Science Plan for CLEX-10

The Cloud Layer EXperiments (CLEX) are a series of field experiments whose goals are to better understand the microphysics and dynamics of non-precipitating, mid-level, mixed-phase clouds (i.e. altocumulus, altostratus and "altostratocumulus"), which we will refer to as "CLEX clouds". Studies have shown that these clouds cover ~22% of the Earth (Warren et al. 1986, 1988). They have been called the "forgotten clouds" (e.g. Vonder Haar et al. 1997; Fleishauer et al. 2002) since there is comparatively little literature on them. These complex clouds should not be forgotten. They are important for several reasons:

    - They routinely contain supercooled liquid droplets, which are the source of aircraft icing. This is particularly problematic for small civilian aircraft and Unmanned Aerial Vehicles (UAVs).

    - They occur at mission critical altitudes for U.S. military aircraft. As a result, they obscure ground targets, prevent mid-air refueling efforts and prohibit flights by UAVs, which have little to no de-icing capabilities.

    - Altocumulus clouds have the ability to produce significant turbulence.

    - Many mid-level clouds are mixed-phase and provide a unique testing area for microphysical theories (e.g. ice initiation from supercooled liquid layers, liquid - ice phase interactions).

    - The radiative properties of mixed-phase clouds are not well known. There is large uncertainty in how these clouds are related to global climate. Clouds are perhaps the largest source of uncertainty in simulations of global warming (e.g. Cess et al. 1993).

The Tenth CLEX field campaign (CLEX-10) is designed to improve our knowledge of mid-level, mixed-phase clouds both microphysically and dynamically. The overall goals of this research are:

    - improve the understanding of ice initiation in supercooled liquid layers and how liquid and ice particles interact;

    - improve/validate satellite cloud phase detection algorithms;

    - characterize the large scale evironment where these clouds form and dissipate;

    - validate the use of CloudSat and CALIPSO for studying mid-level and mixed-phase clouds;

    - characterize the background cloud condensation nuclei (CCN) and ice nuclei (IN) concentrations over the study region; and

    - improve numerical forecasts of mid-level clouds.

CLEX-10 will be an extension of the Canadian CloudSat/CALIPSO Validation Project (C3VP) that will take place from 31 October 2006 to 1 March 2007 over Southern Ontario and Southwestern Quebec, Canada. The primary goal of C3VP is to provide in-depth validation of CloudSat data products at mid- to high latitudes during the cold season. Primary focus will be given to stratiform cold cloud systems over Southern Ontario. Preliminary satellite climatologies performed by the authors have revealed a relatively high rate of occurrence of mid-level, mixed-phase clouds over this region and timeframe compared to the U.S. Great Plains where previous CLEX studies have been located. C3VP offers a unique opportunity to observe mid-level and/or mixed-phase clouds through a combination of satellite, aircraft and ground based instruments.

  1. Satellite Instruments

As stated previously, the primary objective of C3VP is to validate measurements and retrieved products from the CloudSat and CALIPSO satellites, which are a part of the "A-Train" (Stephens et al. 2002). The A-Train is a series of satellites in a line in a sun synchronous orbit. The first (Aqua) and last (Aura) satellites pass over the same point on the Earth nearly 15 minutes apart, with CloudSat and CALIPSO in between. The reader is referred to Stephens et al. (2002) for detailed information about the satellites within the A-Train.

A short list of instruments on these satellites includes: MODIS (visible radiometer), AIRS (infrared sounder), AMSU-A and AMSR-E (microwave sounder and radiometer), 94 GHz cloud radar, 532nm and 1064nm channel lidar, POLDER (visible and near-infrared polarimeter), HIRDLS and MLS (infrared and microwave limb sounders) and OMI (ultraviolet spectrometer).

The 94 GHz cloud radar and the 2-channel lidar make up CloudSat and CALIPSO, respectively. They are designed to make the first global satellite climatology of cloud profiles.

  2. Aircraft Instruments

C3VP and CLEX will use the National Research Council's (NRC) Convair-580 research aircraft. This aircraft will be instrumented with a suite of microphysical probes and remote sensing instruments including: Passive Cavity Aerosol Spectrometer Probe (PCASP), several Forward Scattering Spectrometer Probes (FSSPs) including FSSP-100, FSSP-100X and FSSP-300, Particle Measuring Systems optical array probes: 2DS, 2D2C, 2D-C and 2D-P, HVPS, Nevzorov TWC probe, King LWC probe, Rosemount Icing Detector, chilled mirror hygrometer, Counterflow Virtual Impactor, upward and downward looking 532nm and 1064nm channel lidar, upward and downward looking 35 GHz cloud radar, up/down/side looking 94 GHz dual pol Doppler radar, visible and infrared radiometers, and the CSU IN counter. A complete list of aircraft instrumentation available is located here.

Aircraft operations will be based in Ottawa, ON. The Convair-580 has an in-flight ceiling of 7km and a range of about 5 hours of flight time.

CLEX flights will utilize in equal proportions a combination of Eulerian and Lagrangian flight tracks, as well as horizontal and vertical (slant or spiral) flight legs. Significant time will be given to sample the atmosphere above and below cloud, which will optimally utilize measurements of the lidar and IN counter, as well as shed light on the broader environment in which these clouds form and dissipate.

  3. Ground Measurements

Surface observations will be taken from the Center for Atmospheric Research Experiments (CARE), located about 45 miles NNW of Toronto, ON (roughly one hour of flight time from Ottawa). The CARE site is a climate reference station and part of the Canadian Air & Precipitation Monitoring Network (CAPMon). Research groups from several universities and government agencies will set up a wide array of instruments at CARE including: W-, X-, and multi- (W-Ka-Ku) band radars, ceilometer, two video disdrometers, two Parsivel laser disdrometers, Precipitation Occurrence Sensor System (POSS), Radio Acoustic Sounding System (RASS), 89 & 150 GHz profiling radiometer, 915 MHz wind profiler, Geonor precipitation gauge, hot plate precipitation rate sensor, visibility meter, 10m meteorological tower, several broadband radiometers and a Vaisala radiosonde system.

Observations will be taken daily/continuously at CARE from 1 October 2006 to 31 March 2007.

  4. Putting it all together

The sun synchronous orbit of the A-Train has a 16 day repeat cycle and passes over Southern Ontario twice daily at approximately 1:30 AM LT and 1:30 PM LT. Because of this, C3VP has planned four intensive observation periods (IOPs) of 10-12 days each that cover every other repeat cycle of the satellite orbit. The dates of the IOPs are as follows:

Each IOP contains 6 identical satellite overpasses (5 daytime and 1 nighttime) over Southern Ontario and/or Southwestern Quebec that will be the targets of C3VP flights. IOPs II, III and IV will also have a sixth daytime flight. Due to weather considerations and flight hour limitations, one of the satellite overpasses in each IOP will be skipped at the discretion of the flight scientists and forecasters. This is (and will be) shown graphically on the daily operations page. CLEX does not currently plan on participating during IOP IV, but this may change depending on weather and flight time considerations. (Note: CLEX did participate in IOP IV - remotely. The C3VP aircraft crew did a good job keeping their eyes open for good CLEX cases and, with our guidance, flew during several good cases for us while we stayed in Fort Collins.)

On each flown satellite overpass day, the aircraft will be targeting cloud systems along and near the subsatellite track, with the goal being to take measurements directly underneath the A-Train as it passes overhead. Additional flight time will be used by CLEX to target any mid-level or mixed phase clouds along or near the flight path. Also, on non-overpass days, flights dedicated to CLEX may be flown, weather permitting.

Top priority for flights will be given to those times with satellite overpasses and suitable clouds nearest the CARE site. Secondary priority for flights will be given to non-overpass days with suitable cloud near the CARE site. This is followed by, in order of decreasing priority, flights with suitable cloud under a satellite overpass, and flights with suitable cloud within the Convair-580's flight range on non-overpass days. Data from CARE on days with suitable cloud but no flight due to practical considerations (i.e. airport closed due to snow, aircraft grounded for repairs, etc.) will also be collected.

Previous CLEX field campaigns have allowed for significant insight into the nature of mid-level, mixed-phase clouds. See our list of publications for more information. These field campaigns have all occurred over the Great Plains of the United States, with all flights having taken place during daylight hours. Several questions leftover from these experiments have yet to be answered:

    - What conditions are favorable for the formation of mid-level, mixed-phase clouds?

    - How do mid-level, mixed-phase clouds over the Great Plains differ from those that form elsewhere?

    - How is the supercooled water in mixed-phase clouds characterized? What are the mechanisms that sustain and deplete supercooled liquid water? And how important are each of those mechanisms?

    - What processes are involved in the formation of ice particles? How does the ice phase evolve in time and space? How important is the ice phase to the overall morphology and lifecycle of mixed-phase clouds?

    - How is the morphology of mixed-phase altered by the presence or absence of solar radiation?

    - What rates of aircraft icing can be expected in mid-level, mixed-phase clouds?

    - To what extent do ice nuclei affect the microphysics and overall morphology of mixed-phase clouds?

Joining CLEX with C3VP will allow for a rigorous investigation into the answers of these questions. Dedicated nighttime flights will give CLEX its first look at mid-level, mixed-phase clouds in the absence of solar radiation. The use of the CSU IN counter, PCASP probe and airborne lidar will give an unprecedented view of the effects of aerosols on the microphysics of mixed-phase clouds. The suite of microphysical probes allow for a detailed, quantitative examination of the microphysical properties that previous CLEX campaigns have been lacking.

Overall, the comprehensive collection of airborne, satellite and ground-based instruments offer a unique opportunity to study mid-level, mixed-phase clouds in a way that has never been done before.

Cess, R. D., M-H Zhang, G. L. Potter, H. W. Barker, R. A. Colman, D. A. Dazlich, A. D. Del Genio, M. Esch, J. R. Fraser, V. Galin, W. L. Gates, J. J. Hack, W. J. Ingram, J. T. Kiehl, A. A. Lacis, H. Le Treut, X-Z Liang, J. F. Mahouf, B. MaAveney, V. P. Meleshko, J-J Morcrette, D. A. Randall, E. Roeckner, J-F Royer, A. P. Sokolov, P. V. Sporyev, K. E. Taylor, W-C Wang and R. T. Wetherald, 1993: Uncertainties in carbon dioxide radiative forcing in atmospheric general circulation models. Science, 262, 1252-1255.

Fleishauer, R. P., V. E. Larson and T. H. Vonder Haar, 2002: Observed microphysical structure of mid-level, mixed-phase clouds. Journal of the Atmospheric Sciences, 59, 1779-1804.

Stephens, G. L., D. G. Vane, R. J. Boain, G. G. Mace, K. Sassen, Z. Wang, A. J. Illingworth, E. J. O’Connor, W. B. Rossow, S. L. Durden, S. D. Miller, R. T. Austin, A. Benedetti, C. Mitrescu, and the CLOUDSAT Science Team, 2002: The CLOUDSAT Mission and the A-Train: A New Dimension of Space-Based Observations of Clouds and Precipitation. Bulletin of the American Meteorological Society, 83, 1771-1790.

Vonder Haar, T. H., S. K. Cox, G. L. Stephens, J. M. Davis, T. L. Schneider, W. A. Peterson, A. C. Huffman, K. E. Eis, D. L. Reinke and J. M. Forsythe, 1997: Overview and Objectives of the DoD Center for Geosciences-sponsored "Complex Layered Cloud Experiment". Cloud Impacts on DoD Operations and Systems Conference, preprint, Newport, RI, September 1997.

Warren, S. G., C. J. Hahn, J. London, R. M. Chervin and R. Jenne, 1986: Global Distribution of Total Cloud Cover and Cloud Type Amount Over Land. NCAR TN-273 STR, 216 pp.

Warren, S. G., C. J. Hahn, J. London, R. M. Chervin and R. Jenne, 1988: Global Distribution of Total Cloud Cover and Cloud Type Amount Over the Ocean. NCAR TN-317 STR, 212 pp.