Forecasting weather over the ocean presents a unique challenge due to the lack of ground observations that can survey the area. Buoys and ships can give data that can help with forecasts and warnings. However, these are sparse and cannot effectively cover the entirety of the ocean. Satellites help mitigate this with the use of scatterometers and altimeters. We use altimeters to retrieve significant wave heights and wind speed, and scatterometers provide information about wind speed and direction. Both are satellite microwave radars, and they send energy towards the ocean’s surface, measuring the energy that returns to the satellite. They fly on Low Earth Orbiting (LEO) satellite and provide global coverage, flying over the same location twice in a single day, with more frequent observations closer to the poles.
Specifically, altimeters can help measure significant wave heights over the ocean. The National Weather Service defines this as the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period. When there are waves present, the return pulse takes a longer time to reach the satellite and changes the shape of the pulse. Using this, the altimeter can measure the wave height at a spacing of 7 km over the ocean’s surface. Significant wave height could have impacts on coastal erosion and ships attempting to avoid dangerous waters.
Scatterometers work similarly by sending a pulse towards the ocean surface and measuring the energy received by the instrument. They use the Bragg Scattering principle where scatterometers send a pulse with a similar wavelength of tiny ripples embedded within the ocean’s wave. This causes a resonance that causes energy to be scattered towards the satellite. This allows scatterometers to measure wind speed and direction. Scatterometers measure at a particular angle called an incident angle to allow it to accurately measure radar backscatter. Radar is most sensitive to wind speeds for incidence angles between 30° and 60°. Having an angle outside this range would cause the satellite to miss the radar backscattering. Also, to get wind direction, scatterometers must view the embedded waves from different horizontal directions. This allows for it to narrow down the possible directions given a particular wind speed.
There are a few hindrances behind altimeter and scatterometer readings. Rain can be detrimental to observations as rain can absorb the initial radar pulse lowering wind speed measurements and can scatter energy back to the satellite which would overestimate wind speeds. These effects are significant because when there is heavier rain the measurements would be more sensitive to the rain’s effects.
Altimeters and scatterometers provide a great resource in monitoring significant wave heights and wind speed/direction estimates. This can help in marine forecasting, aiding ships in where to travel, monitoring weather over the ocean such as high and low centers of pressure, and coastal warnings regarding significant wave heights and the threats that come with it.
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Photo Credit: The Comet Program
©2019 Weather Forecaster Dakari Anderson