Notice

CASES 97: Observing and Modeling the Effects of Soil Moisture on the Diurnal Behavior of the Atmospheric Boundary Layer

From 22 April to 22 May 1997 the first expedition to the CASES site was made by boundary layer and radar meteorology scientists from Universities and government laboratories. For further information on the CASES site, collaborators, and philosophy, please contact the CASES website ( http://www.mmm.ucar.edu/cases). The broad objectives of the expedition were two-fold: a) observe and model the effects of soil moisture on the fair weather boundary layer's diurnal cycle and b) Begin a determination of the relationship between S-band polarized radar signals, NEXRAD radar signals, and observed rainfall. The two objectives covered most of the weather events that occurred during the expedition.

Data from the following instrumentation will be available by 22 May 98 at the latest:

1. Winds and RASS virtual temperature from three 915 MHz/sodar profilers arranged in a triangle covering the southern two-thirds of the Walnut River Watershed apprx. 20 miles east of Wichita, KS. These Profilers and two surface flux stations are part of the Argonne Boundary Layer Experiment (ABLE; see http://gonzalo.er.anl.gov/ABLE/, which is the backbone of CASES. These profilers have continued to operate after CASES97.

2. Cross-chain Loran Sounding System (NCAR) rawindsondes were collocated with the ABLE Profilers and launched every 90 minutes during 24-hour Intensive Operation Periods (IOP). CLASS sondes were released twice a day during non-IOP days. These sondes measured wind, temperature, humidity, and pressure. Sondes were equilibrated with local surface conditions before release.

3. 12 Eddy correlation surface flux stations located on surfaces which comprised most of the primary land-use over the watershed. The top land surface types were: prairie grass (grazed and ungrazed) which dominated the land area east of the north-south running Walnut River, winter wheat crop-land and tilled but unplanted crop-land which dominated the land area west of the Walnut River. In one or two cases, stations were located on similar land surface types at similar and different elevations to check for similarity and elevation differences. The surface fluxes measured were: sensible heat, moisture, and momentum. One station measured CO2 flux. All of the stations observed the surface energy balance including radiative terms, soil temperature, infra-red surface temperature, and SOIL MOISTURE. Two of eight NCAR stations measured soil moisture profiles to approx. 0.8 m depth; one of those NCAR stations, over a greening up field of prairie grass, measured a greenness index related to NDVI. Two stations (one from ABLE and one from ARM/CART) continue to operate after the expedition.

4. Two aircraft platforms, the NSF-Univ. Wyoming King Air and the NOAA/Air Resources Lab Twin Otter were present. These aircraft carried similar instrument packages to measure atmospheric temperature, humidity, pressure, winds, up and down-welling short and longwave radiation, IR "surface" temperature, and video records of ground surface overflown. These aircraft also carried "turbulence" packages capable of observing rapid fluctuations of vertical wind, horizontal wind, temperature, and humidity that can be combined to estimate vertical fluxes of sensible heat, moisture, and momentum; the NOAA aircraft also measured CO2 fluctuations, CO2 flux, and cloud condensation nuclei particle distributions. The two aircraft flew coordinated missions on IOPs 1-5 while the NSF King Air flew alone on remaining IOPs.

5. S-band Polarized Radar system (SPOL) from NCAR was present. It has the capability to determine precipitation type as well as rate and Doppler air motion. Using insects as targets, air motions in clear air were also measured. The radar was able to observe sharp gradients in humidity and thus was able to give estimates of boundary layer height.

6. Digital all-sky imager/laser ceilometer combination from NOAA/ETL was near one of the surface flux towers located in a winter wheat field. This instrument operated continuously.

7. Satellite retrievals of NDVI and standard images.

8. Stream-flow information routinely collected by NOAA within the watershed.

Experimental Design was as follows:

1. IOPs were carried out only for fair-weather situations; there were four full IOPs (each covering an entire 24-hr period) and 3 partial IOPs (covering either the morning transition from stable to unstable conditions or the evening transition from unstable to stable conditions). Except for the last IOP, NO LOWER CLOUDS WERE OBSERVED. However, scattered to broken cirrus was present for most of the IOPs.

2. IOPs were separated by convective weather events ranging from scattered showers to mesoscale convective systems containing tornadoes. This gave CASES97 a wide range of soil moisture conditions ranging from water standing in fields to spotty conditions during dry-down periods.

3. Substantial greening of prairie grass was observed over the month the expedition was in the watershed. Winter wheat was already green but grew taller during the period. For the first two IOPs considerable field burning was observed resulting in a very hazy situation compared to later IOPs. The intensity of the burning was unexpected.

4. During an IOP, CLASS soundings were launched every 90 minutes.

5. Aircraft flight tracks sampled the horizontal and vertical structure of the atmosphere. Horizontal structure was observed by flying constant pressure (pressure altitude) between the Profiler/CLASS sites making up a triangle covering the lower two-thirds of the watershed; an intercomparison leg was flown from the aircraft base in Ponca City, OK (50-60 nm south of the watershed) to the southern-most profiler. This will allow for comparison of aircraft and profiler winds as well as estimates of gradients to the south (important during south wind periods). Vertical structure was sampled in three ways:
   1. ramp and spiral soundings to observe mixed layer height changes with time as well as detailed vertical structure.

   2. stacks within the triangle of profilers of 3-5 level legs depending upon the depth of the mixed layer. This was done to observe the shape of the flux profile and estimate the flux divergence term of the budget equations for sensible heat, moisture and momentum. Some legs were flown in the inversion layer to obtain fluxes and conditions above the mixed layer as well as sample portions of the inversion layer with observed radar "structures".

   3. vertical race-track patterns which sampled only the lowest level (apprx. 30m above ground) and a level just below the mixed layer height (varied from 200m to 1.7 km). This pattern supplemented the stack pattern in order to track time changes of the near-surface fluxes and near-inversion fluxes (also known as entrainment flux).

For further information, please contact either Bob Grossman or Peggy LeMone.

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