1. FORECASTING HEATWAVE, DROUGHT, FLOOD and FROST DURATION Bernd Becker
Growing experience with probabilistic weather forecasts for the medium range has generated interest in exploring forecast applications out to one month ahead. This paper investigates forecasts of risk related to the duration of periods of adverse weather. Examples showing the risk of periods with heat- wave conditions, floods, droughts and temperatures below freezing are presented. The risk forecast is derived from operational coupled monthly forecast ensembles at the Met Office. Counting the longest period in each member of a forecast ensemble describes the distribution of threshold exceedance duration. Risk is defined as the predicted change with respect to a climate reference forecast. Monthly forecasts of threshold exceedance may find a substantial role with regional resilience team preparedness and business continuity planning for the effects of extreme weather on public health and safety.
Methodology
The European Centre for Medium range Weather Forecasts (ECMWF) monthly forecasting system MFS (Vitart, 2004) is used to predict the states of the atmosphere and ocean for a month ahead. The MFS consists of a 51-member ensemble of 32-day integrations. Each ensemble member is initialized from an observed state of the atmosphere that is slightly perturbed in regions of meteorological activity to initialise spread in the ensemble (Leutbecher et al., ). Long-range forecasts tend to drift toward a model climatology that may differ from an observed climatology. The model drift may be corrected for by assessing forecasts with reference to a model climatology. In the monthly forecast system an ensemble of 51 forecasts is combined with 5-member ensembles produced for each of the previous 18 years to establish the model reference climatology (hindcast) for the forecast period. Daily precipitation, maximum and minimum 2-metre temperature are used to describe heat-wave, drought, flood and cold weather duration on a global grid with 1.125° resolution.
Weather anomalies
Weather anomalies persisting for an extended length of time increase weather risk. The products presented here not only show the likelihood of occurrence of adverse weather, but also show for how long adverse weather may persist during the coming month. Threshold exceedance duration is derived from the ensemble forecast and from the hindcast. Threshold values are chosen to describe rare events with respect to the resolved space scales of the ensemble prediction system. Heatwave conditions are defined as two consecutive days with maximum temperatures above 30°C bounding a night with minimum temperatures above 15°C. Drought conditions are defined as consecutive days with precipitation less than 0.1 mm/day. Flood is defined as consecutive days with precipitation above 5 mm/day and cold periods are defined where the daily minimum temperature does remain below the freezing level. Threshold exceedance duration is derived in the following way: In each ensemble member exceedance events are identified [1]. The number of consecutive events is counted. Periods of exceedance are measured in number of days n that a daily forecast value is above a threshold, thus describing one threshold exceedance of n days duration [2]. Counting the longest exceedance duration in each ensemble member yields the distribution of exceedance events [3]. Applying the 3-step procedure described above to the 51 member forecast yields the predicted distribution of threshold exceedance duration. Applied to the 90 member hindcast the procedure yields the model baseline exceedance duration typical for the forecast period (colours in 3). Contrasting the baseline with the prediction infers the severity of risk described by threshold exceedance duration (30% contours in 4). Products may be presented as maps [4], showing the risk of threshold exceedance for a fixed period or as threshold exceedance duration probability charts for all durations up to a month at a fixed location [5].
Members
Days 1
Flag daily events in each ensemble member
Each event is measured in number of days n that the event persists, thus describing one threshold exceedance of n days duration.
(on consecutive days)
Indian Flood
Cold US/Canadian Border
Almost 20 million people have been displaced as some of the worst floods for years have hit a wide swathe of northern India in August 2007
and
cold spell during November / December 2007 left thousands of homes in US and Canada without electricity.
6 UK Cold weather payment planning 2 3
Counting the longest exceedance duration in each ensemble member yields the cumulative distribution of events.
Probability of event duration up to d days in each Forecast and Hindcast set.
Duration 7d 4
Could Greek Wild Fires in 2007 be predicted?
Climatology of 10 day exceedances of heat and drought in colour
+30% shift of 10-day exceedances contoured 2d -3 -2
30% -1
30%
obs
Sheffield
7 UK Floods:
People living in areas inside the 30% contours are likely to suffer both heat- wave and drought conditions lasting for up to 10 days during the coming month
Heat wave and Drought risk increment (red) forecast at grid point near Volos 5
precipitation above 5 mm/d for 2 days during the periods when heavy flooding
occurred in Ottery St. Mary and in Morpeth.
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