4.2.1 Duplicates
Many entries in the table occur within 24 hours of a higher SSI value. These are rejected, so one storm event has only one date and time under consideration with the maximum SSI value retained. For example, storm Klaus has the highest SSI value (55.47) at 1200UTC on 2009-01-24, as well as the fourth and sixth highest values (45.16, 40.49) at 0600UTC and 0000UTC on the same day. As these are related to the same storm, the highest value is retained, and the fourth and sixth rejected as duplicates. The 48 hour period (24 hours before the storm, 24 hours after) is extended for some storms, when examining surface pressure charts (not shown) indicate that two high SSI values are caused by the same system. Overall, 49 out of the 120 analysis times are rejected for this reason.
4.2.2 Mediterranean Lows
Mediterranean lows have different dynamics from the North Atlantic cyclones of interest to this work (Trigo et al., 1999), but they cause relatively high wind speeds in the area where SSI is calculated. As these are rare events, the Mediterranean region has a lower 98th percentile of wind speed. Therefore, the wind speed does not need to be as high as in northern Europe for the SSI to be large. One example of this is 30th March 1995 at 1200UTC. High values of SSI are concentrated near Venice and Genoa, Italy (Figure 4.1c), and the associated high wind speeds occur entirely in the Mediterranean region (Figure 4.1b). Although the surface pressure chart is dominated by the high pressure over western Europe, a small low is present in the Adriatic Sea (Figure 4.1a). The high and low cause a strong pressure gradient, notably in the Gulf of Genoa and northern Italy, inducing high winds there (Figure 4.1b) and so high values of SSI. As the aim of this study is to investigate North Atlantic cyclones, the Mediterranean cyclones are eliminated, which means that thirteen of the 120 high SSI events are rejected.
(a) (b)
(c)
F 4.1: (a) Mean sea level pressure (hPa), (b) wind speed (ms−1) and (c) SSI for 1200UTC
Chapter 4. The Storms 81
4.2.3 Polar Lows
There are three Polar Lows identified, which behave unlike the other Atlantic cyclones. Figure 4.2a shows storm Petra, who caused high SSI at 1200UTC on 22nd Febuary 1999. The track starts very far north and moves southeastwards into Europe. The pressure plot (Figure 4.2c) shows that Petra is filling throughout. Though this track is generally parallel to the jet stream (Figure 4.2a), the intense wind speeds are so far away from the storm that influence from the jet stream is probably small. The θefield (Figure 4.2b) shows that the air is unusually warm and moist for such a high latitude, indicating the potential for convective energy to be released, as is typically the case for polar lows (Rasmussen, 2003). The surface pressure charts during the days before the track of the storm is detected (not shown) indicate that the cyclone is the result of cyclogenesis between Newfoundland and Greenland. The resulting disturbance then crosses the northern North Atlantic, passing over the tip of Greenland, and moving north of Iceland. This low pressure then remains in the Norwegian Sea for three days, before forming a sufficiently coherent low for the tracker (Section 3.3) to detect it. This cyclone and the others like it are different to the systems of interest in a number of ways: no deepening phase, no clear interaction with the jet stream, and originating very far north. Therefore, they are probably polar lows, and will not be considered further.
(a)
(b) (c)
F 4.2: Polar low Petra at 1200UTC on 22nd Febuary 1999: (a) wind speed at 300hPa [ms−1],
(b) θe at 850hPa [K ], (c) cyclone core pressure [hPa] against time [h]. (a) and (b) are plotted as
Chapter 4. The Storms 83
4.2.4 High Pressure
The fourth reason for rejecting some high-SSI days is the presence of high pressure over most of Europe, with high winds and SSI in some parts of the domain. In the case from 5th March 1990, there are strong westerlies across northern Scotland and southern Scandinavia (Figure 4.3b), induced by a pressure gradient between the unusually high pressure of over 1040hPa across Europe and the stationary low pressure in the Norwegian Sea (Figure 4.3a). There is a short-wave trough visible in the pressure field, which could increase the wind speeds enough for them to generate a high-SSI, but this kind of system is not of interest to this study for both dynamic and practical reasons: it has different factors affecting its deepening to midlatitude cyclones, and it is difficult for the tracker to detect a trough without closed pressure contours. Examining the SSI field (Figure 4.3c) indicates an additional orographic influence on the winds, as the highest SSI values occur as the westerlies reach the Norwegian mountains. There are nineteen occasions where a strong large- scale pressure gradient is the main reason for a high SSI value, and these were all checked by hand to ensure no mobile North Atlantic cyclones are present in these cases, so they are rejected.
(a) (b)
(c)
F 4.3: (a) Mean sea level pressure (hPa), (b) wind speed (ms−1) and (c) SSI for 1200UTC
Chapter 4. The Storms 85
4.2.5 Orography
The inclusion of the 1000m criteria on the calculation region for SSI (Section 3.2) filters out many cases with high SSI values over orography. This means that only one of the top SSI days is rejected on orographic grounds, on 18th February 1997. Figure 4.4a shows a strong pressure gradient between Iceland and the rest of Europe. This produces exceptional wind speeds off the Norwegian Coast (Figure 4.4b) and associated high values of SSI (Figure 4.4c). These exceptional wind speeds could be due to orographic blocking, where the wind is forced to speed up so it can move over the mountains or form a barrier jet parallel to the mountain range. As this is not a clear-cut case of a North Atlantic cyclone, this high-SSI day is rejected.
(a) (b)
(c)
F 4.4: (a) Mean sea level pressure (hPa), (b) wind speed (ms−1) and (c) SSI for 0000UTC