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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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Shu-Hsien Chou and Eueng-Nan Yeh

Abstract

Airborne measurements of atmospheric turbulence spectra and cospectra made at the 50 m level above the western Atlantic Ocean during cold air outbreaks have been studied. The data cover nearshore areas of cloud streets or roll vortices. In the inertial submnge, the results are in good agreement with previous measurements made over both land and sea, except for those of temperature.

The high-frequency behavior is consistent with local isotropy. In the inertial subrange, a near 4/3 ratio is observed between velocity spectra normal to and those along the aircrat~ heading. Also, the normalized spectra and cospectra of vertical velocity, humidity, and temperature are independent of sampling directions. The normalized cospectrum of the humidity flux appears to have a -4/3 cospectral slope, while the normalized cospectrum of stress appears to have -4/3 and -5/3 cospectral slopes in the alongwind and crosswind samples, respectively.

The shapes of the spectra and cospectra vary with sampling direction. Except for crosswind velocity, the normalized spectra and cospectra appear to have more energy in the crosswind samples at the dimensionless frequency (f) ~ 0.2 and more in the alongwind samples at f< 0. I. This is mainly due to the stretching action ofthe mean wind shear on convective elements and is in good agreement with previous aircraft measurements.

For the alongwind samples, the normalized velocity spectra for f< 0.1 appear not to be in good agreement with the spectral models derived from the cloud-free, highly convective Minnesota data. According to the latter, the normalized horizontal velocity spectra are nearly equal for low frequencies, while our results show that the low-frequency convection is significantly suppressed in the alongwind direction by the vertical wind shear.

The three dissipation estimates, derived from the high-frequency part of the velocity spectra, appear to be in good agreement with a 9% mean discrepancy. The normalized dissipation is systematically smaller than those derived from the convective Kansas and Minnesota data. The turbulent kinetic energy budget is also different. For the same total energy production by wind shear and buoyancy, energy is available for exporting out of the surface layer in this case, whereas it must be imported into the Kansas and Minnesota surface layers.

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Shu-Hsien Chou, Wenzhong Zhao, and Ming-Dah Chou

Abstract

The daily mean heat and momentum fluxes at the surface derived from the Special Sensor Microwave/Imager and Japan’s Geostationary Meteorological Satellite radiance measurements are used to study the temporal and spatial variability of the surface energy budgets and their relationship to the sea surface temperature during the Coupled Ocean–Atmosphere Response Experiment intensive observing period (IOP). For three time legs observed during the IOP, the retrieved surface fluxes compare reasonably well with those from the Improved Meteorological Instrument (IMET) buoy, RV Moana Wave, and RV Wecoma. The characteristics of surface heat and momentum fluxes are very different between the southern and northern warm pool. In the southern warm pool, the net surface heat flux is dominated by solar radiation, which is, in turn, modulated by the two Madden–Julian oscillations. The surface winds are generally weak, leading to a shallow ocean mixed layer. The solar radiation penetrating through the bottom of the mixed layer is significant, and the change in the sea surface temperature during the IOP does not follow the net surface heat flux. In the northern warm pool, the northeasterly trade wind is strong and undergoes strong seasonal variation. The variation of the net surface heat flux is dominated by evaporation. The two westerly wind bursts associated with the Madden–Julian oscillations seem to have little effect on the net surface heat flux. The ocean mixed layer is deep, and the solar radiation penetrating through the bottom of the mixed layer is small. As opposed to the southern warm pool, the trend of the sea surface temperature in the northern warm pool during the IOP is in agreement with the variation of the net heat flux at the surface.

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Shu-Hsien Chou, Robert J. Curran, and George Ohring

Abstract

The effects of two different evaporation parameterizations on the sensitivity of simulated climate to solar constant variations are investigated by using a zonally averaged climate model. One parameterization is a nonlinear formulation in which the evaporation is nonlinearly proportional to the sensible heat flux, with the Bowen ratio determined by the predicted vertical temperature and humidity gradients near the earth's surface (model A). The other is the formulation of Saltzman (1968) with the evaporation linearly proportional to the sensible heat flux (model B). The computed climates of models A and B are in good agreement except for the energy partition between sensible and latent heat at the earth's surface. The difference in evaporation parameterizations causes a difference in the response of temperature lapse rate to solar constant variations and a difference in the sensitivity of longwave radiation to surface temperature which leads to a smaller sensitivity of surface temperature to solar constant variations in model A than in model B. The results of model A are qualitatively in agreement with those of the general circulation model calculations of Wetherald and Manabe (1975).

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Shu-Hsien Chou, David Atlas, and Eueng-nan Yeh

Abstract

The structure and kinetic energy budget of turbulence in the convective marine atmospheric boundary layer as observed by aircraft during a cold air outbreak have been studied using mixed layer scaling. The results are significantly different from those of previous studies under conditions closer to free convection. The normalized turbulent kinetic energy and turbulent transport are about twice those found during the Air Mass Transformation EXperiment (AMTEX). This implies that, for a given surface heating, the present case is dynamically more active. The difference is mainly due to the greater importance of wind shear in the present case. This case is closer to the roll vortex regime whereas AMTEX observed mesoscale cellular convection, which is closer to free convection. Shear generation is found to provide a significant energy source in addition to buoyancy production to maintain a larger normalized turbulent kinetic energy and to balance a larger normalized dissipation. The interaction between turbulent pressure and divergence (i.e., pressure scrambling) is also found to transfer energy from the vertical to the horizontal components, and expected to be stronger in roll vortices than in mesoscale cells.

The sensible heat flux is found to fit well with a linear vertical profile in a clear or subcloud planetary boundary layer (PBL), in good agreement with that of Lenschow. The heat flux ratio between the PBL top and the surface, derived from the linear fitted curve, is approximately −0.14; this is in good agreement with that derived from the lidar data for the same case. Near the PBL top, the heat flux profiles are consistent with those of Deardorff and Deardorff et al.

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David Atlas, Bernard Walter, Shu-Hsien Chou, and P. J. Sheu

Abstract

The combination of vertical lidar and in situ meteorological observations from two aircraft provide an unprecedented view of the marine atmospheric boundary layer (MABL) during a cold air outbreak. To a first approximation, the lidar reflectivity is associated with the concentration of sea salt aerosols. Across the capping inversion, the lidar reflectivity contours approximate isentropes and streamlines thereby defining the inversion. Within the mixed layer, high reflectivity cores are associated with updrafts carrying aerosol-rich air upward and conversely. These effects are enhanced by increasing humidity in updraft and decreasing humidity in downdrafts that operate to increase and decrease aerosol sizes. Narrow high reflectivity columns extend upward from the ocean indicating that organized flow exists all the way to the surface. Entrainment across the inversion is manifested by small scale perturbations (∼200–500 m) superimposed upon the large scale (&sim 1–2 km) undulations of the inversion. These occur where the local entrainment zone is sharpest; generally, this is on the upshear side of the, convective. domes where Kelvin-Helmholtz instability is triggered by local compression of the inversion,

The MABL on 20 January 1983 is highly organized. The organization takes the form of 1–2 km scale roll vortices and corresponding undulations of the inversion with amplitude of 150–200 m peak to trough. The roll circulation is very strong with up and downdrafts of 2–4 in s−-1 at the 210 m level. The axes of the rolls are essentially north-south along the direction of the strong northerly low-level winds. The rising arm of the roll coincides with a column of high lidar reflectivity and with the updraft which transport aerosols, moisture, and heat up from the surface. The presence of the rolls, driven mainly by the combination of strong transverse sheer and buoyancy, serves to produce low-level convergence which concentrates the small-scale buoyant eddies to form a single well-ordered updraft in the manner previously postulated by LeMone.

The fluxes measured by the covariance method in the undulating inversion are unreliable because of the sensitivity to detrending and inadequate sampling of the exchanges across the interfaces of the dames and troughs. The partitioning method of Wilczak and Businger provides improved insight as to the mechanisms responsible for the downward flux in the inversion. However, unlike Wilczak and Businger, who find the downward flux dominated by cold updrafts we find that it is due mainly to the entrainment of warm eddies which are then transported downward by the larger-scale roll circulations on the downshear side of the domes.

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Anastasia Romanou, William B. Rossow, and Shu-Hsien Chou

Abstract

In the first part of the paper, a high space–time resolution (1° latitude/longitude and daily) dataset of the turbulent fluxes at the ocean surface is used to estimate and study the seasonal to annual near-global maps of the decorrelation scales of the latent and sensible heat fluxes. The decorrelation scales describe the temporal and spatial patterns that dominate the flux fields (within a bandpass window) and hence reveal the dominant variability in the air–sea interaction. Regional comparison to the decorrelation scales of the flux-related variables such as the wind stress, the humidity difference, and the SST identifies the main mechanism responsible for the variability in each flux field.

In the second part of the paper, the decorrelation scales are used to develop a method for filling missing values in the dataset that result from the incomplete satellite coverage. Weight coefficients in a linear regression function are determined from the spatial and temporal decorrelations and are functions of zonal and meridional distance and time. Therefore they account for all spatial and temporal patterns on scales greater than 1 day and 1° latitude/longitude and less than 1 yr and the ocean basin scale. The method is evaluated by simulating the missing-value distribution of the Goddard Satellite-Based Surface Turbulent Fluxes, version 2 (GSSTF2) dataset in the NCEP SST, the International Satellite Climatology Project (ISCCP)-FD (fluxes calculated using D1 series) surface radiation, and the Global Precipitation Climatology Project (GPCP) datasets and by comparing the filled datasets to the original ones. Main advantages of the method are that the decorrelation scales are unrestricted functions of space and time; only information internal to the flux field is used in the interpolation scheme, and the computation cost of the method is low enough to facilitate its use in similar large datasets.

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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, Eric Nelkin, Joe Ardizzone, and Robert M. Atlas

Abstract

The ocean surface latent heat flux (LHF) plays an essential role in global energy and water cycle variability. In this study, monthly LHF over global oceans during 1992–93 are compared among Goddard Satellite-Based Surface Turbulent Fluxes, version 2 (GSSTF2), Hamburg Ocean–Atmosphere Parameters and Fluxes from Satellite Data (HOAPS), NCEP–NCAR reanalysis (NCEP), and da Silva et al. (da Silva). To find the causes for discrepancies of LHF, monthly 10-m wind speed (U 10m), 10-m specific humidity (Q 10m), and sea–air humidity difference (Q SQ 10m) are also compared during the same period. The mean differences, standard deviations of differences, and temporal correlation of these monthly variables over global oceans during 1992–93 between GSSTF2 and each of the other three datasets are analyzed. The large-scale patterns of the 2-yr-mean fields for these variables are similar among these four datasets, but significant quantitative differences are found.

The temporal correlation is higher in the northern extratropics than in the south for all variables, with the contrast being especially large for da Silva as a result of more missing ship observations in the south. The da Silva dataset has extremely low temporal correlation and large differences with GSSTF2 for all variables in the southern extratropics, indicating that da Silva hardly produces a realistic variability in these variables. The NCEP has extremely low temporal correlation (0.27) and large spatial variations of differences with GSSTF2 for Q SQ 10m in the Tropics, which causes the low correlation for LHF. Over the Tropics, the HOAPS mean LHF is significantly smaller than GSSTF2 by ∼31% (37 W m−2), whereas the other two datasets are comparable to GSSTF2. This is because the HOAPS has systematically smaller LHF than GSSTF2 in space, while the other two datasets have very large spatial variations of large positive and negative LHF differences with GSSTF2 to cancel and to produce smaller regional-mean differences. Based on comparison with high-quality flux observations, we conclude that the GSSTF2 latent heat flux, surface air humidity, and winds are likely to be more realistic than the other three flux datasets examined, although those of GSSTF2 are still subject to regional biases.

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Shu-Hsien Chou, Ming-Dah Chou, Pui-King Chan, Po-Hsiung Lin, and Kung-Hwa Wang

Abstract

Seasonal to interannual variations of the net surface heating (F NET) and its relationship to sea surface temperature tendency (dT s/dt) in the tropical eastern Indian and western Pacific Oceans are studied for the period October 1997–September 2000. The surface heat fluxes are derived from the Special Sensor Microwave Imager and Japanese Geostationary Meteorological Satellite radiance measurements. It is found that the magnitude of solar heating is larger than that of evaporative cooling, but the spatial variation of the latter is significantly larger than the former. As a result, the spatial patterns of the seasonal and interannual variability of F NET are dominated by the variability of evaporative cooling. Seasonal variations of F NET and dT s/dt are significantly correlated, except for the equatorial western Pacific. The high correlation is augmented by the high negative correlation between solar heating and evaporative cooling.

The change of F NET between the 1997/98 El Niño and 1998/99 La Niña is significantly larger in the tropical eastern Indian Ocean than that in the tropical western Pacific. For the former region, reduced evaporative cooling arising from weakened winds during El Niño is generally associated with enhanced solar heating due to reduced cloudiness, leading to enhanced interannual variability of F NET. For the latter region, reduced evaporative cooling due to weakened winds is generally associated with reduced solar heating arising from increased cloudiness, and vice versa. Consequently, the interannual variability of F NET is reduced. The correlation between interannual variations of F NET and dT s/dt is weak in the tropical western Pacific and eastern Indian Oceans, indicating the importance of ocean dynamics in affecting the interannual SST variation.

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