EXPLORING SEVERE THUNDERSTORM EVENTS OVER GREECE

USING LOCAL INSTABILITY INDEX (LII) AND LOW ORBIT

SATELLITE RAINFALL PRODUCTS

KOROLOGOU M. (1), VLASSI Ad. (1) et SARRAS Chr. (2)

(1) Hellenic National Meteorological Service, Hellas, marina.korologou@hnms.gr(2) Regional Meteorological Center Macedonia, Hellas.

Summary: Due to the climate change, the severity of thunderstorms along with the intensity of the corresponding rainfall over northwest Peloponnese shows an increasing trend over the last years. It is then a challenge for scientists to use all data available as well as new emerging tools and products to study them. In this work, the Local Instability Index (LII), a threshold function considering the low level moisture, a practical approximation of the CAPE, the terrain heating effect and a formalized operational experience; and the low orbit satellite rainfall products (NASA/3B41RT) were used together with traditional tools to assess three severe thunderstorm events over the area in question along with an attempt to test their performance, utility and warranty. Similarities and differences between the events were pointed out and discussed. The 3B41RT enabled the calibration of the LII, which was found to have satisfactory performance and proved operationally useful; in addition, it gave quite a representative spatiotemporal rainfall distribution being of great value for areas not covered by other methods.

Key words: TRMM, GPM, Local Instability Index

Résumé : La violence des orages et l'intensité des précipitations présente, sur le nord-ouest du Péloponnèse, une tendance augmentant pendant les dernières années due au changement climatique. Les météorologistes répondent au défi d’utiliser toutes les données disponibles ainsi que les outils et produits nouveaux pour les étudier. En particulier, l'Indice d’Instabilité Locale (LII), une fonction de point limite, une approche du CAPE, l'effet du réchauffement de la terre, une expérience opérationnelle formalisée et les précipitations produites du satellite a orbite basse (NASA/ 3B41RT) ont été utilisés pour mieux évaluer trois cas d'orages sur la région en question. Du même qu’une tentative pour tester leurs performance, utilité et garantie. Les similitudes et les différences ont été ponctuées et discutées. Le 3B41RT a permis l'étalonnage du LII, qui a présente une performance satisfaisante et s'est avéré utile sur le plan opérationnel et il a fourni une répartition des précipitations représentative au temps et à l’espace ayant alors une grande valeur pour les régions non couvertes par d’autres méthodes.

Mots clés: TRMM, GPM, Indice d’Instabilité Locale.

Introduction

The Earth's climate has changed throughout the years, mainly because of small variations in the Earth's orbit changing the amount of solar energy it receives. However, the current trend of warming has specific characteristics, as it is much likely (over 95% probability) to be mainly due to the human activity since the mid-20th century (NASA, 2019). Among the evidence depicting the climate change, is the variation in the world’s total precipitation distribution (NASA 1, 2019). It is most probable that an average global temperature increase may cause a change in the atmospheric moisture. As a result, more intense precipitation causes more frequently hazardous floods over the land while in other areas, higher temperatures may lead to increased drying and consequently to drought.

In the Mediterranean area, the hazardous floods are mainly due to thunderstorms with heavy rainfall. The increase in the frequency and the severity of flashfloods in hydrological basins has been a matter of great concern for hydro-meteorologists because of their both short

and long term consequences on the environment, the properties and even on human lives. Forecasting such type of thunderstorms early enough, is of high importance in order for the necessary protection and mitigation measures to be taken. Hydro-meteorologists use sophisticated computing tools and all available data related in order to forecast, detect and monitor such phenomena. Nevertheless, it is of high importance that these tools are tested and validated for their performance over a specific area in order for them to become more reliable and operationally useful. One of these new emerging tools is the Local Instability Index (LII) (Korologou et al., 2014). It is an alternative methodological tool developed for forecasting thunderstorms with heavy rainfall taking place in a specific hydrological basin. The index takes into account parameters concerning the thunderstorm triggering point and the available potential energy. The LII was tested for a period of 5 years to northwestern (NW) Peloponnese and forecasted all thunderstorms with heavy rainfall occurred. Three of the above cases were selected for presentation in this work. The Quantity Precipitation Estimation (QPE) was calculated using satellite data derived from the Tropical Rainfall Measuring Mission (TRMM)/ 3B41RT product (NASA 2, 2019).

In the framework of this analysis, an attempt was made to identify the similarities between the selected events and the available observations, the synoptic weather patterns and the local meteorological conditions in order to examine the performance of the TRMM/ 3B41RT product for the specific area and its potential operational use.

1. Data and methodology

Three thunderstorm events accompanied with heavy rainfall, during the period 1 January 2006 to 30-6-2011 (Korologou, M. et al., 2014), were selected for study, based on the number of lightning events detected. More specifically the first event, where the greatest number of lightning events was detected (2137 lightnings within 06:00-12:00UTC), took place on 15/9/2008, the second (313 lightnings within 25/9/2009 06:00-12:00UTC, 352 lightnings within 25/9/2009 12:00-18:00UTC and 828 lightnings within 26/9/2009, 00:00-06:00UTC) on

25-26/9/2009 and the third (371 lightnings during 06:00-12:00UTC) on 17/10/2010. All these cases refer to a hydrological basin located in the NW Peloponnese (Figure 1) defined by the Peiros, Parapeiros, Vergas and Pinios rivers (2500km2). The area including the basin was defined as 20.50o

to 22.00o E and 38.50o to 37.25o N and marked in a grid of 6 x 5 cells. Data were mapped on a three-dimension space for each case. A 3-hour time step was considered as the time dimension. The spatiotemporal cells were selected in accordance with Real-Time TRMM Multi-Satellite Precipitation Analysis (TMPA-RT) data grid (NASA 3, 2019).

The Hellenic National Meteorological Service (HNMS) provided the necessary lightnings raw data for the events’ periods and a Matlab script was developed in order for the data to be mapped in the suitable cells. A second script was also developed using as basic parameter the number of strokes per cell.

The 3-hour surface synoptic observations (SYNOP) were also provided by HNMS’s closest available meteorological stations of NW Peloponnese i.e. Andravida (LGAD), Araxos (LGRX), Pyrgos and Zakynthos (LGZA). Data from the Agrinio (LGAG) and Kefalonia (LGKF) stations were also taken into account. Missing SYNOP data were filled in by implementing the Acock method (Korologou, M. et al., 2014; Acock, M. C. and Pachepsky, Ya. A., 2000).

Figure 1. TMRA-RT cells over NW Peloponnese

The relevant precipitation amounts were estimated using the Tropical Rainfall Measuring Mission (TRMM)/ 3B41RT product. The TRMM satellite rotated in a semi-equatorial orbit with an inclination of 35o to the Equator. This type of orbit precessed approximately 7o per day allowing for TRMM Microwave Imager (TMI) to sample the surface at 16 times per day compared with the twice per day windows of the polar-orbiting radiometers (RSS, 2019). The main reason for choosing the specific satellite is the very low operating average altitude i.e. 403 km while polar orbiting weather satellites rotate around the Earth at altitudes of 850 km. The 3B41RT product is of 1-hour temporal resolution and consequently more compatible to the temporal scale of flash floods compared with other products’ time scales. Data were extracted with the aid of an algorithm developed in Matlab for each cell.

Meteosat SEVIRI / Channel 9 (9.8-11.8 Thermal Infrared) and Channel 5 (5.35-7.15 Mid-IR / Water Vapour) satellite images, derived from the archive of Dundee University, were used in order to monitor the cloudiness associated with the events and to identify any stratospheric dry air intrusion downwards. Temperature and relative humidity values at the isobaric surfaces of 850, 700, 500, 300hPa were provided from the ECMWF re-analysis of 0.125 deg resolution with a 6-hour time step (Veremei, N. E. et al., 2013).

2. Analysis

2.1 Synoptic, Dynamic context and Local patterns

The synoptic scale features and patterns were similar for all three cases. At the level of 500hPa, the Azores High associated with warm and dry air masses, influences the weather in the Mediterranean Sea. The equatorial flow of the Polar Jet stream is interrupted while shifted S-SE guiding relatively colder air masses around the edges of the High. The situation gradually weakens the anticyclone, especially around its edges, allowing the S-SE movement of the masses and reducing gradually the geodynamic heights. In the cases examined the air masses reached Italy resulting in a southwestern current establishment over NW Peloponnese implying at its turn a huge probability for severe thunderstorm triggering. In all events the synoptic scale situation shown at 500 hPa was well corresponding with the one at the 850 hPa level, where the Azores High was also prevailing i.e. the colder air masses reached the central Mediterranean enriching the mid-level atmosphere over the area with moisture. Similarly, the lowest atmospheric levels were also enriched with moisture, which is a prerequisite for thunderstorm initiating and developing (Moncrieff, M.W. and Miller, M.J, 1976) assisted by the surface wind field driven by the topography of the area. The aforementioned moisture was then transferred either from the Ionian Sea or the Golf of Patras to the hydrological basin examined, fact that was confirmed by the ECMWF reanalysis data. In contrast at the higher levels of the atmosphere (300 hPa) the relative humidity was relatively decreased, implying the intrusion of dry air masses downwards, as it can be confirmed by the SEVIRI/ Channel 5 (WV6.2) satellite images.

2.2 TRMM/ 3B41RT product, SYNOP and Lightnings data

The TRMM/ 3B41RT product successive images (Figures 2-6) show the per hour precipitation on 15/09/2008. As it can be seen, in the period 03:00-05:00 UTC, a thunderstorm cell was developed over the northwestern part of the area (Kefalonia island) (Figure 2(a, b)) and then moved southerly, over Zakynthos island, where heavy rainfall took place (Figure 2(c)). The lightnings detected in the cells defined in Fig.1 (Table 2), were compatible enough with the corresponding 3B41RT data (Table 1). Afterwards (06:00-07:00 UTC), the rainfall rate was intensified and the cell moved east northeasterly spreading over

almost the whole NW Peloponnese (Figure 3(a)). A second cell, developed over Araxos area, caused heavy but short- term rainfall and dissipated promptly soon (Figure 3(b, c), 4(a)). The former cell persisted to stay over Zakynthos island causing heavy precipitation until 11:00 UTC, when it started moving easterly (Figure 4(b, c)) triggering the development of a new, third, cell over Andravida area (Figure 5(a)) moving quickly south southeasterly towards

Ryrgos area (Figure 5(b, c), 6(a, b)). After 17:00 UTC no precipitation took place over the examined area. As it is shown at the Tables 3-9, the detected lightnings distribution found to be very similar with the maximum 3B41RT values and their trajectories.

Table 7. 3B41RT precipitation per cell, 12:00-15:00 UTC

Table 8. Lightnings per cell, 12:00-15:00 UTC

Table 6. Lightnings per cell, 09:00-12:00 UTC

Table 1. 3B41RT precipitation per cell, 03:00-06:00 UTC

(c) 11:00-12:00 UTC(a) 09:00-10:00 UTC

Figure 4: TRMM/3B41RT precipitation, 09:00-12:00 UTC

(a) 12:00-13:00 UTC

Figure 5: TRMM/3B41RT precipitation, 12:00-15:00 UTC

Table 2. Lightnings per cell, 03:00-06:00 UTC

(a) 06:00-07:00 UTC

Figure 3: TRMM/3B41RT precipitation, 06:00-09:00 UTC

(b) 10:00-11:00 UTC

Table 5. 3B41RT precipitation per cell, 09:00-12:00 UTC

(b) 13:00-14:00 UTC (c) 14:00-15:00 UTC

Table 9. 3B41RT precipitation per cell, 15:00-18:00 UTC

(b) 07:00-08:00 UTC (c) 08:00-09:00 UTC

Table 3. 3B41RT precipitation per cell, 06:00-09:00 UTC

Table 4. Lightnings per cell, 06:00-09:00 UTC

cells 1 2 3 4 5 6A 10,70 8,21 5,14 2,36 2,02 1,76B 7,50 7,78 5,68 2,76 2,01 1,41C 7,31 7,65 5,55 3,65 2,38 1,98D 11,1 8,48 3,97 2,88 2,00 1,94E 7,85 2,19 2,28 2,01 1,93 1,74

cells 1 2 3 4 5 6A 61 45 0 0 0 0B 0 0 0 0 0 0C 0 0 0 0 0 0D 0 0 0 0 0 0E 0 0 0 0 0 0

cells 1 2 3 4 5 6A 8,67 9,13 13,00 14,30 14,80 13,90B 8,51 11,00 14,40 16,90 17,80 11,80C 9,97 19,30 13,90 13,40 10,30 7,54D 10,10 18,30 11,30 8,89 7,57 4,96E 3,74 9,02 9,02 7,19 5,57 3,73

cells 1 2 3 4 5 6A 0 0 0 0 0 0B 0 23 47 28 0 0C 190 410 0 0 0 0D 7 0 0 0 0 0E 111 11 0 0 0 0

cells 1 2 3 4 5 6A 1,89 6,88 15,70 15,30 12,10 9,71B 2,69 10,50 17,60 15,60 12,30 10,40C 2,73 13,90 17,20 13,60 15,10 12,50D 2,10 17,90 22,40 15,70 14,00 9,20F 6,13 7,67 10,10 6,48 5,50 3,81

cells 1 2 3 4 5 6A 45 12 19 0 0 0B 0 55 41 57 0 0C 24 34 20 0 6 0D 22 310 43 7 0 0F 168 390 547 16 0 0

cells 1 2 3 4 5 6A 0,00 0,63 2,62 5,33 8,39 6,95B 0,00 0,58 5,54 10,20 9,07 9,47C 0,00 0,44 7,77 12,50 9,07 7,34D 0,00 0,22 10,60 16,70 9,06 7,48F 0,00 0,31 11,60 15,80 13,60 12,40

cells 1 2 3 4 5 6A 0 0 0 9 0 0B 0 0 18 0 0 0C 0 0 23 0 0 0D 0 26 0 6 0 11F 0 13 383 143 54 42

3. Exploring high-orbit satellite products, TRMM products and Lightnings

In this study, a major concern was whether the rainfall associated with the case studies was captured and measured with a sufficient accuracy. The sparseness of ground gauges, the insufficient radar coverage over the hydrological basin and the low accuracy of the geostationary satellite precipitation products were obstacle for a detailed representation of the spatiotemporal precipitation distribution. To overcome this obstacle, new products and tools were exploited. Namely, the microwave- calibrated IR precipitation 3B41RT product, available at 1-hour temporal resolution and 0.25ox0.25o latitudinal x longitudinal resolution for the belt from 60oN to 60oS, was additionally used to monitor the initiation, the trajectory, the possible feedback, the pick, the dissipation and the spatiotemporal precipitation distribution. The information aggregated from these data, was fusioned in 3D cells (log, lat, time), enabling a detailed representation of the spatiotemporal precipitation distribution. An attempt was made to correlate the 3B41RT precipitation values with the number of lightning events detected for each cell and the available ground observations. Data were summarized in 3 -hour intervals because in shorter duration cases, the number of lightning events was very few to be correlated with the rest of the data. According to the spatiotemporal data, the number of lightning events were found to be very similar with the precipitation distribution taking into account the location and the trajectory of these maximum values. The results were a promising sign of a high similarity between the two parameters, concluding that the precipitation distribution derived by 3B41RT was quite satisfactory.

Conclusions and Discussion

In this study, a detailed spatiotemporal distribution of precipitation for an area with sparse grid of ground gauges and no radar coverage (NW Peloponnese) was presented using TRMM/ 3B41RT data and lightnings data for three cases of severe thunderstorm events with heavy rainfall. The information assisted in improving the near real-time thunderstorm monitoring and increased the reliability of the accumulated precipitation estimation. The analysis carried out, shown that the 3B41RT data were very similar with the distribution of lightnings detected, validating its accuracy. It was proven that both the 3B41RT, when available - and the lightning data can be used with high confidence in studying such type of events.

In addition, the aforementioned detailed spatiotemporal precipitation distribution enabled the adjustment of the localized instability indices parameters for such areas e.g. the LII. For all three cases of this study the LII performed quite good forecasting of all the thunderstorms. Likewise, the calibrated LII using the TRMM successor's satellite (Global Precipitation Measurements (GPM)) products (NASA 4, 2019) can be successfully applied to other Mediterranean regions as well.

As a conclusion, the LII can be considered as a new methodological tool which after being tested and calibrated by suitable satellite products, can be useful to operational forecasters for the forecasting of severe thunderstorms events and the issuance of timely warning bulletins contributing to human and environmental protection.

(a) 15:00-16:00 UTC

Figure 6: TRMM/3B41RT precipitation, 15:00-17:00 UTC

(b) 16:00-17:00 UTC

cells 1 2 3 4 5 6A 0,00 0,00 0,00 0,00 0,23 1,82B 0,00 0,00 0,00 0,00 0,46 1,96C 0,00 0,00 0,00 0,00 1,76 2,17D 0,00 0,00 0,00 0,00 1,94 2,17E 0,00 0,00 0,00 0,00 1,67 2,73

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