Table of Contents

Introduction

A severe convective outbreak with big hail and thunderstorms occurred over parts of Western and Central Europe on Monday, May 25th 2009. Overnight, a strong squall line that produced gusts in excess of 35 m s-1 developed over Northern France, Belgium and the Netherlands, uprooting trees and causing considerable damage to property. In addition, hail 10 cm in diameter was recorded. The severity of the associated weather events is also shown in the animation of the Meteosat 9 IR10.8 series of images overlaid with the weather reports of the ESWD. (courtesy of ESSL)

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Fig 1.1. Meteosat 9 IR10.8 with ESWD overlay

In this case study of this first severe convective outbreak of the 2009 season, we will look at the various stages of the convection and describe them with derived products to place extra emphasis on their use for operational nowcasting. The EUMeTrain community would like to thank the respected authors for their ideas and knowledge invested in this case study.

Synoptic Analysis

On this day the synoptic maps showed conditions favourable for severe thunderstorms over an extensive area.

In Western Europe an upper-level trough moved in from the Bay of Biscay on the 25th, pushing forward an unusually warm humid air mass, covering NE-France, Belgium, Luxembourg and the Netherlands with minimal dew point depressions.

Dust RGB
Fig 2.1. Meteosat 9 Dust RGB of 25 May 2009 0000UTC showing the moist airmass over central France in a bluish tone. A positive brightness temperature difference between the channels IR12.0 and IR10.8 used in this RGB indicates high relative humidity. The cirrus shield part of the anvil shows up black in the picture.

The following four images show the Airmass RGB overlaid with the geopotential height at 500 hPa for 25th May 2009 at 00, 06, 12 and 18UTC, respectively. Click your way back and forward to obeserve that a cut-off low with the center over Western Spain slowly connects with a short wave trough travelling in a strong westerly flow over the Atlantic.

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Fig 2.1. Meteosat 9 Airmass RGB overlaid with Geopotential Height 500 hPa.

During the course of the day this trough deepens and a blocking situation establishes as a significant ridge of high pressure is building up, stretching from the central Mediterranean to southern Scandinavia. At west side of this omega block a strong southerly flow prevails.

Radio Sounding - Trappes
Steep lapse rates, generated by an elevated mixed layer and a moist boundary layer below, were responsible for extreme amounts of convective available potential energy (CAPE) over Western Europe. Additionally, moderate to high amounts of wind shear were in place, emphasising the possibilities for a convective event with large hail (given the high CAPE) and a serious threat of wind gusts (see Fig 2.3).

A long-lived cluster of thunderstorms evolved over the Bay of Biscay during the evening hours of the 24th and entered the western English Channel on the 25th around midnight. After that, the convective cells that developed were smaller due to lacking insolation. On the 25th, in the afternoon, new storms initiated over the northern and western parts of France and across Belgium. A small cluster including a massive supercell crossed Belgium, moving into Germany.

Further initiation took place over central France at roughly 15UTC and this activity organized rapidly into an MCS moving in a northeasterly direction with numerous, discrete supercells ahead of it. Hail around 10 cm in diameter causing widespread damage was reported with these supercells. To the east, another trough stagnated over Eastern Europe, defining a large warm sector with a subtropical airmass reaching far into Central Europe with only a weak high pressure influence. In spite of a strong ridge aloft, we also see some thunderstorms developing in this part of Europe. They appear to be mainly diurnally driven.

The following four links provide you with the satellite images and numerical data in which you can combine the various parameters.

Preconvective Environment

Global Instability Index

The Global Instability Index (GII) product is one of the MPEF meteorological products and describes the instability of the clear atmosphere by a number of airmass parameters. Although of highly empirical nature, the instability indices often indicate the potential for convection a few hours prior to the actual onset of convection.

A detailed description on how the GII product functions, is found in the following animation.

For the 25 May 2009 case, the unstable airmass is well visible in several ways: (a) in the K-Index, where values exceed 30 °C; (b) in the Lifted Index, with values around -6 K (and in some areas even below -8 K); and (c) in the Total Precipitable Water (TPW) content, which shows the associated humidity of around 30 mm. The TPW content of the warm, humid airmass over Central France that was discussed previously and shown in the Dust RGB appears particularly clearly in the derived product GII, with values around 40mm.
The Lifted Index is (by definition) driven by near-surface heating and thus shows the expected diurnal cycle, while both the K-Index and the TPW show much smaller temporal changes.

NWCSAF Statistical Physical Retrieval

The SEVIRI Physical Retrieval (SPhR) algorithm retrieves either the atmospheric temperature and moisture profiles and the surface temperature for one clear sky SEVIRI pixel, or a Field-Of-Regard (FOR) which contains M x M pixels. The main purpose of the SPhR is to provide information on the water vapor contained in a vertical column of unit cross-section area in several layers in the troposphere and to provide a range of instability indices. These parameters are calculated from the retrieved profiles of temperature and humidity. For your interest you can find more information on the product in this article.

The algorithm functions much like a GII with a neural network. The algorithm, based on physical retrieval, obtains retrieved Temperature (T) and humidity (q) profiles on clear pixels. The parameters are calculated from these retrieved profiles.

Summary

In this chapter two similar products for a preconvective environment were presented: (1) the GII product with its origin in the MPEF production chain, and (2) the PGE13 (Physical Retrieval) as it is produced and developed by the Nowcasting SAF. These products are useful mainly in the morning hours. Identifying the regions with the highest potential for thunderstorms makes it easier to focus on them later that day. This idea was also the starting point of a study by ZAMG in 2008, where the early morning Lifted Index and K-index were compared with lightning occurences later that day. Below we see the K-index for the GII (left) and the Physical Retrieval, or PGE13, as it is also known (right).

GIIPGE13
Fig 3.1. K-index as obtained by MPEF GII (left) and from the Nowcasting SAF Physical Retrieval (right) for 25 May 2009 0600UTC

Both images use the same colour scale for the K-index values over 20. The differences are found in Baden-Württemberg (SW Germany), Southern Austria (Carinthia and Styria) and along the Adriatic Coast. All these areas apprear to be less stable in the GII retrieval scheme with all K-index values topping 30°C or higher. Both maximum values for K-index are found along the coast of the Adriatic.

Further north, in the Netherlands, stable conditions are found over the central part in the PGE13 retrieval scheme, with the GII listing values up to around 20°C.

In the IR10.8 images we can see the result of convection of that day and making it able to do a subjective(!) comparison.

IR108
Fig 3.1. Meteosat 9 IR10.8 - 25 May 2009: 1500UTC

In the image several convective areas are to be recognised; Brittany (FR), Netherlands, Baden-Württemberg (GER), Carinthia (AT), Adriatic Coast and the center part of Italy. The performance between the two preconvective products is listed in the table below.

  GII NWCSAF: PGE13
1 - Brittany + ++
2 - Netherlands + -
3 - Baden-Württemberg ++ +
4 - Carinthia ++ +
5 - Adriatic Coast ++ +
6 - Central Italy + -


Table 3.1. Table listing the performance of the GII and the PGE13 product for several regions in Europe. A '+' is sign of good performance, with an increase of '+' indicative for a higher K-index. A '-' in this table means that the algorithm failed to nowcast the convective event.

In this case it seems that the GII performed better, however a longer comparison and a longer testing of the algorithm is necessary.

Convective Initiation

Geostationary satellites such as Meteosat Second Generation provide observations of developing thunderstorms at high temporal and spatial resolutions. The wide electromagnetic spectrum makes it possible to sense various physical characteristics of convective clouds before they could be detected by radar. The nowcasting method named Convective Initiation relies on this and describes the process where a cumulus cloud evolves from an immature "fair weather" state to a mature cumulonimbus. In the course of this animation we will further explain the process of Convective Initiation and the nowcasting algorithms that help to detect these potential convective clouds.

Imagine a small cloud at time 0. It is characterised by 0 dBZ, and unless you have a truly state-of-the-art radar system, you would not be able to detect it, let alone pay attention to the cell from your work station. In 30 minutes, however, this cell will grow from a small innocent fair weather cumulus into a large cell with a high reflectivity of 35 dBZ.

It will be only at this stage that you would pay attention to it and you probably would wait another 30 minutes to see what the cell will do before you issue a warning to media and public. Convective Initiation would save you this time!

It is only at this stage that you would pay attention to it, and even then you would probably wait another 30 minutes to see what the cell would do before issuing a warning to the media and public. Convective Initiation would save you this time!

For the purpose of a larger evaluation study, the CI algorithm was adapted for MSG SEVIRI channels in the SATCAST algorithm. The SATCAST algorithm is designed to detect initial cumulus clouds which are likely to eventually produce radar echoes greater than 35 dBZ. To achieve this, 17 interest fields are evaluated. They distinguish three features representing physical properties of the growing clouds: cloud depth, updraft strength and cloud top glaciation. In the version of the algorithm used in this study, 14 interest fields must exceed individual thresholds for a nowcast being issued. Since many of these interest fields are based on temporal evolution, it is essential to monitor the same cloud(s) over a time period of 30 min. No tracking or motion vector functionality is available in the present algorithm, so all data is averaged in a box of 7×7 pixels around the target pixel.

The total lightning data (cloud and ground flashes) is used as a proxy for strong convection, since radar data for entire Europe was not available for the study.

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Fig 4.1. Meteosat 9 IR10.8 SATCAST Composite for 25 May 2009.

In the MSG composite images above, the following colour coding is applied: pixels in red denote a SATCAST-warning. Warnings are based on data from t, t-1 and t-2, where t is the number of the SEVIRI scan. Green pixels denote lightning in the following hour, meaning 60 min starting from the indicated start of the scan plus 10 min (the approximate scanning time for Europe). So keep in mind that for a developing thunderstorm, green pixels will appear 45-60 min before the actual onset of lightning. Ideally, SATCAST detections should have appeared by then as well. Pixels in which SATCAST and lightning detections coincide are marked in cyan. Please note that the datasets are only considered over land.

In the following, we highlight three different cases: one in which initiation of a later thunderstorm was detected by SATCAST (over France, Italy and Switzerland; case 1), one with a missed storm detection (Austria; case 2), and one false alarm (Ukrain and Romania; case 3). The following text hopes to elicit future improvements of satellite-based CI algorithms.

Case 1 (Hit)

Case 1A small band of convective clouds moves in from the southwest. In front of this band a thunderstorm develops over this alpine region. Lightning first occurs shortly after 13 UTC. The first nowcast can be seen in the 1200 UTC image, which would have been transmitted via EUMETCast by 1230 UTC. So in this case the algorithm does provide a usable lead time of some 25-30 min with respect to lightning initiation. This could be improved to 45-50 min if the SATCAST processing were done directly at EUMETSAT upon MSG data reception. The cloud band itself is accompanied by SATCAST detections, but no lightning occurred in it. Two synop stations in the region of the cloud band also reported no precipitation. However, since the terrain is mountainous, we cannot exclude the possibility of small rain showers with radar echoes greater than 35 dBZ nearby.

Case 2 (Miss)

Case 2Since the algorithm does not account for cloud motion, it is not surprising to find mismatches between nowcasts and the actual location of the lightning quite frequently. These misses are easily found on a pixel-by-pixel basis. However, on this day we could not find a showcase miss exhibiting lightning without any nowcast nearby in the imagery. What we will demonstrate as a miss is a case in which the first warning came after the first stroke of lightning.

Over central Austria, several thunderstorms developed. The one regarded here first produced lightning around 1220 UTC. There is a SATCAST detection in the 1230 UTC scan. However, this scan covered central Europe at about 1240 UTC - 20 min after the first flash. In the 1215 UTC scan, still sweeping across Europe roughly 5 min after the first flash, and in all the previous scans, there was no SATCAST detection, although convection can be seen in the IR10.8 imagery from at least 1000 UTC on.

Case 3 (False Alarm)

Case 3There were several bands of Cu/Sc over eastern Europe which led to many SATCAST detections. On the easternmost edge of the domain, scattered lightning pixels can be seen. However, in the marked area virtually no flashes occurred. Some stations reported no rain, but we cannot definitely exclude the possibility that some of the clouds may have produced rain showers. The cities of Buzau, Khmel (Khmelnytskyi?), Ternopil and Odessa are marked by accordant letters. Synop reports are available from these cities. Of these, only Odessa at the Black Sea coast reported light rain showers at 1030 and around 1600 UTC. The SATCAST detections are mainly triggered at the leading edge of the advected clouds. Therefore, one explanation for those SATCAST detections may be "advective cooling" - that is, the rapid cooling of a SEVIRI-pixel or pixel group by the advection of clouds over warm terrain - which the algorithm then interprets as vertical cloud growth or glaciation. There are a few control steps implemented in SATCAST to mitigate this problem, but it seems that it can still cause some false alarms.

Mature Phase

Overshooting tops are distinct features of mature thunderstorms. They are formed when the updraft is strong enough to penetrate the equilibrium level, which often coincides with the tropopause. The overshooting tops can be long- or short-lived, depending on the severity of the thunderstorm. Overshooting and long-lasting overshooting tops in particular indicate heavy rainfall and dangerous regions for aviation. Therefore it is important to detect those regions.

sandiwch
Fig 5.1. Meteosat 8 IR10.8 and HRV Sandwich: 25 May 2009 1045UTC

A distinct overshooting top can be seen in the so called sandwich product of IR window BT and visible bands at the French-Belgian border (Fig. 5.1). The sandwich product is a multilayer image, in which the bottom layer or background is a HRVIS image and the upper layer a IR10.8 BT image.


Due to the high solar angle at this time the HRVIS alone is rather inconspicuous (Fig. 5.2).

hrv
Fig 5.2. Meteosat 8 HRVIS: 25 May 2009 1045UTC

A better VIS-picture of overshooting tops can be found later that day with lower sun-angle resulting in higher contrast, over the western part of Germany, where also an above-anvil plume appears (fig.5.3).

germany
Fig 5.3. Meteosat 8 HRVIS: 25 May 2009 1645UTC

With even lower sun-angle, In figure 5.4, over France not only some distinct overshooting tops over France, but also gravity waves due to the pertubation caused by the Overshooting Tops over Normandy are visible. These gravity waves are a strong signal for turbulence.

hrvlate
Fig 5.4. Meteosat 8 HRVIS: 25 May 2009 1845UTC

Bedka et al. developed a method to detect overshooting tops numerically. This method uses IR window temperature gradients and NWP tropospheric information. On May 25th 2009 this method revealed good results as shown in Figure 5.5.

bedka
Fig 5.5. Overshooting Tops Detection using Bedka etal. IR10.8 algorithm: 25 May 2009

Comparing the red dots, indicating overshooting tops, with the severe weather reports from ESWD (Fig. 5.6; courtesy of ESSL) one can find a similar structure.

essl
Fig 5.6. Severe weather reports on May 25th 2009 (red: tornado, yellow: severe wind gusts, green: large hail, blue: heavy rain, white: funnel cloud)

Beside overshooting tops also the enhanced IR10.8 shows some remarkable features. Nearly all the thunderstorms developed cold ring shapes. A somewhat different structure is in literature often denoted as Cold U/V shapes. An example of this is seen in Figure 5.7.

uv
Fig 5.2. Meteosat 8 IR10.8: 25 May 2009 1550UTC

The appearance of this shape is however rare because the wind speed at 200 hPa., respectively at tropopause level, is just up to 30 or 40 knots with a correpsonding low upper-level wind shear.

In the following animations one can get an overview about the spatial and temporal scale of this convective outbreak. The thunderstorm cells show a cross-section dimension from 300 to 500 km and exist for up to 12 hours. With a south-westerly flow enough moisture from the Gulf of Biscay is transported to France to produce new cells along the day over.


Meteosat 8 - WV7.3 Meteosat 8 RSS - HRV (black and white)
Especially with the smaller scale cells the high resolution channel allows us a very good monitoring of the convective development. In a sequence of 5 minutes the satellite images are presented and described. Try to focus on subtle signals such as overshooting tops (which casts a shadow) and V-shaped storms.
Meteosat 8 - WV7.3 Meteosat 8 RSS - IR10.8 (color enhanced)
Meteosat 8 infrared 10.8 µm is shown, but the images have been artificially color enhanced using the color scheme from blue to turqois to yellow and red applied over a fixed interval from the temperatures of 200 to 240 K. This will improve the discrimination of where most ice particles form during the several convective stages. In a sequence of 5 minutes the rapid scan satellite images are presented.
Meteosat 8 - WV7.3 Meteosat 8 RSS - Sandwich Product
Combination of both the HRVIS and the enhanced IR10.8 to correlate the fine structures that the HRVIS offers in combination with the tempareture information.

Summary

A blocking situation with an upper level trough over Western Europe and a ridge over Central Europe caused a strong southerly flow that brought up very warm and humid air mainly to France, Belgium, the Netherlands and the western parts of Germany. Those were the ingredients providing for CAPE values of up to 1900 J kg-1 K-1 and the constant generation of new thunderstorms producing large hail.

Satellite-based nowcasting facilities were used in order to forecast these thunderstorms. Two methods were applied for capturing the general instability: the GII (global instability indices) and the NWCSAF Statistical Physical Retrieval algorithm. Both algorithms compute the K-Index, Lifted Index and Total Precipitable Water (TPW). The Physical Retrieval also provides the Showalter-Index and the TPW in three layers (boundary layer, medium layer, high layer). In this special case the GII performed a little bit better than the Physical Retrieval, which does not make the GII better in general. The GII detected the humid and warm air mass with the TPW particularly well. A slight disadvantage of both algorithms is that they require cloud-free areas, so they are best used in the morning hours.

After capturing the most unstable regions the task was to forecast where convection would start. For this purpose a convective initiation (CI) algorithm adapted for MSG SEVIRI channels in the SATCAST algorithm was used. This method proved to be still lacking as it produced a hit, a false alarm and a miss. Therefore, its results should not be taken at face value until further improvements have been made.

Several cold ring shapes occurred in the mature stage of the thunderstorms, producing not only heavy rainfall and large hail, but also a tornado. The best method to detect those shapes is the IR window BT channel.

This case study has proven that satellite data can help forecast the whole life-cycle of thunderstorms from the formation to the mature stage. Especially SAFs (Satellite Application Facilities) like the GII have proven to be helpful.

In the final picture (Fig. 6.1) one can see how severe and huge some of the thunderstorm cells were on this day. In this special case, the Belgian and Dutch meteorologists were not to be envied.

uv
Fig 6.1. Meteosat 8 IR10.8: 25 May 2009 1300UTC

Appendix

Here is a short "guideline" which of the products shown in this case study that are best to be used in order to forecast convection.

When forecasting convection one should always have in mind that thunderstorms require three ingredients for their formation: moisture, lability and a lifting mechanism.

In order to evaluate the moisture content of the prevailing airmass the Dust RGB was used in this case study. As a first hint it may be helpful for the advanced forecaster, but to go into detail NWP products and observations are more useable. For forecasting the lability in the case study GII and MPEF Physical Retrieval were presented. Especially in the morning hours, when the sky is still clear, a look at both methods can help to locate possible unstable regions with the comprehension of NWP products. Also radiosoundings give a good and accurate survey over stability especially when looking at the lapse rates. Moreover the instability indices determined with the sounding data can be compared to the accordant numerical values of GII and MPEF Physical Retrieval.

Last but not least convection needs a lifting mechanism that initiates the upward motion. Therefore again NWP data is useful to locate fronts, Airmass RGBs can show whether the timing of the front is correctly forecasted. Also other synoptic scale forcing mechanisms as outflow boundaries can facilitate enough lifting to form new thunderstorms.

The lifting mechanism for air mass thunderstorm is mainly solar insolation. In order to evaluate the solar input nearly every (visible) satellite picture can be used. Also a Natural Colour RGB should be checked because it gives a hint of the soil conditions, which may be important due to differential heating.

As mentioned in the case study for forecasting the strength of convection from radiosoundings and, for a more widespread view, from NWP data should be used.

After the formation of thunderstorms the position of overshooting tops is of importance as it indicates heavy rainfall and dangerous regions for aviation. During the daytime the best method to locate overshootings is with the sandwich product of IR window BT and visible bands. Also HRVIS images alone can be very useable. After sunset one has to use IR10.8 images.

Cold ring and U/V shapes can be detected with either an enhanced IR10.8 or the sandwich product, as shown in the case study.