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- Author or Editor: S. A. Hsu x
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Abstract
Along the United States Gulf coast and over the northern Gulf of Mexico, frontal overrunning occurs frequently. Cyclogenesis over the Gulf is often associated with this type of weather system. Effects of baroclinic fields on frontal overrunning are investigated from synoptic and climatological points of view. It is found that, from October through April, the orientation of the shelf break is a very important baroclinic characteristic because fronts tend to stall there rather than at the physical coastline. To further substantiate this deduction dynamically, the local geostrophic vorticity field over the western Louisiana-upper Texas shelf region is estimated monthly. The correlation coefficient between the vorticity field and the frequency of frontal overrunning along the central Gulf coast was .86. For forecasting applications, a simple formula is provided to estimate this local vorticity from the temperature difference between Lake Charles, Louisiana, and buoy station 42002 in the deep Gulf.
Abstract
Along the United States Gulf coast and over the northern Gulf of Mexico, frontal overrunning occurs frequently. Cyclogenesis over the Gulf is often associated with this type of weather system. Effects of baroclinic fields on frontal overrunning are investigated from synoptic and climatological points of view. It is found that, from October through April, the orientation of the shelf break is a very important baroclinic characteristic because fronts tend to stall there rather than at the physical coastline. To further substantiate this deduction dynamically, the local geostrophic vorticity field over the western Louisiana-upper Texas shelf region is estimated monthly. The correlation coefficient between the vorticity field and the frequency of frontal overrunning along the central Gulf coast was .86. For forecasting applications, a simple formula is provided to estimate this local vorticity from the temperature difference between Lake Charles, Louisiana, and buoy station 42002 in the deep Gulf.
Abstract
Under geostrophic and hydrostatic conditions, the Margules equation for the equilibrium slope of a stationary front is applied to study the relationship between monthly frontal overrunning and the temperature difference (ΔT) across the central Gulf Coast. Data employed were 10 years of frontal overrunning statistics, 30 years of onshore temperature and wind records at New Orleans, Louisiana, and 86 years of offshore temperature and wind conditions. Monthly frontal overrunning correlates both meteorologically and statistically with ΔT, as expected. However, the high correlation coefficient of 0.91 was unexpected. The contribution of wind difference across the coastal zone is smaller by far than that of ΔT. The results may therefore be applied for operational planning and to supplement local forecasting of frontal overrunning.
Abstract
Under geostrophic and hydrostatic conditions, the Margules equation for the equilibrium slope of a stationary front is applied to study the relationship between monthly frontal overrunning and the temperature difference (ΔT) across the central Gulf Coast. Data employed were 10 years of frontal overrunning statistics, 30 years of onshore temperature and wind records at New Orleans, Louisiana, and 86 years of offshore temperature and wind conditions. Monthly frontal overrunning correlates both meteorologically and statistically with ΔT, as expected. However, the high correlation coefficient of 0.91 was unexpected. The contribution of wind difference across the coastal zone is smaller by far than that of ΔT. The results may therefore be applied for operational planning and to supplement local forecasting of frontal overrunning.
Abstract
Hourly measurements of temperature and wind speed (at 10 and 33 m) for a one-year period were made on the flat south coast of St. Croix, U.S. Virgin Islands. Values of the exponent p used in the power-law wind profile were obtained by applying criteria of Golder's (1972) nomogram, the U.S. Nuclear Regulatory Commission's (1974) temperature difference method, and Sedefian's (1980) formulation. It was found that p ranges from 0.15 for stability class A to >0.40 for class F. Thus the familiar 1/7 power law will not be useful for a tropical coast. Since expected values of p as determined from Sedefian's nomogram were in good agreement with the observations, it is recommended for use in a tropical environment.
Abstract
Hourly measurements of temperature and wind speed (at 10 and 33 m) for a one-year period were made on the flat south coast of St. Croix, U.S. Virgin Islands. Values of the exponent p used in the power-law wind profile were obtained by applying criteria of Golder's (1972) nomogram, the U.S. Nuclear Regulatory Commission's (1974) temperature difference method, and Sedefian's (1980) formulation. It was found that p ranges from 0.15 for stability class A to >0.40 for class F. Thus the familiar 1/7 power law will not be useful for a tropical coast. Since expected values of p as determined from Sedefian's nomogram were in good agreement with the observations, it is recommended for use in a tropical environment.
Abstract
On the basis of hourly measurements of wind and air and sea surface temperatures for at least 6 yr at three buoy stations in the eastern Gulf of Mexico, the onset of the free convection regime, which coincides with the commencement of stability class C (for slightly unstable conditions in the Pasquill stability classification) at approximately R
b
= −0.03, −Z/L = 0.4, and −Z
i
/L = 5, is verified over the ocean, where R
b
is the bulk Richardson number, Z (= 10 m) is the height above the sea, L is the Monin–Obukhov stability length, and Z
i
is the height of the convective boundary layer (CBL). Datasets for the CBL are analyzed in the context of the boundary layer physics of Garratt. It is found that Z
i
is linearly proportional to the surface buoyancy flux—that is, (
Abstract
On the basis of hourly measurements of wind and air and sea surface temperatures for at least 6 yr at three buoy stations in the eastern Gulf of Mexico, the onset of the free convection regime, which coincides with the commencement of stability class C (for slightly unstable conditions in the Pasquill stability classification) at approximately R
b
= −0.03, −Z/L = 0.4, and −Z
i
/L = 5, is verified over the ocean, where R
b
is the bulk Richardson number, Z (= 10 m) is the height above the sea, L is the Monin–Obukhov stability length, and Z
i
is the height of the convective boundary layer (CBL). Datasets for the CBL are analyzed in the context of the boundary layer physics of Garratt. It is found that Z
i
is linearly proportional to the surface buoyancy flux—that is, (
Abstract
A mechanism is proposed for a physical explanation of the increase in wind stress (drag) coefficient with wind speed over water surfaces. The formula explicitly incorporates the contribution of both winds and waves through the parameterizations of an aerodynamic roughness equation. The formula is consistent with measurements from the field and with results obtained by numerical models for storm surges and water level fluctuations.
Abstract
A mechanism is proposed for a physical explanation of the increase in wind stress (drag) coefficient with wind speed over water surfaces. The formula explicitly incorporates the contribution of both winds and waves through the parameterizations of an aerodynamic roughness equation. The formula is consistent with measurements from the field and with results obtained by numerical models for storm surges and water level fluctuations.
Abstract
A formula that linearly relates the difference in wind speed between onshore and offshore regions, as tested successfully in the Great Lakes region, has been revised and extended to other parts of the world. This formula is further substantiated theoretically by using an approximation of the equations of motion. Contribution of air–sea temperature difference to wind speed and direction, as well as the meteorological conditions under which this formula way be applied, are also evaluated.
Abstract
A formula that linearly relates the difference in wind speed between onshore and offshore regions, as tested successfully in the Great Lakes region, has been revised and extended to other parts of the world. This formula is further substantiated theoretically by using an approximation of the equations of motion. Contribution of air–sea temperature difference to wind speed and direction, as well as the meteorological conditions under which this formula way be applied, are also evaluated.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
At the air–sea interface, estimates of evaporation or latent heat flux and the Monin–Obukhov stability parameter require the measurements of dewpoint (T dew) or wet-bulb temperature, which are not routinely available as compared to those of air (T air) and sea surface temperature (T sea). On the basis of thermodynamic considerations, this paper first postulates that the quantity of (q sea − q air) for the difference in specific humidity between the sea surface and its overlying air is related to the quantity of (T sea − T air). Using hourly measurements of all three temperatures, that is, T sea, T air, and T dew from a buoy in the Gulf of Mexico under a severe cold air outbreak, a linear correlation between (q sea − q air) and (T sea − T air) does exist with a compelling high correlation coefficient, r, of 0.98 between these two quantities. Second, based on this Clausius-Clapeyron effect, the Bowen ratio B is proposed to relate to the quantity of (T sea − T air) only such that B = a(T sea − T air) b . Using all data for these three temperatures available from four stations in the Gulf from 1993 through 1997 reveal that for deepwater a varies from 0.077 to 0.078, b from 0.67 to 0.71, and r from 0.85 to 0.89. Similar equations for the nearshore region are also provided. Limited datasets from the open ocean also support this generic relationship between B and the quantity of (T sea − T air).
Abstract
At the air–sea interface, estimates of evaporation or latent heat flux and the Monin–Obukhov stability parameter require the measurements of dewpoint (T dew) or wet-bulb temperature, which are not routinely available as compared to those of air (T air) and sea surface temperature (T sea). On the basis of thermodynamic considerations, this paper first postulates that the quantity of (q sea − q air) for the difference in specific humidity between the sea surface and its overlying air is related to the quantity of (T sea − T air). Using hourly measurements of all three temperatures, that is, T sea, T air, and T dew from a buoy in the Gulf of Mexico under a severe cold air outbreak, a linear correlation between (q sea − q air) and (T sea − T air) does exist with a compelling high correlation coefficient, r, of 0.98 between these two quantities. Second, based on this Clausius-Clapeyron effect, the Bowen ratio B is proposed to relate to the quantity of (T sea − T air) only such that B = a(T sea − T air) b . Using all data for these three temperatures available from four stations in the Gulf from 1993 through 1997 reveal that for deepwater a varies from 0.077 to 0.078, b from 0.67 to 0.71, and r from 0.85 to 0.89. Similar equations for the nearshore region are also provided. Limited datasets from the open ocean also support this generic relationship between B and the quantity of (T sea − T air).
Abstract
A dynamic roughness equation including major wind and wave interaction parameters (wind shear velocity, wave height, and phase velocity) is derived. Because this equation implicitly incorporates the effects of wave steepness, relative water depth, and wind duration and fetch, it may be applied to a wide variety of natural conditions. This equation was used to construct a nomogram which can be utilized to determine the wind stress at the sea surface, given phase velocities, wave heights, and the wind speed at any height in the atmospheric boundary layer. The proposed relationships are verified by the available field and laboratory data under near-neutral atmospheric stability conditions from which the appropriate parameters could be determined.
Abstract
A dynamic roughness equation including major wind and wave interaction parameters (wind shear velocity, wave height, and phase velocity) is derived. Because this equation implicitly incorporates the effects of wave steepness, relative water depth, and wind duration and fetch, it may be applied to a wide variety of natural conditions. This equation was used to construct a nomogram which can be utilized to determine the wind stress at the sea surface, given phase velocities, wave heights, and the wind speed at any height in the atmospheric boundary layer. The proposed relationships are verified by the available field and laboratory data under near-neutral atmospheric stability conditions from which the appropriate parameters could be determined.
