2011.7.27 IGARSS2011@Vacouver, Canada Orographically-forced coastal wind fields around Hokkaido, Japan Osamu Isoguchi (RESTEC) ● Masanobu Shimada (JAXA/EORC)

2011.7.27 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 IGARSS2011@Vacouver, Canada

Orographically-forced coastal wind fields around Hokkaido, Japan

Osamu Isoguchi (RESTEC)●Masanobu Shimada (JAXA/EORC)


1.Characteristic of ocean surface winds in the lee of an isolated islandisoguchi 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


Characteristic of ocean surface winds in the lee of an isolated islandObjective: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.


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


Temperature & wind vertical profile from rawinsonde soundings @ Wakkanai

tem

pera

tu

re

Pote

nti

al te

mpera

ture

Temperature inversion at 200-500m layer: stable condition at lower layer


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


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 19-24

• 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



Statistical Characteristics of Atmospheric Island Wakes

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


Classification of 115 SAR-estimated wind fields (ERS-1,2 & PALSAR) around Rishiri IslandType 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%)


Theoretical flow regimes on the non-dimensional mountain height

Non-dimensional mountain height:

U

Nhhˆ

U: Upstream wind speedN: Buoyancy frequencyh: 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 windsĥ > 1: upstream stagnation, flow splitting and lee vortices


neutral stable

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


Surface winds

Wind 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 < 1.75

1.0 < 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


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


Temperature & wind vertical profile from rawinsonde soundings @ Sapporo & Wakkanai

Wakkanai

Sapporo

Southeasterly stratified flows in the atmospheric boundary layer caped by westerlies in the above later.

tem

pera

tur

ePo

tenti

al te

mpera

ture

Mountain height ~100m => Non-dimensional mountain height ~1.9Condition for wave breaking (downslope winds) regime


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


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.

Documents

2011.7.27 IGARSS2011@Vacouver, Canada Orographically-forced coastal wind fields around Hokkaido, Japan Osamu Isoguchi (RESTEC) ● Masanobu Shimada (JAXA/EORC)