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Alan E. Strong


An air-water interaction study was performed near Waukegan, Ill., to determine the effect of the cold waters of Lake Michigan on warm air flowing over them. Four days in late May and early June were chosen to analyze the physical nature of the interaction. Many times during spring and early summer, a cold temperate-latitude lake produces intense inversions that limit vertical turbulent energy transfers. Typically these inversions average 1 km in thickness. Coupled with such an inversion there often develops a shallow overwater mesoscale high pressure system, the lake anticyclone.

The observations of the cold lake's effect on the atmosphere were taken on an east-west line through Waukegan for a distance 5 km inland (west) and ranging as far as 40 km over the lake (east). Hourly wind soundings to a height of approximately 1500 m were made and air and dew point temperature profiles were measured by wiresonde from the R/V (research vessel) Mysis. Pilot balloon observations at the shoreline and 5 km inland were analyzed together with balloon soundings over the water to depict the diurnal development of the lake's effect on the gradient wind near the shoreline.

The low-level wind near the lake was always altered by the prevailing regional wind above the inversion. This lake influence was observed on every sunny day. The major orthogonal gradient winds and their interaction with the lake anticyclones were compared with results from Estoque's numerical model of “sea effect”. The most dramatic effect occurred when a warm offshore wind moved out over a developing inversion above the lake. As the air approached the shoreline it experienced a pronounced upward component of motion, followed by strong subsidence as it moved out over the lake.

Parallel-to-shore wind data from Waukegan demonstrated increased vertical motions from the balancing forces of the northerly gradient wind and the lake high over the reverse case of the southerly gradient wind and the lake high. Results under onshore gradient winds, while least dramatic in vertical motion induced by this lake, demonstrated low-level cloud suppression for several tens of kilometers over land downwind.

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Irregularities in sea-surface sunglint patterns have been frequently noticed in photographs from earth-orbiting satellites. High-resolution color photographs from low-altitude manned spacecraft missions have shown small-scale detail in many of the sunglint pictures. At the much higher altitude of the Applications Technology Satellites (ATS) the reflection pattern of the sun is spread over such a large area that varying sea-surface conditions can be inferred in many areas within a single sunglint region.

Of particular interest are patches or swaths of ocean surface that appear dark within the brighter sunglint region. Short-period time sequences of photographs from ATS III exhibit reversals in brightness when the horizontal specular point moves into the area of the anomalous dark feature.

Modeling statistics of sea-surface slope for increasing near-surface wind velocities show 1) a rapid drop in maximum sunglint radiance and 2) an increase in the area covered by the total glint pattern. It is shown, by combining calm surface conditions with higher background sea states, that sunglint patterns can be obtained which are very similar to those observed from satellites. Consequently, anomalous dark swath observations from ESSA satellites can be used to infer sea-state variations. The streaklike anomalies in many cases correspond to calm waters beneath high-pressure ridges or, when paralleling coastlines, the seaward limit of local sea-breeze circulations.

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Alan E. Strong and E. Paul McClain

Multi-window infrared measurements, together with visual channel observations, enable relatively high-resolution and accurate local, regional, and global retrievals of ocean surface temperatures to be repetitively and routinely obtained from operational environmental satellites. Drifting buoys appear to be the best means to date of validating the satellite estimates. Root mean square differences of about 0.6°C are found between satellite and drifter, whereas with ships-of-opportunity they are 1.8°C. Fixed buoy comparisons fall between these extremes.

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A case is made for the detection of melting snow or ice using multispectral remote sensing from earth satellites. Snow and thick ice are highly reflective in both the visible and the near-infrared portions of the electromagnetic spectrum. During thaw conditions, however, near-infrared radiation is absorbed strongly, while reflection of visible radiation is only slightly affected. Simultaneous visible and near-infrared imagery from the Nimbus 3 satellite illustrates how these reflectance differenecs can be used to obtain information of hydrologic usefulness. Two examples of such use are presented.

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