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Thomas A. Schroeder

Abstract

A detailed study has been made of the severe Oahu rainstorm 19 April 1974 and its implications for the Hawaii flood forecast problem. Data from Qahu's extensive recording raingage network were analyzed to determine the mesoscale structure of the precipitating cloud systems. Study of a special rain and stream gage network in Moanalua Valley showed that the current Hawaiian detection scheme based on one telemetered raingage per watershed is inadequate. Raingage data were incorporated with available conventional meteorological observations to construct a conceptual model of the flood-producing convective regime of 19 April. It was concluded that orography played an essential role in the persistence of major thunderstorm activity.

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Thomas A. Schroeder

Abstract

During the period 22–29 June 1978, meteorological data were collected at six stations arranged in nearly linear transection extending from the coast at Anaehoomalu, Hawaii to Waimea Airport, 25 km inland and 800 m higher. Sea breeze response to synoptic-scale weather patterns was documented.

Within the six days of 24 h measurements, two disturbances embedded in the prevailing trade winds passed over the island. Three different types of sea breeze development ensued. Sea breeze variability appears to be related to both synoptic-scale cloud and wind distributions and to the thermal properties of lava surfaces situated within the transection. Days of maximum insolation corresponded to minimum sea breeze advance through the network. It is suggested that numerical models be employed with this data set to test the sensitivity of sea breezes to soil and cloud conditions.

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Thomas A. Schroeder

Abstract

The most recent 15 years of records were examined to compare the nature of Hawaiian funnel clouds to south Florida waterspouts. The majority of Hawaiian funnel clouds were found to be small vertices which may represent the minimum vortex possible in cumulus-scale systems. Hawaiian funnels are less intense and occur less frequently than south Florida waterspounts. However, the climatology is influenced by several nonmeteorological factors.

Examination of major tornadic events showed that Hawaiian tornadoes are mainly associated with intense mid-latitude cyclones. Conventional stability indices appear to be useful indications of tornado potential.

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C. S. Ramage and Thomas A. Schroeder

Abstract

Very large coast to mountain rainfall gradients have been observed in the trade winds and winter monsoons. Since the surface moist layer is usually capped by a subsidence inversion near 2 km, the rain is “warm.” On the top of a mountainous island, which is generally below the inversion, such as Kauai, trade wind rainfall can be very great and the coast to top rainfall gradient very large. Autographic rainfall measurements at the top of Mount Waialeale (1598 m MSL, one of the wettest spots on the earth) on Kauai together with surface and upper-air measurements made at Lihue, 20 km to the southeast, and weather satellite images confirm and expand on earlier descriptions of the nature of mountain rainfall in the trade winds.

Significant rain results from moderate or fresh trade winds being lifted up the eastern escarpment of Waialeale, but only when a band or area of cumulus extends upwind of the mountain. Small wind shear in the vertical and a sharp upper limit to the moist layer reduce entrainment and facilitate growth of cloud droplets. At the mountaintop rain is usually light or moderate, with drops smaller than 2 mm, but persisting long enough to produce large accumulations. Along the windward coast, drops usually evaporate before reaching the ground. Divergence and upward motion east of an upper-tropospheric trough barely affect the moist trade wind layer. Cloud lines associated with shear line extensions of cold fronts or with dying tropical cyclones to the south account for much of the rain though short-lived mesoscale cloud systems are also important.

Thunderstorms are very rare with surface flow from a trade wind direction. The wind then curves cyclonically on the northwest sides of sharp troughs or small cyclones. Upper-tropospheric southwesterlies usually prevail. The nocturnal rainfall maximum at Waialeale probably stems largely from radiational cooling at the top of the moist layer causing clouds over and upwind of the island to increase. Other trade wind islands of about the same size and height as Kauai, but with no mountaintop rain gauges, probably also have large coast to mountaintop rainfall gradients.

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P. Anders Daniels and Thomas A. Schroeder

Abstract

Low-level winds in the central valley of the island of Maui were investigated in a field program during August 1976. Forty-one sites were occupied using three mobile stations during a period of persistent trade winds. Contemporaneous data from the Kahului Weather Service Office as well as other Hawaiian stations were collected to relate field observations to large-scale events. Streamline analyses reflect the diurnal variation of the low-level circulation which is profoundly influenced by Haleakala and West Maui volcanoes. The field survey was utilized in planning new fixed stations to monitor wind characteristics for wind power applications. Preliminary fixed station results are discussed. The importance of diurnal mesoscale patterns on wind power planning was emphasized.

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James H. Churnside, Thomas A. Stermitz, and Judith A. Schroeder

Abstract

A neural network is used to obtain vertical profiles of temperature from microwave radiometer data. The overall rms error in the retrieved profiles of a test dataset was only about 8% worse than the overall error using an optimized statistical retrieval. In certain cases, such as one with a large temperature inversion, the neural network produced better reproductions of the profiles than did the statistical inversion.

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Yongxin Zhang, Yi-Leng Chen, Thomas A. Schroeder, and Kevin Kodama

Abstract

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.

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Ryan E. Lyman, Thomas A. Schroeder, and Gary M. Barnes

Abstract

On 29 October 2000, the Hana region of Maui received 700 mm of rain in 7 h. Radar analyses revealed that the storm consisted of seven cells that were initiated along the southeast slope of Haleakala volcano. One of these cells survived for nearly 4 h and was responsible for 80% of the volumetric rainout from the storm. The interaction of low-level flow distorted by the island of Hawaii located farther east, the passage of a trough, and the topographic forcing caused by Haleakala volcano were major factors responsible for the evolution of the storm.

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Pao-Shin Chu, Ying Ruan Chen, and Thomas A. Schroeder

Abstract

For the first time, trends of five climate change indices related to extreme precipitation events in the Hawaiian Islands are investigated using daily observational records from the 1950s to 2007. Four indices [simple daily intensity index (SDII), total number of day with precipitation ≥25.4 mm (R25), annual maximum consecutive 5-day precipitation amount (R5d), and the fraction of annual total precipitation from events exceeding the 1961–90 95th percentile (R95p)] describe the intensity (SDII), frequency (R25), and magnitude (R5d and R95p) of precipitation extremes, and the fifth index [consecutive dry days (CDD)] describes drought conditions. The annual probability density functions (PDFs) of precipitation indices for two epochs (i.e., 1950–79 and 1980–2007) are analyzed. Since the 1980s, there has been a change in the types of precipitation intensity, resulting in more frequent light precipitation and less frequent moderate and heavy precipitation intensity. The other three precipitation-related indices (R25, R5d, and R95p) demonstrate a shift toward the left of the distribution over time, suggesting shorter annual number of days with intense precipitation and smaller consecutive 5-day precipitation amounts and smaller fraction of annual precipitation due to events exceeding the 1961–90 95th percentile in the recent epoch relative to the first epoch. The changes of PDF distribution for SDII, R25, R5d, and CDD are significant at the 5% level according to a two-sample Kolmogorov–Smirnov test.

A nonparametric trend analysis is then performed for four periods, with different starting years (e.g., the 1950s, the 1960s) but the same ending year (2007). Long-term downward trends are evident for four precipitation-related indices, and long-term upward trends are observed for CDD. Geographically, Kauai and Oahu are dominated by long-term decreasing trends for four precipitation-related indices, while increasing trends play the major role over the island of Hawaii. The upward trends of drought conditions in the long run are predominant on all the major Hawaiian Islands.

To investigate whether the trends are stable throughout the time, the derivatives of trends for each of the 30-yr running series are calculated (e.g., 1950–79, 1951–80, … , 1978–2007) for four precipitation-related indices at each station. For Kauai and Oahu, positive derivatives prevail for all indices in the presence of long-term negative trends, suggestive of a phase change in precipitation extremes and such extremes showing an upswing recently. For the island of Hawaii, there is also an indication of phase reversal over the last 60 yr, with negative derivatives occurring in the presence of the background positive trends.

A positive relationship is found between the precipitation indices and the Southern Oscillation index (SOI), implying more precipitation extremes during La Niña years and vice versa for El Niño years. Spatial patterns of standardized anomalies of indices are presented for the La Niña/−PDO minus El Niño/+PDO composites.

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Guangxia Cao, Thomas W. Giambelluca, Duane E. Stevens, and Thomas A. Schroeder

Abstract

Using 1979–2003 radiosonde data at Hilo and Līhu‘e, Hawaii, the trade wind inversion (TWI) is found to occur approximately 82% of the time at each station, with average base heights of 2225 m (781.9 hPa) for Hilo and 2076 m (798.8 hPa) for Līhu‘e. A diurnal pattern in base height of nighttime high and afternoon low is consistently found during summer at Hilo. Inversion base height has a September maximum and a secondary maximum in April. Frequency of inversion occurrence was found to be higher during winters and lower during summers of El Niño years than non–El Niño years. Significant upward trends were found for inversion frequency at Hilo for March–May (MAM), June–August (JJA), and September–November (SON) seasons, and at Līhu‘e for all seasons and for annual values.

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