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During the early summer rainy season over Taiwan, three types of low-level jets are observed, including a synoptic low-level jet (SLLJ) situated in the 850–700 hPa layer in the frontal zone, a marine boundary layer jet (MBLJ) embedded within the southwesterly monsoon flow over the northern South China Sea at approximately the 925 hPa level, and an orographically induced jet at approximately the 1 km level off the northwestern Taiwan coast (e.g., barrier jet (BJ)). The characteristics and physical processes of the formation of these three types of low-level jets are reviewed, and their roles in the development of heavy rainfall are discussed.

From 0200 to 1000 LST 2 June 2017, the shallow, east–west-oriented mei-yu front (600 mm) occurs over the northern side of the Yang-Ming Mountains. From 1100 to 1200 LST, with the gradual deepening of the postfrontal cold air, the front finally passes over the Yang-Ming Mountains and arrives at the Taipei basin, which results in an east–west-oriented rainband with the rainfall maxima over the northwestern coast and Taipei basin. From 1300 to 1400 LST, the frontal rainband continues to move southward with rainfall over the northwestern slopes of the Snow Mountains. In the prefrontal southwesterly flow, the orographic lifting of the moisture-laden low-level winds results in heavy rainfall on the southwestern slopes of the Snow Mountains and the Central Mountain Range. With the terrain of the Yang-Ming Mountains removed in the high-resolution model, the mei-yu front moves quickly southward without a rainfall maximum over the northern tip of Taiwan.

Drought is a prominent feature of Hawaiʻi’s climate. However, it has been over 30 years since the last comprehensive meteorological drought analysis, and recent drying trends have emphasized the need to better understand drought dynamics and multi-sector effects in Hawaiʻi. Here, we provide a comprehensive synthesis of past drought effects in Hawaiʻi that we integrate with geospatial analysis of drought characteristics using a newly developed 100-year (1920–2019) gridded Standardized Precipitation Index (SPI) dataset. The synthesis examines past droughts classified into five categories: Meteorological, agricultural, hydrological, ecological, and socioeconomic drought. Results show that drought duration and magnitude have increased significantly, consistent with trends found in other Pacific Islands. We found that most droughts were associated with El Niño events, and the two worst droughts of the past century were multi-year events occurring in 1998–2002 and 2007–2014. The former event was most severe on the islands of O’ahu and Kaua’i while the latter event was most severe on Hawaiʻi Island. Within islands, we found different spatial patterns depending on leeward versus windward contrasts. Droughts have resulted in over $80 million in agricultural relief since 1996 and have increased wildfire risk, especially during El Niño years. In addition to providing the historical context needed to better understand future drought projections and to develop effective policies and management strategies to protect natural, cultural, hydrological, and agricultural resources, this work provides a framework for conducting drought analyses in other tropical island systems, especially those with a complex topography and strong climatic gradients.

Short-lived afternoon heavy rainfall events may form over central Oahu during seasonal transition periods (June and October) under favorable large-scale settings. These include a deep moist layer with relatively high precipitable water (>40 mm), blocking pattern in midlatitudes with a northeast–southwest moist tongue from low latitudes ahead of an upper-level trough, absence of a trade wind inversion, and weak (−1) low-level winds. Our high-resolution (1.5 km) model results show that immediately before the storm initiation, daytime land surface heating deepens the mixed layer over central Oahu and the top of the mixed layer reaches the lifting condensation level. Meanwhile, the development of onshore/sea-breeze flows, driven by land–sea thermal contrast, brings in moist maritime air over the island interior. Finally, convergence of onshore flows over central Oahu provides the localized lifting required for the release of instability. Based on synoptic and observational analyses, nowcasting with a lead time of 2–3 h ahead of this type of event is possible. In the absence of orographic effects after removing model topography, processes that lead to heavy rainfall are largely unchanged, and subsequent development of heavy showers over central Oahu are still simulated. However, when surface heat and moisture fluxes are turned off, convective cells are not simulated in the area. These results indicate that daytime heating is crucial for the development of this type of heavy rainfall event under favorable large-scale settings.

During the wintertime, easterly (E) to southeasterly (SE) flow in the Hawaiian coastal waters is frequent. These wind regimes alter the location and magnitude of channel and tip jet accelerations and the orientation and horizontal extent of the wake zones from east-northeast (ENE) trade wind conditions. The differences are the result of changes in orographic blocking by the Big Island and Maui, with respect to the prevailing wind. During an E wind event, the fastest winds over the ‘Alenuihāhā Channel (>9 m s−1) occur in the channel exit with sinking of the inversion, which rises again downstream. Although the upstream wind speed is similar to typical summer ENE trade winds (7–8 m s−1), the maximum channel wind speed is 3–4 m s−1 slower in the exit. The SE flow is characterized by maximum (~6 m s−1) northeasterly (NE) channel winds along Maui’s south shore and at the channel exit. These winds are the result of orographic blocking on the eastern end of Maui as the northwestern tail of a tip jet off the northeastern coast of the Big Island impinges on Mount Haleakalā. Channel wind speeds are modulated by the speed and direction of this tip jet, which itself varies diurnally and throughout the approach of a midlatitude cold front. Removal of the Big Island shows how the tip jet speed and orientation modulate the pressure gradients and winds in the ‘Alenuihāhā Channel. Removal of the Maui County terrain reveals the impact of orographic blocking on the occurrence of channel winds off Maui’s south shore.

Tracks of tropical cyclones (TCs) forming in the South China Sea (SCS) are examined in terms of their seasonal variability and associated modulations by seasonal and intraseasonal circulations. Three major TC tracks appear in the SCS: northeastward (NE) toward the Philippines and Taiwan, northward (N) toward southern China, and westward-northwestward (W-NW) toward the Indochina Peninsula and southwestern China. Seasonally, the dominant tracks are NE in May, from NE into W-NW in June, W-NW and N in July to September (JAS), and W-NW in October to November (ON). Formation of TCs with W-NW tracks is associated with a southeastward-extending monsoon trough (MT) and a northwestward-expanding western Pacific subtropical high (WPSH) in May to June (MJ), a southward-intensified MT and a westward-strengthened WPSH in JAS, and a northward and westward enhanced equatorial trough in ON. Formation of TCs with NE tracks in MJ is related to an eastward-intensified MT and an eastward-retreated WPSH. For TCs with N tracks, their formation is associated with an eastward and northward intensification of the MT and a northwestward extension of the WPSH. In terms of modulations of intraseasonal oscillations (ISOs), the 10-24-day and 30-60-day ISOs exert different effects on TC movement. For all track types, TCs tend to follow the propagation of the 10-24-day cyclonic anomaly and move along different tracks. The above movement comes under the influence of favorable environments in terms of moisture convergence provided by 30-60-day cyclonic anomalies distributing along TC tracks. The 10-24-day and 30-60-day ISOs exhibit comparable intensity to jointly modulate TC activity in the SCS.

Satellite observations and high-resolution modeling during July–August 2013 are used to study the effects of trade wind strength on island wake circulations and cloudiness over O‘ahu, Hawai‘i. O‘ahu is composed of two northwest–southeast orientated mountain ranges: the Wai‘anae Range (~1227 m) along the western leeside coast and the Ko‘olau Range (~944 m) along the eastern windward coast. At night, the flow deceleration of the incoming northeasterly trade winds on the eastern windward side is more significant when trades are stronger.In the afternoon hours, effective albedo and simulated cloud water are greater over the Ko‘olau Range when trades are stronger, and clouds are advected downstream by the trade winds aloft. Over the Wai‘anae Range, orographic clouds are more significant when trades are weaker due to less moisture removal by orographic precipitation over the Ko‘olau Range and the development of both upslope flow on the eastern slope and upslope/sea-breeze flow along the western coast, the latter of which brings in warm, moist air from the ocean. When trades are weaker, cloudiness off the western leeside coast is more extensive and originates from orographic cloud development over the Wai‘anae Range, which drifts downstream due to a combination of trade winds and the easterly return flow aloft. The latter is associated with the low-level sea-breeze/upslope flow.

Drought is a prominent feature of Hawaii's climate, however, the biological, ecological, cultural, and socioeconomic impacts of drought in Hawaii are not well understood. This paper provides a comp...

The marine boundary layer jets (MBLJs) over the northern South China Sea during the early summer rainy season over Taiwan are analyzed using 5-yr (2008–12) National Centers for Environmental Prediction Climate Forecast System Reanalysis data with a 6-h interval. The MBLJ is distinctly different from the low-level jets associated with the subsynoptic frontal systems. During this period, the MBLJ events over the northern South China Sea mainly occur during the second half of the monsoon rainy season over Taiwan (after 1 June) and have a wind speed maximum around the 925-hPa level. The MBLJs are mainly related to the subsynoptic-scale pressure gradients related to a relatively deep mei-yu trough over southeastern China and a stronger-than-normal west Pacific subtropical high. Within the MBL, there is a three-way balance among pressure gradients, Coriolis force, and surface friction, with cross-isobar ageostrophic winds pointing toward the mei-yu trough throughout the diurnal cycle. At the jet core, the vertical wind profile resembles an Ekman spiral with supergeostrophic winds >12 m s−1 near the top of the MBL. The MBLJs are strongest at night and close to geostrophic flow in the late afternoon/early evening. This is because the friction velocity and ageostrophic wind decrease during daytime in response to mixing in the lowest levels. The MBLJs play an important role in horizontal moisture transport from the northern South China Sea to the Taiwan area. In the frontal zone, the moisture tongue extends vertically upward. The rainfall production is related to vertical motions in the frontal zone or localized lifting due to orographic effects.

During 11–12 June 2012, heavy precipitation occurred over the northwestern Taiwan coast (~435 mm) and within the Taipei basin (~477 mm). With the presence of a midlatitude omega-blocking pattern, a persistent cold northerly wind component west of the northeast China low and west of the mei-yu frontal cyclone extends all the way to the subtropics and up to the 700-hPa level. At 2000 LST 11 June, the total precipitable water ahead of the front is elevated (>70 kg m−2) with horizontal southwesterly moisture fluxes >360 g kg−1 m s−1 at the 950-hPa level. The rainfall maximum along the northwestern coast mainly occurs before 0200 LST 12 June, as the convective activities in the frontal zone are enhanced by the localized convergence between the prefrontal southerly barrier jet and environmental airflow. After landfall, the relatively deep (~1.5 km) mei-yu front moves over the mountains (with peaks ~1121 m) along the northern coast and into the Taipei basin. During 0200–0800 LST 12 June, it stalls at the foothills of the Snow Mountains (with peaks ~3886 m) south of the basin under the postfrontal west-northwesterly flow. Rain cells associated with the mei-yu front are enhanced as they move southeastward toward the Snow Mountains. The barrier jet and the rainfall maxima over the northwestern coast and within the Taipei basin are well simulated using the high-resolution WRF Model. With the model terrain removed, the simulated mei-yu front continues to move southward after landfall without reproducing the barrier jet and both observed rainfall maxima.

Two trapped lee-wave events dominated by the transverse mode downstream of the island of Oahu in Hawaii—27 January 2010 and 24 January 2003—are simulated using the Weather Research Forecasting (WRF) Model with a horizontal grid size of 1 km in conjunction with the analyses of soundings, weather maps, and satellite images. The common factors for the occurrences of these transverse trapped mountain-wave events are 1) Froude number [Fr = U/(Nh)] > 1, where U is the upstream speed of the cross-barrier flow, N is stability, and h is the mountain height; 2) insufficient convective available potential energy for the air parcel to become positively buoyant after being lifted to the top of the stable trade wind inversion layer; and 3) increasing cross-barrier wind speed with respect to height through the stable inversion layer, satisfying Scorer’s criteria between the inversion layer and the layer aloft. Within the inversion layer, where the Scorer parameter has a maximum, the wave amplitudes are the greatest. The two trapped mountain waves in winter occurred under strong prefrontal stably stratified southwesterly flow. On the other islands in Hawaii, where the mountaintops are below the base of the inversion, transverse trapped lee waves can occur under similar large-scale settings if the mountain height is lower than U/N. The high-spatial-and-temporal-resolution WRF Model successfully simulates the onset, development, and dissipation of these two events. Sensitivity tests for the 27 January 2010 case are performed with reduced relative humidity (RH). With a lower RH and less-significant latent heating, trapped lee waves have smaller amplitudes and shorter wavelengths.

A cycling run, which began 36 h before the model forecast, was employed to assimilate special Terrain-influenced Monsoon Rainfall Experiment (TiMREX) soundings, Global Telecommunications System (GTS) data, and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) global positioning system (GPS) radio occultation (RO) refractivity profiles to improve the model initial conditions provided by the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) to study a coastal, heavy rainfall event over southwestern Taiwan during 15–16 June 2008. The 36-h cycling run with data assimilation (DA_ALL_DATA run) has a positive impact on the depiction of subsynoptic flow in the model initial conditions at 1200 UTC 15 June, including the warm moist tongue and southwesterly monsoon flow over the open ocean. Furthermore, the cold pool caused by the evaporative cooling of antecedent rains and orographic blocking over southwestern Taiwan are better resolved in the nested high-resolution domain in the DA_ALL_DATA run as compared to the initial conditions provided by the NCEP GFS. As a result, the heavy rainfall along the southwestern coast and afternoon localized heavy rainfall over northern Taiwan are better predicted in the DA_ALL_DATA run. Model sensitivity tests are also performed to diagnose the effects of terrain and rain-evaporative cooling on the intensity and depth of the cold pool and degree of orographic blocking on the southwesterly flow over southwestern Taiwan. It is apparent that including rain-evaporative cooling from antecedent rains and orographic effects in the model initial conditions are important to account for the predicted rainfall distribution of this coastal rainfall event.

Global positioning system radio occultation (GPS RO) refractivity data obtained from the first Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) for the years 2007 to 2012 were used to estimate an overall climatology for the height of the marine boundary layer (MBL) over the central North Pacific Ocean including the Hawaiian Island region (10°N–45°N; 125°W–175°W). The trade wind days are identified based on the six-year National Centers for Environmental Prediction (NCEP) global analysis for the same period. About 87% of the RO soundings in summer (June–July–August, JJA) and 47% in winter (December–January–February, DJF) are under trade wind conditions. The MBL height climatology under trade wind conditions is derived and compared to the overall climatology. In general, MBL heights are lowest adjacent to the southern coast of California and gradually increase to the south and west. During the summer (JJA) when the northeasterly trade winds are the dominant surface flow, the median MBL height decreases from 2.0 km over Kauai to 1.9 km over the Big Island with an approximate 2 km maximum that progresses from southwest to northeast throughout the year. If the surface flow is restricted to trade winds only, the maximum MBL heights are located over the same areas, but they increase to a median height of 1.8 km during DJF and 2.1 km during JJA. For the first time, the GPS RO technique allows the depiction of the spatial variations of the MBL height climatology over the central North Pacific.

The complex wave climate of Hawaii includes a mix of seasonal swells and wind waves from all directions across the Pacific. Numerical hindcasting from surface winds provides essential space-time information to complement buoy and satellite observations for studies of the marine environment. We utilize WAVEWATCH III and SWAN (Simulating WAves Nearshore) in a nested grid system to model basin-wide processes as well as high-resolution wave conditions around the Hawaiian Islands from 1979 to 2013. The wind forcing includes the Climate Forecast System Reanalysis (CFSR) for the globe and downscaled regional winds from the Weather Research and Forecasting (WRF) model. Long-term in-situ buoy measurements and remotely-sensed wind speeds and wave heights allow thorough assessment of the modeling approach and data products for practical application. The high-resolution WRF winds, which include orographic and land-surface effects, are validated with QuickSCAT observations from 2000 to 2009. The wave hindcast reproduces the spatial patterns of swell and wind wave events detected by altimeters on multiple platforms between 1991 and 2009 as well as the seasonal variations recorded at 16 offshore and nearshore buoys around the Hawaiian Islands from 1979 to 2013. The hindcast captures heightened seas in interisland channels and around prominent headlands, but tends to overestimate the heights of approaching northwest swells and give lower estimates in sheltered areas. The validated high-resolution hindcast sets a baseline for future improvement of spectral wave models.

A strong coastal boundary layer jet (CBLJ) (~8 m s−1) off the southeastern coast of China (around 28°N, 122°E) is found from the July 2006–11 hourly model data simulated by the Advanced Research Weather Research and Forecasting Model (WRF-ARW) with a 9-km horizontal grid. The southerly CBLJ has a jet core at the 925-hPa level, located along the western periphery of the west Pacific subtropical high (WPSH). The CBLJ is mainly contributed by large-scale enhancement by diurnal forcing and orographic effects by the coastal terrain along the southeastern China coast and the terrain of Taiwan. Although the geostrophic winds offshore are faster in the afternoon due to the larger east–west pressure gradient caused by land surface heating over the China plain, the CBLJ has a nocturnal (~0200 LST) maximum. In the afternoon hours, easterly ageostrophic winds driven by differential land–sea thermal heating develop at low levels. After sunset, with the disappearance of land surface heating, the ageostrophic winds offshore veer southward by the Coriolis force and combine with the southerly geostrophic flow resulting in a nocturnal maximum in the CBLJ. Furthermore, from two model sensitivity experiments (NoTW and LowFJ), it is apparent that the terrain of Taiwan and Fujian exerts a secondary influence (1–2 m s−1) on the strength of the CBLJ. The orographic blocking by the terrain of Taiwan and Fujian is more significant with a larger (~1 m s−1) southerly wind component north of the Taiwan Strait at night than in the afternoon hours.

During the summer, sustained winds in the ‘Alenuihāhā Channel, Hawaii, may exceed 20 m s−1 with higher gusts. The Advanced Research Weather Research and Forecasting model is used to diagnose airflow in the Hawaiian coastal waters. High-resolution (2 km) runs are performed for July 2005 covering the ‘Alenuihāhā Channel and nested in a 6-km state domain. Under normal trade wind conditions (7–8 m s−1), winds at the channel entrance are 1–2 m s−1 faster than upstream due to the convergence of the deflected airflows by the islands of Maui and Hawaii, and accelerate through the channel due to along-gap pressure gradients and lower pressure in the wakes of both islands. The acceleration is accompanied by descending airflow (>9 cm s−1) in the exit region with lowering of the trade wind inversion. Deceleration occurs downstream of the channel exit with a rapid change from sinking motion to rising motion (>3 cm s−1). Under normal or strong trade wind conditions, the flow is subcritical [Froude number (Fr) < 1] upstream of the channel, supercritical (Fr > 1) in the exit region, and subcritical again (Fr < 1) downstream with a weak hydraulic jump. The localized sinking motion on the lee side of bordering ridgelines (>1 m s−1) is most significant in the afternoon hours and results in warming and lowering of surface pressure on the lee side, into the channel, and farther downstream. As a result, the channel winds and the wind speed maximum along the southeastern coast of Maui exhibit an afternoon maximum.

This study makes improvements to the tropical cyclone (TC) initialization method introduced by Nguyen and Chen (i.e., the NC2011 scheme). The authors found that prescribing sea level pressure associated with the initial vortex using a modified Fujita formula has very little impact on the vortex structure and intensity during a series of 1-h model integration and relocation. On the other hand, inserting an artificial warm core makes the vortex spin up much faster. When a warm core is inserted during the initial spinup process, the computational time required for model initialization is reduced by ½–⅓. Because prescribed sea level pressure is not required to spin up the vortex, information on vortex size, such as radius of maximum wind, is no longer needed. The performance of the improved NC2011 scheme with an initial prescribed warm core during the initial spinup process is tested for typhoons that made landfall over southern China or Vietnam in 2006. Before landfall, these storms were over the open ocean where conventional data were sparse, without special observations. Two sets of model runs, with (NC2011-CTRL) and without (CTRL) vortex initialization, are performed for comparison. The initial and time-dependent boundary conditions are from the NCEP Final Analyses (FNL). There are twelve 48-h simulations in each run set. Results show that the vortex initialization improves TC track and intensity simulations.

The detailed spatial distributions and diurnal variations of low-level jets (LLJs) during early summer (May–July) in China are documented using 2006–11 hourly model data from the Weather Research and Forecasting (WRF) Model with a 9-km horizontal resolution. It was found that LLJs frequently occur in the following regions of China: the Tarim basin, northeastern China, the Tibetan Plateau (TP), and southern China. The LLJs over China are classified into two types: boundary layer jets (BLJs, below 1 km) and synoptic-system-related LLJs (SLLJs, within 1–4 km). The LLJs in the Tarim basin and the TP are mainly BLJs. The SLLJs over southern China and northeastern China are associated with the mei-yu front and northeast cold vortex (NECV), respectively. The BLJs in all regions show pronounced diurnal variations with maximum occurrences at nighttime or in the early morning, whereas diurnal variations of SLLJs vary, depending on the location. From the analysis of model data, the diurnal variation of BLJs is mainly caused by inertial oscillation at nighttime and vertical mixing in the boundary layer during daytime. Over northeastern China, SLLJ occurrences show little diurnal variation. Over southern China, two diurnal modes of SLLJs, propagation and stationary, exist and have seasonal variations, which is generally consistent with diurnal variations of precipitation.

Two contrasting localized heavy rainfall events during Taiwan’s early summer rainy season with the daily rainfall maximum along the windward mountain range and coast were studied and compared using a combination of observations and numerical simulations. Both events occurred under favorable large-scale settings including the existence of a moisture tongue from the tropics. For the 31 May case, heavy rainfall occurred in the afternoon hours over the southwestern windward slopes after a shallow surface front passed central Taiwan. The orographic lifting of the prevailing warm, moist, west-southwesterly flow aloft, combined with a sea breeze–upslope flow at the surface provided the localized lifting needed for the development of heavy precipitation. On 16 June before sunrise, pronounced orographic blocking of the warm, moist, south-southwesterly flow occurred because of the presence of relatively cold air at low levels as a result of nocturnal and rain evaporative cooling. As a result, convective systems intensified as they moved toward the southwestern coast. During the daytime, the cold pool remained over southwestern Taiwan without the development of onshore/upslope flow. Furthermore, with a south-southwesterly flow aloft parallel to terrain contours, orographic lifting aloft was absent and preexisting rain cells offshore diminished after they moved inland. Over northern Taiwan on the lee side, a sea breeze/onshore flow developed in the afternoon hours, resulting in heavy thundershowers. These results demonstrate the importance of diurnal and local effects on determining the location and timing of the occurrences of localized heavy precipitation during the early summer rainy season over Taiwan.

The scheme developed by Nguyen and Chen (2011) is used to produce 18 TCs (2004−2013) over the Northwestern Pacific that are well adjusted to the environment. The environment, including SST, in which the storm is embedded has a significant effect on the intensity and rainband patterns of these TCs. During the early season, TCs have a tendency to exhibit a “9” type asymmetric structure with an upper-level outflow channel extending southwestward from the southeastern quadrate of the storm. At low levels, the convergence area between the TC circulation and the southwesterly monsoon flow is a favorable location for the development of spiral rainbands. Late season TCs have a tendency to produce a “6” type storm structure with an outflow channel extending northeastward from the northwestern part of the eyewall, especially when an upper-level cold low or trough is present to the northwest of the storm. At low levels, the convergence of the northeasterly monsoon flow and the TC circulation is favorable for the occurrences of spiral rainbands. For intense TCs that underwent an eyewall replacement cycle, the scheme also shows considerable skill in reproducing the concentric eyewall structure. (Citation: Chen, C., Y. Chen, and H. V. Nguyen, 2014: The spin-up process of a cyclone vortex in a tropical cyclone initialization scheme and its impact on the Initial TC structure. SOLA, 10, 93−97, doi:10.2151/sola.2014-019).

Hawaii's access to the ocean and remoteness from fuel supplies has sparked an interest in ocean waves as a potential resource to meet the increasing demand for sustainable energy. The wave resources include swells from distant storms and year-round seas generated by trade winds passing through the islands. This study produces 10 years of hindcast data from a system of mesoscale atmospheric and spectral wave models to quantify the wind and wave climate as well as nearshore wave energy resources in Hawaii. A global WAVEWATCH III (WW3) model forced by surface winds from the Final Global Tropospheric Analysis (FNL) reproduces the swell and seas from the far field and a nested Hawaii WW3 model with high-resolution winds from the Weather Research Forecast (WRF) model capture the local wave processes. The Simulating Waves Nearshore (SWAN) model nested inside Hawaii WW3 provides data in coastal waters, where wave energy converters are being considered for deployment. The computed wave heights show good agreement with data from satellites and buoys. Bi-monthly median and percentile plots show persistent trade winds throughout the year with strong seasonal variation of the wave climate. The nearshore data shows modulation of the wave energy along the coastline due to the undulating volcanic island bathymetry and demonstrates its importance in selecting suitable sites for wave energy converters.

nteraction among trade winds,terrain, land thermal effects, andthe trade-wind inversion give theHawaiian Islands one of the mostvaried rainfall patterns on Earth.Distinct, persistent patterns of upliftlead to dramatic rainfall gradientsand, together with elevation-relatedtemperature differences, producenearly the full range of climate types.This microcosm of global environ-mental diversity provides a uniquenatural laboratory for world-classresearch on topics such as terres-trial ecosystem carbon dynamics, soilgeochemistry, and the mechanics ofspecies invasion. Knowledge of meanrainfall patterns in Hawai'i is criticallyimportant in support of these researchendeavors as well as for managing andprotecting groundwater and surfacewater resources, controlling and eradicating invasivespecies, protecting and restoring native ecosystems,and planning for the effects of global climate change.The Rainfall Atlas of Hawai'i is a set of digitalmaps of the spatial patterns of 1978-2007 meanmonthly and annual rainfall for the major Hawaiian

This study investigates a long-duration mesoscale system with extremely heavy rainfall over southwest Taiwan during the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). This mesoscale convective system develops offshore and stays quasi-stationary over the upstream ocean and southwest coast of Taiwan. New convection keeps developing upstream offshore but decays or dies after moving into the island, dropping the heaviest rain over the upstream ocean and coastal regions. Warm, moist, unstable conditions and a low-level jet (LLJ) are found only over the upstream ocean, while the island of Taiwan is under the control of a weak cold pool. The LLJ is lifted upward at the boundary between the cold pool and LLJ. Most convective clusters supporting the long-lived rainy mesoscale system are initiated and develop along that boundary. The initiation and maintenance is thought to be a “back-building–quasi-stationary” process. The cold pool forms from previous persistent precipitation with a temperature depression of 2°–4°C in the lowest 500 m, while the high terrain in Taiwan is thought to trap the cold pool from spreading or moving. As a result, the orography of Taiwan is “extended” to the upstream ocean and plays an indirect effect on the long-duration mesoscale system.

The anomalous circulation patterns during an unusually prolonged stormy-weather period in Hawaii from 19 February to 2 April 2006 are analyzed and are compared with those of two previously known prolonged heavy-rainfall periods (March 1951 and February 1979). The circulation patterns for these three periods are characterized by 1) a negative Pacific–North American (PNA) pattern in the midlatitudes with a blocking high southwest of the Aleutian Islands, 2) retraction and splitting of the zonal jet into a polar jet north of 50°N and a persistent subtropical jet to the south over the central Pacific Ocean, 3) an anomalous low west of the Hawaiian Islands embedded in the subtropical jet, and 4) a weaker-than-normal Hadley circulation in the mid-Pacific. The moisture advected from low latitudes by the southerly wind component east of the persistent anomalous low, combined with upward motion, provides the large-scale setting for the unusually prolonged unsettled weather across the Hawaiian Islands. For all three cases, the prolonged stormy weather started after the onset of large-scale blocking and a negative PNA pattern over the North Pacific and the occurrence of a persistent anomalous low embedded in the subtropical jet west of the Hawaiian Islands. Furthermore, the persistent low was located at the optimal position to bring moisture from the central equatorial Pacific to Hawaii. The stormy weather ceased after the midlatitude blocking pattern weakened and the anomalous low in the subtropics decayed and/or shifted westward. There are no apparent common precursors in the 2-week period prior to the prolonged stormy weather among these three cases, however.

An unusual heavy coastal rainfall event (>231 mm day−1) occurred during the period of 24–25 June 1987 over the lowland (elevation less than 200 m) and coastal areas in northwest and central Taiwan. The Weather Research and Forecasting (WRF) model is used to investigate the role of synoptic forcing, orographic effects and the diurnal heating cycle on the generation of a prefrontal localized low-level convergence zone offshore leading to the observed coastal rainfall maximum. This case is well simulated by the control experiment initialized at 0000 UTC (0800 LST) 24 June 1987 using the European Centre for Medium-Range Weather Forecasts data. A model sensitivity test without Taiwan’s terrain fails to reproduce the observed coastal rainfall maximum. It is apparent that for this case, synoptic forcing by the Mei-Yu jet/front system is inadequate to initiate deep convection leading to the development of coastal heavy precipitation. The generation of the localized low-level convergence zone is closely related to the simulated strong winds with a large southwesterly wind component (or the barrier jet) along the northwestern coast as the surface front approaches. The development of the simulated barrier jet is due to a 50–60% increase in the meridional pressure gradient as a result of orographic blocking. The diurnal heating cycle also impacts the strength of the simulated barrier jet over the northwestern Taiwan coast. The simulated barrier jet is stronger (~3 m s−1) in the early morning than in the afternoon as orographic blocking is most significant when the surface air is the coldest. The representation of the terrain in the model impacts the simulated barrier jet and rainfall. With a coarse horizontal resolution (45 km), orographic blocking is less significant than the control run with a much weaker meridional wind component over the northwestern coast of Taiwan. The coarse resolution model fails to reproduce the observed rainband off the northwestern coast. Thus, to successfully simulate this type of event, high-resolution mesoscale models adequately depicting Taiwan’s terrain are required.

During the 2006 wet period, as eastward-moving transient disturbances passed through a semipermanent low pressure system west of Hawaii, southerly winds east of the low strengthened bringing in higher than usual amounts of moisture from the deep tropics to Hawaii. All five heavy rainfall episodes during the wet period occurred during a southerly wind regime. Favorable conditions for the development of the Kahala Mall flood case on 31 March 2006 are examined. A high low-level θe axis across Hawaii indicated the existence of convective instability over Hawaii. A transient midlatitude trough extending southward merged with the semipermanent subtropical trough. The tropopause folding associated with the deepening subtropical trough contributed to the spinup of the Kona low. The advection of high-PV air in the upper troposphere enhanced upward motion downstream over Hawaii. The Weather Research and Forecasting Model (WRF) simulation shows that latent heat release contributed to an eastward shift of the moisture tongue and enhanced moisture convergence at low levels. The horizontal distributions of instability indices, especially the K index, from WRF modeling can provide useful forecast guidance for the development of heavy rainfall. On 31 March, heavy rainfall occurred on the lee side of the Ko’olau Mountain Range with maximum rainfall at the summit as a convective line followed by an intense storm moved inland along the south shore and continued to advance northward through the range. As the convective cells moved across the mountain range, radar echoes intensified with deeper echo tops and higher vertically integrated liquid water content.

A model self-bogus vortex is constructed by cycle runs using the Weather Research and Forecasting (WRF) model to provide high-resolution initial conditions for tropical cyclone (TC) simulations. The vortex after 1 h of model simulation is used to construct the vortex structure for the initial conditions for the next cycle run. After about 80 cycle runs, the TC structure is well adapted to the model employed and well adjusted to the given large-scale conditions. Three separate simulations using three different initial conditions including global analysis (CTRL), the bogus package from WRF (WB), and the new initialization package (NT) are performed for Typhoon Morakot (2009). The NT scheme shows advantages in generating realistic vortex features including sea level pressure, winds, a warm core, and correct TC size with the meteorological fields away from the observed TC center consistent with the global analysis. The NT scheme also shows significant improvements in TC simulations including asymmetric structure, track, intensity, strength of low-level winds, radar reflectivity, and rainfall. For other runs, such as WB and CTRL, the unbalanced initial vortex needs to adjust to the changing environment during the first 2–3 days of model simulations, which is likely to have negative impacts on the track, intensity, and rainfall forecasts in most cases. For all three different types of model initializations, the model is capable of simulating heavy orographic precipitation over southern Taiwan. However, with a better track forecast, only the NT run simulates the high-reflectivity band associated with the convergence zone between Morakot’s circulations and the southwest monsoon off the southeast coast. In addition to Morakot’s slow movement and relatively large size, Typhoons Goni and Etau were embedded within a moist monsoon gyre. The combined circulations associated with the monsoon gyre and tropical storms bring in moisture-laden flows toward the western slopes of southern Taiwan.

Hawaii is subject to direct approach of swells from distant storms as well as seas generated by trade winds passing through the islands. The archipelago creates a localized weather system that modifies the wave energy resources from the far field. We implement a nested computational grid along the major Hawaiian Islands in the global WaveWatch3 (WW3) model and utilize the Weather Research and Forecast (WRF) model to provide high-resolution mesoscale wind forcing over the Hawaii region. Two hindcast case studies representative of the year-round conditions provide a quantitative assessment of the regional wind and wave patterns as well as the wave energy resources along the Hawaiian Island chain. These events of approximately two weeks each have a range of wind speeds, ground swells, and wind waves for validation of the model system with satellite and buoy measurements. The results demonstrate the wave energy potential in Hawaii waters. While the episodic swell events have enormous power reaching 60 kW/m, the wind waves, augmented by the local weather, provide a consistent energy resource of 15–25 kW/m throughout the year.

In this study, characteristics of summer trade wind rainfall over Oahu, Hawaii, are analyzed. In the early morning before sunrise, flow deceleration on the windward coastal area is the greatest when the island land surface is the coldest. Furthermore, relatively calm winds are found over central Oahu between the Ko’olau Mountains and the Waianae Mountains, with weak westerly katabatic winds on the windward side of the Waianae Mountains. Most windward stations have an early morning rainfall maximum with a secondary rainfall maximum in the early evening. Morning (afternoon) land (sea) breezes dominate under variable winds, and are more pronounced over leeward Oahu. Precipitation on the western leeside coast has a slight peak in the afternoon due to an increase of cloud development from the afternoon sea breezes. Daily orographic rainfall over Oahu is greater under the strong trade wind regime, and less under weak trade wind and variable-wind flow regimes. However, the maximum correlation between daily trade wind rainfall and trade wind speed is

The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) coupled with the Noah land surface model (LSM) is employed to simulate island-scale airflow and circulations over Maui County, Hawaii, under summer trade wind conditions, during July–August 2005. The model forecasts are validated by surface observations with good agreement. In this study, it is shown that a previously known closed circulation over the Central Valley of Maui, or the Maui vortex, represents the northern cyclonic vortex of the dual-counter-rotating vortices in the lee of Haleakala, which extend up to the base of the trade wind inversion with a westerly reversed flow (>2 m s−1). At low levels, the northern cyclonic vortex is more pronounced than the southern anticyclonic vortex. The asymmetric structure of the dual vortices is related to the shape of Haleakala and the flow deflection by the West Maui Mountains. The Maui vortex has a relatively narrow east–west extent in the lowest levels, especially at night, due to the deflected strong northerly/northeasterly winds from the windward foothills of the West Maui Mountains. Unlike the lee vortices off the leeside coast of the island of Hawaii, the Maui vortex and the westerly return flow in low levels are mainly over land and are strongly modulated by the diurnal heating cycle. In addition, the location and horizontal and vertical extent are affected by the trade wind speed and latent heat release. Over the West Maui Mountains, with their height below the trade wind inversion, dual-counter-rotating vortices are present below the 1-km level in the wake, with strong downslope flow on the leeside slopes followed by a hydraulic jump. In the afternoon, downslope winds are weak, with combined westerly return/sea-breeze flow along the leeside coast. Orographic blocking is also evident over eastern Molokai with strong downslope winds, especially at night.

The leeside circulations and weather of the island of Hawaii were studied from the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) land surface model simulations for eight strong (∼7.9 m s−1) and eight weak (∼5.2 m s−1) trade-wind days and for five days with southeasterly trades (∼7.1 m s−1) during summer 2004. The objective is to investigate the effects of trade-wind strength and directions on the leeside circulations and rainfall and the modification of these effects by the land surface thermal forcing. For the small wake on the lee side of the Kohala Mountains (1700 m, lower than the trade-wind inversion at 2000 m) over northern Hawaii, the hydraulic jump is present with stronger downslope winds and warmer and drier conditions on the lee side and a weaker westerly reversed flow offshore when trades are stronger. In contrast, the westerly reversed flow along the large wake axis off the central Kona leeside coast (behind massive mountains with tops >4000 m) is 1–1.5 m s−1 stronger and 200–300 m deeper with higher moisture content when trades are stronger. Over the Kona slopes, the daytime thermally direct circulation cell is more significant when trades are stronger because of descending airflow aloft with less cloudiness. In the evening, the convergence between the westerly reversed flow offshore along the wake axis and the offshore/katabatic flow in the Kona coastal region is more significant with higher evening rainfall when trades are stronger. During the day, the lee side of the Kohala Mountains is characterized by a reversed flow (∼4 m s−1) merging with sea-breeze circulations along the coast. When trades are stronger, the convergence between the anabatic winds and the descending flow from the upper slopes is greater. However, the simulated cloud water there is less under strong trades because of warmer and drier conditions due to significant adiabatic descent in the lee. At night, when trades are stronger, the combined downslope/katabatic flow prevails without a reversed flow offshore. Under a southeasterly trade-wind flow with a lower trade-wind inversion (1.5 km), the westerly reversed flow is shallower; the adiabatic descent aloft on the southwestern leeside areas is more significant with warmer temperatures (0.5 K), a larger negative potential vorticity maximum [0.2 potential vorticity units (PVU), 1 PVU = 10−6 K m2 s−1 kg−1], and a more pronounced anticyclonic vortex offshore. The westerly reversed flow off the Kona coast shifts northward.

The effects of trade wind strength and the diurnal heating cycle on the production of summer trade wind rainfall on the windward side of the island of Hawaii are examined from the data collected from the Hawaiian Rainband Project (HaRP) during 11 July–24 August 1990 and from National Weather Service Hydronet and National Climatic Data Center rain gauge data during 11 July–24 August for the years 1997–2000. For strong trades, the daily rainfall totals on the windward lowlands west of Hilo are higher with a nocturnal maximum there due to the convergence of the katabatic flow and the incoming decelerating trade wind flow, and orographic lifting aloft. The maximum rainfall axis shifts farther inland when trades are stronger. Except in the late afternoon hours, rainfall amounts on the windward side are higher when trades are stronger. For weak trades (≤5 m s−1), the rainfall distributions have a pronounced late afternoon maximum on the windward slopes due to the development of anabatic winds. The nocturnal rainfall over the windward lowlands and the early morning coastal rainfall are lower when trades are weaker.

Islands in Hawaii have different sizes and terrain heights with notable differences in climate and weather. In this study, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) land surface model (LSM) is used to conduct numerical simulations for the island of Hawaii with different model terrain heights and sizes during the diurnal cycle for the Hawaiian Rainband Project (HaRP) period. In addition to island blocking and orographic lifting, terrain heights also affect the land surface thermal forcing throughout the diurnal cycle by the variations of orographic clouds during the day and by the variations of the longwave radiation heat loss at night. The simulated rainfall distributions and amounts throughout the diurnal cycle are closely related to rising motions caused by nonlinear interactions among island blocking, orographic lifting, and land surface processes. Besides the terrain/mountain height, island size is another factor that affects rainfall production and distribution in Hawaii, because island size affects orographic lifting, surface forcing, and the advection time scale for an air parcel to reach the mountaintop. The heavy rainfall maximum on the mountaintop of the island of Kauai is due to its suitable height and size. This study confirms that the evening rainfall maximum along the western Kona leeside coast is caused by the convergence between the westerly return flow and the offshore flow. For a lower model terrain, the westerly return flow is weaker; as a result, there are smaller evening rainfall amounts.

The seasonal variations of heavy rainfall days over Taiwan are analyzed using 6-yr (1997–2002) hourly rainfall data from about 360 rainfall stations, including high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System stations and 25 conventional stations. The seasonal variations and spatial variations of nontyphoon and typhoon heavy rainfall occurrences (i.e., the number of rainfall stations with rainfall rate >15 mm h−1 and daily accumulation >50 mm) are also analyzed. From mid-May to early October, with abundant moisture, potential instability, and the presence of mountainous terrain, nontyphoon heavy rainfall days are frequent (>60%), but only a few stations recorded extremely heavy rainfall (>130 mm day−1) during the passage of synoptic disturbances or the drifting of mesoscale convective systems inland. During the mei-yu season, especially in early June, these events are more widespread than in other seasons. The orographic effects are important in determining the spatial distribution of heavy rainfall occurrences with a pronounced afternoon maximum, especially during the summer months under the southwesterly monsoon flow. After the summer–autumn transition, heavy rainfall days are most frequent over northeastern Taiwan under the northeasterly monsoon flow. Extremely heavy rainfall events (>130 mm day−1) are infrequent during the winter months because of stable atmospheric stratification with a low moisture content. Typhoon heavy rainfall events start in early May and become more frequent in late summer and early autumn. During the analysis period, heavy rainfall occurrences are widespread and dominated by extremely heavy rainfall events (>130 mm day−1) on the windward slopes of the storm circulations. The spatial distribution of typhoon heavy rainfall occurrences depends on the typhoon track with very little diurnal variation.

A high wind event (14–15 February 2001) over the Hawaiian Islands associated with a cold front is simulated using the National Centers for Environmental Prediction (NCEP) Mesoscale Spectral Model (MSM) coupled with an advanced land surface model (LSM). During this period, a strong high pressure cell moved to the northeast of the Hawaiian Islands following the passage of the cold front. The cell then merged with the semipermanent subtropical high and resulted in windy conditions across the state of Hawaii. Analyses of soundings from Lihue on Kauai and Hilo on the Big Island reveal a mean-state critical level below 400 hPa, a strong cross-barrier flow (∼13 m s−1), and the presence of a trade wind inversion. The MSM–LSM predicts downslope windstorms on the lee sides of mountains or ridges with tops beneath the trade wind inversion and within ocean channels between islands. In the case of high mountains with a peak height above the trade wind inversion, weak winds are simulated on the lee side. Around the corners of the islands and in gaps between mountains, gap winds and downslope windstorms are both important for the development of localized leeside windstorms. The localized windstorms over the Hawaiian Islands develop as a result of interactions between large-scale airflow and the complex local topography. Since the terrain is not adequately resolved by the 10-km RSM–LSM, it is no surprise that these windstorms are better simulated by the high-resolution nonhydrostatic MSM–LSM than the 10-km RSM–LSM.

Validations of the 10-km operational Regional Spectral Model (RSM) and the coupled Mesoscale Spectral Model (MSM) with an advanced land surface model (LSM) forecasts during a 1-month period from 20 May through 20 June 2002 are performed at three surface sites on the island of Oahu. One heavy rainfall case over the Hawaiian Islands is also simulated using the MSM–LSM. Over land with adequate representation of the terrain, the 1.5-km MSM provides better forecasts of surface variables than the 10-km operational RSM. However, there are still appreciable discrepancies between the MSM simulations and observations. Further improvements are achieved by coupling the MSM with the LSM. In particular, overestimation of the surface wind speed and daytime cold biases experienced by the MSM are largely corrected in the coupled MSM–LSM. Composite analyses of surface variables at three surface sites under different trade wind conditions show that the observed diurnal cycles in 2-m temperature, 2-m dewpoint temperature, and 10-m wind are better forecasted by the MSM–LSM than by the MSM. The observed daytime minima in 2-m dewpoint temperatures during the strong trade wind days at two urban sites are reproduced by the MSM–LSM. The heavy rainfall case studies presented herein indicate that the high-resolution MSM–LSM has better capability in simulating localized rainfall distributions and airflows associated with the heavy rainfall event than the 10-km RSM–LSM. A major model bias is that the MSM–LSM produces excessive rainfall on the windward side of the island of Oahu with no rainfall downstream of the mountain ridges, in contrast to the observed rainfall distribution that shows the maximum rainfall axis occurring slightly downstream of the mountain ridges.

Sea-breeze cases during 23–28 June 1978 over northwest Hawaii are simulated using the National Centers for Environmental Prediction (NCEP) Mesoscale Spectral Model (MSM) coupled with an advanced Land Surface Model (LSM) with 3-km horizontal resolution. Subjective analyses show that except for 27 June, the MSM–LSM-predicted onset time, duration, and vertical extent of the sea breezes agree well with observations. The largest mean absolute errors for surface air temperature occur at the coastal stations under strong trade wind conditions (e.g., 23 and 27 June). The model-simulated rainfall distribution in association with sea-breeze fronts is consistent with observations. Sensitivity tests demonstrate the modulation of sea-breeze behavior by surface properties. High-resolution (1 km) MSM–LSM simulations for 23 and 27 June show improvements over the 3-km MSM–LSM in reproducing the observed sea breezes through a better representation of local terrain and a better simulation of orographically enhanced trades channeling through the Waimea Saddle. Deficiencies noted in the model simulations include 1) sea-breeze speeds are more than 2–3 m s−1 weaker than observations, and 2) horizontal penetration of sea breezes is generally overestimated. These deficiencies in the model simulations are primarily related to two factors: one is the underestimation of the trade wind speeds in the initialization from the NCEP–NCAR reanalysis data that is favoring the farther penetration of the sea breezes, and the other is the uncertainties in the thermal properties of the lava rocks that affect the surface temperature and the sea-breeze speed.

The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)/land surface model (LSM) is used to simulate the diurnal island-scale circulations over the island of Hawaii during the Hawaiian Rainband Project (HaRP, 11 July–24 August 1990). The model is initialized with the NCEP–NCAR reanalysis data. The diurnal variations of the land–sea thermal contrast at the land surface and the planetary boundary layer at Hilo, Hawaii, are well simulated. The main discrepancy occurs on the leeside areas of mountains or ridges below the trade wind inversion (2 km), where the simulated afternoon land–sea thermal contrast at the surface is 1°–3°C lower than observed mainly due to the misrepresentation of lava rocks by the bare ground category in the U.S. Geological Survey (USGS) data and stronger than observed simulated sea breezes bringing in relatively cool maritime air. The flow deceleration and splitting of the incoming trade wind flow and the evolution of the diurnal circulation cells on the windward side, the thermally driven diurnal winds, and the wake circulations on the lee side are well simulated. The simulated diurnal variations in rainfall are also in good agreement with observations. However, the simulated winds in areas well exposed to the trade wind flow are weaker (1–3 m s−1) than observed mainly due to the underestimation of trade wind flow in the NCEP–NCAR reanalysis. The simulated rainfall over windward lowlands at night is underestimated and the maximum rainfall axis shifts farther toward the coast as compared with observations, due to an underestimation of orographic lifting aloft and a relatively large horizontal extent of the simulated katabatic flow because of the weaker- than-observed trade wind flow in the initial conditions. In the afternoon hours on the windward side, the strongest winds (anabatic/sea breeze and trade wind flow) are simulated in low levels over land in response to the surface heating, with a westerly wind deviation beneath the mean trade wind inversion (2 km) and sinking motion over the adjacent oceans. The simulated low-level flow deceleration of the incoming trade wind flow is most significant in the early morning as a combination of island blocking and nocturnal cooling over land. At that time, the simulated upward motion representing the rising branch of the thermally direct circulation extends more than 40 km offshore. Sensitivity tests show that with better surface conditions in the model coupled with the Oregon State University (OSU) LSM, the simulated thermal forcing over land is improved. The improvements in simulated ground temperature, land–sea thermal contrast at the land surface, and mixing ratio lead to better simulation of the strength of land/sea breezes over the island.

The National Centers for Environmental Prediction regional spectral model and mesoscale spectral model (NCEP RSM/MSM) use a spectral computation on perturbation. The perturbation is defined as a deviation between RSM/MSM forecast value and their outer model or analysis value on model sigma-coordinate surfaces. The horizontal diffusion used in the models applies perturbation diffusion in spectral space on model sigma-coordinate surfaces. However, because of the large difference between RSM/MSM and their outer model or analysis terrains, the perturbation on sigma surfaces could be large over steep mountain areas as horizontal resolution increases. This large perturbation could introduce systematical error due to artificial vertical mixing from horizontal diffusion on sigma surface for variables with strong vertical stratification, such as temperature and humidity. This nonnegligible error would eventually ruin the forecast and simulation results over mountain areas in high-resolution modeling. To avoid the erroneous vertical mixing on the systematic perturbation, a coordinate transformation is applied in deriving a horizontal diffusion on pressure surface from the variables provided on terrain-following sigma coordinates. Three cases are selected to illustrate the impact of the horizontal diffusion on pressure surfaces, which reduces or eliminates numerical errors of mesoscale modeling over mountain areas. These cases address concerns from all aspects, including unstable and stable synoptic conditions, moist and dry atmospheric settings, weather and climate integrations, hydrostatic and nonhydrostatic modeling, and island and continental orography. After implementing the horizontal diffusion on pressure surfaces for temperature and humidity, the results show better rainfall and flow pattern simulations when compared to observations. Horizontal diffusion corrects the warming, moistening, excessive rainfall, and convergent flow patterns around high mountains under unstable and moist synoptic conditions and corrects the cooling, drying, and divergent flow patterns under stable and dry synoptic settings.

Orographic influences on the development of two heavy rainfall events over southwestern Taiwan during the Mei-yu season were analyzed. For the first event (5 June 1997), the maximum daily rainfall accumulation (∼371 mm) occurred over the southwest coast of Taiwan, whereas for the second event (20 May 2001), the maximum daily rainfall accumulation (∼535 mm) occurred over the southwestern windward slopes. The important processes for the development of heavy rainfall for these two events were studied from gridded data from the European Center for Medium-Range Weather Forecasts and from surface rainfall data and numerical modeling results using the nonhydrostatic fifth-generation mesoscale model from the National Center for Atmospheric Research and the Pennsylvania State University (MM5).

Using aircraft data and portable automated mesonet (PAM) data during the Hawaiian Rainband Project (HaRP; 11 July‐24 August 1990), the circulation on the lee side of the island of Hawaii is analyzed. The largest temperature deviations (2‐3 K) and negative mixing ratio deviations (2 2t o2 4gk g 21) are found in the northwestern and southwestern leeside areas of the island because of the descending trade winds aloft in the lee of low (,2 km) mountain ridges and tops. Rainfall in these areas is much less compared to the central leeside coast (e.g., Kona coast). In these areas, the sea-breeze duration is short compared to the Kona coast because of the presence of opposing trade winds; the wind speed is greater because of larger land‐sea thermal contrast. During the stronger trade wind days, adiabatic descent in the lee is more significant and the LCL is higher. As a result, in the northern and southern leeside areas, localized trade wind rain showers driven by local circulations occur mainly under weak trades. In the Kona area, the mean surface winds are calm behind the massive mountains, with a daytime upslope (onshore) and nighttime downslope (offshore) flow. Along the axis of the return flow in the large wake off the Kona coast, the water vapor content is higher than the ocean upstream, possibly caused by the low-level convergence between the counterrotating eddies along the wake axis. Along the Kona coast, the rainfall in the afternoon hours continues in the early evening and reaches the daily maximum after the onset of the land breezes, possibly because of the lifting of stronger moist return flow by the land-breeze front. Under stronger trade winds, the return flow would be stronger because of stronger island blocking. More rainfall occurs there when trades are stronger.