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					                                                           Partitioning of precipitation into rain and snow in distributed hydrologic simulations
                                                                                   in the Western Cascades, Oregon, USA.
                                                                                                                                     Edwin P. Maurer1, Jasmine Cetrone2, Socorro Medina2, and Clifford Mass2
                                                                                                                                                         1. Civil Engineering Department, Santa Clara University, Santa Clara, CA 95053-0563
       AMS Annual Meeting 2004                                                                                                                        2. Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195-1640
      Poster Session 3: Poster P3.5

                                                                                                                                                 3     Meteorology in South Santiam Basin during IMPROVE Observation Period
ABSTRACT                                                                                                                                                                                                                                                                                                                    Elevation and surface air temperature at each pixel (interpolated from
One of the greatest challenges in hydrologic modeling in areas with significant orographic influences is accurate simulation of the                                                                                                                                                                                         observations with a lapse rate of -5.5 °C/km) were combined with the radar-
                                                                                                                                                             Examination of P/T/SWE relationships at 2 SNOTEL sites in basin
precipitation fields, since this drives the streamflow response. In the northwest United States, where most of the precipitation occurs
during the cool season, another major factor in streamflow simulation is the determination of whether the precipitation is falling as rain                                                                                                                                                                                  observed freezing level to illustrate the variability in surface temperatures
or snow, since these strongly influence the timing of the resulting runoff. The partitioning of precipitation in distributed hydrologic models                                                                                                                                                                              associated with rain and snow. At each time step, the radar detected 0° level was
into rain, snow, or a mixture of the two is often based on surface air temperature, since this is included in the station observation records                                                                                                                                                                               projected across the basin. The surface air temperatures for pixels with elevations
that provide the precipitation and other meteorological data used to force the model. This study examines the adequacy for hydrologic                                                                                                                                                                                       within 10m of this 0° level, were taken as Surface Air Temperatures for Snow/Rain
modeling of using surface air temperature to determine this partitioning of rain and snow in the Santiam River basin, Oregon. The
western slopes of the Cascade mountain range in Oregon, specifically an area including the South Fork of the Santiam River, was the
                                                                                                                                                                                                                                                                                                                            samples of the minimum surface air                  Inferred from Radar 0°C Level
geographical focus for the second phase of the research effort dubbed Improvement of Microphysical PaRameterization through                                                                                                                                                                                                 temperature at which any rain occurs,
Observational Verification Experiment (IMPROVE-2). This intensive field observation campaign was carried out from 26 November                                                                                                                                                                                               Tmin(rain). At a distance 300 meters
through 22 December 2001, with measurements used to perform comprehensive verification of cloud and precipitation microphysical                                                                                                                                  Sample reflectivity data from the NOAA/ETL S-              below the 0° level all melt is assumed
processes parameterized in mesoscale models. Included in the suite of IMPROVE-2 observations were both scanning and vertically                                                                                                                                   band vertically pointing radar. The data shown are         complete, and pixels with elevations
pointing radar. While scanning radar observations in areas of complex terrain, such as the western Cascades, are problematic due to                         JUMP_OFF_JOE                                           LITTLE_MEADOWS
                                                                                                                                                                                                                                                                 for 2215 UTC 13 December - 0115 UTC 14                     close to this are representative of the
ground clutter and beam blocking, vertically pointing radar does not suffer from this. We show that, by replacing the surface air
temperature-based algorithm in a distributed hydrologic model with a freezing level determined with S-band radar supplemented by
                                                                                                                                                                                                                                                                 December 2001. Note the bright band in red, the top        maximum surface air temperature at
other observations, significant improvement in the simulated hydrograph can be obtained.                                                                                                                                                                         of which is typically associated with 0°C                  which snow occurs, Tmax(snow). A linear
                                                                                                                                                                                                                                                                 temperatures. (Houze and Medina, 2002)                     mixture is assumed between these levels.
1     Focus of This Study
    In many hydrologic models, determination of precipitation type is indexed to surface air temperature, and the                                                                                                                                                                                                                                            Minimum during                 Maximum during    Average during
                                                                                                                                                                                                                                                                   Observed 0° Level Based on Bright Band                                                   IMPROVE-2 period               IMPROVE-2 period IMPROVE-2 period
       selection of the maximum snow and minimum rain thresholds are chosen empirically, by calibration or using                                                                                                                                                                Identification
       published values (e.g. U.S. Army Corps of Engineers, 1956), or are selected arbitrarily (e.g., Bowling et al.,                                                                                                                                                                                                          Tmin(Rain)                              -9.7                          -0.6                          -4.9
       2003). Some models use one fixed temperature as a division between rain and snow, rather than using a
                                                                                                                                                                                                                                                                                                                               Tmax(Snow)                              -6.7                           1.7                          -2.4
       range with mixed (frozen and solid) precipitation (e.g. Bicknell et al., 2002). The NWSRFS implementation
       of the Sacramento model (Office of Hydrologic Development, 2002) is rare in allowing the incorporation of
       freezing level data.                                                                                                                                                                                                                                                                                                                                                                                   This example, from
    The following questions take advantage of the availability of radar-based freezing level observations in the                                                                                                                                                                                                                                                                                              LITTLE_MEADOWS,
       study region to look for opportunities for improving streamflow simulations in regions of complex topography                                                                                                                                                                                                                                                                                           highlights 2 periods where
       and strong orographic influence:                                                                                                                                                                                                                                                                                                                                                                       the air temperature
                                                                                                                                                                                                                                                                                                                                                                                                              indexing and radar 0°C
    1) How well do surface temperature-based methods work for determining whether precipitation is falling as                                                                                                                                                                                                                                                                                                 levels can give different
       rain, snow, or a mixture?                                                                                                                                                      Net decrease in snow water equivalent (swe)                                                                                                                                                                             results. Here two periods
                                                                                                                                                                                      Net accumulation of snow water equivalent                                                                                                                                                                               are highlighted where the
    2) Does the radar-detected 0°C level differ substantially from the air-temperature-based method?
                                                                                                                                                     Observations show:                                                                                                                                                                                                                                       freezing level is well
    3) Can the observed radar-based 0°C level be used to improve streamflow simulations during the events                                            • Surface air temperature is not a good indicator of whether precipitation is falling as rain or snow.
                                                                                                                                                                                                                                                                                                                                                                                                              above the station elevation
       studied during IMPROVE-2?                                                                                                                     • This is especially evident for lower elevation JUMP_OFF_JOE site, closer to valley bottom, where
                                                                                                                                                                                                                                                                 The average 700 - 925 mb wind speed for the entire                                                                                           (indicating rain), while the
                                                                                                                                                       there is essentially no correlation between air temperature during a precipitation even and whether
                                                                                                                                                       snow is accumulating or melting.                                                                          IMPROVE-2 period is 13.5 ms-1, thus applying the 0°                                                                                          surface air temperature is
2     IMPROVE-2 Overview and River Basin for Study                                                                                                   • Even at LITTLE_MEADOWS, closer to the ridge, at air temperatures between 2° and 4°C during                level at the S-Band location to the entire basin                                                                                             below zero (indicating at
    IMPROVE is aimed at
                                                                                                                                                       this period, air temperature is a poor indicator of precipitation type.                                   introduces at most a 1 hour timing error for any point                                                                                       least partial snow).
                                                                                                                                                     • There is a wide discrepancy between the 2 locations in the air surface temperatures associated            in the basin on average.
    comprehensively checking and                                                                                                                       with both rain and snow, indicating the use of one index for the basin could be problematic.
    improving the parameterization
    schemes currently implemented in
    the Penn State/NCAR Mesoscale
    Model (MM5), a mesoscale model                                                                                                               4      Effect of Precipitation Type Determination on Hydrologic Simulations                                                                                                                5      Summary
    that has been extensively used for                                                                                                                 Given the above differences in air temperature-based versus radar-based discrimination                    RMSE for peak events (observed flows > 60 m3/s)                                • Based on surface observations of air temperature and
    both research and operational                                                                                                                      of rain and snow, we investigate the sensitivity of streamflow simulations in
                                                                                                                                                                                                                                                                                        Gauge 14185000     Gauge 14185900
                                                                                                                                                                                                                                                                                                                                                  snow accumulation, the surface air temperature is not a
    forecasting. The primary goal of                                                                                                                   IMPROVE to incorporation of freezing level data, using the DHSVM model                                                                                                                     strong indicator of accumulation or melt of snowpack
    IMPROVE is to utilize quantitative                                                                                                                 (Wigmosta et al., 1994), modified to ingest freezing level data.                                           Trial #1                    43                 57
                                                                                                                                                                                                                                                                                                                                                  (or whether precipitation type is rain or snow).
    measurements of cloud microphysical                                                                                                                                                                                                                           Trial #2                    40                 40
                                                                          South Santiam River Basin                                                     Trial #1 – Using air temperature-based indexing of 2°C Tmax(snow) and                                                                                                                   • The first method of partitioning precipitation into rain
    parameters in a variety of mesoscale
                                                                                                                                                        0°C Tmin(rain) (following Bowling, et al., 2003; Office of Hydrologic                                     Trial #3                    39                 39                               and snow used radar detected 0° elevation as that below
    features to improve the
                                                                                                                                                        Development, 2002)
    representation of cloud and                                                                                                                                                                                                                                                                                                                   which snow begins to melt; 300 m below this elevation
    precipitation processes in mesoscale                                                                                                                                                                                                                      Using the radar detected 0°C level, either directly (Trial #3) or
                                                                                                                                                                                                                                                                                                                                                  marked that below which all precipitation is rain.
    models. The IMPROVE-2 field study,                                                                                                                                                                                                                        combining it with the basin DEM and surface air temperatures to
    focused on orographic clouds and                                                                                                                                                                                                                          estimate a Tmax(snow) and Tmin(rain) (Trial #2) produces simulated                • A second method combined radar data with the
    precipitation in the Oregon Cascade                                                                                                                                                                                                                       improved hydrographs compared to literature-based Tmax(snow)                        elevations and surface air temperatures at each pixel and
    Mountains, was conducted 26                                                                                                                                                                                                                               and Tmin(rain) values (Trial #1), as reflected in the RMSE values                   time step in the basin. Populations of Tmin(rain) and
                                                                                                                                                                                                                                                              above. Trial #2 achieves most of the decrease in RMSE, implying
    November through 22 December                                                                                                                                                                                                                                                                                                                  Tmax(snow) (see Box 3 for definitions) were derived, the
    2001. For more details, see:                                                                                                                                                                                                                              that “calibrating” Tmax(snow) and Tmin(rain) using radar data may
                                                                                                                                                                                                                                                              be possible and beneficial to hydrologic simulations.                               average of which provided temperature indices for
                                                                                                                                                                                                                                                                                                                                                  partitioning precipitation into rain, snow, or a mixture.
                                                                                                                                                                                                                                                              The simulation of snow at the 2 SNOTEL sites in the basin is more
                                                                                                                                                                                                                                                              problematic. The use of the variable radar-detected freezing level                • The use of the second method in a hydrologic model
     The IMPROVE-2 domain overlaps largely with the South Santiam River basin, shown here, which has a total basin                                     Trial #2 – Using air temperature-based indexing, as in Trial #1, but with
                                                                                                                                                                                                                                                              (trial #3) improves the snow water equivalent simulations at these                  (see Trial 2, Box 4) produced measurable improvements
     area of 1,440 km2. This catchment provides a spatial integrator for the observed and modeled precipitation, and a                                 average vales inferred from radar-detected level (see box 3 above):
                                                                                                                                                                                                                                                              points compared to trial #1, while trial #2 is not a consistent                     in simulated peak flows over using values of Tmin(rain)
     valuable validation tool for assessing precipitation fields simulated by forecast models.                                                         -2.4°C Tmax(snow) and -4.9°C Tmin(rain)
                                                                                                                                                                                                                                                              improvement. None of the trials could reproduce the complete
                                                                                                                                                                                                                                                                                                                                                  and Tmax(snow) from literature.
                                                                                                                                                                                                                                                              removal of snow at the Jump_off_Joe site, indicating that other
     The South Santiam basin, during the IMPROVE-2                                                                                                                                                                                                            local factors are important.                                                      • Using the first method (see Trial 3, Box 4) produced
     period, included many observational assets. The
     subset of observations used in this study included
                                                                                                                                                                                                                                                                                                                                                  further improvements in the simulation of one flood
                                                                                                                                                                                                                                                              Trial #1                                                                            peak, but the incremental improvement over using the
     a vertically pointing S-Band radar, daily and hourly
     cooperative observer stations, SNOTEL stations,                                                                                                                                                                                                                                                                                              second method was small.
     precipitation gauges installed for this study                                                                                                                                                                                                                                                                                              • At two locations where snow was observed, the
     (labeled “IMPROVE”), and USGS streamflow                                                                                                                                                                                                                                       JUMP_OFF_JOE              LITTLE_MEADOWS
                                                                                                                                                                                                                                                                                                                                                  simulations by the hydrologic model were better with
     gauges, shown on this map.
                                                                                                                                                                                                                                                                                                                                                  the first method than the second, though local effects
                                                                                                                                                        Trial #3 – Using elevation indexing: Observed 0° Level Based on                                       Trial #2
                 Vertically-pointing S-Band                                                                                                             Bright Band Identification (see plot in Box 3 above), varies with time:
                                                                                                                                                                                                                                                                                                                                                  complicate the accurate simulation of snow at one site.
                 Radar                                                                                                                                                                                                                                                                                                                          REFERENCES
                                                                                                                                                                                                                                                                                                                                                Bicknell, B.R., J.C. Imhoff, J.L. Kittle Jr., A.S. Donigian, Jr. and R.C. Johanson. 1997. Hydrological
                 Daily Cooperative Obs.                                                                                                                                                                                                                                                                                                         Simulation Program -- FORTRAN, User's Manual for Version 11. EPA/600/R-97/080. U.S. EPA, National
                                                                                                                                                                                                                                                                                                                                                Exposure Research Laboratory, Athens, GA.

                 Hourly Cooperative Obs.                                                                                                                                                                                                                                                                                                        Bowling, L.C., D.P. Lettenmaier, B. Nijssen, L.P. Graham, et al. 2003, Simulation of high latitude
                                                                                                                                                                                                                                                                                                                                                hydrological processes in the Torne-Kalix basin: PILPS Phase 2(e) 1: Experiment description and
                                                                                                                                                                                                                                                                                                                                                summary intercomparisons, Journal of Global and Planetary Change, 38(1-2), 1-30.
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                                                                                                                                                                                                                                                                                                                                                Preprints: 10th Conference on mountain meteorology and MAP meeting, Park City, UT, 17-21 June, 2002.
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                                                                                                                                                                                                                                                                                                                                                Section 3.3-RSNWELEV, National Weather Service.
                 USGS Stream Gauge                                                                                                                                                                                                                                                                                                              U.S. Army Corps of Engineers, 1956: Summary Report of the snow investigations – Snow Hydrology,
                                                                                                                                                                                                                                                                                                                                                North Pacific Division, Portland, OR, June 1956.
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