David Oc. Starr

Cirrus Clouds. Part I: A Cirrus Cloud Model

Abstract A two-dimensional (x, z), time-dependent, numerical cloud model is developed for the purpose of investigating the role of various physical processes involved in the maintenance of cirriform clouds. In addition to accounting for dynamic and thermodynamic processes including phase changes of water, effects due to microphysical composition and radiative processes are also explicitly incorporated into the model. Diagnostic parameterizations for the local radiative properties of cloudy volumes and the gravity induced relative fall speed of the contained ice water are presented. Results of a simulation of a thin cirrostratus cloud are given. Features of the simulated cloud structure are quite realistic. Quantitative agreement is found between the simulated ice water contents and vertical motions and comparable observations. It is shown that radiative effect may be very significant in the maintenance of cirrus. The effects of the gravity-induced relative fall speed of ice crystals are found to be of cri...

Cirrus Clouds. Part II: Numerical Experiments on the Formation and Maintenance of Cirrus

Abstract The numerical cirrus cloud model of Starr and Cox is used to investigate the role of various physical processes in the formation and maintenance of cirrus. Effects due to microphysical composition, i.e., crystal habit and size distribution, are found to be quite important in determining the overall cloud water budget. Radiative processes are also shown to affect the organization and bulk properties of the cloud. Substantial differences between simulations of thin cirrus under midday and nighttime conditions are found with the cloud being less dense overall (∼20%) but more persistently cellular during the day with all other environmental factors being the same. Cloud-scale interactions and feedbacks between dynamic, thermodynamic and radiative processes and the microphysical composition are significant and strongly modulate the properties of the simulated clouds. A comparison is made between simulations of weakly forced cirrostratus and nonprecipitating altostratus (liquid phase) under comparable ...

Mesoscale and Microscale Structure of Cirrus Clouds: Three Case Studies

Abstract The structure and composition of three basic cirrus cloud types are examined through coordinated aircraft and ground-based polarization lidar and radar measurements. The cloud systems consist of a multilayered orographic cirrus, a 6-km deep cirrostratus, and a group of fibrous cirrus bands at the tropopause. The data reveal the presence of mesoscale generating regions with horizontal dimensions ranging from ∼15 km in narrow cloud bands up to ∼100 km in cirrostratus. These generating regions appear to be composed of complexes of much smaller convective structures, presumably on the ∼1-km scale of cirrus uncinus cells, and so are termed Mesoscale Uncinus Complexes (MUC). Accumulations of ice particles within cirrus, commonly referred to as precipitation trails, are associated with generating regions at or near cloud tops, but are also created by the local production of ice crystals within embedded convective impulses. Supercooled cloud droplets large enough to be detected by aircraft probes (≳5 μm ...

The 5–6 December 1991 FIRE IFO II Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols

Abstract In presenting an overview of the cirrus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5–6 December 1992 Project FIRE IFO II case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations incl...

Mission investigates tropical cirrus clouds

It has been 18 months since NASA conducted the highly successful Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment Study (CRYSTAL-FACE). The measurement campaign was designed to investigate the physical properties and formation processes of tropical cirrus clouds. CRYSTAL-FACE was sponsored by NASAs Earth Science Enterprise as an integral component of its Earth observation research strategy and included substantial collaboration with the National Oceanic and Atmospheric Administration, the National Science Foundation, the Department of Energy and the Naval Research Laboratory. During July 2002, the missions six aircraft (NASA ER-2, NASA WB-57, Scaled Composites Proteus, University of North Dakota Citation, NSF-supported NRL P-3,and NRL Twin Otter) operated from the Key West Naval Air Facility.

An Observational and Theoretical Study of Highly Supercooled Altocumulus

Abstract Two altocumulus clouds, which formed at a temperature of −30°C, were sampled using the NCAR King Air aircraft and coincident lidar during the 1986 FIRE cirrus experiment in Wisconsin. The clouds were structurally and thermodynamically similar to stratocumulus, with extensive cloudtop entrainment, a capping temperature inversion, and a dry layer above. The microphysical and radiative properties of both clouds were characterized and modeled numerically. Calculations of droplet concentration and mean diameter profiles compare favorably with the measurements when entrainment effects are incorporated in the model. Radiative transfer calculations suggest radiation played an important role in driving convection in the more dynamically unstable of the two clouds. A simple model shows that radiative cooling causes sufficient negative buoyancy in cloudtop parcels to produce convective instability and to reproduce the observed downdraft velocities. Entrainment of warmer, drier air near cloudtop is shown to ...

A Cirrus-Cloud Experiment: Intensive Field Observations Planned for Fire

Plans for an intensive cirrus-cloud field experiment are described. The Cirrus Intensive Field Observations (Cirrus IFO) is a major component of the First ISCCP3 (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). The field campaign was conducted in Wisconsin during October 1986. Observing systems include satellites, “cloud” lidars, a very high-altitude, satellite-simulator aircraft platform, two research aircraft instrumented for detailed in situ microphysical and radiometric observations, a Doppler lidar, numerous passive surface-radiation sites, and a rawinsonde network. This is the first cirrus experiment involving such a comprehensive observing system.

The 27-28 October 1986 FIRE IFO cirrus case study: Comparison of radiative transfer theory with observations by satellite and aircraft

Abstract Observations of cirrus and altocumulus clouds during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) are compared to theoretical models of cloud radiative properties. Three tests are performed. First, Landsat radiances are used to compare the relationship between nadir reflectance at 0.83 μm and beam emittance at 11.5 μm with that predicted by model calculations using spherical and nonspherical phase functions. Good agreement is found between observations and theory when water droplets dominate. Poor agreement is found when ice particles dominate, especially if scattering phase functions for spherical particles am used. Even when compared to a laboratory measured ice particle phase function (Volkovitskiy et al. 1980), the observations show increased side scattered radiation relative to the theoretical calculations. Second, the anisotropy of conservatively scattered radiation is examined using simultaneous multiple-angle views of the cirrus from Landsat and E...

An Indirect Effect of Ice Nuclei on Atmospheric Radiation

A three-dimensional cloud-resolving model (CRM) with observed large-scale forcing is used to study how ice nuclei (IN) affect the net radiative flux at the top of the atmosphere (TOA). In all the numerical experiments carried out, the cloud ice content in the upper troposphere increases with IN number concentration via the Bergeron process. As a result, the upward solar flux at the TOA increases whereas the infrared one decreases. Because of the opposite response of the two fluxes to IN concentration, the sensitivity of the net radiative flux at the TOA to IN concentration varies from one case to another. Six tropical and three midlatitudinal field campaigns provide data to model the effect of IN on radiation in different latitudes. Classifying the CRM simulations into tropical and midlatitudinal and then comparing the two types reveals that the indirect effect of IN on radiation is greater in the middle latitudes than in the tropics. Furthermore, comparisons between model results and observations suggest that observational IN data are necessary to evaluate long-term CRM simulations.

Examination of Coupling between an Upper-Tropospheric Cloud System and Synoptic-Scale Dynamics Diagnosed from Wind Profiler and Radiosonde Data

Abstract The evolution of synoptic-scale dynamics associated with a middle and upper tropospheric cloud event that occurred on 26 November 1991 is examined. The case under consideration occurred during the FIRE Cirrus-II Intensive Field Observing Period held in Coffeyville, Kansas, during November–December 1991. Using data from the wind profiler demonstration network and a temporarlly and spatially augmented radiosonde array, emphasis is given to explaining the evolution of the kinematically derived ageostrophic vertical circulations and correlating the circulation with the forcing of an extensively sampled cloud field. This is facilitated by decomposing the horizontal divergence into its component parts through a natural coordinate representation of the flow. Ageostrophic vertical circulations are inferred and compared to the circulation forcing arising from geostrophic confluence and shearing deformation derived from the Sawyer–Eliassen equation. It is found that a thermodynamically indirect vertical ci...