APPENDIX A

A.1. Introduction

A.2. Relevant Issues

• The GCIP Program Office remains committed to the support of activities in the LSA- NC. However, resources are extremely limited and unlikely to increase substantially in the near term. Therefore, maximum use must be made of existing infrastructure and projects in this region.

• Airborne gamma radiation measurements of snow water equivalent (SWE) are made routinely by the National Operational Hydrologic Sensing Center. These measurements provide rather accurate estimates of SWE (+1 cm) and surface (0-20 cm) soil moisture (+2 -3%). There are several hundred flight lines within the LSA- NC, each covering an area of approximately 5 km2. There are 50 flight lines in the Minnesota River Basin, one proposed focus area of study. Although these data provide a good sample of SWE over the region, they do not provide complete areal coverage of any single sub-basin or model grid square.

• Although WSR-88D data can provide the high resolution precipitation estimates required to intensively study subgrid-scale heterogeneity, the operational algorithms used to estimate precipitation from the radar returns were developed for warm season precipitation systems. A presentation of NEXRAD precipitation totals from the March 19, 1996 snowstorm in the Midwest emphasized that these estimates are completely inappropriate for snowfall accumulations. However, an enhanced software package called the Warning and Decision Support System will be installed at the Minneapolis- St. Paul (MSP) National Weather Service office. This system includes enhanced processing algorithms for WSR-88D data, including those appropriate for snowfall accumulation.

• An intercomparison of two land surface models using Alaska data shows that substantial differences among models can be expected, especially in the disposition of surface water among evapotranspiration, sublimation, and runoff. The freeze-thaw cycle has received little attention in these models.

• The University of Minnesota operates an experimental site near Rosemount, 15 miles south of St. Paul. Detailed measurements are obtained during the cold season at a 40- acre site. These measurements include the surface energy budget, soil moisture and temperature, standard meteorological variables (pressure, wind profile, relative humidity, precipitation), latent and sensible heat flux, and snow pack changes. The list of variables that are now measured includes nearly all those that have been identified as needed for model intercomparison studies. This site thus is a prime candidate for model process studies. The Illinois State Water Survey operates an automated weather station network called the Illinois Climate Network (ICN). The most notable feature of this network is the routine long-term monitoring of soil moisture using the neutron probe technique. One of the ICN sites is at Bondville. This is also a location of a SURFRAD site and is part of the EPA wet/dry deposition network. In addition, NOAA ATDL may soon install an eddy correlation system for routine monitoring of sensible and latent heat fluxes. The combination of these facilities makes the Bondville site a second prime candidate for model process studies.

• Another candidate site is the Walnut Creek ARS facility in the Des Moines River Basin. This site is also well-instrumented. There are a few drawbacks. Part of the basin is urbanized and snowfall is not as dependable as at the Rosemount site. Also, there may be few unique scientific objectives that can be addressed at this site and not at the other two listed above. Nevertheless, since only two cold season observing periods are planned and unusual weather (e.g., lack of snowfall) may make the observations less typical at the Rosemount site, observations at additional sites can decrease that risk and the Walnut Creek site appears to be a viable candidate.

• A fourth site is the Shingobee River watershed in north-central Minnesota. This is the site of the U.S. Geological Survey's Interdisciplinary Research Initiative (IRI). The objectives of the IRI are to provide the "opportunity for fundamental interdisciplinary research of interactions within the hydrologic system on a watershed scale, to increase the basic understanding of watershed hydrology, and to provide information necessary for better management of our nation's water resources." (Water Fact Sheet, 1994, U.S. Geological Survey). The site is mostly forested, providing a contrast with the mostly agricultural (or urban) nature of the other three sites. Standard climatological measurements are routinely obtained. Additional research initiatives at this site are encouraged by USGS.

A.3 Scientific Themes

A.3.1 Land Surface Model Physics

A.3.2 Land Surface Modeling of SubGrid

A.3.3 Monitoring of the Land-Surface State

A.4. Recommended Activities

A.4.1 Land-Surface Model Physics

(a) radiative, sensible, and latent energy fluxes at the surface-atmosphere interface

(b) snowpack maturation and melt

(c) ground frost, soil temperature, and soil water movement. To foster improvements in modeling, it is recommended that appropriate data sets be collected at more than one site. The validation and development of land surface process modeling will require the set of measurements shown in Table A-1.

Table A-1  Variables Required for Land Surface Model Intercomparison Studies
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Forcing measurements (30 minute resolution)
     U component wind speed at 10 m
     V component wind speed at 10 m
     Temperature at 2 m
     Specific humidity at 2 m
     Surface pressure
     Surface skin temperature
     Precipitation
     Surface Radiation - downward shortwave 
     Surface Radiation - downward longwave
Validation
     Surface Radiation - upward longwave
     Surface Radiation - net radiation (measured)
     Streamflow 
     Soil moisture (profiles)
     Soil temperature (profiles)
     Surface latent heat flux
     Surface sensible heat flux
Set up for Experiment
     Vegetation type and characteristics Site
     Site Description
     Surface Radiation - upward shortwave (albedo)
     Soil characteristics 
     Wilting point 
     Rooting zone 
     Field capacity

Specific activities that are recommended are:

• During ESOP-97, the above data sets should be collected at one or more sites. Based on information presented at the task group meeting, the Rosemount site and the Bondville ICN site appear to be particularly suitable for such efforts. Only minor improvements in measurement capabilities are necessary to meet all of the above data requirements. There are significant and relevant differences in the climates of these two sites that provide different challenges to process models. The Rosemount site is characterized by an extended cold season with near-surface soil temperatures remaining below 0C for months and snowcover usually remaining intact for that period. By contrast, at Bondville several episodes of snowmelt and ground thawing typically occur through the winter months. Additionally, frost depths are greater at Rosemount than at Bondville; therefore, the spring surface thaw will have different physical processes, i.e., the Rosemount soil will have a thick frozen layer below the thawed surface.

• The above datasets should also be collected during ESOP-98 at the same sites. Other suitable sites such as the Walnut Creek and Shingobee River watersheds can be considered for additional data collection efforts.

• An intercomparison of land surface models should be undertaken, possibly as part of a PILPS initiative, based on the data collected at all of the selected sites.

A.4.2 Validation of Land Surface Modeling of Subgrid-Scale Heterogeneity

(a) One critical element is the measurement of the spatial distribution of SWE within the focus area immediately prior to and at frequent intervals during the spring snowmelt. Airborne gamma radiation measurements can supply data at a resolution of 300 m x 300 m by doing multiple passes over a flight line. However, flight lines cover only a small fraction of any of the candidate focus areas. These will probably need to be supplemented by ground-based in situ measurements of SWE. In addition, satellite measurements of snow areal extent and fractional coverage within the focus area should be obtained.

(b) Another critical element is the surface energy budget. This is likely to depend on a number of variables including snow cover, land use (forest vs. agriculture), topography, soil moisture, etc. The per site cost of the installation and operation of surface energy budget stations is high, effectively limiting the number of sites that can be instrumented. However, it is likely that the dominant factors will be snow cover (albedo) and soil moisture. The influence of the secondary factors can be minimized by choosing a focus area of small topographic variability and one predominant use (i.e., agriculture). In addition, soil moisture variability may have a minor effect on the surface energy budget during the snowmelt phase because meltwater will maintain wet conditions at the top of the soil profile. Thus, representative measurements of grid- averaged surface energy budget properties may be possible by sampling one snow-covered and one bare site during the partial snow cover stage. To accomplish this, there should be two mobile systems. Immediately prior to the expected beginning of the spring snow melt, one can be deployed in an area of high SWE and the other in an area of low SWE.

(c) Soil moisture variability will have a major effect on the disposition of meltwater and the threat of flooding. Airborne gamma radiation measurements may be the most effective technique to sample soil moisture in a grid-scale area.

• The GCIP should investigate the suitability of several sites, including the Le Sueur and Cottonwood River Basins, for a study of subgrid-scale variability.

• During ESOP-98, the above data collection effort should be undertaken with an emphasis on the spring snowmelt period.

• A second model intercomparison study should be conducted, validated against areally- averaged values of relevant variables.

A.4.3 Monitoring of the Land Surface State

(a) cooperative observer data - the most serious problem is the well-known low bias of liquid water equivalent because of wind-sensitive under-catchment. A second limitation is the low (compared to the scale of spatial heterogeneity) spatial resolution of the network.

(b) satellite - Areal extent of snow cover and fractional cover within a pixel may be feasible, but snow water equivalent is more difficult.

(c) airborne gamma radiation - these measurements cover only a small percentage of the surface area and are not frequent in time. For soil moisture, data sources include airborne gamma radiation measurements, experimental satellite estimates, and a few ground based in-situ measurements. Since existing in situ measurements are few and fiscal constraints will limit the number of additional sites that can be added or upgraded, it will be necessary to rely heavily on remote sensing.

• A corrected set of the cooperative observer data of snowfall, snowdepth, and SWE should be developed for the LSA-NC both for ESOP-97 and ESOP-98 and for the historical record. It should be feasible to extend it back to 1948. This set should be compatible with the corrected Canadian snow data (i.e., contours should match at the international boundary).

• Optimal methods to combine cooperative observer, satellite, and airborne gamma radiation snow data should be developed. These methods should produce snow fields with acceptable accuracy both for research studies (when all data can be used) and for operational applications (when only a subset of cooperative observer data are available).

• The GCIP office should investigate whether more applicable radar algorithms like those to be used at the MSP radar can be implemented operationally before ESOP-98 for those radar systems covering the LSA-NC.

• NEXRAD data from the MSP site should be archived for ESOP-97 and ESOP- 98. Studies of snow water variability using these and other relevant data should be encouraged.

• The development of methods to combine remotely sensed and in situ soil moisture should be encouraged. Of particular interest are methods that are accurate at the beginning of the cold season, just before the soil freezes and snow cover commences, and just after snow cover has disappeared.

• To the extent possible, GCIP should encourage and support routine soil moisture measurements at several sites within LSA-NC.

• Satellite estimates of fractional snowcover should be obtained for the surface sites of interest (Rosemount, Bondville, Walnut Creek, Shingobee, etc.)