FIRE Cirrus IFO-II addressed the problem of cirrus cloud development and impact over a wide range of scales through the integration of models and observations. The field experiment provided a unique observational basis for improving the quantitative utilization of remote sensing observations from satellites and from the surface for investigating and monitoring cirrus clouds. It must be emphasized that the true value of the observations will be derived from their use in evaluating and improving models and observing systems (data analysis algorithms) which can be used to extend the knowledge gained into the context of the global climate system.
The Cirrus IFO-II was conducted in southeastern Kansas from November 13 to December 7, 1991. The primary subjects were the extended large-scale cirrus systems associated with the subtropical and midlatitude jet streams. The observational elements are summarized.
Satellite data was collected at full spatial, temporal, and spectral (all channels) resolution from all geosynchronous and polar orbiting meteorological satellites viewing the region from 25N to 50N latitude and 75W to 125W longitude. Data from other satellites were also obtained for this region, including Landsat. The ISCCP analysis products (CX) were acquired as part of the Cirrus IFO-II data sets. These data were integral to achieving the experiment objectives.
Four surface-based remote sensing sites were fielded. The stations were closer to each other than in IFO-I (50 km versus 135 km) to better resolve the dominant scales of cloud organization. A greater diversity of the most sophisticated cirrus cloud remote sensing instrumentation were deployed including polarization lidar, scanning cloud lidar, high spectral resolution lidar, infrared (CO2) Doppler lidars, short-wavelength (mm) Doppler radars, high spectral resolution infrared interferometer spectrometers, and other radiometric sensors. A much greater effort to monitor ambient meteorological conditions using active and passive sensors was also performed. The strategy was to obtain coincident observations from as broad a range of complementary observing systems as possible. This facilitated analyses that combined data from multiple systems. In this way, the aggregate information content was maximized in that the limitations and uncertainties of individual systems were compensated by others whose characteristics are quite different. Exploring this potential for synergism was a main theme for Cirrus IFO-II. A complete suite of observing systems was only possible at one location (the Hub) since some of these systems were one of a kind. The instrument complement at the second site (R2) was also extensive, while the third site (R3) was quite limited. A fourth site (R1a) near the Hub location also accommodated the operational requirements of the volume imaging (scanning) lidar (VIL). This was different than the IFO-I strategy of evenly spreading resources over the sites (Starr, 1987a). It should also be noted that the scanning systems provided some coverage over the other sites given the closer spacing adopted for Cirrus IFO-II. In addition, the closer site spacing facilitated greater coincidence with airborne remote sensing and in situ observations since the site separation distance was compatible with the scales of typical aircraft sampling patterns.
Four aircraft participated in the experiment, including one remote sensing platform and three in situ platforms. The remote sensing platform was the high-altitude NASA ER-2, which provided a "satellite-view" with significantly greater observing capability than available from satellites, including a down-looking cloud lidar for the unambiguous detection of cloud top and internal cloud structure. Three in situ platforms were considered highly desirable given that cirrus cloud systems were often quite complex occurring in multiple layers over appreciable depths of up to 5 km (Starr and Wylie, 1990) and typically exhibited great spatial variability in physical structure, especially in microphysical structure (Heymsfield et al., 1990). Cloud microphysical structure represented the key linkage between dynamical and radiative processes. Knowledge of this relationship was fundamental for understanding cirrus cloud development and for evaluating and improving parameterizations of cirrus clouds and their effects for use in large-scale models (GCM's). Accurate knowledge of cloud microphysical properties was essential for achieving the FIRE-II scientific objectives. Three in situ platforms (NCAR Saberliner, NCAR King Air, and University of North Dakota (UND) Citation) significantly improved the sampling of microphysical cloud properties and allowed a greater degree of flexibility in mission planning and execution in comparison to the Cirrus IFO-I (two platforms).
A number of key improvements were made in aircraft instrumentation in comparison to IFO-I. Ice particle number density size distributions were measured down to 10 um particle size. The lack of knowledge of the concentrations of particles smaller than 25 to 50 um represented a major uncertainty in the analysis of the IFO-I observations with important ramifications in the areas of cirrus cloud radiative interactions and remote sensing possibilities (Wielicki et al., 1990; Ackerman et al., 1990). The capacity for collecting actual crystals was also significantly enhanced. These observations were essential for assessing ice crystal habit and morphology which impacted the processing of data from airborne optical microphysical probes and was an important factor in determining cloud development and radiative properties. These instruments were developed and installed on the NCAR King Air and Sabreliner aircraft. In addition, accurate observations of ambient water vapor concentrations in and around cirrus clouds was possible using a new cryogenic frost point hygrometer on the NCAR Sabreliner. These observations were critical for understanding cloud development and testing cloud models. The data obtained was helpful to resolve a number of pivotal uncertainties in our knowledge of cirrus clouds.
The primary motivation for locating the Cirrus IFO-II in the lower midwestern U. S. was the availability of nearly-continuous observations from a mesoscale network of surface-based wind profilers recently constructed by the NOAA Environmental Research Laboratory (ERL). These data served to greatly enhance the value of both the synoptic-scale observations and the in situ and remotely-sensed cloud observations for developing, testing, and validating models of cirrus cloud development on scales ranging from very high resolution cloud models to regional mesoscale models and, ultimately, to cirrus parameterizations in large-scale forecast models and GCM's. In particular, the data permitted derivation of upper tropospheric vertical motions at scales comparable to the mesoscale cirrus cloud features that were such a prevalent characteristic of extended large-scale cirrus cloud systems.
A substantial increase in the collection of rawinsonde data was realized for the Cirrus IFO-II. Three large-scale case studies were conducted where six-hourly soundings were taken for a 36-hour period over much of the continental United States (43 stations). During the final twelve hours, three-hourly soundings were made over a more limited region of the central U.S. (15 stations). These data were absolutely essential for evaluating and improving large-scale models of cirrus cloud development. Supplemental soundings (1800 UTC) from the inner 15 stations were also performed on most other experiment days (10 additional days). Five Cross-Chain Loran Atmospheric Sounding System (CLASS) rawinsonde stations were used to support remote sensing studies and to resolve thermodynamic structure over an area defined by three NWS wind profiler stations.
Forecast experiments with both mesoscale and global-scale models were used to test current and new cirrus parameterization schemes and to examine the predictability of cirrus. Specific models that were used include the Colorado State University/Regional Atmospheric Model (CSU/RAMS), Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR), NASA Ames Research Center/NCAR, and National Meteorological Center (NMC) eta mesoscale models. Experiments using nested grids were performed with the mesoscale models and drew heavily from the highly detailed process models. Large-scale data sets with increased temporal resolution over a wide area were required for improved definition of the development of the dynamic and thermodynamic environment of extended cirrus cloud systems, especially the distribution of upper tropospheric water vapor. This was particularly true for the mesoscale models that served as the bridge between the very detailed models and the global-scale models and provided the framework for investigating scale interactions that were central to the problem of relating cloud response and feedback to large-scale control.
Other collaborative experiments participating in the Cirrus IFO-II were the Spectral Radiation Experiment (SPECTRE) and the Surface Radiation Budget (SRB). Approximately 150 researchers from 40 research institutions from the U. S.,Russia, Japan, Canada, Germany, and Switzerland participated.
During the 25-day period, observations were performed on cirrus clouds associated with the midlatitude jet stream, including baroclinic leaf, ridge crest, and shortwave phenomena; subtropical jet stream, subtropical storms, and tropical sources; and pre-warm frontal systems. Observations were made of the regional development and dissipation of cirrus cloud systems; remote and in situ sensing of microphysical, radiative, and dynamical cirrus cloud properties; and clear sky conditions. For the first time, cirrus clouds were measured from aircraft, satellite, and surface-based instruments over a significant part of the diurnal cycle, including sunrise, daytime, sunset and midnight conditions. A summary of the operational aspects are as follows: 118 platforms, instruments, or models participated; operations were conducted every day; 4 aircraft flew 51 missions for 158 hours; 1524 special rawinsondes were released; and 88 satellite overpasses occurred at times and locations when sondes or aircraft were operating. The science team has selected the periods November 25-26 and December 5-6 for priority data reduction and analysis.
The data analysis phase for the Cirrus IFO-II will include both individual and multi-investigator analyses. End data products for both field experiments are expected to be quantitative measurements of the radiative, physical, microphysical, optical, dynamical, thermodynamical, and meteorological properties of cirrus clouds
Schematic of Cirrus IFO-II Operations Area and NWS Wind Profilers and Rawinsonde Networks
