Rosenfeld proposed a conceptual model of microphysical characteristics of clouds and vertical profiles of effective cloud particle radius depending on the updraft speed (Figure 1). These profiles express effective cloud particle radius (re) dependence on the cloud top temperature (T), they help to estimate the updraft strength inside clouds and then to forecast storm severity. The influence of the updraft strength can be explained by the fact that higher updraft speed delays cloud particles growth to larger sizes and postpones glaciation of clouds. If there is the strong updraft in clouds, the particles raise upwards through the cloud very fast, so they have not enough time to grow to larger sizes. Retrieved T-re profiles indirectly reflect updraft speed of vigorously growing convective clouds showing small particles at the cloud tops. Storm intensity is considered to be the result of the updraft strength. CAPE, wind shear and cloud rotation still remain important factors of severe storm forming. Described T-re profiles express the effective particle size dependence on cloud top temperature (height respectively), they provide one of the possible nowcasting methods. To understand the microphysical processes please see e.g. [1]. T-re plots allow finding individual microphysical zones inside clouds according to the Figure 2. 88

2 Figure 1. T-re plots conceptual models. A maritime clouds, B continental clouds, temperature 38 C (homogenous freezing) and size 14 µm (precipitation threshold) are stressed. According to [7]. Figure 2. Individual microphysical zones inside the clouds. According to [3]. Basic theory application We use MSG data of Central Europe for observing cloud top temperature and cloud particles effective radius using described T-re plots. These plots show some features useful in severe weather nowcasting. First of all the evolution of cloud top effective radius (re) with cloud top temperature (T, or height) for cloud group at specific time is similar to T-re evolution of each cloud in this location. The re near the cloud top is similar to that inside clouds at the same height before precipitation falls through these clouds. Constructed T-re plot of a convective cloud area is stable in time and depends mainly on the thermodynamic and aerosol properties of the air mass [1]. These assumptions mean that we can use T-re plot for whole chosen area instead of tracking T-re evolution for each individual cloud. Cumulonimbus clouds have nearly the same T-re relation such as clouds that did not reach the Cb stage. T-re plot is not only one individual line but it is the full set of lines expressing 5 th to 100 th percentile of re in 1 C interval (Fig. 3). Mostly the 15 th percentile is analyzed, because it represents younger vigorously growing clouds in the given height (temperature). Temperature of 38 C expressing homogenous freezing threshold and particle radius size of 14 µm showing precipitation threshold are taken in account. Analysis is done according to the shape and steepness of the T-re plot, position of described temperature and size to 15 th percentile line is also very important indicator in the analysis and forecasting. Some other values such as TL (temperature of the liquid phase top), Tg (temperature of glaciation phase bottom) and their relations are studied and can help to find cloud areas prone to grow into severe storms. In this paper we only give a short overview of this analysis. For more details please see e.g. [7]. 89

3 Figure 3. T-re profile, 15 th (r15), 50 th (r50) and 85 th (r85) percentile plotted. According to [1]. Figure 4. Analysis of three different Cbs, different storm intensity Tornadic storm left, hail storm middle, nonsevere storm right. Microphysical zones vertical lines: yellow diffusional, green coalescence, pink mixed, red glaciated zone. Taken from [5]. T-re plots testing Testing of the T-re plots was done for example in North America, South America, South Africa or Israel. Different satellite data (MSG included) were used depending on location. Evaluation was done with help of radar, radiosonde measurement and special cloud scanner aboard jet aircraft [3,6]. Fig. 4 shows the differences in shape and steepness of T-re plots during three convective storms with the different intensity and attendant phenomena. Such figures significantly help to understand forecasting essence of the plot in context of the shape and steepness of the plot, microphysical zones are marked by vertical lines in different colors. T-re plots in the Czech Republic and the vicinity The assumptions described above were tested in several countries and now they are evaluated for the Czech Republic and its vicinity on archive data for days when intensive accompanying phenomena of storms were reported. Received profiles are very similar to those from USA so this method seems to be applicable for severe storm nowcasting in the Czech Republic as well. Two chosen analysis (due to extent of this paper) for Central Europe region follow, dangerous phenomena and T-re features are 90

4 described. Analysis of these situations is shown in Figures 5 and 6 which are based on RGB Storm products gained from EUMETSAT archive and processed in MSG_RGB software [8]. 12 th June 2010 Central Europe was influenced by low pressure trough and accompanied cold front that was moving to south-east, temperature maxima were about 26 C, after 15 UTC near Olomouc and Tábor mighty Cbs were observed, after 16 UTC in the vicinity of Šumperk thunderstorms developed. Near Tábor a supercell formed, rotation of the cloud system was observed and funnel cloud was reported, after 17 UTC 3 cm hail and heavy rain occurred. Strong wind gusts about UTC followed by heavy precipitation in southern Moravia was reported, cellars were flooded. After 20 UTC strong wind gusts damaged some roofs in southern Moravia. In Austria strong wind was observed about UTC, it raised sand and dust and was followed by heavy rain. Roof and chimney damage was reported, wind speed on the squall line was 25 m/s. Analysis in Figure 5 shows that T-re plots of clouds include very small particles (less than 15 µm in radius) up to temperature about 20 C, from this reason it seems there must be the strong updraft in the lower part of clouds. Namely in the plot signed Area 1, we can recognize long and nearly linear part of plot indicating severe weather occurrence possibility. Plots in Figure 5 are very similar to that profiles gained by Rosenfeld during his test in USA etc. when hail and other severe weather events occurred. This may be confirmed by hail report from the south of Bohemia and other dangerous phenomena nearby. 25 th June 2006 Low pressure area with a cold front was moving to Central Europe. There was F1 T3 category tornado documented in Germany, path length was 3 km and the maximum path width was 300 m, funnel cloud was and suction vortices were observed, gradually hail size was 3 cm, heavy rain 50 mm/h and strong wind gusts 35 m/s with visibility 100 m were reported, crop and forest damages and injury was noted. Large hail were observed in south Bohemia near Tábor and Soběslav about 16 UTC, unfortunately the size was not reported. Analysis of the Area 1 in Figure 6 shows again intensive updraft in large part of cloudiness, cloud particles are slowly growing to larger sizes. Strong updraft can be found in Area 3 up to level of temperature 23 C as well. Gained T-re plots again respond to those with hail occurrence in the south of Bohemia even 1 hour in advance. This case also supports the presumption of using T-re plot hypothesis for severe weather nowcasting in the Central Europe. Figure 5. Analysis of the 12 th June 2010, UTC. 91

5 Figure 6. Analysis of the 25 th June 2006, UTC. Conclusion The basic principle of described theory is the satellite observation of clouds in the different spectral channels that enables to generate T-re plots of microphysical constitution of clouds. These profiles were already successfully tested in different countries. If we confirm their validity in the Central Europe, we would get useful nowcasting tool for severe convective storms and their dangerous accompanying phenomena, improving our skills of forecasting that may help to protect estate as well as people s lives. Our analysis was done on archived satellite data chosen according to dangerous weather reports. Applicability of this theory has to be confirmed by analyzing of nonsevere situations to separate weather events that endanger people and their property from the nonsignificant weather phenomena. From this reason the next plan is the operative testing of this theory on real-time MSG data during convective season. Objective criterion based on these analyses should be established and in case of interest of national weather service automatization of this tool will be constructed. References [1] Lensky, I., Rosenfeld, D., The Time-Space Exchangeability of Satellite Retrieved Relations Between Cloud Top Temperature and Particle Effective Radius, Atmos. Chem. Phys., 6, p , [2] Lindsey, D. T., Hillger, D. W., Grasso, L., Knaff, J. A., Dostalek, J. F., GOES Climatology and Analysis of Thunderstorms with Enhanced 3.9 µm Reflectivity, Monthly Weather Review, 134, [3] Martins, J. V., Marshak, A, Remer, L. A., Rosenfeld, D., Kaufman, Y. J., Remote Sensing The Vertical Profile of Cloud Droplet Effective Radius, Thermodynamic Phase, and Temperature. Atmos. Chem. Phys. Discuss., 7, p , [4] Rosenfeld, D., Lensky, I., Satellite-Based Insights into Precipitation Formation Prosesses in Continental and Maritime Convective Clouds, The Bulletin of American Meteorological Society 79, p , [5] Rosenfeld, D., Woodley, W. L., Kelman, G., Golden, J. H., Short-Term Forecasting of Severe Convective Storms Using Quantitative Multi-Spectral Satellite Imagery: Results of the Early Alert Project, from D. Rosenfeld [6] Rosenfeld, D., Woodley, W. L., Krauss, T. W., Makitov, V., Aircraft Microphysical Documentation from Cloud Base to Anvils of Hailstorm Feeder Clouds in Argentina, Journal of Applied Meteorology and Climatology, 45, [7] Rosenfeld, D., Woodley, W. L., Lerner, A., Kelman, G., Lindsey, D. T., Satellite Detection of Severe Convective Storms by Their Retrieved Vertical Profiles of Cloud Particle Effective Radius and Thermodynamic Phase, J. Geophys. Res., 113, D04208, doi: /2007JD008600, [8] User Guide for MSG_RGB, from Itmar Lensky. 92