 |
|
|
System Explanation of Storms and Storm Surges
|
Storms are atmospheric disturbances accompanied by severe weather conditions affecting the earth's surface such as strong winds, gusts or heavy precipitation. In meteorology an individual storm is identified as a low pressure complex associated with characteristic wind, cloud and precipitation conditions etc. Storms range from rather small scales such as dust devils, tornados and the like through tropical cyclones (e.g., typhoons, hurricanes) to large-scale extra-tropical cyclones with typical dimensions of about 1000 km. Often, storms are identified by their accompanying most destructive weather phenomenon, e.g. windstorms, rainstorms, snowstorms etc.
 Figure 1: Wind map of hurricane Katrina on August 28, 2005 - 15:50 UTC, prior to making landfall at the US coast. The wind map was computed from Advanced Synthetic Aperture Radar (ASAR) data of the European satellite ENVISAT, using the WiSAR Program from the GKSS Research Center, Germany.
|
Over sea storms are associated with marine hazards, the most destructive of which are wind waves and storm surges. Wind waves are wind-generated waves at the ocean surface. Their height and period are primarily determined by wind speed, fetch and duration. Generally speaking wind waves become higher and longer with increasing wind speed, fetch and duration. Here wind speed refers to the speed of the wind acting on the sea surface and duration represents the time the wind is acting on the sea surface. Fetch refers to an ocean area over which the wind blows at constant direction and speed. It may also be called the generating area. When the length of the waves exceeds twice the water depth, the impact of the sea bottom on the waves becomes recognizable (shallow water waves). In shallow areas, water depth limits the heights of the waves.
As the sea surface is highly variable it remains difficult to describe the sea state by characterizing individual waves. Generally, integrated parameters such as the significant wave height or wave period are used. Traditionally these parameters refer to the average height and period of the highest third of waves passing an observing point within a given time span. When wave heights and periods are reported in forecasts or analyses, usually significant wave condition are meant. Within those conditions sometimes unexpectedly high individual waves of great steepness may occur. These are commonly known as freak or rogue waves and may cause serious damage to ships, platforms and offshore operations among others.
While the hazards caused by wind waves are large for both the open ocean and coastal areas, the hazards created by storm surges are most virulent for low lying coastal areas. The word surge refers to a rise or drop of water levels primarily caused by winds pushing water masses towards or away from the coast and secondarily by a direct response to atmospheric pressure: if the air pressure increases the water levels are depressed and vice versa. Storm surges are associated with atmospheric low pressure systems. Here air pressures are relatively low and the water level will increase in direct response to atmospheric pressure. The increase is typically in the order of 1 cm per 1 hPa atmospheric pressure decrease and may be enhanced when the storm is moving over a continental shelf with a speed close to that of a shallow water wave. Usually the rise of the water level in direct response to atmospheric pressure is, however, small compared to the wind set-up caused by winds pushing water masses towards the coast. Depending on area and storm type, surge levels may vary from a few centimetres to more than 9 metres. The spatial dimension of the phenomenon may range from a few tens of kilometres as in the case of tropical cyclones to several hundreds of kilometres such as for extra-tropical cyclones approaching a long shallow coast.
|
The frequency and intensity of storms, storm surges and severe wave events varies from year to year and on longer time scales. An example is provided in Figure 2 for storminess over the Northeast North Atlantic and the North Sea. Storm activity was rather weak around 1960 and increased afterwards until about 1990-1995 where it reached levels comparable to that at the beginning of the 20th century. After 1995 storm activity decreased again. The 1960-1995 increase has been overemphasized by numerous studies because of limited time series length and/or inhomogeneous data. Long-term variability of storm surges and severe wave events closely resemble that of storm activity in the area.
 Figure 2: Storm index representing the storm activity in the Northeastern North Atlantic and the North Sea. The index is based on geosptrophic wind speed percentiles derived from station pressure data. Updated version of diagram provided by Alexandersson et al. (2000).
|
|
In the course of anthropogenic climate change an intensification of the storm, wave and storm surge climate is discussed. Scenarios prepared by a chain of assumed emissions, global and regional climate models point to a slightly more violent future of storminess, storm surges and waves in the North Sea. For the end of the century an intensification of up to 10% is envisaged, mostly independently of the emission scenario used. When not only the change in windiness but also the enlarged volume of the ocean is considered, then, for extreme water levels, an increase of 20 cm in 2030 and of 70 cm in 2085 along the German Bight coast line are reasonable guesses for future conditions.
References
Alexandersson, H., T. Schmith, K. Iden and H. Tuomenvirta, 2000: Trends of storms in NW Europe derived from an updated pressure data set. Clim. Res. 14:71-73
 |  |
| |
|
|
