Forecasting Applications of Synoptic Observation for Some Selected Parameters at Addis Ababa. Part I : Frequency of Weather Events against Wind, Dew-point Temperature and Pressure Yitaktu T.Tsion, Meteorological Officer, Research & Studies Department, National Meteorological Services Agency, P. O. Box 1090, Addis Ababa, Ethiopia. ( Tel. (251-1) 51 22 99, Fax. (251-1) 51 70 66, e-mail: nmsa@telecom.net.et) Abstract Keywords: Kiremt Season, Synoptic hours, Weather category, Weather parameters, Weather prediction. Prediction of weather on daily, short-term or even long-term basis is important due to the fact that weather influences every human activity. The main objective of this study is to investigate important features of atmospheric parameters observed at Addis Ababa and to identify forecasting potentials if any. In this study the data used is for the Kiremt Season (June - September) for the period 1990 to 1994. The parameters are: wind (speed & direction), dew point depression (DPD), present weather, and pressure. For all parameters observations are based on standard synoptic hours, that is: 00, 03, 06, 09, 12, 15, 18 and 21 UTC for the station Addis Ababa Bole. This Season- Kiremt is the main rainy season for most parts of the country and it is during this period that there is strong challenge in preparing forecasts based on users' interests. Simple frequency analysis based on a contingency table and time series plotting are used. For the sake of simplification the details in weather type are merged into bigger classes, that is, all weather categories related to rain are grouped into 'Rain' category, etc.

The cumulative frequency distribution indicates that 'No Weather- No Wx' category has by far the highest value in decreasing order it is be followed by 'Rain,' except in June during which 'Mist' takes precedence; 'Mist,' and 'Thunder Showers-TS.' In general terms 'No Wx' category has the highest frequency around 2100 UTC, 'Mist' around 0600 UTC, 'Rain' around 1500 UTC, and 'TS' around 1200 UTC. As such this general picture varies at particular months and observation hours. Analyses done with the help of contingency tables indicate that weather deterioration occurs when there is high humidity or low dew-point depression--DPD. Almost in all the Kiremt-months weather deterioration occurs when the DPD is below 5°C. In some months at around mid-day and the afternoon, weather deterioration occurs for DPD in the range between 5 to 10°C. Of all the relationships, the association of wind direction with weather is the most complex one. In general terms results of the analyses based on contingency tables indicate that weather activity could be associated with winds varying from southeasterly to northwesterly (southeasterly, southerly, southwesterly, westerly, and northwesterly). However, closer look at particular months and observation hours portray interesting trends, that is to say weather activity could be associated with any wind direction. In general, results of the analyses agree with other observations made at tropical inland stations during summer. Some of the details during night-time could, however, be misleading as there are higher possibilities for human-error.