Coastal Wind Speeds: Eritrea

Technical Note:

In short, this map was produced utilizing a binned, adaptive length, gaussian-weighted averaging of data from 1971-1993 of the Comprehensive Ocean-Atmosphere Data Set (COADS) of the National Oceanic and Atmospheric Administration. More specifically, data from the COADS data set were binned into four six-hour periods for two different seasons (producing a total of 8 temporal bins). A map was produced for each temporal bin, and the maps produced from the binned data were averaged to produce the annual map. (This is done to take out biases from uneven temporal sampling in the data). The six hour periods are 0-6 hours local time, 6-12 hours local time, 12-18 hours local time, and 18-24 hours local time. The two seasons are the High-wind season (October-April) and the Low-Wind season (May-September).

Then for each point on the map, two quantities are calculated. The first quantity is a gaussian-weighted wind:

WIND-SUM = SUM OF (WS*EXP(-(R^2)/(R0^2)))

where WS is the wind speed, R is the distance between the data point and the point of interest, and R0 is the averaging distance. The second quantity is the sum of the weights:

WEIGHT-SUM = SUM OF (EXP(-(R^2)/(R0^2)))

These two quantities are calculated for progressivly larger R0 starting from R0= 10 kilometers to R0 = 150 kilomters. When the weight-sum exceeds 30, it is considered that there is enough data to calculate an average, and the average is calculated as the ratio of the wind-sum and weight-sum for that location.

Hence, an average wind speed map is calculated for four periods of the day for both the High-wind and Low-wind seasons. The average of the four periods is taken to get the average for each season. To obtain the annual average, 5/12 of the Low-wind season average is added to 7/12 of the High-wind season average.

-Robert Van Buskirk