Storms and Storm Surges - Development of the storm climate

How to determine decadal and longer variations in the storm climate?

A major problem with determining changes in windiness represents the homogeneity, or more precisely the lack of homogeneity, of observed time series. The term 'inhomogeneity' refers to the presence of contaminations in a data set, so that the meteorological data, which are supposed to describe the meteorological conditions and their changes over time, are actually a mix of the looked-after signal and a variety of factors reflecting changing environmental conditions, changing instruments and observation practices.

For instance, pressure readings usually depend not very much on the specifics of the location (apart of the height) and have been recorded over long periods of time with rather similar instruments, namely the mercury barometer. A rather different example represents wind measurements which depend very strongly on the details of the surrounding, in particular the exposition and obstacles. Also instruments and observation practices have changed frequently. This is in particular so with wind observations and wind estimates over sea.

Figure 1a displays a simple example. It shows the frequency of strong wind events in the city of Hamburg (Germany) per decade of years. Obviously a very strong decline took place from the 1940s to the 1950s. The explanation is that the instrument was moved from the harbour to the airport. Another less obvious example are marine wind speeds over the Pacific (Figure 1b). The stationary weather station (ship) P is taking quality controlled wind observations. From the COADS data set, many other wind reports are available from ship (of opportunity) observations originating from a series of grid boxes surrounding the position of the weather station. However, when the ship observations are averaged for each year, and compared to the quality controlled data form the ocean weather station, a discrepancy emerges – the ship data indicate an upward trend, while station P reports variable but by and large stationary conditions.

Figure 1: (a) Reported number of days per years with wind speeds of 7 Beaufort and more in Hamburg. In the early 1950s the observation was moved form the harbour to the airport (after Schmidt, pers. communication). (b) Estimated changes in mean wind speed over the North Pacific in the area of ocean weather station OWS P. Data from the ocean weather station are marked as 'OWS' (ocean weather ship), and those from the ships of opportunity in the vicinity of OWS as 'COADS' (after Isemer, pers. communication).

Thus, direct observations of wind are almost never helpful to assess changes in windiness for decades of years. As an alternative, a number of proxies representative for the strength of windiness or storminess in a season or a year have been suggested and tested. They are mainly based on pressure readings. Specifically spatial and temporal pressure differences are in use, but also the frequency of low pressure occurrences. A totally different proxy is derived from short-term water variations at a tide gauge. Water levels at tide gauges are often changed by local water works but also by slow variations related to geological phenomena. Therefore, first the annual mean high tide is determined, and then the variations of the high tide relative to this mean high tide are considered.