The factors that contribute to the flash flood hazard on tropical Pacific Islands include (1) orographic influences - flooding in Hawaii often occurs when convective cells, triggered or enhanced by orographic effects, become anchored to the mountains, and (2) small watersheds - peak streamflow typically occurs less than one hour after peak rainfall, thus respose time is short. Since the response time for hazardous flood events is very short, research concentrates on the mesoscale/nowcasting aspects of the problem.
The goals of the collaborative research effort are to:
(1a) Deriving integrated water vapor from the GOES-9 satellite.
The GOES-9 satellite has IR channels at 11 and 12 um that provide the opportunity to derive the total column water vapor wmounts. Figure 3 shows a day with southwesterly synoptic-scale flow at low levels at the time of a bow echo development near Kauai. Integrated water vapor measurements from soundings and GPS will be used to valibrate/calibrate PW measurements made by the new GOES-9 algorithms.
(1b) Improving application of the WSR-88D in the tropics.
Since installation the WSR-88D radars have documented a plethora of mesoscale severe weather signatures. WSR-88D reflectivity and winds data (Figures 4a and b) led to issuance of the the first severe weather warnings in the history of the Honolulu WSFO. Shortly afterwards winds of 80 knots were recorded at Nawiliwili, on the south shore of Kauai.
Rainfall estimation from radar reflectivity is dependant on the particulars of the drop size distribution, which in turn varies with air mass and climate. Therefore, it is not surprising that the performance of the WSR-88D 's rainfall algorithms has been lacking in the tropical Pacific (Figure 5). To construct appropriate rainfall algorithms for the contrasting weather conditions in the tropical Pacific (deep convection vs. shallow warm rain), 15-minute data telemetered from Limited Automatic Remote Collection (LARC) rain gauges are compared with computed 1km values from radar algorithms. The resulting improved radar_derived rainfal estimates will be used to produce storm totals (Figure 6) and to validate precipitation predictions made by our regional mesoscale numerical model and as input for radar-based forecasting tools such as AMBER discussed below.
(1c) Precipitable water Data from Global Positioning System (GPS)
receivers based in Hawaii.
L-Band raido signals transmitted by GPS satellites are delayed (refracted) by atmospheric water vapor as they propagate to ground-based GPS receivers. This 'wet delat' is nearly proportional to the quantity of water vapor integrated along the signal path. The all-weather ability of GPS to accurately measure integrated water vapor has been demonstrated. A network of continuously operating GPS recievers, including approximately 25 receivers outfitted with surface barometers, is being constructed across the state of Hawaii for a combination of geodetic, navigational, and meteorological purposes (Figure 2). Precipitable water data from these sites will be used in weather analysis and modeling applications.
(1d) Telemetered rain gauges and basin data.
There are 100 LARC telemetered rain gauges in Hawaii that serve as a verification and multi-use data source. High resolution geographic informations systems data, such as slope angle, aspect, soil and vegetative cover, are being collected to provide more intelligent flood inundation information to the forecasters and end users.
To address the shortcomings of the AVN, a hydrostatic version of the regional spectral model (RSM), developed at NCEP, is currently being run at UH with a synoptic domain (25 km resolution) and a nested domain (10 km resolution) covering the main islands of the Hawaiian chain. Sample output from the 25 and 10 km funs of the RSM are shown in Figures 7a and 7b, respectively, for the case in which a bow echo formed off Kauai (Figure 4).
RSM data enhancement
Due to the need for mesoscale observational data over the central Pacific Ocean, wind profiles and precipitable water data from GOES-9 satellite and WSR-88D radar and precipitable water from the Earth-based GPS receiver network will be used to enhance the initial state of the RSM.
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Last Updated 14 October 1997