„EXTREME CONVECTIVE CASES - THE USE OF SATELLITE PRODUCTS FOR STORM NOWCASTING AND MONITORING” Monika Pajek1, Rafal Iwanski1, Marianne König2, Piotr Struzik1 1 Institute of Meteorology and Water Management, P. Borowego Str.14, 30-215 Krakow, Poland 2 EUMETSAT, Am Kavalleriesand 31, D-64205 Darmstadt, Germany Abstract A detailed storm nowcasting is still a very demanding activity for operational activities of forecasting offices. Proper prediction of exact location and intensity of the initial convection, estimation of storm intensity based on its development and storm trajectory monitoring and forecasting are very important for warning purposes. Use of dedicated satellite products may improve operational storm prediction and monitoring. Early detection of the unstable air and assessment of the potential of deep convection were already presented on EUMETSAT Conferences in 2006 and 2007. In the frame of co-operation between EUMETSAT and IMWM, further works on storm nowcasting were done. This paper focuses on selected case study, where extreme convective case were used to demonstrate usefulness of satellite products for analysis of pre-storm conditions, convection detection, characterisation of convective cells and nowcasting future storm behaviour. The area of Poland used for this analysis suffers from many severe storms between April and September with highest storm activity in the May to August period. Selected case was connected with tornado, intense lightning activity (including stratospheric sprite registered in Poland), heavy wind and rainfall damages. Possibilities and weaknesses of used satellite products for storm nowcasting and monitoring were discussed. Introduction. Process of storm development consists of pre-storm conditions leading to the development of convection followed by development of deep convective clouds, which became storm cells after the first lightning. Main goal of this paper is presentation of various satellite products for analysis of the whole convective process and possible use of them for warnings. Selected case was a good example of storms development connected with extreme weather phenomena, such as: tornado, large hail and heavy precipitation. The satellite products used for analysis are both operational ones available for forecasters and products which are still under investigation and testing.

Short description of analysed synoptic process. Presented analysis of satellite data use for storm nowcasing and monitoring is based on 20.07.2007 case. On this day Poland was in the area of low pressure with the centre over the British Isles with warm front horizontally splitting country (Fig.1). Very warm tropical air was coming from SE at low level and SW inflow occurred at higher levels. High level jet was crossing fronts over Germany and moving over Poland. F2/T4 tornado occurred in Częstochowa region (Huby φ 50,52 λ 19,21) at 16:10 UTC with wind speed peeking 50-60 m/s. Tornado path was 14 km long and up to 500m wide. On the edges of the areas of funnel cloud path, intensive hailstorms were reported before (1515 UTC) and after (1625 UTC) tornado. Convective clouds were classified as a MCS from it early stage at the 1300 until about 2200 when the cell weakens and disappears.

Fig.1 Synoptic situation on 20.07.2007 and resulted by hail and tornado damages. Synoptic situation (upper left) 1500UTC with marked localization of tornado (T) and radiosounding (star). Radiosounding profile for Wroclaw 1200 UTC (upper right). Vertical wind profile indicates jet stream at a high of 9,2 – 12 km and the wind shear at the low layer of the atmosphere. Synoptic map (bottom right) 1600 UTC (tornado time). The surface convergence zone over in the area of forming tornado is clearly visible. Air temperature reached 32,3º C and dew point temperature was very high – exceeded 21ºC. Map of tornado and hail occurrence (bottom left) : Source G.Beblot.

Large damages were registered: 783 houses and 1361 farm buildings were damaged by hail with size 5-7 cm. Tornado damages cover: 111 houses, 151 farm buildings 120 ha of forest, 3 high voltage pylons 24 m tall (Fig.2). NWP mesoscale models (COSMO, ALADIN with spatial resolution -14 km) and other data available at the morning did not suggest extreme meteorological phenomena. Expected conditions were: wind 2-4 m/s, precipitation 0-4 mm.

Fig.2. Funnel cloud and resulted by hail and tornado damages. Source www.IMGW.pl, G.Beblot.

Introduction to applied satellite products. The variety of satellite products was used at different stages of storm development. These ranges are not strict and use only for order purposes (Tab.1). Detailed description of products is available via the internet distributed documents (references).

T

Pre-convective situation Convection initiation Storm development MPEF/GII: KI, LI, TPW, LPW, pseudo-profiles

NWC-SAF: SAI, LPW

MPEF: AMA, DIV

TOVS/NOAA sounding (T, Td, Geopotential height, geostrophic wind)

Met9/Images: HRV, IR, RGB compositions,

Convection Initiation (CI),

NWC-SAF: RDT, CT,

MPEF: CTH

TOVS /NOAA sounding

Rapid Scan: HRV, WV

Met9: Overshooting Tops (WV-IR)

Met9/Images: IR Colour Enhancement (M. Setvak palette), WV

MPEF: CTH, MPE

NWC-SAF: CRR,

Table 1. List of satellite products used for analysis at the different stages of process. Pre-storm conditions. (0600 UTC – 1300 UTC) Conditions leading to the development of deep convection and resulted sever weather events are characterised generally by: unstable air, high moisture at low and mid-level and force which stimulate convection. This force may be caused for example by ground heating, orography, convergence, atmospheric front, jet stream etc. Use of satellite data and specialized products make possible to detect and characterise many of mentioned features. What is very important, state of the atmosphere can be determined several hours before the beginning of convection and dynamically traced in 15 minutes steps (5 min with use of Meteosat-9/Rapid Scan). On selected day, high air instability in southern part of Poland was observed since early morning hours, while northern part of Poland was stable. Within the next hours higher instability was retrieved, especially on GII/Lifted Index field (below -8 deg) presenting regional maxima located just in the places where the strongest storms were eventually developing. (Fig.3.). Similar results were obtained for the same GII indices (Lifted Index and K index) but with use of Aladin NWP mesoscale model (instead of ECMWF global model) used as a first guess. Fig.3 Stability Indices GII/Lifted Index-upper row. GII/KIndex-bottom row. Resolution 3x3 SEVIRI pixels. First guess - ECMWF forecast. 0600UTC (left)-preconvection, 1300 UTC (middle)-convection beginning, 1500 UTC (right)- convection developing. Both indices present unstable situation since early morning hours. An hour before development of tornadic storm, well depicted area of storm development especially by LI. During 0600-1645 UTC hours increased values of moisture water content was detected, the amount of precipitable water (GII/PW) in the area of future storm developing exceeded 45 mm in the whole column of the atmosphere at 1300 UTC. Availability of high moisture in the lower troposphere,

reaching 25 mm at 1500 UTC (NWCSaf/LPW product on Fig.3) presented favourable conditions for tornado development.

Fig.4 Precipitable Water – amount of precipitable water in the atmosphere in cloud free areas. Upper row - GII/Precipitable Water Content, resolution 3x3 SEVIRI pixels, physical retrieval, and first guess - ECMWF forecast. Bottom row - NWCSAFv.2.1/ Layer Precipitable Water in Boundary Layer (1013hPa -840 hPa), resolution 1x1 SEVIRI pixels. Obtained with neural network algorithm. 0900UTC (left)-preconvection, 1300 UTC (middle)-convection beginning, 1500 UTC (right)- convection developing

High level jet stream strengthen the updraft motion on the left side of its axis and let convection develop really high. Presence and exact location of jet stream could be depicted with use of satellite soundings. Geopotential height and geostrophic wind retrieved from NOAA/TOVS soundings shows increased wind speed in area of interest and indicates probable jet stream (unfortunately TOVS calibration cycle is in the same place). At 1515 UTC NOAA/TOVS retrieved temperature presents high horizontal gradient of temperature between hot airmass at south and cold one at the north of Poland. Also content of moisture is much higher (42 mm) in the area of interest then in nearby. The advantage of this calculations with use of polar satellite NOAA/TOVS data is that they could be made on different geopotential high, also in cloudy areas, but unfortunately only few times per day (depending on NOAA satellite passes).

Fig.5. NOAA/TOVS/AVHRR retrieval. NOAA 17 - 1515 UTC. Left - geostrophical wind field. 500 hPa level. Middle up - Temperature retrieval on different geopotential levels.

Middle down – Total precipitable Water (compare to Fig.4 – good coincidence). Right – NOAA/ AVHRR RGB composition, channels 1/2/4.

Convection initiation. (1300 UTC – 1600 UTC) Fast development of convective clouds occurred at 1300 – 1600 UTC. Clouds were continuously moving towards north-east forming ‘MCS’ – Mezoscale Convective System. In the area of tornado two convective cells merged into one object at the time of funnel cloud forming 1610 UTC. In case of such a big amount of convective clouds, important issue is recognition and tracing of convective cells responsible for future severe events (storm, hail and tornado).

Fig. 5 RDT (Rapid Developing Thunderstorm) – NWCSAF v.2.1, background Meteosat9 ch. IR 10.8 µm mask (see/land) enhanced palette. Location of tornado event (1610 UTC) marked. Left 1400 UTC - first detection of rapidly developing convective cloud – part of later MCS. Cirrus clouds embedded to the convective cell. Middle 1600 UTC and right 1715UTC - good and wrong recognition of convective cloud’s size and movement, respectively.

Rapid Developping Thunderstorms (RTD) product was designed for this purpose. Unfortunately, in some cases proper recognition of cells is questionable. It makes difficult to use this product operationally by forecasters. The idea of another product - the Convective Initiation - was to identify strongly growing cumulus clouds even before the radar picks them up. Modified algorithm developed for GOES satellites (Mecikalski, J. R., and K. M. Bedka, 2006) was implemented for European conditions and satellites by EUMETSAT. First the convective cloud mask is computed mostly on the base of Met9/HRV cloud classification process (daylight algorithm). Then for clouds classified as "cumulus" the final classification is performed with use of two criteria: change of IR cloud top temperatures from above to below 0 deg. Celsius within last 15 minutes (between two Met9 slots), cooling rates (computed for 25 by 25 MSG IR pixel boxes averages) have exceeded -4 K/15 minutes). (Fig.6). Clouds meeting those conditions are identified as strongly growing cumulus clouds. In this case, improvement of storm cell recognition comparing to radar image is not high (about 10 minutes lead time). But presented "very first results" looks promising. More research and adaptations are definitely needed. The thresholds were simply taken from US work and probably should be slightly modified for European conditions. The advantage of CI nowcasting product is simplified analysis of large sets of individual products, and as a results give very clear and easy to interpretation output, which can be useful in forecasters work.

Fig. 6 Tests of CI nowcast algorithm: channel 10,8 um images of satellite MET9 and calculated cooling rate, convective cloud mask based on HRV image analysis. CI Nowcast product at 1400 UTC detected the most active part of future MCS. Clouds classified as potential extreme convection, have only 27 dBZ radar reflectivity, which turned to more 45 dBZ within next ten minutes.

The lightning are close connected with severe convection and could give information about convective cell phase. Cloud to ground discharges appeared usually when the storm cell turn into mature stage, amount of positive lightning- responsible for most damages, - shows storm severity (positive lightnings are typically six to ten times more powerful then the negative ones). On 20.07.2007 high electric activity of analyzed MCS comparing to the other storm cells was observed.( Fig.7.)

Fig.7 Spatial and time distribution of lightning on 20.07.2007 Left – Lightning registered by PERUN (safir) system between 1600- 1610 UTC Superimposed over Met9/HRV image. Middle- Spatial distribution of lightning during the period 0800 – 2100 UTC in four temporal classes. Right - Temporal distribution of lightning in the period 0900 – 1900 UTC in hourly classes.

The system of lightning detection PERUN/safir registered first, single Cloud to Ground (CG) discharges about noon, Then the number of discharges rapidly increased till more then 3000 between 1600- 1700 UTC. The peak of lightning –CG discharges in the area of moving tornado was just after the occurrence of funnel cloud at 1625-1630 UTC when the storm cell become matured. General high lightning activity in the evening hours creates good condition to Sprites observations during late hours. On the night of 20th of July 2007 during the SPARTAN Sprite-Watch campaign 2007 (Odzimek,A., et al, 2008) the first red sprite events were scientifically observed from Poland. Red sprites belong to a bigger group of phenomena collectively termed Transient Luminous Events (TLEs). They occur above thunderstorms in the upper atmosphere, emitting visible (mostly red) light. A sprite discharge and related light emission starts at about 65-75 km altitude and proceeds downwards. It also develops upwards in the form of blurred red or purple glow, reaching levels of up to 90 km altitude. On the night of interest two very powerful storms emerged – one from East Germany and the second one over Czech Republic providing suitable conditions for strong electric activity in the vicinity

Cloud to cloud CC Cloud to ground negative CG- Cloud to ground positive CG+

of the developing cells and also in the perimeter of the recording system installed on ŚnieŜka Mount. The optical system for TLE observations at ŚnieŜka consisted of a low light TV camera, a fixed 16mm lens and joint peripheral processing equipment connected to notebook endowed with night sky inspection software. It was capable to detect five TLE events during one night from 20:10 till 00:18 for the very first time from Poland. (Fig. 8.) As a conclusion we can state that in Central Europe sprites can be produced by massive storm cells build on warm fronts supplied by warm and humid tropical air masses, during local summer thunderstorm season.

Fig.8. The red sprites recorded at ŚnieŜka Peak (Sudety Mountains) after 20:10:08 UTC on 20.07.2007. Different cell

then tornadic one.

Storm cell development. (1600 – late evening) At 1610 UTC funnel cloud near Czestochowa region occurred. In this time vertical extend of storm cloud was very deep. On the radar image representing the height of the clouds’ tops EHT, values exceeded 15 km. Similar values are shown on satellite product CTH (Cloud Top High). Clouds development up to 16 km corresponds well to radar crossections. (Fig. 9)

Fig.9 Echo Top radar product - Radar Brzuchania (left) 1610 UTC, MPEF/Cloud Top High product (right) 1645

UTC.Coresponding area marked. Storm cell analysis. At 1515 UTC severe hail was reported in the area of Mykanow (near Czestochowa). It is considered, that this local circumstances were a “trigger” which substantially contributed to the formation of tornado. Considerable air cooling in this area caused by intense hailstorm after 1500 UTC might lead to formation of big thermal contrast. Thunderstorms may develop along a distinct dry/moist boundary. The hailstorm started at 1515 time and after 5 minutes the ground was covered with thick layer of ice, in some places reaching the knees of the grown man. Such a big hail indicate very strong updraft. Just before hail event, at 1445 UTC the dark stripe was observed on the Met9/WV image (Fig.10). This dark stripe is a sign of dry-adiabatic descent initially occurring within rear inflow. The descending air would accelerate development of the circulation in the cloud. Strong mesoscale downdrafts are most

likely created by sublimating snow or ice, so the hail could be the reason of starting mesoscale vorticity (Schulz, D.M, Schumacher, P.N, 1999).

Fig. 10 Evolution of convective cell (s) in MET9/WV 6,2 um imagery, enhanced palette. 20.07.2007 Left – 1445 UTC WV dark stripe first appearance, close to first hail event. (1515UTC) Middle – 1615 UTC Well developed dark stripe – close to tornado event (1610 UTC) Right – 1815 UTC Ddark stripe disappeard Additionaly scheme of clasical look of MCS In WV band: source http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?/docu/Manual/SatManu/CMs/CbC/structure.htm Also at 1445 UTC, for the first time on this day, the positive difference of brightness temperature channels WV6,2 and IR 10,8.(Overshooting Tops) was noted. Within next few hours Overshooting Tops were registered on every slot indicating area of clouds where the strongest updraft exist. Also the IR 10.8 brightness temperature (T below -63 deg C) confirms, that severe weather and heavy rainfall may be expected. The interesting feature is correlation between Overshoting Tops Image and the brightness temperature of IR 10,8 and WV 6,2. At 1715, at a later stage of storm’s life cycle the large positive values of Overshooting Tops (+3 C) in a South East part of cloud does not correspond to the lowest value of IR 10,8 and WV 6,2. (Fig 11).

Fig. 11. Evolution of storm cloud in different channels of Met9. 20.07.2007. Upper row 1600UTC – tornado event, bottom row 1715 UTC – MAture mesoscale Convective System From left to the right, IT 10,8 um enhanced Martin Setvak palette, WV 6,2 enhanced palette, VW-IR „Overshooting Tops”Image, HRV Image (arrow points Tornado area.), respectively.

Conclusions On 20.07.2007 big instability, enough moisture and convergence at the low level let the deep convection developed. Jet stream in high atmosphere, high dew point temperature and inflow of different air mass at the higher level (wind shear) were favourable conditions to funnel cloud and tornado development. Satellite products help to recognize, diagnose and monitor of the whole process. Specialized satellite products improve analysis of convective process development and provides valuable information of possible future phenomena. Some products present their weakness, other products (often in development phase) are promising operational tools. Use of satellite products increase lead-time for storm nowcasting by 30 min – 2 hours. More studies concerning relations between satellite products and physical phenomena at different stages of convection process are highly required. Number of useful satellite products requires „automatic expert system” for preliminary data analysis and forecaster warning. Further use of various satellite products for case studies is recommended, to properly understand their added value. Due to limited size of papaer only part of available satellite tools were presented. Acknowledgments: Authors would like to thank Grazyna Beblot, Irena Tuszynska, Danuta Serafin-Rek, Małgorzata Szczech-Gajewska Aleksandra Zawadzka, Joanna Kozakiewicz, Witold Wiazewski, Piotr Drzewiecki, Leokadia Zagajewska, Magdalena Wells, Roman Kaseja, Józef Warmuz for contribution with data analysis and preparation. The paper is a results of cooperation between IMGW/Poland and EUMETSAT organization. References Beblot, G.,Holda, I, Korbek K., (2008), Traba powietrzna w rejonie Częstochowy w dniu 20 lipca 2007

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Validation Studies on product Quality and analysis of sensibility to input model data., In: Joint 2007 EUMETSAT and 15th AMS Conference, P.50

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Odzimek, A., L. B. N. Clausen, V. Kanawade, I. Cnossen, N. Edberg, F. Faedi, A. Del Moro, U. Ural, K. Byckling, P. Krzaczkowski, R. Iwanski, P. Struzik, M. Pajek and W. Gajda (2008), SPARTAN Sprite-Watch Campaign 2007, In: 15th Young Scientists Conference on Astronomy and Space Physics. Proceedings of Contributed

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EUMETSAT Meteorological Satellite Conference, Dubrovnik, Croatia, EUMETSAT, P.46. 460-466. Internet: http://nwcsaf.inm.es/ - NWCSAF products description http://www.eumetsat.int/HOME/Main/Access_to_Data/Meteosat_Meteorological_Products/Product_List/index.htm?l=en – EUMETSAT MPEF products description