Mesoscale Weather in Hawaii

Yi-Leng David ChenDepartment of Meteorology

University of HawaiiNovember 20, 2012

Outlines

• 1. Introduction

• 2. Cool season (October-April) mesoscale weather in Hawaii

• 3. Tropical storms

• 4. Summer trade-wind weather

Wang et al. (1998)

• Heavy rains and floods – about 6 per year. In Hawaii, with small watersheds and narrow basins, the time between peak rain and peak discharge may be as little as 15 minutes. Thus, timely detection is extremely important.

• High winds - most of the damaging winds are orographically enhanced.

• High surfs - accounts for most of the weather related fatalities in Hawaii.

• Thunderstorms –Destructive convective winds, lightning strikes, tornadoes, water spouts and hail storms do occur in Hawaii. Most of the threat to life and property during thunderstorm activity is due to heavy rains and flash flooding.

Weather Threats We FaceFlash Flooding

March 3, 2006 Windward Oahu

On the night of January 12, 2004 and lasting through the day of January 13, Hawaii was struck by winds nearly 100 mph during the passage of a cold front.

Other ThreatsHigh Surf

Severe Squall Line

Produced wind gust of 90 kt at Nawili Harbor on Kauai (Businger, S., T. Birchard, K. R. Kodama, P. A. Jendrowski, and J.-J. Wang, W&F 1998).

A tornado took shape and then touched down in a pineapple field in Central O'ahu on January 25, 2005. There were reports almost a dozen funnel clouds in the same area. Lightning strikes, and floodings were also reported in several areas.

• Most of Hawaii storms during the cool season (October-April) are related to one of three classes of synoptic-scale disturbances (Blumenstocks and Price 1967):

*Cold fronts (or windshear lines)

*Kona lows (subtropical cyclones)

*Upper-level troughs

2. Cool Season Mesoscale Weather in Hawaii2a Cold Fronts

Cold fronts (windshear lines) (Larson 1976) extending southwestward to the subtropics (about 9 per years) (Worthly 1967):

Heavy rains and southerly winds frequently occur in the pre-frontal zone.

Strong, gusty northeasterly trade winds dominate after the frontal passage as the postfrontal high pressure system merges with the semipermanent subtropical high and centers north of the Hawaiian Islands (Zhang et al. 2005a).

Blizzards and winter storms at altitude.

(a) (b)

Surface analyses for 1400 HST on (a) 13 February and (b) 14 February 2001 (adapted from the subjective analyses by forecasters at the Weather Forecast Office, Honolulu). Isobars are plotted for every 4 hPa. A strong subtropical high moved to the north of the Hawaiian Islands following the passage of a cold front (a). This high, reaching its maximum strength of 1038 hPa by 1400 HST 14 February (b), brought in strong trade winds across the state of Hawaii. (Zhang et al. 2005a)

Hawaiian Islands

the MSM domain for Oahu at a 1.5-km resolution; (The terrain contour interval is 100 m

Δx=Δy=1.5 km

Honolulu Intl. Airport

24-h accumulated rainfall (mm) from 0000 HST 14 February to 0000 HST 15 February on the island of Oahu: (a) observed, and (b) simulated by the 1.5-km MSM/LSM. Contour interval is 25 mm in (a) and 10 mm in (b). The MSM/LSM was initialized at 1400 HST 13 February 2001. The MSM/LSM captured the observed rainfall on the windward side, but it failed to bring the heaviest rain towards leeside as indicated in observations. Little rainfall was observed along the windward slope of the Waianae Range. The 10-km RSM/LSM predicted negligible rainfall for the entire island. (Zhang et al. 2005a)

Waianae Range Koolau Range

(a) (b)

Skew-T chart for Lihue, Kauai at 1400 HST 14 February 2001 (Zhang et al. 2005a).

Trade-wind inversion

Deep layer of trades

Critical level

(a) (b)

(a) 10-m wind (m s-1) over the Oahu domain and (b) longitude-height cross section along MOA-MOB showing horizontal wind (black lines), equivalent potential temperature (red lines) and pressure vertical velocity (green lines) constructed from the 1.5-km MSM/LSM forecasts valid at 1800 HST 14 February. Isotaches (isentropes) are drawn for every 4 m s-1 (2 ºK). Pressure vertical velocity is drawn for every 5 Pa s-1. The model terrain in this cross section is characterized by two mountain ranges with a similar peak height of 0.6 km. Notice the strong downslope winds (~ 22 m s -

1) along the lee slopes of both mountains with an associated weak hydraulic jump. The MSM/LSM was initialized at 1400 HST 13 February 2001. (Zhang et al. 2005)

2b Kona Storms (Subtropical cyclones)

• Kona means leeward in reference to the leeside of the prevailing northeasterly trades.

• These disturbances are either cut off lows in the westerlies or upper-level cyclones develop in the subtropics (Simpson 1952).

• Kona Storms affecting Hawaiian Islands usually form in the region bounded by 15-350N and 175oE-1400W and move erractically with a slow tendency to the west.

• These systems are persistent, able to last up to two weeks (Ramage 1962).

• These are cold core systems with the strongest circulations in the upper troposphere.

• These storms are notorious producers of heavy rains, high surfs, strong winds (Wang et al. 1998; Businger et al. 1998).

• Erratic track, persistence of these systems and mesoscale organization of precipitation – present a considerable forecasting challenges (Kodama and Businger 1998).

Kona lows contribute >50% of the heavy rain events on the Island of Hawaii

•Monthly distribution of heavy rain events on the SE slope of Mauna Loa by synoptic type (Kodama and Barnes 1997).

Weather Hazards in Kona Lows

•flash floods•high winds•waves and swell•blizzards at altitude•severe thunderstorms

– high winds– large hail– Tornados

(Businger et al. 1998)

Climatology of Kona Lows

•Monthly Variability (1980-2002) (Caruso and Businger 2006)

Morrison and Businger (2001)

•Quickscat Surface WindsGOES Water Vapor loop

October 30, 2004 Manoa Flood with more than 100 million

dollars damage to the Univ. of Hawaii Manoa

campus (A Kona low).

GOES IR at 7 pm 10/30/2004

Radar Image at 7 pm October 30, 2004

24-h rainfall accumulation

2c. Upper-level toughs

• These systems overlie lower troposphere trade-wind flow without a low-pressure center at the surface.

• Could produce an increase in upper-level cloudiness without much effect on Hawaiian weather.

• Downward development of these systems could eliminate trade-wind inversion, allowing deep convection to occur.

• With orographic effects and local winds, localized heavy rains and flash floods could occur (Schroeder 1977)

Upper-level troughs

Kodama and Barnes (1997)

(a)(b)

(a) 250-hPa geopotential height (gpm, solid lines) and winds (m s-1, barbs and dashed lines) analyses based on the NCEP/NCAR Reanalyses data for 1400 HST November 1 2000; (b) surface analyses for 1400 HST November 1 2000 (adapted from the subjective analyses by forecasters at the Weather Forecast Office, Honolulu). Isotaches in (a) are plotted for every 20 m s-1. Isobars in (b) are plotted for every 4 hPa. This flash flood event brought heavy rains to the island of Hawaii (Big Island). Notice that the surface southeasterly was subject to orographic lifting on the southeastern portion of the Big Island (Zhang et al. 2005b).

Hawaiian Islands

Relative humidity (%) at 850 hPa for 1400 HST 1 November 2000 based on the NCEP/NCAR Reanalysis data. RH > 90% is associated

with remnants of tropical storm Paul (Zhang et al. 2005b)

The composite radar image at 0912 HST 2 November 2000 from WSR88D at South Point of the Island of Hawaii

Observed 24-h accumulated rainfall (mm) from 1400 HST 1 November to 1400 HST 2 November 2000. Contours are drawn for 50, 100, 300, 500, 700, and 900 mm. The southeast part of the island and the Hilo area reported rainfall of more than 700 mm with amounts in excess of 900 mm recorded on the southeast rift of the Mauna Loa. (Zhang et al. 2005b)

Hilo area

Mauna Kea

Mauna Loa

3. Tropical Storms over the Central North Pacific (0-30 0N and 140-1800 W)

Tropical Storms over CNP• In Central North Pacific (CNP) the tropical cyclone frequency is 3.5

cyclones per year with tropical storm intensity or greater (Shaw 1981).

• Most tropical cyclones that occur in CNP form and intensify in the eastern North Pacific. Only storms that persist long enough can travel more than 3000 km across 140 W into CNP.

• Most of the tropical cyclones that move across CNP are south of the Hawaiian Islands. Occasionally, the presence of trough in the westerlies can resulting in the turning of storm northeastward or northeastward (Iniki in 1992). In addition to high winds and heavy orographic rains, storm surges are significant threat.

• There are instances that tropical storms move westward into the Hawaiian Islands in the trade-wind flow and dissipate perhaps due to cold SST and the presence of vertical wind shear. These dissipated systems can bring significant rainfall.

3. Summer Trade-Wind Weather 3.1 Island Effects on Airflow

Mechanical effects: orographic blocking (Smolarkiewicz et al. 1988; Rasmussen et al. 1989; Carbone et al. 1998), orographic lifting (Leopold 1949, Garrett 1980), lee vortices (Patzert 1969; Nickerson and Dias 1981;Rotunno and Smolarkiewicz, 1989; Smith and Grubisic 1993), hydraulic jump (Lavoie 1974; Smith and Grubisic 1993), lee waves (Burroughs and Larson 1979); downslope winds (Zhang et al. 2005a); Maui vortex (Leopold 1949; Ueyoshi et al. 1996; Carlis 2007) and etc.

Thermal effects: Land/sea circulations (Leopold 1949; Schroeder 1981; Chen and Nash 1994; Chen and Wang 2004; Yang and Chen 2003; Zhang et al. 2005b), mountain/valley winds (Leopold 1949; Garrett 1980; Feng and Chen 1998; Frye and Chen 2001), rain evaporative cooling (Carbone et al. 1995; 1998; Chen and Wang 1995; Feng and Chen 2001), convective feedbacks (Chen and Feng 2001; Yang et al. 2005; 2007).

The first aircraft probing of Hawaiian wake was done by Nickerson and Dias (1981).

Patzert (1969) first reported the airflow over ocean from 15-yr ship observations with the existence of the lee vortices.

From HaRP data, Smith and Grubisic (1993) show that the wake consists of two elongated counter-rotating eddies and a reverse flow along the wake axis. They also observed decelerating trades, and splitting airflow on the windward side, accelerating winds around the northern and southern tips of the island with weak hydraulic jump.

Chen and Nash (1994; MWR)

Garret (1980)

3.2 Island Effects on Rainfall

• Effects of trade-wind Inversion

• Effects of wind speed

• Effects of diurnal heating cycle

• Effects of trade-wind inversion height on rainfall during the summer months

Big Island (Island of Hawaii) HARP 1990 (Chen and Feng 1995)

HARP (Chen and Feng 1995)

TWI HEIGHT CATEGORIES(days with synoptic disturbances removed)

Low1250<TWI<1500

Normal1750<TWI<2250

High2500<TWI<3500

Note: TWI Height was determined from 12 UTC soundings over Lihue

Effects of TWI height on rainfall for Oahu and Kauai (tops below TWI). (Period of Study 2002 May – July, 2003 May – September)

Hartley and Chen (2010; WAF)

Rainfall Data – 57 Stations

• For the high trade-wind inversion-height category, daily rainfall is greater over most areas of Oahu than for the low inversion-height category with the maximum daily rainfall (> 6 mm) over the ridge tops and daily rainfall frequencies ~100% on the windward slopes of the Ko’olau Mountains. However, days with a higher trade-wind inversion may not produce more rainfall over the Ko’olau Mountains as the maximum correlation between the daily rainfall and inversion height is only about 0.2.

• Effects of trade-wind speed on rainfall during the summer months

Windward side of Hawaii

Daily rainfall totals (mm day-1) for weak (4.8-5.6 m s-1) trade-wind days. Terrain contours every 1 km. (11 July - 24 August 1997-2000) (Esteban and Chen (2008; MWR)

Daily rainfall totals for very strong trade-wind (> 8.2 m s-1) days.

Effect of trade-wind strength on the lee-side rainfall of the island of Hawaii (Yang and Chen 2003).

WIND CATEGORIES(days with synoptic disturbances removed)

ENE Trade winds70º < WD < 91º

Variable WindsWS < 3 m/s

Strong8 < WS < 10

Normal6 < WS < 8

Weak4 < WS < 6

Note: Wind Data from NCEP/ NCAR at (22.5ºN 157.5ºW)

Period of study (2002 May – July, 2003 May – September(Loos 2004)

Effects of trade-wind speed on Oahu and Kauai rainfall

2002 May – July, 2003 May – September(Loos 2004) (Oahu)

Normal

High Inversion Low inversion

• Daily rainfall over Oahu exceeds 8 mm over the Ko’olau Mountains under the strong trade-wind regime, and is significantly less under weak trade-wind (4 mm) and variable-wind (2 mm) flow regimes. However, the maximum correlation between daily trade-wind rainfall and trade-wind speed is only > 0.2. Days with high rainfall generally occur under strong trades, but not all strong trade-wind days produce significant rainfall.

• When forecasting summertime rainfall over Oahu, wind speed and inversion height observations are valuable tools only when upstream cloudiness and trade-wind showers conditions are also considered. The scatter plots between daily rainfall and daily wind speed, and daily rainfall and daily inversion height suggests that trade-wind clouds upstream of the islands are a prerequisite for the development of island-scale rainfall over the islands. The use of satellite and radar observations to monitor upstream cloudiness and trade-wind showers is imperative for short-term rainfall forecast over Oahu.

Effects of the diurnal heating cycle

vertical axis

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•

0510152025303540455055

1 3 5 7 9 11 13 15 17 19 21 23

STN. 24STN. 25STN. 31

a. Windward Foothills

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1 3 5 7 9 11 13 15 17 19 21 23

STN. 7STN. 19STN. 57STN. 58STN. 59

b. Waianae

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1 3 5 7 9 11 13 15 17 19 21 23

STN. 13STN. 20STN. 21STN. 22STN. 28STN. 30

c. Ko'olau Mountains

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1 3 5 7 9 11 13 15 17 19 21 23

STN. 10STN. 16STN. 34STN. 60

d. Central Oahu

 

Hourly Rainfall Frequencies under variable winds(%)