The Cirrus IFO will gather data to support high spatial and temporal resolution studies of cirrus cloud fields. The observations will be of sufficient accuracy and resolution for validation of radiative transfer models, ISCCP cloud property retrievals, cloud models, and GCM cloud parameterizations. Diagnosis of cloud processes requires careful measurement of environmental conditions and cloud structure as the cloud field evolves.The two new and unique aspects of this field study for FIRE are inclusion of simultaneous "cloud truth" observations to validate retrievals of cloud properties from satellite measurements and the observations of upwelling and downwelling radiation at the surface, near the cloud boundaries, and at other levels in the atmosphere in conjunction with simultaneous satellite radiance measurements. The intensive measurements must be coordinated with satellite observations so that diagnostic results can be extended to scales of 100 km and days.
In addition, the intensive phase will provide the opportunity for a three-platform intercomparison study. Ground, aircraft, and satellite measurements will be combined to infer the effects of processes that are extremely difficult to measure directly. For example, aircraft radiation measurements taken above the clouds will be combined with satellite measurements to deduce free-atmospheric radiative heating/cooling rates; likewise, ground and aircraft data will be used to deduce free troposphere and whole boundary layer heating rates.
Radiative modeling is required in this analysis to test the accuracy of extending the "point" measurements to larger scales. Cloud process models are needed to attempt simulations of the observed time history and to understand the significance of the larger scale variations. GCM studies should concentrate on diagnostic studies to forecast studies to determine both the sensitivity of such diagnoses to measured variables and the crucial statistical quantities which are needed to constrain the parameterization and which should therefore be determined from the observations. Model studies should also be used to plan subsequent observation sequences.
PLATFORMS, INSTRUMENTS, AND MEASUREMENTS
The cloud lidar/radiometric instrumentation will be deployed at four field sites. A list of this surface-based instrumentation in included in the section Participating Instruments.
Because of the importance of relating the observed cloud and radiative fields to the large scale meteorological conditions and the difficulty of obtaining good estimates of large scale vertical motion at cirrus cloud levels, it is proposed that two independent wind sounding systems be deployed over the intensive field observing region.
The first is the conventional rawinsonde network that will be operated by NOAA/NWS. Coordinated on-demand launches will be required of all these sites during times when aircraft missions are in progress. The affected stations would be St. Cloud, MN; International Falls, MN; Green Bay, WI; Omaha, NB; Peoria, IL; St. Marie, MI; and Flint, MI. In addition, at least one and possibly two stations will be required in the observing region (western edge). Data from the supplementary rawinsonde sites will allow the analysis of these data to resolve subsynoptic scale (mesoscale) special structure in the larger scale forcing, e.g., vertical motion, in association with observed variations in the cloud field at this scale. In addition, these data will provide a tie-on point between the aircraft observations and the sonde observations.
The second upper-level wind system is a VHF wind profiling system being developed by Astronautics, Inc., Madison, Wisconsin for possible use by NOAA/ERL. This system is unaffected by problems of sonde tracking and drift and may provide an accurate direct measurement of ambient vertical motion over each unit. In addition, horizontal wind speeds and directions can be observed which may then be used to infer larger scale vertical motions in much the same way as is done for rawinsonde observations. Continuous sampling allows time averaging that ensures a much greater degree of representativeness than rawinsonde observations offer. To achieve high accuracy at cirrus cloud levels wind profilers should be operated at a frequency of 50 MHz. Ideally, three of these sites would be operated within the field region provided resources can be found to support this activity. At the very least, one profiler would be very useful to provide a detailed description of the convective structure of the overlying cirrus cloud. Astronautics, Inc. of Madison, Wisconsin is currently developing a prototype microwave wind profiler operating at 200 MHz. Astronautics expects it will be operational in time for the IFO. Discussions are underway to include it within the IFO operations.
The preliminary site selected for the first experiment will be a (-150 km) area in central Wisconsin.
Wisconsin was chosen for a number of reasons. Climatologically, this area has a relatively high and reliable frequency of cirrus cloud occurrence (-35%), especially pre-warm frontal cirrus during this fall time period. The northerly location together with the selected time period will ensure that cloud overflights are possible with at least two research aircraft, i.e., relatively low cirrus layers (<12 km) will occur. Furthermore, complications in the analysis of vertical motion fields due to orographic effects are minimized by using this site. This site is within the view of both GOES satellites and is close to Lake Superior and Lake Michigan, which provide uniform backgrounds for aircraft-satellite cloud data intercomparisons.
