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Extreme Convective Cases

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Extreme Convective Cases „EXTREME CONVECTIVE CASES - THE USE OF SATELLITE PRODUCTS FOR STORM NOWCASTING AND MONITORING” Monika Pajek 1, Rafal Iwanski 1, Marianne König 2, Piotr Struzik 1 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. T 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). Pre-convective situation Convection initiation Storm development MPEF/GII: KI, LI, TPW, LPW, Met9/Images: HRV, IR, RGB Met9: Overshooting Tops pseudo-profiles compositions, (WV-IR) NWC-SAF: SAI, LPW Convection Initiation (CI), Met9/Images: IR Colour Enhancement (M. Setvak MPEF: AMA, DIV NWC-SAF: RDT, CT, palette), WV TOVS/NOAA sounding (T, Td, MPEF: CTH MPEF: CTH, MPE Geopotential height, TOVS /NOAA sounding geostrophic wind) NWC-SAF: CRR, Rapid Scan: HRV, WV 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.
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