TERRAIN-INDUCED TURBULENCE OVER LANTAU ISLAND - 7 JUNE 1994 TROPICAL STORM RUSS CASE-STUDY

Numerical simulations of terrain-induced turbulence associated with ai rflow over Lantau Island of Hang Kong are presented. Lantau is a relat ively small island with three narrow peaks rising to between 700 and 9 50 m above mean sea level. This research was undertaken as part of a p roject to better understand and predict the nature of turbulence and s hear at the new airport site on the island of Chek Lap Kok, which is l ocated to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (L ANTEX). This paper focuses on flow associated with the passage of Trop ical Storm Russ on 7 June 1994, during which severe turbulence was obs erved. The nature of the environmental and topographic forcing on 7 Ju ne 1994 resulted in the turbulence and shear being dominated by the co mbination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were pe rformed using the Clark model. The simulation results indicate that gr avity-wave dynamics played a very minor role in the flow distortion an d generation of turbulence. As a result of this flow regime, relativel y 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 su rface and with horizontal resolutions of bath 125 and 62.5 m over loca l, nested domains of about 13-24 km on a side. The 125-m model resolut ion simulated highly distorted flow in the lee of Lantau, with streaks emanating downstream from regions of sharp orographic gradients. At t his resolution the streaks were nearly steady in time. At the higher h orizontal resolution of 62.5 m the streaks became unstable, resulting in eddies advecting downstream within a distorted streaky mean flow si milar to the 125-m resolution simulation. The temporally averaged fiel ds 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 temp oral average. Overall, the higher resolution simulations compared quit e well with the observations, whereas the lower resolution cases did n ot. The high-resolution experiments also showed a much broader horizon tal and vertical extent for the transient eddies. The depth of orograp hic influence increased from about 200 m to over 600 m with the increa se in resolution. A physical explanation, using simple linear argument s based on the blocking effects of the eddies, is presented. The natur e of the flow separation is analyzed using Bernoulli's energy form to display the geometry of the separation bubbles. The height of the 80 m (2) s(-2) energy surface shows eddies forming in regions of large orog raphic gradients and advecting downstream. Tests using both buoyancy e xcitation and stochastic backscatter to parameterize the underresolved dynamics at the 125-m resolution are presented, as well as one experi ment testing the influence of static stability suppressing turbulence development. All these tests showed no significant effect. Implication s of these results to the parameterization of mechanically induced tur bulence in complex terrain are discussed.