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Orographically -forced coastal wind fields around Hokkaido, Japan

Orographically -forced coastal wind fields around Hokkaido, Japan. Osamu Isoguchi (RESTEC) ● Masanobu Shimada (JAXA/EORC). Orographically-forced coastal wind phenomena revealed by SAR wind data. Characteristic of ocean surface winds in the lee of an isolated island

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Orographically -forced coastal wind fields around Hokkaido, Japan

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

  2. Orographically-forced coastal wind phenomena revealed by SAR wind data • 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

  3. 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.

  4. 1. Characteristic of ocean surface winds in the lee of an isolated island • 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

  5. A Case Study of a Low-level Jet Formed in the Lee of Rishiri Island on 22 May 2009 Temperature & wind vertical profile from rawinsonde soundings @ Wakkanai PALSAR-derived wind fields temperature Potential temperature Temperature inversion at 200-500m layer: stable condition at lower layer Black arrows：Reanalysis winds White arrows：In situ winds 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.

  6. In situ stations Motodomari Kutsugata (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 19-24 PALSAR acquisition • 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

  7. 1. Characteristic of ocean surface winds in the lee of an isolated island • 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

  8. 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 22 2009 (19%) • Type D • Wind shadows in the lee of the island without jets, different from Type B (36%)

  9. A non-dimensional parameter describing a stratifiedflow passing over a mountain 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) Theoretical flow regimes on the non-dimensional mountain height 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

  10. Low wind speeds U (wind speed)-Nh (buoyancy frequency multiplied by mountain height) diagram for the SAR wind patterns Histograms of occurrence frequency for each type as a function of ĥ. Stratification neutral stable • 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) Wake types and their ĥ -dependent transition are qualitatively consistent with the theoretical regimes

  11. Summary D C B nonlinear nature Hunt and Snyder (1980JFM) • The behavior of the wind field around the island, being sensitive to ĥ, changed drastically around transition zones (1.0 < ĥ < 2.5).

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

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

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

  15. 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.

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