Select Committee on Environment, Food and Rural Affairs Minutes of Evidence


Supplementary memorandum submitted by the Met Office (FL 120a)

  1.  The Met Office is grateful for the opportunity to provide oral evidence to the Environment, Food and Rural Affairs Select Committee inquiry on the summer 2007 floods. At the session on 12 December, and also through Marek Kubala's letter of 18 December 2007 to Professor Mitchell, you invited witnesses to provide further written evidence, either to clarify or build on the oral evidence submitted. The Met Office would like to offer the following in support of its oral evidence.

Did anyone provide advice on the impact of large amounts of rainfall falling on already saturated ground?

  2.  The Met Office always considers the impacts of the weather when issuing a forecast, so does take into account previous meteorological events. For example, Met Office broadcast meteorologists will often refer to the impact of rain falling on already saturated ground.

  3.  However, the Met Office does not have access to detailed hydrological models or maps and so is unable to add any specific or local detail over and above the general statement that rain falling on already saturated ground is potentially a problem. Better integration of hydrological and meteorological models would provide better information that would show, in advance, how catchments will react to weather events.

  4.  At present only limited use is made of precipitation forecast data in relation to river flooding. As the resolution of rainfall forecasts improves as planned in the coming years, better integration with hydrological models and mapping of river catchments should allow more timely and accurate warnings of river flooding.

  5.  As highlighted in earlier evidence, no organisation currently has responsibility for providing a pluvial flood warning service. Such a service would also need to integrate detailed hydrological and meteorological models in order to make a useful and timely assessment of risk. This capability could be developed on the back of our high resolution weather forecasting model to be deployed in 2009. To provide timely warnings would require a 24/7 operational capability and robust communication mechanisms.

Do we need to re-assess our definition of return periods to take into account intense rain fall in such short periods of time?

  6.  There was some discussion at the evidence session regarding the relationship between return periods and rainfall intensity. It might be useful to clarify the fact that return periods do take into account rainfall intensity.

  7.  Return periods are calculated for a range of weather events, including rainfall events, which are specified in terms of duration and quantity (giving an average figure of intensity over a known period). So, for every rainfall event it is possible, in theory, to calculate the associated return period either for the whole event or for a particularly intense part of it.

  8.  The concept of a 1 in 200 year rainfall event being squeezed into a shorter time period is therefore misleading as the more intense event will itself have its own return period.

  9.  For example, the table below displays data for two locations from 20 July 2007. The table broadly shows that short periods of intense rainfall are more common than long periods of intense rainfall. It is important to note that these return periods relate to rainfall rather than flooding. As already understood by the Committee, even a short period of intense rainfall can lead to flash flooding if it is falling onto already saturated ground or impermeable surfaces.


Location
Date
Start time
(GMT)
Duration
(Hours)
Rainfall
amount
(mm)
Return period
(Years)

Brize Norton
20/7/07
13:00
1
17.2
7.8
Brize Norton
20/7/07
11:00
3
48.6
83
Brize Norton
20/7/07
09:00
6
90.6
>200* (497)
Brize Norton
20/7/07
06:00
12
115.4
>200* (728)
Brize Norton
20/7/07
00:00
24
126.2
>200* (640)
Pershore College
20/7/07
15:00
1
16.4
6.3
Pershore College
20/7/07
14:00
3
37.8
26
Pershore College
20/7/07
11:00
6
70.8
144
Pershore College
20/7/07
07:00
12
113.8
>200* (568)
Pershore College
20/7/07
00:00
24
142.4
>200* (860)

* The fact that return periods are calculated using historical data sets typically stretching back 100 years means that extrapolation to produce return periods beyond 200 years is probably not justified. Our statistical modelling techniques can produce return periods in excess of 200 years—we have shown these in brackets for completeness— but we quote these as >200 years.


  10.  To illustrate the timing and intensity of the rainfall on the 20 July, hourly rainfalls for Pershore College (Worcestershire) and Brize Norton (Oxfordshire) are given below.




  11.  The models and statistical analysis used to produce the return periods above work well in a stable climate system.

  12.  However, we know that climate is changing and therefore the past is no longer a useful guide for the future, in terms of weather patterns. In order to translate the impacts of climate change into weather patterns and return periods, more work is required. The Met Office Hadley Centre continues to conduct world-leading research in these areas, but we do already know enough to suggest that the decisions we make now and in the future must be made in the context of a changing climate.

  13.  In the future, average winters are likely to be wetter than they are now and summers are likely to be drier. However, the expected intensity of severe weather increases in both seasons, which will have an impact on our flood defence strategies and warning services.

Met Office

January 2008





 
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