Canada Orographically-forced coastal wind fields around Hokkaido, Japan Osamu Isoguchi (RESTEC) ● Masanobu Shimada (JAXA/EORC)

Canada 1.Characteristic of ocean surface winds in the lee of an isolated island isoguchi et al. (2011),Characteristics of Ocean Surface Winds in the Lee of an Isolated Island observed by Synthetic Aperture Radar, Mon. Wea. Rev. 2.Coastal meteological phenomena forced by headlands Orographically-forced coastal wind phenomena revealed by SAR wind data

Canada Characteristic of ocean surface winds in the lee of an isolated island Objective: Characteristic of ocean surface winds around an isolated island is examined using Synthetic Aperture Radar (SAR) and rawinsonde sounding 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.

Canada A Case Study of a Low-level Jet Formed in the Lee of Rishiri Island on 22 May 2009 Statistical Characteristics of Atmospheric Island Wakes 1. Characteristic of ocean surface winds in the lee of an isolated island

Canada Temperature & wind vertical profile from rawinsonde Wakkanai temperature Potential temperature Temperature inversion at m layer: stable condition at lower layer A Case Study of a Low-level Jet Formed in the Lee of Rishiri Island on 22 May 2009 PALSAR-derived wind fields 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. Black arrows ： Reanalysis winds White arrows ： In situ winds

Canada Kutsugata Motodomari In situ stations PALSAR acquisition (a) Time series of potential temperature and wind vertical profiles at Wakkanai and (b) wind speed and direction at In situ stations in Rishiri Island on May Wind speeds at the leeward foot of the island (Motodomari) rapidly intensify, which is concurrent with an increase in atmospheric stability in the atmospheric boundary layer. Low-level jet behind the island is connected to downslope winds

Canada A Case Study of a Low-level Jet Formed in the Lee of Rishiri Island on 22 May 2009 Statistical Characteristics of Atmospheric Island Wakes 1. Characteristic of ocean surface winds in the lee of an isolated island

Canada Classification of 115 SAR-estimated wind fields (ERS-1,2 & PALSAR) around Rishiri Island Type A No significant island wakes (17%) Type B Wind shadows in the lee of the island accompanying low-level jets on both sides of the shadow area (27%) Type C Low-level jets formed in the lee of the island, same as the case study on May (19%) Type D Wind shadows in the lee of the island without jets, different from Type B (36%)

Canada Theoretical flow regimes on the non-dimensional mountain height Non-dimensional mountain height: U: Upstream wind speed N: Buoyancy frequency h: Mountain height Mountain height normalized by a scale for the wavelength of a linear two- dimensional mountain wave Inverse Froude number (Fr) A non-dimensional parameter describing a stratified flow passing over a mountain ĥ < 1: small amplitude waves (quasi-linear and weakly nonlinear ranges) ĥ ~ 1: wave breaking → a wave-induced stagnant layer → onset of downslope winds ĥ > 1: upstream stagnation, flow splitting and lee vortices

Canada neutralstable Histograms of occurrence frequency for each type as a function of ĥ. Stratification Low wind speeds Type A : weak ambient flow condition. Types B, C, and D : non-dimensional mountain height (ĥ ) dependence Type B: under the large ĥ (>2.0) flows Type D: under relatively small ĥ (< 1.75) flows Type C: in the transition range of types B and D (1.0 < ĥ < 2.5) U (wind speed)-Nh (buoyancy frequency multiplied by mountain height) diagram for the SAR wind patterns Wake types and their ĥ -dependent transition are qualitatively consistent with the theoretical regimes

Canada Surface windsWind shadows behind the island Low-level jets behind the island Wind shadows behind the island and low-level jets on both sides of the shadow area Non- dimensional mountain height ( ĥ ) ĥ < < ĥ < 2.5 ĥ > 2.0 Theoretical regimes small amplitude waves (quasi-linear and weakly nonlinear ranges) wave breaking ( → downslope winds) flow splitting and lee vortices Schematic view Summary The behavior of the wind field around the island, being sensitive to ĥ, changed drastically around transition zones (1.0 < ĥ < 2.5). Hunt and Snyder (1980JFM) nonlinear nature D C B

Canada 2. Coastal meteological phenomena forced by headlands PALSAR sometimes images strong NRCS streaks extending from headlands under south-easterly ambient flows. PALSAR ScanSAR images on February 2, 2008 and March 13, 2009

Canada Temperature & wind vertical profile from rawinsonde Sapporo & Wakkanai Wakkanai Sapporo Southeasterly stratified flows in the atmospheric boundary layer caped by westerlies in the above later. temperature Potential temperature Mountain height ~100m => Non-dimensional mountain height ~1.9 Condition for wave breaking (downslope winds) regime

Canada Headland heights~1000m at lower layer (~500m): southeasterly => gap winds Hypothesis: layer-dependent orographic forcing at 500~1000m layer: south-southeasterly interacts with mountains in the headlands => onset of downslope winds

Canada Numerical simulations by MM5 The model reproduces intensified wind streak patterns but not co-existence of gap winds and wind streaks. More investigation is needed to clarify the mechanism of orographic forcing.