Island Effects on Rainfall for the Hawaiian Islands with Mountaintops below the Trade Wind Inversion

dc.contributor.advisor Chen, Yi-Leng Hsiao, Feng
dc.contributor.department Atmospheric Sciences 2020-07-07T19:08:32Z 2020-07-07T19:08:32Z 2020 Ph.D.
dc.subject Atmospheric sciences
dc.title Island Effects on Rainfall for the Hawaiian Islands with Mountaintops below the Trade Wind Inversion
dc.type Thesis
dcterms.abstract The goal of this research is to investigate the island-scale rainfall, weather, and climate over various local communities in Hawai‘i, especially for islands (O‘ahu and Kaua‘i) with mountain heights below the typical trade wind inversion. High-resolution modeling and satellite observations during July–August 2013 are used to the physical processes responsible for wake circulation and clouds over O‘ahu in the lee-side wake zone under trade wind conditions. In the morning, orographic clouds are more significant on the windward side of the Koʻolau Range when trades are stronger, and are advected downstream by trade winds aloft. During the daytime, warmer air over the island interior is advected off the western leeside coast by downslope winds under strong trades. In contrast, upslope/sea-breeze flow occurs along the leeside coast under weak trades with orographic clouds on the western leeside slopes after sunrise under weak trades. These clouds are advected westward by the combined trade wind and the easterly return flow aloft, resulting in more significant cloudiness in the wake zone with larger horizontal extent in the early afternoon when trades are weaker. Short-lived afternoon heavy rainfall events may form over Central Oʻahu during seasonal transition periods (June and October) under favorable large-scale settings. These include a deep moist layer with relatively high total precipitable water (TPW, > 40 mm), a blocking pattern in mid-latitudes with a northeast–southwest moist tongue from low latitudes ahead of an upper-level trough, the absence of a trade wind inversion, and variable (< 3 m s-1) low-level winds. Our high-resolution (1.5 km) model results show that daytime land surface heating deepens the mixed layer over Central Oʻahu while the top of the mixed layer reaches the lifted 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 the onshore flows over Central Oʻahu provides the localized lifting required for the release of instability. Based on synoptic and observational analyses, nowcasting with a lead time of 2–3 hours ahead of this type of event is possible. In the absence of orographic effects after removing model topography, the destabilizing effects of daytime heating, horizontal advection of moist maritime air inland by the onshore/sea-breeze flows, convergence over Central Oʻahu, and the subsequent development of the heavy showers over land are still simulated. However, when surface fluxes are turned off in the NF run, 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 typical summer trade wind conditions, orographic precipitation occurs frequently over the mountainous Hawaiian Islands. The mountaintop of Kaua‘i, one of the wettest spots on earth, has the highest rainfall amount during the summer months among the Hawaiian Islands. Based on model sensitivity test, our results show that the orographically induced moisture flux convergence and orographic lifting at the slope surface are the two most dominant terms for the rainfall production. In addition, rainfall production also related to TPW in the environment. Under undisturbed summer trade-wind weather, the moisture flux convergence is related to orographic and local effects, including: flow deceleration on the windward side due to orographic blocking, interactions between the incoming flow and offshore flow due to nocturnal and/or evaporative cooling, and convergence between the incoming winds and opposing sea breezes in the leeside. Furthermore, moisture flux convergence is enhanced by latent heat release. The vertical lifting by winds on the slope surface could be attributed to mechanical lifting and modified by daytime upslope and nighttime downslope winds. For previous theoretical studies on orographic precipitation based on Froude number and orographic rainfall index, the orographically induced moisture convergence and convective feedbacks as well as diurnal variations in land surface heating are largely ignored. The El Niño-Southern Oscillation (ENSO) is a prominent mode of climate variability at the inter-annual time scale. During the negative phase of ENSO (El Niño), Hawai‘i frequently experiences droughts in winter, which continue into the following spring. Less is known about the impact of El Niño on local spatial patterns of rainfall, temperature, moisture and winds. Recent studies have shown that there are two different flavors of El Niño [east Pacific type/Cold Tongue (CT) and central Pacific type/warm pool (WP)]. With large spatial variations of local climate, better understanding of the impact of El Niño favors on different island communities is investigated with regional and island-scale simulations. During El Niño winters, especially under CT events, synoptic subsidence is greater with lower TPW over Hawai‘i than long-term seasonal mean due to a merged inter-tropical convergence zone and South Pacific convergence zone and eastward shift of the Walker Circulation. During El Niño springs, the enhanced tropical convergence weakens with less significant impacts on subsidence over the Hawai‘i than during winters. The results from 6-km simulations during 1979–2017 shows drier conditions during the CT events than during the WP events, and the dry conditions are more significant in winter. During El Niño winters, the Hawaiian region has positive temperature anomalies at the 700 hPa level and are more significant (1.0 K) during the CT winters than during the WP winters (0.5 K). Five CT, six WP, and six neutral events during winter and spring are simulated with an island-scale model over O‘ahu and Hawai‘i. The simulated rainfall patterns are comparable to the Rainfall Atlas of Hawai‘i, however, the high-resolution model overestimates the rainfall amount. During both types of El Niño events, the nighttime cooling accentuated by less atmospheric water vapor affects the simulated surface temperatures over the regions below the trade wind inversion. For island regions above the trade wind inversion, surface warming is accentuated by increased subsidence associated with the Hadley circulation. For Hawai‘i Island, drought conditions are expected during the CT winters over the mountaintops of Mauna Kea and Mauna Loa and in the leeside of the Kohala Mountains due decreased trade wind and low TPW. During the CT springs, persistently weak trade wind speed and low TPW result in maximum rainfall deficiencies over the windward slopes and mountaintops below the trade wind inversion. However, the dry conditions over the windward slopes are less significant during the CT springs than during the CT winters. The only region with greater daily rainfall during the CT springs is off the Kona coast, before sunrise, resulting from the convergence of katabatic flows with the westerly winds offshore. The rainfall distributions during the WP events are quite similar to those during the CT events, however, the dry conditions are less significant due to higher TPW in the environment. For islands with mountaintops below the trade-wind inversion, during the CT winters, increased synoptic subsidence and low TPW are crucial in resulting in drier or even drought conditions over the mountaintops, e.g. the Ko‘olau Mountains on O‘ahu. Daily rainfall over the Ko‘olau Mountains and windward coast are lower during the CT winters than the neutral winters. During the CT springs, the dry conditions are still simulated over mountainous regions. For WP events, mountainous regions with drier than neutral conditions are less significant. During the WP springs, more rainfall is only simulated over the southwestern coasts in the afternoon hours than during the neutral springs due to enhanced thermally driven circulations over the leeside coasts due to warmer surface daytime temperature over land than during the neutral springs.
dcterms.extent 279 pages
dcterms.language eng
dcterms.publisher University of Hawai'i at Manoa
dcterms.rights All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
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