Abstract:

Characteristics of the ocean surface winds in the lee of Rishiri Island were investigated linking to atmospheric stability and flow speeds, using a number of high-spatial resolution wind images derived by SAR and in situ atmospheric observations. Rishiri Island, located west of Hokkaido, Japan, is an almost cone-shaped isolated island with a diameter of about 16 km and height of 1719 m. We obtained the following results:

1) The SAR-derived wind map found case evidence of the low-level jet formed in the lee of the island under the ambient stably stratified flow. The time series of wind measurements in the islands and the rawinsonde soundings suggested that the jet was connected to downslope winds because the wind speeds observed in the leeward foot of the island rapidly intensified, which is concurrent with an increase in atmospheric stability in the atmospheric boundary layer.

2) The 115 SAR wind images were classified into the following four types in terms of wind pattern around the island. a) Type A showed no significant island wakes, which accounted for 17 percent. b) Type B represented wind shadows in the lee of the island accompanying low-level jets on both sides of the shadow area, which accounted for 26 percent. The left side jet is generally stronger than the right one. c) Type C depicted low-level jets formed in the lee of the island, which accounted for 20 percent. d) Type D indicated wind shadows in the lee of the island, but showed no jets different from Type B, which accounted for 36 percent. The examples of Types B, C, and D are shown in Fig.1.

Fig.1 Examples of SAR-derived wind speed patterns around Rishiri Island, (a) Types B, (b) C, and (c) D.

3) Type A corresponded to the weak ambient flow condition. On the other hand, Types B, C, and D with significant island wakes indicated the non-dimensional mountain height (є=Nh/U) dependence. Fig. 2(a) depicts a scatter diagram for all SAR observations as functions of Nh and U, where the four types are shown with difference marks. Fig. 2(b) represents histograms of each type, as a function of є. Whereas Type B occurred under the large є (>2.0) flows, Type D tended to occur under relatively small є (< 1.75). Type C, in general, occurred within their transition range (1.0 < є < 1.5). These wake types and their є-dependent transition were qualitatively consistent with the previous laboratory, numerical, and theoretical studies: small amplitude waves (Type D), wave breaking (Type C), and flow splitting (Type B).

Fig.2 (a) Inertia (U: horizontal wind speed)-buoyancy (Nh: the Brunt-Väisälä frequency multiplied by mountain height) diagram for the SAR-indentified ocean surface wind patterns (Types A to D) around Rishiri Island. Type A is plotted with black circles, Type B with green triangles, Type C with red triangles, and Type D with blue squares. Dashed lines indicate contours of the non-dimensional mountain height, є. (b) Histograms of occurrence frequency for each type as a function of є.

4) Intensification of the low-level jets of Type C that had passed over the mountain was approximately proportional to the Brunt-Väisälä frequency (N) within the cases of the present study, which was roughly consistent with the linear lee wave theory by Smith (1980). Although nonlinear effects, such as wave breaking, were not verified in the present study, its importance was strongly implied due to the discrepancies between the linear theory and the observations in terms of the position and extent of the low-level jets.

5) Occurrence frequencies of each type showed seasonal dependence being associated with seasonal characteristics of atmospheric stability. Type B came up mainly in boreal summer from May to September, when the intrusion of the cold easterly known as the Yamase frequently occurs letting the low-level flow stabilize. Type D came up mainly in winter from October to March, when well-mixed low-level flows are common due to a monsoon, cold air heated by the relatively warmer sea surface. Type C tended to occur though the year, falling upon a transition between Types B and D.

The present study demonstrated using actual atmospheric measurements that the behavior of the wind field around the island changed drastically, being sensitive to . In terms of surface winds in the lee of the island, they changed up and down with the monotonic increasing of . We could not determine a critical number that defines explicitly transition boundaries between the wind patterns because the rawinsonde soundings we used were not strictly representative of the atmosphere over Rishiri Island in a spatiotemporal sense. We did not consider the critical level of incident flows or the vertical shear of atmospheric stability, neither due to the limitation of the representativeness of the soundings. Although these problems are beyond the scope of the present study, numerical simulations as well as simultaneous and intensive atmospheric measurements are considered of value to solving them. The general results of the present study could provide reliable observational evidence for the future model studies. In any case, understanding the vertical structure of atmospheric condition is critical to predicting the low-level jets revealed by the present study.