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- Author or Editor: S. A. Hsu x
- Journal of Applied Meteorology and Climatology 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
For overwater diffusion estimates the Offshore and Coastal Dispersion (OCD) model is preferred by the U.S. Environmental Protection Agency. The U.S. Minerals Management Service has recommended that the OCD model be used for emissions located on the outer continental shelf. During southerly winds over the Gulf of Mexico, for example, the pollutants from hundreds of offshore platforms may affect the gulf coasts. In the OCD model, the overwater plume is described by the Gaussian equation, which requires the computation of σ y and σ z , which are, in turn, related to the turbulence intensity, overwater trajectory, and atmospheric stability. On the basis of several air–sea interaction experiments [the Barbados Oceanographic and Meteorological Experiment (BOMEX), the Air-Mass Transformation Experiment (AMTEX), and, most recently, the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE)] and the extensive datasets from the National Data Buoy Center (NDBC), it is shown that under neutral and stable conditions the overwater turbulence intensities are linearly proportional to the gust factor (G), which is the ratio of the wind gust and mean wind speed at height z (U z ) as reported hourly by the NDBC buoys. Under unstable conditions, it is first shown that the popular formula relating the horizontal turbulence intensity (σ u,υ /u∗, where u∗ is the friction velocity) to the ratio of the mixing height (h) and the buoyancy length (L) (i.e., h/L) suffers from a self-correlation problem and cannot be used in the marine environment. Then, alternative formulas to estimate the horizontal turbulence intensities (σ u,υ /U z ) using G are proposed for practical applications. Furthermore, formulas to estimate u∗ and z/L are fundamentally needed in air–sea interaction studies, in addition to dispersion meteorology.
Abstract
For overwater diffusion estimates the Offshore and Coastal Dispersion (OCD) model is preferred by the U.S. Environmental Protection Agency. The U.S. Minerals Management Service has recommended that the OCD model be used for emissions located on the outer continental shelf. During southerly winds over the Gulf of Mexico, for example, the pollutants from hundreds of offshore platforms may affect the gulf coasts. In the OCD model, the overwater plume is described by the Gaussian equation, which requires the computation of σ y and σ z , which are, in turn, related to the turbulence intensity, overwater trajectory, and atmospheric stability. On the basis of several air–sea interaction experiments [the Barbados Oceanographic and Meteorological Experiment (BOMEX), the Air-Mass Transformation Experiment (AMTEX), and, most recently, the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE)] and the extensive datasets from the National Data Buoy Center (NDBC), it is shown that under neutral and stable conditions the overwater turbulence intensities are linearly proportional to the gust factor (G), which is the ratio of the wind gust and mean wind speed at height z (U z ) as reported hourly by the NDBC buoys. Under unstable conditions, it is first shown that the popular formula relating the horizontal turbulence intensity (σ u,υ /u∗, where u∗ is the friction velocity) to the ratio of the mixing height (h) and the buoyancy length (L) (i.e., h/L) suffers from a self-correlation problem and cannot be used in the marine environment. Then, alternative formulas to estimate the horizontal turbulence intensities (σ u,υ /U z ) using G are proposed for practical applications. Furthermore, formulas to estimate u∗ and z/L are fundamentally needed in air–sea interaction studies, in addition to dispersion meteorology.
Abstract
On the basis of 30 samples from near-simultaneous overwater measurements by pairs of anemometers located at different heights in the Gulf of Mexico and off the Chesapeake Bay, Virginia, the mean and standard deviation for the exponent of the power-law wind profile over the ocean under near-neutral atmospheric stability conditions were determined to be 0.11 ± 0.03. Because this mean value is obtained from both deep and shallow water environments, it is recommended for use at sea to adjust the wind speed measurements at different heights to the standard height of 10 m above the mean sea surface. An example to apply this P value to estimate the momentum flux or wind stress is provided.
Abstract
On the basis of 30 samples from near-simultaneous overwater measurements by pairs of anemometers located at different heights in the Gulf of Mexico and off the Chesapeake Bay, Virginia, the mean and standard deviation for the exponent of the power-law wind profile over the ocean under near-neutral atmospheric stability conditions were determined to be 0.11 ± 0.03. Because this mean value is obtained from both deep and shallow water environments, it is recommended for use at sea to adjust the wind speed measurements at different heights to the standard height of 10 m above the mean sea surface. An example to apply this P value to estimate the momentum flux or wind stress is provided.
Abstract
The mathematical formulation of Paulson’s Ψ m (Z/L) that applies to the unstable atmospheric boundary layer has been simplified. The authors propose that Ψ m (Z/L) = a(−Z/L) b , where the coefficients a and b have been determined. Based on data provided in Panofsky and Dutton, a = 1.0496 and b = 0.4591. The correlation coefficient between Ψ m (Z/L) and (−Z/L) is 0.99, so this equation can directly account for (0.99)2 = 98% of the variation in Ψ m (Z/L). Comparisons between Paulson’s and this proposed formula show that the difference between the two is negligible for overwater applications.
Abstract
The mathematical formulation of Paulson’s Ψ m (Z/L) that applies to the unstable atmospheric boundary layer has been simplified. The authors propose that Ψ m (Z/L) = a(−Z/L) b , where the coefficients a and b have been determined. Based on data provided in Panofsky and Dutton, a = 1.0496 and b = 0.4591. The correlation coefficient between Ψ m (Z/L) and (−Z/L) is 0.99, so this equation can directly account for (0.99)2 = 98% of the variation in Ψ m (Z/L). Comparisons between Paulson’s and this proposed formula show that the difference between the two is negligible for overwater applications.
