CRYSTAL-FACE Science Overview

1.0 CRYSTAL-FACE Science Overview

CRYSTAL-FACE is a measurement campaign designed to investigate tropical cirrus cloud physical properties and formation processes.  Understanding the production of upper tropospheric cirrus clouds is essential for the successful modeling of the Earth’s climate.

1.1 Background

1.11 Tropical Cirrus Clouds

Cirrus clouds are high, cold clouds composed of ice crystals. In the tropics, cirrus form at altitudes of about 30,000 to 60,000 feet (9-18 km).  Among other mechanisms, tropical cirrus are generated at the tops of cumulonimbus clouds. These deep convective clouds pump water vapor and ice crystals to the upper troposphere creating the stratiform cloud seen as the top of an anvil. The cirrus anvils can spread to cover vast areas and persist for several hours.  Tropical cirrus are also frequently observed in locations remote from deep convection, perhaps existing as remnants of convective storms or perhaps formed by other processes.  In the top few kilometers of the tropical tropopause, laminar, optically thin (often subvisible) cirrus occur frequently.

Tropical cirrus clouds play an important, but complex role in the earth’s climate system.  Cirrus ice crystals scatter incoming sunlight, reducing the solar radiation reaching the earth’s surface and resulting in a surface cooling effect. Cirrus clouds also absorb upwelling infrared radiation emitted by the surface and lower atmosphere, effectively reducing the infrared energy escaping the earth-atmosphere system.  The interaction between cirrus and infrared radiation heats the upper troposphere and, indirectly, has a surface warming effect.  The net effect of tropical cirrus on surface temperatures depends on several factors including cloud height, cloud thickness, and ice crystal sizes.  A thorough understanding of the radiation budget in the tropics is critical to better model the global climate since solar energy absorption in the tropics is the heat engine driving the entire atmospheric circulation.

The ultimate role of tropical cirrus in future climate change involves feedback effects.  For example, anthropogenic greenhouse gases can increase the surface temperature, presumably resulting in increased frequency and intensity of convective storms.  The increased convection intensity will likely result in altered tropical cirrus cloudiness, with corresponding effects on the earth radiation budget and additional surface temperature changes. Hence, the net effect of increased greenhouse gas concentrations on surface temperature depends on the response of convection and cirrus to the changing environment.  Prediction of these feedback effects requires understanding of the full cirrus lifecycle from generation in deep convection to horizontal spreading and ultimate dissipation.

Tropical cirrus may also be changing in response to anthropogenic aerosols.  Particles from industrial activity or biomass burning may affect ice nucleation in the convective updrafts, ultimately changing the numbers and sizes of cirrus ice crystals.  These cirrus modifications would ultimately affect radiation budgets and climate.

1.12 Cirrus Clouds and Water Vapor

Tropical cirrus can also affect climate through their role in the water vapor budget. Water vapor is the dominant greenhouse gas in the earth’s atmosphere. The majority of the temperature change predicted in response to increasing greenhouse gas concentrations (e.g., CO2, CH4, etc.) is attributed to the water vapor feedback effect: increasing concentrations of CO2 and other greenhouse gases warm the surface, resulting in increased moisture in the atmosphere, ultimately leading to additional surface warming. Water vapor in the upper troposphere has a particularly strong infrared radiative effect due to the low temperatures there. The mass of water deposited in the upper troposphere by deep convection depends on how effectively water is removed from the rising air by precipitating droplets and ice crystals.

Water vapor in the stratosphere is important not only for its radiative forcing, but also for its role in stratospheric chemistry. Stratospheric water vapor concentrations affect both the production of OH radicals and the formation of polar stratospheric clouds.  These polar stratospheric clouds play an integral role in polar ozone destruction.

Water vapor enters the stratosphere almost exclusively through the tropical tropopause. The dryness of the stratosphere is caused by freeze-drying of air as it crosses the cold tropical tropopause. Water vapor in excess of saturation condenses on ice crystals that fall out of the rising air, preventing the condensed water from getting into the stratosphere. The result of this freeze-drying is extremely dry air in the lowermost tropical stratosphere. Water vapor concentrations increase slowly due to methane oxidation as air is transported upward and poleward by the stratospheric circulation.

Remote sensing and in situ measurements indicate a trend of increasing water vapor concentrations in the stratosphere in recent decades.  This trend cannot be explained by trends in tropical tropopause temperature or methane concentrations. Given the importance of stratospheric water vapor there is a need to understand the detailed processes controlling water vapor concentrations entering the stratosphere in the tropics.

1.13 Clouds and Climate Models

The reliable prediction of climate change ultimately depends upon the accuracy of general circulation models (GCMs). Evaluations of several GCMs have identified cloud feedbacks as the leading source of uncertainty in attempting to predict the response to increasing greenhouse gas concentrations over the next century.  Representing clouds in GCMs is challenging for two reasons: First, there is a severe scale mismatch. GCM horizontal grid box dimensions are typically at least 100 km. In contrast, cloud properties and the vertical motions that drive cloud formation occur on scales as small as a few meters. This scale discrepancy forces climate modelers to represent clouds with parameterizations predicting the grid-box cloud properties given only the large-scale meteorological fields.

A second problem facing climate modelers is the limited knowledge of tropical cirrus physical properties and processes. Relatively few measurements of tropical cirrus ice crystal size distributions or radiative properties have been made. The sensitivities of cirrus anvil size and lifetime to the strength of convective forcing are not known. The basic physical properties of cirrus and the processes controlling their lifecycle must be known before realistic parameterizations can be developed for use in GCMs.

2. CRYSTAL-FACE Science Goals

The CRYSTAL-FACE mission is designed to address several specific science questions related to tropical cirrus clouds. The objectives of the mission are to make progress on these issues using a combination of measurements and modeling.

2.1 Cirrus Anvil Sensitivity to Convective Intensity

Recent studies have shown that the response of surface temperature to increasing greenhouse concentrations depends sensitively on the processes controlling tropical cirrus anvil production. As greenhouse gases drive up the sea surface temperature, convection will become more intense. However, it is not clear that increased convective intensity implies larger, longer-lived cirrus anvils. In stronger convective systems, the removal of water by droplet and ice crystal precipitation may be more efficient, resulting in decreased ice mass outflow into the anvil. Evaluation of this sensitivity using satellite data has proven challenging because of problems determining convective intensity and cirrus anvil properties from satellite measurements.

A key objective of CRYSTAL-FACE is to evaluate the sensitivity of cirrus anvils to their generating convective systems using a case-study approach. We plan to characterize the convective systems (structure, mass fluxes, updraft velocities) using airborne and ground-based Doppler radar. Then, in situ and remote sensing instruments will be used to characterize the ice crystal size distributions, cloud structure, radiative properties, and the evolution of cirrus anvils produced by these convective systems.  Past field experiments have generally focused on either the convection and precipitation production processes or the properties of the cirrus anvils. In CRYSTAL-FACE, an attempt will be made to relate the convective and stratiform stages of the cumulonimbus storm system development. The goal is to sample several cumulonimbus systems during the deployment.

These case studies will be extremely useful for modelers attempting to simulate cirrus anvil generation. Several modeling groups will use sophisticated dynamical / microphysical models to simulate the convective systems and cirrus anvils sampled during CRYSTAL-FACE.  The objective here is to improve understanding of the processes controlling the cirrus anvil production and evolution. These processes include the dynamics of the convection and the outflow anvil, cloud microphysics (droplet activation, ice crystal nucleation, coalescence, precipitation, etc.), and interactions between dynamics, microphysics, and radiation. These case-study modeling efforts will serve both to improve the detailed cloud models and to provide insights for development of GCM cloud parameterizations.

2.2 Evolution of Cirrus Anvils

In addition to investigating cirrus anvil production processes, we also hope to improve understanding of cirrus anvil evolution processes. The coverage of cirrus in the tropics depends on anvil lifetimes and spreading by wind shear. Solar and infrared radiative heating in cirrus anvils can drive thermal instability and small-scale convection within the anvils. It is not known to what extent these secondary convective motions extend the lifetime of tropical anvils. Other factors likely to affect cirrus anvil lifetime include upper tropospheric humidity, large-scale dynamics, and wind shear. Extremely strong convective systems can generate cirrus with tops in the highest few kilometers of the troposphere. The final stage of these very high cirrus is unclear. As the larger ice crystals fall out, leaving behind optically thin cirrus, the clouds may be lofted by radiative heating, resulting in persistent thin cirrus as often observed near the tropopause.  These thin tropopause layer clouds can also be formed in situ due to slow ascent in the vicinity of the tropopause.

Our goal is to address these issues by measuring cirrus anvil properties through as much of the cloud lifecycle as possible using airborne, ground-based, and satellite instruments. These measurements will characterize the cloud structure, ice crystal size distributions, ice water content, ice crystal single-scattering properties, radiative fluxes, relative humidity, and wind velocities. Along with the cloud measurements, modeling studies will be undertaken to understand the processes controlling the evolution of cirrus anvils.

Much of the cirrus cloud cover in the tropics is not directly attached to (or necessarily originating from) deep convective systems. We anticipate sampling many such layers during CRYSTAL-FACE. Using in situ measurements of trace gases transported to the upper troposphere by convection (e.g., CO, C2H4, etc.), along with trajectory analyses, we hope to improve our understanding of the origin of these isolated cirrus in the tropics.

2.3 Upper Tropospheric Water Vapor

In spite of the climatic importance of water vapor in the upper troposphere, the processes controlling moisture levels in this region are poorly understood. A particularly important issue is how deep convection affects upper tropospheric humidity. The direct effect must be to moisten the upper troposphere since deep convection injects large amounts of moisture into the upper troposphere. However, increased convective activity may alter tropical circulations, effectively accelerating the subsidence in the vast majority of the tropics outside convection. Hence, increased convective intensity may actually result in a drier upper troposphere.

Resolution of this issue will ultimately require understanding of processes on scales ranging from individual clouds to the global circulation. We hope to gain insight into the relationship between upper tropospheric humidity and convection during CRYSTAL-FACE using in situ measurements of humidity and tracers of convection. The tracer measurements, along with trajectory studies will allow us to determine the history of air parcels sampled. For example, we can determine whether layers of high relative humidity often observed in the upper troposphere originate from convection and how long these high humidity layers last after convective injection.

2.4 Lower Stratospheric Water Vapor

Understanding the processes controlling stratospheric humidity is important not only for its greenhouse forcing, but also because water vapor in the stratosphere plays important role in stratospheric chemistry.

Several issues regarding dehydration of air as it enters the stratosphere across the cold tropical tropopause remain unresolved.  In particular, the relative importance of fast processes (deep convection into the tropopause region) and slow processes (gradual ascent across the tropopause and thin cirrus formation) is not known. The effectiveness of dehydration associated with thin cirrus formation in slowly rising air depends upon poorly understood ice nucleation processes as well as the presence of wave-driven temperature oscillations on a variety of scales.

Resolution of these issues has remained elusive in part due to the lack of accurate measurements of relative humidity in the tropical tropopause region. The combination of accurate in situ water vapor, water vapor isotopes, temperature, cloud particle, aerosol, and tracer measurements during CRYSTAL-FACE should allow us to make progress on this problem.

2.5 Validation of Remotely Sensed Cirrus Cloud Properties

Resolution of many issues regarding cirrus production and interactions with the global circulation will require remote sensing measurements from ground-based and satellite instruments with large spatial and temporal coverage. For example, understanding how cirrus clouds impact regional and global upper tropospheric humidity clearly requires analysis of large-scale cloud and humidity fields. Remote sensing will constitute an important part of the measurement campaign by providing the horizontal distributions of cloud properties and gas concentrations at a variety of spatial and temporal scales.

Satellite remote sensing has been a central theme of CRYSTAL-FACE’s scientific predecessors throughout the First ISCCP Radiation Experiment (FIRE) investigations of cloud systems. CRYSTAL-FACE will play a crucial role in providing validation opportunities for Terra, Aqua and TRMM cloud property retrieval algorithms. Beyond validation of existing algorithms, the data set generated by the CRYSTAL-FACE field campaign will support development of future satellite retrieval schemes (CALIPSO, CloudSat, EOS-Aura) for the retrieval of cirrus cloud properties and upper tropospheric water vapor.

In situ measurements of ice crystal size distributions, ice mass, and water vapor will be used for validation of remote sensing measurements of these quantities. The remote sensing measurements will include both ground-based and airborne lidars, radars, and radiometers.

3. Science Implementation

The CRYSTAL-FACE mission design was driven by the science goals described above. The mission will occur during the month of July, 2002 in the Florida region. Measurements from ground sites, aircraft, and satellites will be made. The mission will also include extensive modeling efforts.

3.1 Deployment Site

Aircraft will be based at Key West Naval Air Station. Ground-based instruments will be located on the southwest coast of Florida. The primary target region is southern Florida and the surrounding waters where deep convection is known to occur frequently in July. The south Florida region offers extensive assets in the form of rawinsondes, Doppler radar and opportunities for surface-based remote sensing.

3.2 Aircraft

Several aircraft will be used for in situ and remote sensing of aerosols, ice crystals, meteorological fields, radiative fluxes, and gas concentrations.  The ER-2 and WB-57 are NASA aircraft based at Dryden Flight Research Center and Johnson Space Center, respectively.  The Proteus aircraft, owned by Northrop Grumman and operated by Scaled Composites is funded for CRYSTAL-FACE by the National Polar-orbiting Environmental Satellite System (NPOESS) which is a joint NASA, DoD, DoE enterprise.  The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) associated with the Naval Postgraduate School in Monterey, CA is providing the DeHavilland UV-18A, “Twin Otter” aircraft.  The University of North Dakota is providing a Cessna Citation II aircraft and the NSF is supporting deployment of the ELDORA radar onboard the Naval Research Laboratory P-3 aircraft.

The ER-2 and Proteus aircraft will be flown in the lower stratosphere and used primarily for remote sensing but the ER-2 will also include in situ meteorological and water vapor measurements.  The WB-57 will be used for in situ sampling of cirrus anvils, aerosols, gas concentrations, and radiative fluxes in the tropopause region.  The Citation will sample the lower portions of the cirrus anvils including measurements closer to the convective source.  The Twin Otter will sample aerosols and environmental conditions in the boundary layer region feeding into the convective systems.  In addition, the Twin Otter will be the used for measuring radiative fluxes underneath stratiform cirrus anvils.  Finally, the P-3 will carry the ELDORA radar system that will characterize the structure and evolution of convective cloud systems.

3.3 Ground Sites

Instrumented sites that include multi-frequency millimeter radar, lidar, and radiometry will be located at two locations. Since convection is anticipated to occur over the Florida peninsula on a regular basis, and the upper level flow is predominantly northeasterly, a site situated on the southwest coast will allow sampling of the cirrus outflow from diurnally forced convection. Also, a site will be located in southeastern Florida where cirrus from maritime disturbances and local convection will be observed. The eastern ground site will also be optimally situated for observing thunderstorm anvils in situations when the flow in the upper troposphere is more southerly or westerly.

3.4 Cirrus Cloud Modeling Approach

To increase our confidence in cloud/climate feedback predictions, three types of models are required: (1) cloud system models to simulate cloud formation, microphysical, and cloud-scale dynamical processes on local to regional scales; (2) radiative transfer models to determine the effects of a given distribution of cloud ice and liquid water on radiative fluxes and heating rates; (3) general circulation models to simulate the collective effects of an ensemble of such clouds on the large-scale energy balance and general circulation.

Several modeling groups will participate in CRYSTAL-FACE, and models with varying degrees of detail in particular processes (microphysics, dynamics, and radiative transfer) will be employed. In addition to addressing the scientific issues described above, some of the cloud models will be used to forecast the location and timing of convection over southern Florida for flight planning. 

As discussed above, a key objective in the measurement campaign is to thoroughly characterize several cumulonimbus-anvil systems. These case studies should be extremely useful for evaluation of cloud system model performance. CRYSTAL-FACE will make an important contribution to the Global Energy and Water Experiment Cloud System Study through its coordinated cirrus observations and modeling activities.