The
synoptic conditions conducive to heavy precipitation in the tropical Pacific
are broadly understood (Figure 1). In
the vicinity of the state of Hawaii the marine boundary layer generally
has adequate precipitable water for heavy rains at any time. Heavy rainfall
events usually occur in the presence of upper-level forcing and an increase
in mid-level moisture. The presence of moisture in the midtroposphere is
generally an indication of the existence of large-scale ascending motion
associated with synoptic-scale or subsynoptic scale circulations.
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:
The
Pacific Region faces special forecasting challenges that are consequence
of geography and a lack of observational data. Improved observations os
the mesoscale distribution of atmospheric moisture is a key to better forecasting
heavy precipitation. Sources for these moisture data include (1a) GOES-9
satellite, (1b) WSR-88D radar,, (1c) geodetic GPS recievers, and (1d) telemetered
rain gauges (Figure 2 and Figure
6).
(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.
Presently,
operational forecasters in Hawaii use the global spectral model (AVN) from
the National Center for Environmental Prediction (NCEP) as the primary
model guidance for 12- to 72-hour forecasts. The current implementation
of the AVN, with a 1-degree grid resolution, provides inadequate forecast
guidance in Hawaii; the mountainous terrain of the Hawaiian Islands is
not resolved bythe AVN. The inadequacy of the AVN guidance is particularly
apparent for mesoscale tropical systems that form near or over the islands.
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.
Research
opportunities based on archived data sets have been identified and are
being pursued by teams of UH faculty, graduate students and NWS staff.
These inslude cast studies and climatologies of heavy precipitation events,
severe weather events, andf wind storms. This poster depicts results from
two cases under investigation; the bow echo case of 3 November 1995 (Figure
3, Figure 4 and Figure
7) and the flood event of 25 January 1996 (Figure
5, Figure 6 and Figure
8).
Go Back to Steve's Homepage
Original poster graphics and layout by Brooks G. Bays, Jr. (illust@soest.hawaii.edu)
Html formatting by Ray Tanabe
Last Updated 14 October 1997