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Shu-Hsien Chou
and
David Atlas

Abstract

Nomograms of mean column heating, due to surface sensible and latent heat fluxes, have been developed from Stage and Businger's (1981a,b) boundary-layer model for cold air outbreaks over warm water. Mean sensible heating of the cloud-free region is related to the cloud-free path (CFP, the distance from the shore to the first cloud formation) and the difference between land-air and sea-surface temperatures θ1 and θ0, respectively. Mean latent heating is related to the CFP and the difference between land-air and sea-surface specific humidities q 1 and q 0, respectively. Results are also applicable to any path within the cloud-free region. Corresponding heat fluxes may be obtained by multiplying the mean heating by the mean wind speed in the boundary layer. The sensible heating, estimated by the present method, is found to be in good agreement with that computed from the bulk transfer formula. The sensitivity of the solutions to the variations in the initial coastal soundings and large-scale subsidence is also investigated. The results are not sensitive to divergence, but are affected by the initial lapse rate of potential temperature; the greater the stability, the smaller the heating, other factors being equal. Unless one knows the lapse rate at the shore, this requires another independent measurement. For this purpose, we propose to use the downwind slope of the square of the boundary layer height, the mean value of which is also directly proportional to the mean sensible heating. The height of the boundary layer should be measurable by future spaceborne lidar systems. The general behavior of the mean sensible heating, the potential temperature, and the height of the boundary layer as a function of downwind distance within the cloud-free region, and their relations to several important parameters are studied analytically in the Appendix. By-products include the finding that the sensible (latent) heat flux is virtually linear with the contrast in land-air and sea-surface temperatures (specific humidities), thus providing a new kind of flux parameterization in lieu of the classical bulk transfer formulas. The applicability of the results to lake-effect snowstorms is also noted. Finally, the method can be used in reverse to check the validity of boundary-layer models.

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David Atlas
,
Shu-Hsien Chou
, and
William P. Byerly

Abstract

In the case of cold air outbreaks, the combination of the coastal shape and the sea surface temperature (SST) pattern is shown to have a profound effect in establishing a low level mesoscale atmospheric circulation as a result of differential heating due to both variations in overwater path length and the SST. A convergence (or divergence) line then forms along a line exactly downwind of the major bend in the coastline. This is consistent with the structure of the cloud patterns seen in a high resolution Landsat picture of the cloud streets. The major features are also simulated well with a boundary layer model. The dominant convergence line is marked by notably larger clouds. To its cast the convective roll clouds grow downstream in accord with the deepening of the boundary layer. To its west (i.e., coastal side) near the convergence line where the induced pressure field forces a strong westerly component in the boundary layer, the wind shear across the inversion gives rise to Kelvin-Helmholtz waves and billow clouds whose orientation is perpendicular to the shear vector and to the major convergence line. The result is a pattern of cloud streets oriented N–S along the wind direction to the cast of the convergence line, and billow clouds oriented essentially E–W to the west of that line. It is also suggested that the induced mesoscale circulation will feed back on the ocean by intensifying the wind-generated ocean wave growth and altering their orientation.

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Shu-Hsien Chou
,
Robert M. Atlas
,
Chung-Lin Shie
, and
Joe Ardizzone

Abstract

Monthly averages of daily latent heat fluxes over the oceans for February and August 1988 are estimated using a stability-dependent bulk scheme. Daily fluxes are computed from daily SSM/I (Special Sensor Microwave/Imager) wind speeds and EOF-retrieved SSM/I surface humidity, National Meteorological Center sea surface temperatures, and the European Centre for Medium-Range Weather Forecasts analyzed 2-m temperatures. Daily surface specific humidity (Q) is estimated from SSM/I precipitable water of total (W) and a 500-m bottom layer (W B ) using an EOF (empirical orthogonal function) method. This method has six W-based categories of EOFs (independent of geographical locations) and is developed using 23 177 FGGE IIb humidity soundings over the global oceans. For 1200 FGGE IIb humidity soundings, the accuracy of EOF-retrieved Q is 0.75 g kg−1 for the case without errors in W and W B , and increases to 1.16 g kg−1 for the case with errors in W and W B . Compared to 342 collocated radiosonde observations, the EOF-retrieved SSM/I Q has an accuracy of 1.7 g kg−1. The method improves upon the humidity retrieval of Liu and is competitive with that of Schulz et al.

The SSM/I surface humidity and latent heat fluxes of these two months agree reasonably well with those of COADS (Comprehensive Ocean–Atmosphere Data Set). Compared to the COADS, the sea–air humidity difference of SSM/I has a positive bias of approximately 1–3 g kg−1 (an overestimation of flux) over the wintertime trade wind belts and wintertime extratropical oceans. In the summertime extratropical Pacific and summertime eastern equatorial Pacific Ocean, it has a negative bias of about 1–2 g kg−1 (an underestimation of flux). The results further suggest that the two monthly flux estimates, computed from daily and monthly mean data, do not differ significantly over the oceans.

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