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Abstract
Aircraft data from the New England Winter Storms Experiment (NEWSEX) are used to examine the vertical structure of four New England coastal fronts. The aircraft made multiple passes at varying elevations through the coastal fronts. The observations were projected onto cross sections normal to the coastal fronts using frontal orientations determined from surface analyses and frontal velocities determined from a least-squares fit to multiple frontal penetrations.
The frontal zones were found to be roughly 200 m wide, rising sharply from the ground to a height of 150 m to 400 m over distances of 0.5 to 2.5 km before leveling off and rising gradually toward the mountains. The underlying pools of cold air were similar in form and flow patterns to cold-air damming structures observed elsewhere along the East Coast. The coastal fronts formed the leading edges of the cold pools. Air on the warm side of the fronts was neutrally stratified, having been recently heated by the warm Gulf of Maine. This warm air attained vertical velocities of up to 2.5 m s−1 as it rose over the coastal fronts and accelerated toward the mountains, and evidence is found for direct precipitation enhancements by this updraft.
The coastal fronts possessed characteristics of atmospheric density currents, with a low-level inflow within the cold air toward the head and turbulent mixing behind the head. Because the fronts were quasi-stationary and were located in neutrally stratified environments, a direct comparison is made with theoretical predictions of inviscid density current behavior. The observed frontal velocities relative to the warm air were found to be within about 1 m s−1 of the velocities predicted by inviscid density current theory.
Richardson numbers are estimated at the leading edge of the coastal front and along the frontal inversion beyond the region of rapid mixing behind the head. The Richardson numbers are found to be consistent with Kelvin-Helmholtz instability originating at the leading edge of the coastal front.
Abstract
Aircraft data from the New England Winter Storms Experiment (NEWSEX) are used to examine the vertical structure of four New England coastal fronts. The aircraft made multiple passes at varying elevations through the coastal fronts. The observations were projected onto cross sections normal to the coastal fronts using frontal orientations determined from surface analyses and frontal velocities determined from a least-squares fit to multiple frontal penetrations.
The frontal zones were found to be roughly 200 m wide, rising sharply from the ground to a height of 150 m to 400 m over distances of 0.5 to 2.5 km before leveling off and rising gradually toward the mountains. The underlying pools of cold air were similar in form and flow patterns to cold-air damming structures observed elsewhere along the East Coast. The coastal fronts formed the leading edges of the cold pools. Air on the warm side of the fronts was neutrally stratified, having been recently heated by the warm Gulf of Maine. This warm air attained vertical velocities of up to 2.5 m s−1 as it rose over the coastal fronts and accelerated toward the mountains, and evidence is found for direct precipitation enhancements by this updraft.
The coastal fronts possessed characteristics of atmospheric density currents, with a low-level inflow within the cold air toward the head and turbulent mixing behind the head. Because the fronts were quasi-stationary and were located in neutrally stratified environments, a direct comparison is made with theoretical predictions of inviscid density current behavior. The observed frontal velocities relative to the warm air were found to be within about 1 m s−1 of the velocities predicted by inviscid density current theory.
Richardson numbers are estimated at the leading edge of the coastal front and along the frontal inversion beyond the region of rapid mixing behind the head. The Richardson numbers are found to be consistent with Kelvin-Helmholtz instability originating at the leading edge of the coastal front.
Abstract
Numerical simulations of terrain-induced turbulence associated with airflow over Lantau Island of Hong Kong are presented. Lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level. This research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of Chek Lap Kok, which is located to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (LANTEX). This paper focuses on flow associated with the passage of Tropical Storm Russ on 7 June 1994, during which severe turbulence was observed.
The nature of the environmental and topographic forcing on 7 June 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were performed using the Clark model. The simulation results indicate that gravity-wave dynamics played a very minor role in the flow distortion and generation of turbulence. As a result of this flow regime, relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with Tropical Storm Russ.
Results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62.5 m over local, nested domains of about 13–24 km on a side. The 125-m model resolution simulated highly distorted flow in the lee of Lantau, with streaks emanating downstream from regions of sharp orographic gradients. At this resolution the streaks were nearly steady in time. At the higher horizontal resolution of 62.5 m the streaks became unstable, resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125-m resolution simulation. The temporally averaged fields changed little with the increase in resolution; however, there was a three- to fourfold increase in the temporal variability of the flow, as indicated by the standard deviation of the wind from a 10-min temporal average. Overall, the higher resolution simulations compared quite well with the observations, whereas the lower resolution cases did not. The high-resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies. The depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution. A physical explanation, using simple linear arguments based on the blocking effects of the eddies, is presented. The nature of the flow separation is analyzed using Bernoulli’s energy form to display the geometry of the separation bubbles. The height of the 80 m2 s−2 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream.
Tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125-m resolution are presented, as well as one experiment testing the influence of static stability suppressing turbulence development. All these tests showed no significant effect. Implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed.
Abstract
Numerical simulations of terrain-induced turbulence associated with airflow over Lantau Island of Hong Kong are presented. Lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level. This research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of Chek Lap Kok, which is located to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (LANTEX). This paper focuses on flow associated with the passage of Tropical Storm Russ on 7 June 1994, during which severe turbulence was observed.
The nature of the environmental and topographic forcing on 7 June 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were performed using the Clark model. The simulation results indicate that gravity-wave dynamics played a very minor role in the flow distortion and generation of turbulence. As a result of this flow regime, relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with Tropical Storm Russ.
Results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62.5 m over local, nested domains of about 13–24 km on a side. The 125-m model resolution simulated highly distorted flow in the lee of Lantau, with streaks emanating downstream from regions of sharp orographic gradients. At this resolution the streaks were nearly steady in time. At the higher horizontal resolution of 62.5 m the streaks became unstable, resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125-m resolution simulation. The temporally averaged fields changed little with the increase in resolution; however, there was a three- to fourfold increase in the temporal variability of the flow, as indicated by the standard deviation of the wind from a 10-min temporal average. Overall, the higher resolution simulations compared quite well with the observations, whereas the lower resolution cases did not. The high-resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies. The depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution. A physical explanation, using simple linear arguments based on the blocking effects of the eddies, is presented. The nature of the flow separation is analyzed using Bernoulli’s energy form to display the geometry of the separation bubbles. The height of the 80 m2 s−2 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream.
Tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125-m resolution are presented, as well as one experiment testing the influence of static stability suppressing turbulence development. All these tests showed no significant effect. Implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed.
