Estimating Wind Field Offshore Jeddah Based on Land Weather Measurements
SAAD MESBAH ABDEL~MAN
Faculty of Marine Science, KAAU,
Jeddah, Saudi Arabia
ABSTRACT. Offshore meteorological information is essential for several marine studies. However, in the absence of these direct measurements, es- timates of th~ wind field are necessary. Based on the hourly data acquired at the JeddahAirport weather station, the wind speed is adjusted to a nearby coastal station at Ubhur Creek to correct for the location effect. The data are then extended to offshore incorporating the Hsu (1986) model. An op- erational formula is obtained to estimate the winds offshore Jeddah and is successfully verified using the available onshore and offshore world wide data. The obtained formula is adequate for application in the central part of the Red Sea. One of the main advantages of this result is its simplicity in ap- plication and may be useful for many practical marine studies in this region.
l. Introduction
Wind momentum flux transferred fro~ the atmospheric boundary layer to the sea surface is essential information for several 'marine studies such as air-sea interactipn, Wave generation, wave prediction and wind-driven currents. Wind data above the sea surface are generally obtained by direct measurements or by estimates based on the synoptic weather ~apsor utilizing information acquire<;i at a nearby land based station. Measuring data directly over sea surface for marine applications seems costly but is id~al for more $atisfactory results. If marine meteorological information are not available, observations acquired from land based weather stations preferably near coast, are commonly utilized. In these cases, coastal Ai~ort weather stations are frequently used since their anemometers are usually not sheltered by marked to- pographic features.
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(onshore and offshore) are covered by the same atmospheric pressure gradient. In calm wind conditions, the location factor is not defined in the mentioned graph as it may theoretically reach infinity.
A stability correction factor is suggested to take into account the thermal differ- ence between air and sea {~T), considering non-neutral stability cortditions.
As mentioned earlier, the above prescribed procedure was successfully tested in the Great Lakes area and worked normally within 16 km off the lake. The same pro- cedure was applied in a wave hindcasting model off Jeddah and gave reasonable re- sults, however, discrepancies at low Beaufort wind force were reported (Abdel- rahman, 1993). The shortcoming in applying this procedure is the inability to de- scribe adequately the associated offshore wind field in calm land wind conditions.
This is because, if onshore conditions are calm, it is not necessary that offshore winds are also calm i.e. offshore wind speed may not be equal to zero when anemometer9n land reads zero.
Schwab (1978) developed the following formulas extracted from Resio and Vin- cent (1977) graphs :
USea ULand
1.85
uLand .2 +
= ( 4>(AT) (2)
1
3"
~)
1920<f>(~T) = 1 -~ ( (3)
~e = (12.5 .;. 1.5 ~T) -(0.38 -0.03 ~T) uSea I (4) where uSea and uLand in m/sec, ~T in °C and ~e is the clockwise angle between over- land and over-water winds in degrees. These formulas were applied to model the storm surge and current fluctuations in the Great Lakes (Schwab, 1978) and also in wave prediction (Liu et al., 1984) and proved to give good results.
In comparing a simultaneous pair of onshore and offshore wind data, Sethu Raman and Raynor (1980) suggested a ratio USea/ULand to be 1.7, while Hsu (1981) ob- tained a ratio .1.6 j: 0.28 (mean j: standard deviation) when he compared four pairs of NOAA data buoys with eight coastal stations.
Two mathematical expressions were developed by Hsu(1981) to estimate offshore winds based on onshore data; one expression was based on a theoretical considera- tionof the equations of motion which required evaluating several parameters,
ous wind information offshore Jeddah, the data acquired at the local Airport weather station is utilized to estimate the offshore wind. The Airport station is lo- cated at a large clearing distanced 10 km from the shoreline at lat. 21°40'42"N and long. 39008'54"E. The wind field is recorded at z = 3.58 m above mean sea level (MSL), (Fig. 1).
Location of Jeddah Airport weather station (eAp) and Ubhur anemometer (.Vb) FIG.
Hourly values of wind field during the whole year 1990 had been processed. Pre- vailing wind direction are mainly from N-NNW which agrees with the wind pattern over the area. Most wind speeds lie between wind force zero to six on the standard Beaufort scale with occasional gales. This also agrees with the general climatology of this area (Hydrographer of the Navy, 1980). Calm wind condition during that year (where wind speed is defined to be equal or less than 0.50 m/sec) lasted more than 16% of the year (Abdelrahman, 1993).
The stability correction <I>(.1T), as mentioned earlier, depends on the thermal dif- ferences between air and sea. The monthly averaged thermal differences between air and sea (.1 T) at Jeddah are given in Edwards and Head (1987) which show the range to be from -3.0°C to + 1.00C. Therefore, the stability factor may be considered unity according to Hsu (1986). In addition, the resolution of wind direction at Jeddah Air- port vane is only 100 suggesting insignificant deviation in the wind direction (Hsu, 1986).
Thus, according to SPM (1984), the only correction remains to be considered is the location factor which requires a land station adjacent to the shoreline. The Airport
(equation 11) compared to both equations (8) and (9). The equation shows different coefficients that fit the local data.
To verify the obtained results, world wide ocean data presented in Hsu (1986 - Table 1) are reconsidered for the ranges of wind speed that most frequently occur in Jeddah area. The climate of Jeddah throughout the last 30 years shows wind speed that never exceeds 25 m/sec (MEPA, 1992), Table 1. Thus reconsidering the world wide ocean data ranging from the tropics to Alaska for wind speeds less than or equal to 25 m/sec (i.e. excluding storms and hurricanes) and applying linear regression be- tween uSea and uLand' an expression is obtained giving
uSea == 2.73 + 1.07 uLand (12)
Both equations (11 & 12) seem to be in good agreement which prove the validity of the adapted procedure for adjusting Jeddah Ai!p°rt data.
TABLE Climatological normals of extreme wind field during the last 30 years for leddah station (after MEP A, 1992).
The obtained formula (equation 11) is applicable to J~ddah and similar areas of low and moderate wind speeds. This suggested operational formula (equation 11) is more practical when compared with the relative lengthy procedure suggested by SPM (1984) that requires the use of several graphs to adjust the overland data to the offshore conditions. Moreover, equation 11 shows a non-vanishing offshore wind speed during calm over-land wind conditions, while SPM (1984) considered it unde- fined.
IV. Summary and Conclusions
Offshore meteorological information are essential for several marine and coastal
MEPA (1992) Climatological normals-3D years (1961-19!XJ) in Saudi Arabia. Report of Meteorology and Environmental Protection Administration, Climate Directorate.
Neiburger, M., Edinger, J.G. and Bonner, W.D. (1973). Understanding our Atmospheric Environment, W.H. Freeman and Company, San Francisco, p.293.
Hydrographer of the Navy (1980) Red Sea and Gulf of Aden Pilot. Admirality Pilot,.wndon, U.K.
Resio, D. T. and Vincent, C.L. (1977) Estimation of winds over the Great Lakes. Journal of the Waterway, Port, Coastal and Ocean Division, ASCE, 103: No. WW2: 265.
Schwab, D.J.(1978) Simulation and forecasting of lake Erie storm surges. Mon Wea. Rev., 106: 1476- 1487.
Sethu Raman, S. and Raynor, G.S. (1980) Comparison of mean wind speeds and turbulence at a coastal site and offshore location, J. Appl. Meterol., 19: 15-:?1.
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