Theme for 2005:
Precipitation and Flood Forecasting in the American River Basin

Engineering Climatology for California

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Abstract:

This is a progress report on a long running extreme weather archiving project. It has been a cooperative effort of many people and agencies in compiling and searching for better ways to analyze weather records. The weather being a limiting factor in construction project design, building and operation is a major focus with almost everyone. Compiling these data sets has been a major activity for me for decades. The result is compiled on a CD that is available on request at the address shown above. This study describes an archive of weather data (CAClimate CD) intended originally for engineering applications.

This project started fifty years ago when there was much confusion in assigning return periods to extreme rainfall events. This resulted in chaotic variation in design storms due to small sample size and much controversy regarding choices in frequency distributions. There was a need for procedures where all workers would achieve similar results when analyzing small samples of rainfall records that were located near project sites. This earlier confusion led to a regional approach in rainfall studies.

It became apparent that this archive is also a history of our times and very much worthy of serious consideration as historic record. The interesting part of this activity for me has been the analysis of historic trends in all of the various time series data sets.

Validation and Sensitivities of Dynamic Precipitation Simulation for Winter Events over the Folsom Lake Watershed: 1964-1999

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Abstract:

A total of 62 winter storm events in the period 1964-1999 and over the Folsom Lake watershed located at the windward slope of the Sierra Nevada Mountains were simulated with a 9-km resolution using the mesoscale model MM5. Mean areal precipitation (MAP) over the entire watershed and each of four sub-basins was estimated based on gridded simulated precipitation. The simulated MAP was verified with MAP estimated (a) by the California-Nevada River Forecast Center (CNRFC) for the four sub-basins based on 8 operational precipitation stations, and (b), for the period from 1980 to 1986, on the basis of a denser precipitation observing network deployed by the Sierra Cooperative Pilot Project (SCPP).

A number of sensitivity runs were performed to understand dependence of model precipitation on boundary and initial fields, cold vs. warm start, and microphysical parameterization. The principal findings of the validation analysis are: (a) the MM5 model achieves a good percentage bias score of 103% in simulating Folsom basin MAP when compared to MAP derived from dense precipitation gauge networks; (b) spatial grid resolution higher than 9 km is necessary to reproduce the spatial MAP pattern among sub-basins of the Folsom basin; (c) the model performs better for heavy than for light and moderate precipitation. The analysis also showed significant simulation dependence on the spatial resolution of the boundary and initial fields and on the microphysical scheme used.

Understanding the Sierra Nevada Hydrologic Response to Climate Change

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Abstract:

This talk provides an overview from the findings of two publications with analysis of the following topics:

• Changes in temperature, precipitation, streamflow
• Changes in snow accumulation and snow melt
• Distributions by month, elevation, and latitude
• Timing of the cumulative annual streamflow
• Annual peak flow exceedance probabilities
• Temperature, Heat and Mortality Analysis
• Precipitation and Snowpack Projections
• Sea Level Projections and Impacts
• Projected Vegetation Distributions

A Long-Term (50-yr) Historical Perspective on the Meteorology and Hydrology of Flood-Generating Winter Storms in the American River Basin

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Abstract:

Flood-generating winter storms in the American River basin have gained much of their impact from their intense precipitation, their particular orographic distributions of precipitation, and (often) their warm temperatures. Winter storms can be characterized usefully by the large scale patterns of water-vapor transport that support them, with the vapor-transport patterns serving as unified summaries of each day's pertinent atmospheric circulations, tropical and extratropical moisture supplies, and wind fields.

In this talk, 50-yr long daily chronologies of these transports will be used to identify:

1. Storms that yield particularly large amounts of low-altitude (rainy) precipitation, and
2. The notably warm and intense pineapple-express storms

This long-term perspective reveals that, historically, tropical El Nino conditions have favored enhancements of low-altitude precipitation which, falling most often as rain rather than snow, tends to increase flood risks. Pineapple-express patterns, on the contrary, appear to be favored by nearneutral (non-El Nino) tropical conditions during El Nino-rich decades (warm phases of the Pacific Decadal Oscillation). The resulting chronologies of these events, plus events that might have resulted in floodgenerating storms but did not, help put the risks from such storms into perspective. Some hints as to the future of these storms can be gathered by applying similar approaches to recent climate-change projections.

Evaluate the Snow Depletion Curve Theory in the American River Basin with Distributed Snow Model

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Abstract:

In numerical hydrologic snow models the Snow Depletion Curve (SDC) is commonly used to explain spatial variability in the snow pack within the modeling elements. This curve relates the Snow Cover Area (SCA) to the current Snow Water Equivalent (SWE), which is a model derived variable. A primary assumption carried in the exploitation of such curves is that the accumulation and ablation processes that control the areal SWE distribution are mainly related to the physical properties (e.g., topography, land cover) and climatic signals (e.g., prevailing wind, mean monthly precipitation distribution) of a given basin. Since these basin properties are assumed stationary over the years with relatively small inter annual variability the derivation of a single representative SDC curve is feasible. We test the aforementioned assumption in the upper part (i.e., >1500 meter) of the three forks (i.e., North, Middle, and South) of the American River Basin. The NWS snow-17 model which accounts for the current energy and mass states of the pack, was developed in 1-km2 grid cells to provide both the SCA and the SWE as model variables. The model parameters were spatially distributed to account for radiation variation resulting from land cover and aspect, and 6-hour point observed surface temperature and precipitation was interpolated based on climatologically derived lapse rate and mean annual distribution map (PRISM), respectively. The model derived SDC showed uniqueness of SDC in the upper three Forks, however, entail large inter-annual variability. We developed a procedure that estimate SDC in the beginning of the melting season that is based on Snow Course data which reduces the uncertainty of the snow variables (e.g., SWE and melt) resulted from the SDC.

Using Synoptic Climatology and the PRISM Model to Improve Precipitation Assessment and Prediction

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Abstract:

Spatial and elevational patterns of precipitation vary significantly for different synoptic meteorology situations. Some patterns are associated with rather narrow precipitation signatures, while others affect a large area. "Orographic ratio" (OR), which is defined as the average increase in precipitation with elevation for orographic precipitation, varies with different flow regimes. In general, warm storms characterized by abundant subtropical moisture has a relatively low orographic ratio, while cold advection (generally characterized by northwesterly flow) tends to produce much higher ratios. For this reason, "targeted climatologies" were developed. These used the PRISM model, which consists of a local moving-window, regression function between a climate variable and elevation that interacts with an encoded knowledge base and inference engine. PRISM was run for a variety of synoptic categories, including those commonly associated with major rainfall and snowfall events in the Pacific Northwest, in an effort to better understand the spatial and vertical distribution of precipitation during various types of weather events.

Hydrologic Forecasting on the American River

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Abstract:

The California-Nevada River Forecast Center (CNRFC), in partnership with the California Department of Water Resources (DWR), operates a continuous hydrologic model of the American River watershed. The primary purpose of the model is to provide real-time forecasts of inflow to Folsom Reservoir, in terms of both the actual inflow and the full natural flow (FNF). The hydrologic model is capable of making both short range (flooding) and long range (water supply) forecasts.

Previously the American River watershed was modeled as four sub-basins, and the model only calculated full natural flow into Folsom reservoir. During the most recent recalibration, the watershed was divided into nine sub-basins in order to include simulations of inflow into the four largest reservoirs (French Meadows, Hell Hole, Loon Lake, and Union Valley). By simulating the upstream reservoirs, the new model is better able to forecast actual inflows into Folsom reservoir.

Calibration of the new American River watershed model was performed for the period of record of October, 1989, through September, 1999. Full natural flow computations were designed so that they would reflect the operational FNF calculations performed by DWR. Statistical analysis on the model's simulated FNF showed an overall improvement in performance, both for water supply forecasting and flood forecasting.

This presentation will focus primarily on model performance during the most recent high flow event (January 1–3, 1997). Simulations of both full natural flow and actual inflow to Folsom Reservoir will be examined, and areas for future improvement in the modeling system will be proposed. Finally, an explanation of operational forecast products will be presented.

NOAA Efforts to Improve Precipitation and Flood Forecasting by Studying American River

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Abstract:

The National Oceanic and Atmospheric Administration (NOAA) plans to undertake two related research efforts to improve precipitation, flood, and water resources forecasting. The presentation will briefly describe the Hydrometeorological Testbed (HMT) and the Distributed Model Intercomparison Project: Phase 2 (DMIP 2) with emphasis on the research being proposed for the North Fork of the American River.

The HMT program is being developed for the purpose of advancing water resources data assimilation. The general strategy of this effort is to conduct research and development to deploy advanced systems for observed information to support critical decision making for flood mitigation, hydropower energy generation, water resources control, and fisheries management. More specifically, high resolution atmospheric and hydrometeorologic observations (precipitation, soil moisture, snowpack, winds, temperature, and moisture) will be collected and analyzed for several key water resource applications such as distributed hydrologic model validation, quantitative precipitation forecast (QPF), and estimation (QPE) validation.

DMIP 2 continues the work begun under DMIP 1 to conduct research into advanced hydrologic models for river and water resources forecasting. Twelve groups from the hydrologic research community participated in DMIP 1, resulting in a wealth of knowledge for the scientific community and valuable guidance for the National Weather Service (NWS) research program. DMIP 2 is designed around two themes: 1) continued investigation of science questions pertinent to the DMIP 1 test sites, and 2) distributed and lumped model tests in hydrologically complex basins in the mountainous Western US.