3. STUDY PERIODS
3.1.
INTENSIVE OBSERVING PERIODS
3.1.1.
Mid-Winter IOP (DRY)
3.1.2.
Early-Spring IOP (WET)
3.2.
BACKGROUND AND ANCILLARY DATA COLLECTION PERIODS
3. STUDY PERIODS
The experiment will be conducted between the Fall of 2001
and the Spring of 2003, and will focus on two characteristic seasons: 1)
mid-winter, when frozen conditions and dry snow covers prevail, and 2)
early spring, when transitional (e.g. frozen and thawed) conditions and
wet snow covers are prevalent. These seasonal characteristics are critically
important to the hydrology, ecology, and meteorology of cold regions. These
characteristics also have a direct affect on microwave radiative transfer
and measurement by remote sensing. Microwave measurement of snow and soil
characteristics in cold regions is sensitive to the phase of water, ice
or liquid. Dry versus wet conditions have important implications both for
different microwave frequencies and for the type of sensor system (i.e.
either active or passive).
3.1. INTENSIVE OBSERVING PERIODS
Ground and airborne data collection will be focused on
two Intensive Observation Periods (IOP) each year. Each period will last
7-9 days (including contingencies for weather delays). Data collection
is expected to be completed in six working days under ideal conditions.
The first IOP will be in mid-February (dry period), the second in late
March (wet period) (
Table 12
). The IOPs will
be conducted on the same day-of year (DOY) schedule each year. Satellite
data sets will in most cases be collected throughout the snow accumulation
and ablation period, from October through June. Micrometeorological data
sets will also be collected through the full accumulation and ablation
period.
The first IOP each year will be conducted in mid-February, from February 19-25, with a contingency of through February 27 (DOY 50-56, 50-58 contingency). This is well before the seasonal peak snow accumulation for the area, although snow cover is usually widespread at this time of year ( Figure 37 ). Snow and soil conditions tend to be variable due to the effects of wind redistribution and snow metamorphism.
In shallow-snow areas such as the North Park MSA, soils
are typically frozen at this time due to a minimum of insulating snow cover.
Snow packs here tend to be thin, cold, and dry at this time of year. In
deeper snow areas, including the Rabbit Ears and Fraser MSAs, snow packs
provide improved insulation to the soil, and subnivean soils may be either
thawed or frozen depending on conditions prior to snow pack formation and
the elapsed time since snowfall formation. Shallow, early-winter snow packs
in these areas often result in steep temperature gradients within the snow
pack and the formation of large depth-hoar crystals, which in turn significantly
affect microwave radiative transfer. Depth-hoar development is often widespread
in these areas by mid-February. As snow accumulates further, snow pack
temperature gradients tend to decrease, depth-hoar formation is reduced,
and the upper snow layers typically remain finer-grained. Mid-winter conditions
will typically include a wide array of coarse- and fine-grained snow packs,
of varying thicknesses. Absorption of solar radiation near the snow surface
can result in the formation of thin melt-freeze crusts, which may also
affect microwave radiative transfer. In general, however, snow packs in
the MSAs remain dry during this time of year.
The second IOP each year will be conducted in late-March, from March 25-31, with a contingency of through April 2 (DOY 84-90, 84-93 contingency). This is a transitional season. Elevation and topographic gradients within the study areas enhance these transitions (this is one reason that this region was selected for the experiment). By this time of year, snow melt is common (or completed) at lower elevations, and snow cover typically is reduced at the North Park MSA and at the lower elevations of the Rabbit Ears and Fraser MSAs ( Figure 37 ). At the highest elevations, snow packs tend to remain cold well into April or even later. In most years there typically would not yet be any significant snow melt at the highest elevations. Between these extremes, a broad range of conditions may be encountered.
In deeper snow areas experiencing early snow melt, the effects of liquid water are generally confined to the upper portion of the snow pack. The presence of liquid water itself affects microwave radiative transfer. Diurnal melting followed by nocturnal refreezing results in coarser, rounded snow grains and grain clusters. This also affects microwave radiative transfer, even if the snow pack has refrozen and is dry. While the sizes of grains or grain cluster may be similar to depth hoar crystals, the shape of the grains and their location within the snow pack are different, which are expected to have different effects on microwave response.
As snow melt progresses, liquid water penetrates deeper into the snow pack and eventually reaches the soil. The fate of meltwater may include infiltration into the soil, ponding at the soil surface, where it may or may not refreeze, and/or retention within the snow pack, where again it may or may not refreeze. Each of these conditions may uniquely affect microwave response. All are likely to be encountered during the early spring IOP.
Subnivean soils are likely to be thawed during this period,
even though soil temperatures may be at or below 0EC
(organic solutes can depress the freezing point down to -4 to -7EC).
Trees and other vegetation may be active by this time as well.
3.2. BACKGROUND AND ANCILLARY DATA COLLECTION PERIODS
In addition to the intensive observing periods each year, background data collection is required for airborne gamma radiation measurement of snow water equivalent and for radar interferometry. Both require background data over snow-free and preferably dry soil conditions. The background airborne gamma measurements also require in situ soil moisture measurements for calibration (described in Section 4.2.3.). Background gamma radiation measurements and the ancillary soil moisture observations will be collected in the early Fall of 2001. Since the DC-8 aircraft is unavailable at this time, background interferometry data will be collected in the Fall of 2002. The dates of these campaigns are not critical. It is more important to keep dates flexible and select a time period when soil conditions are as dry as possible.