Towards the Verification of Surface Wind Forecasts at Jomo Kenyatta International Airport, Kenya

Abstract

The economic progress of civil aviation industry and aircraft operations is largely reliant on the optimum operationalization of the flight schedules. Bad weather and specifically strong winds at the airport may cause interference of the flight schedule when taking off by holding some time or diversions or making u turn when landing. diversions to other airport lead to enormous financial losses to the aviation operators. Wind speed and direction forecasts are crucial for the flight planning therefore a considerable interest for assessing its accuracy and skill value. The wind speed and direction forecasts are incorporated into the Terminal Aerodrome Forecast (TAF) produced by aerodrome meteorological office. TAF contains the expected conditions of wind speed and direction, visibility, weather and cloud conditions. In this study the forecast of wind speed and direction were verified against the wind speed and direction observation considering for 1hr and 6hr lead times. The highest observed value was compared against the highest forecasted value. Wind speed was sorted into three categories of light covering a range between (1-6) kt, moderate between (7-12) kt and strong above 12kt. Then a 3-category contingency table was developed by considering 1hr and 6hr lead times. Accuracy at 1hr and 6hr lead time was evaluated by using scatter plot and the Root Mean Square Error. Different skill scores were then calculated from the contingency table. For wind direction verification, significant deviations among observations and the forecasts were investigated. Forecast errors are considered significant when the wind speed attains 7 kts and the deviation is more than 600. If the observed wind speed is more or equivalent than the predefined speed threshold, all the forecast errors valid for that hour were calculated. If the observed mean wind speed is less than the speed threshold, the forecast wind direction is regarded as correct since any direction deviation is operationally insignificant. The accuracy of wind direction was assessed by utilizing Percentage Error method. The wind direction was classified into four cardinal directions north, east, south and west. Then a 4-category contingency table was created for both 1hr and 6hr lead times and different skill scores were analyzed. The accuracy and the skill for wind speed and direction forecasts are high at 1hr lead time as compared to 6hr lead time hence the wind speed and direction forecasts should be utilized quickly after production and limited to not more than six hours since its accuracy and skill drops. The new and the amended forecasts should be used with urgency for flight planning since it has more correct forecasts. The verification results should be able show the quality and the weakness of the wind speed and direction forecasts thereby acting as an apparatus to promote further improvement for the strong wind forecasting on the part of the forecasters. The skill scores generated from the contingency table will help the management in evolving ways for improving strong wind forecasting by improving the instrument for observing and forecasting strong wind and also opening further training in strong wind forecasting for the forecasters.