SPECIAL PROJECTS

Appendix

B



In addition to testing the thermostat hypothesis, data collected during CEPEX will be used in a variety of studies, ranging from understanding the origin of water vapor in the stratosphere to testing detailed models of radiative transfer, cloud models, and mesoscale models. These projects include
  1. Radiative heating in thin cirrus and water-vapor transport to the lower stratosphere
  2. Proposal for absolute calibration of VIZ RH sensors used in CEPEX dropsondes
  3. Validation of GFDL line-by-line model estimates of fluxes and heating rates
  4. Cloud-scale numerical simulation
  5. Derivation of cloud physical and microphysical properties using MAS data
  6. Collocating surface and aircraft short-wave radiometric measurements
  7. Regional modeling of the CEPEX region
  8. Interactions between convective systems and radiation—CEPEX studies
  9. Analysis of AVHRR data in support of CEPEX
  10. Three-dimensional observations of water-vapor fluxes in the boundary layer
  11. Vertical transport of liquid water and water vapor in the atmosphere
  12. An investigation of tropical cirrus physical processes and climatic influence
  13. Study of convective super-clusters using calibrated GMS data

B.1 Radiative Heating in Thin Cirrus and Water-Vapor Transport to the Lower Stratosphere: S. Oltmans (NOAA/ERL), D. Kley (Dynamics of the Geosphere Research Center, Juelich), and P. J. Crutzen (Max-Planck Institute for Chemistry, Mainz)

The dryness of the stratosphere remains one of the most intriguing problems in the atmospheric sciences. It is clear that the answer must be sought in the way tropospheric air enters into the stratosphere through the tropopause "cold trap" in the tropics (Brewer, 1949), although some dehydration also occurs in the Antarctic lower stratosphere during wintertime. A clear understanding of the atmospheric processes responsible for this remarkable phenomenon is of great potential importance, as the question must be asked whether in the future the stratosphere may get drier or wetter as a result of the buildup of the greenhouse gases (CO2, CH4, N2O, etc.) in the atmosphere. At this stage, no satisfactory answer can be given to this question. An answer, however, is of considerable practical importance because stratospheric water vapor is an important greenhouse gas and plays a fundamental role in stratospheric ozone chemistry, either through catalytic gas-phase processes or, most important, via heterogeneous reactions of ClX and NX compounds on water-containing polar stratospheric solid particles.

Two major aircraft expeditions (during August—September 1980 over Panama and January—February 1987 over Micronesia) have supplied some highly interesting but still inconclusive information on the processes that determine the water-vapor content of the tropical lower stratosphere. Two opposite mechanisms have thereby been identified in the two regions.
  1. Hydration—probably due to ice-particle evaporation from overshooting turrets, forming large stratospheric anvils above the convective cloud systems in the Panama region (Kley et al., 1982).
  2. Dehydration—due to the fallout of large, extremely cold, radiatively cooled ice particles emanating from overshooting turrets penetrating into the lower stratosphere thereby entraining and drying stratospheric air, in accordance with the basic hypothesis by Danielsen (1982).
Another question remains, however. What determines the overall, net upward movement of air in the tropical stratosphere up to altitudes of 25—30 km, as clearly indicated by the observations of chemical tracers in the stratosphere? It is clear that strong radiative cooling from the tops of the thick anvil clouds, resulting from the heaviest convection regions in the tropics, may be expected to lead to downward motion in the stratospheric regions above the anvil clouds. The question must, therefore, be posed as to whether or not the strong radiative warming occurring in the thinner regions of the anvil clouds—which are exposed to upwelling infrared radiation fluxes from the much warmer lower troposphere and sea surface—is the driving force behind the upward branch of the stratospheric Walker cell, as indicated by Doppler radar observations (Gage et al., 1991). If the answer is affirmative, the next question that must be posed is, what influence would such an induced circulation mechanism have on the transfer of water vapor into the stratosphere, potentially bypassing the coldest regions of the tropopause?

As the CEPEX region is the most convective region on our planet, it is only natural that we take a first look at this issue by obtaining simultaneous measurements of H2O and O3 by the balloon-borne instrumentation of Oltmans and co-workers (Oltmans, 1990). We propose the launching of som 10— 15 balloon packages from the R/V Vickers in the CEPEX region, ranging from the most convective to less convective regions. The radiation flux measurements made onboard the ER-2 and Learjet will be of great value in interpreting the H2O and O3 observations. As a by-product, these measurements will also give highly interesting information on stratospheric ozone photochemistry during this post- Pinatubo period. However, the main reason for including the ozone measurements is that ozone serves as a powerful tracer for stratosphere-troposphere exchange; ozone measurements also serve as a quasi- conservative tracer with the potential to map deep convection. Tropospheric ozone in regions of deep convection can be a necessary and sufficient tracer for the meridional structure of deep convection as well as a diagnostic tool for events of deep convection.

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B.2 Proposal for Absolute Calibration of VIZ RH Sensors Used in CEPEX Dropsondes: D. Kley and H. G. Smit (Dynamics of the Geosphere Research Center, Juelich) and V. Ramanathan and S. Sherwood (Center for Clouds, Chemistry, and Climate, SIO)

The primary means of obtaining water-vapor profiles in the equatorial region during CEPEX will be by dropsonde measurements from the Learjet. The instrument will be the standard NCAR dropsonde, outfitted with the recently developed VIZ temperature-controlled carbon hygristor. Humidity measurements made in the past using carbon hygristor technology have been viewed with suspicion by some of the research community, especially at low temperatures and in cloudy conditions. Although the new sensor carries the promise of significant improvement, it is as yet relatively untested in actual field use.

We are proposing here to test and absolutely calibrate the VIZ RH sensors that will be deployed during CEPEX, both to produce the best possible absolute calibration and to estimate the magnitude of the remaining error. As a preliminary effort, several simultaneous ascents and drops will be performed with balloon-borne cryogenic hygrometers, and drops will also be compared with Vaisala radiosonde ascents. These efforts will occur before and during CEPEX and will ensure that the precision of the instruments is reasonable. For post-experiment calibration, a sample set of hygristors will be tested in the test chamber at the Dynamics of the Geosphere Research Center in Juelich, Germany.

This large (500-l) balloon ascent simulation chamber was recently built and is now operational. The computer-controlled chamber permits us to decrease pressure and temperature in such a way as to mimic radiosonde ascents or descents at, for instance, the 5-ms-1 rate for the NCAR dropsonde. Ozone mixing ratios in the chamber can also be varied according to any preselected profile, down to 20 mb. The chamber is equipped with an ozone reference photometer and the original balloon-borne Lyman- alpha fluorescence instrument (LAFI) by Kley and Stone (private communication) for humidity measurements. Therefore, any type of in-situ ozone instrument can be tested and absolutely calibrated, and the humidity can be measured at any temperature, down to values of less than 1 ppm, corresponding to frostpoints of < -80ºC. The LAFI has been used on many balloon ascents, revealing the existence of the hygropause (Kley et al., 1982), has flown on many U2 and ER-2 missions, including the Stratospheric-Tropospheric Exchange Project (STEP) (1980 and 1987) experiments, and has participated on all major aircraft campaigns in conjunction with the ozone-hole issue. We will perform the following tests:

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B.3 Validation of GFDL Line-by-Line Model Estimates of Fluxes and Heating Rates: F. Valero (NASA Ames Research Center), V. Ramaswamy (Geophysical Fluid Dynamics Laboratory, Princeton University), and C. Weaver (Center for Clouds, Chemistry, and Climate, SIO)

Line-by-line radiation models provide state-of-the-art calculations for estimating heating rates and fluxes in the atmosphere. One of the principal objectives of ARM is to validate such radiation models. During the past two decades, through the efforts of S. Fels, M. D. Schwarzkopf, and V. Ramaswamy, GFDL has maintained a keen interest and dedicated effort (Ramaswamy and Freidenreich, 1991; Ramaswamy and Freidenreich, 1992; Schwarzkopf and Fels, 1991) in developing such radiation codes for the long-wave and solar spectrum.

Vertical profiles of long-wave fluxes, long-wave radiance, and solar broadband fluxes and radiance from the ER-2 and the Learjet will be collected during descent at Fiji. In addition, vertical profiles of the fluxes will be collected from the Learjet during refueling descents at Nauru and at Wallis Island. Further, onboard Lyman-alpha and cryogenic hygrometer instruments and temperature sensors will yield water-vapor and temperature profiles. The temperature and water- vapor profiles will be used as input to the line-by-line model. All in all, we anticipate collecting about 10—20 profiles.

The intercomparison and validation will initially focus on clear-sky conditions. Depending upon the success of this exercise, the validation will be extended to cloudy skies.

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B.4 Cloud-Scale Numerical Simulation: C. Wang and J.-P. Chen (Center for Clouds, Chemistry, and Climate, SIO), P. J. Crutzen (Max-Planck Institute for Chemistry, Mainz), and R. Turco (Department of Atmospheric Sciences, UCLA)

In cloud-scale modeling research of CEPEX, a sophisticated three-dimensional numerical model, the CM4 (climate model, version 4; Wang and Chang, 1993), will be used to interpret cloud-scale dynamical, microphysical, and chemical processes and the interactions among them. The model will be initiated using CEPEX sounding data and evaluated using the cloud thermodynamical, microphysical, and chemical data obtained during CEPEX. The cloud structure parameters in the upper troposphere and lower stratosphere provided by this model can also be used to initiate the stratospheric microphysical and chemical model of R. Turco's group in studying surface processes of ice crystals in the lower stratosphere.

The proposed study will deal with the CEPEX cases and some selected historical cases. The major scientific issues related to CM4 simulations include

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B.5 Derivation of Cloud Physical and Microphysical Properties Using MAS Data: M. D. King (NASA Goddard Space Flight Center), J. A. Coakley (College of Oceanic and Atmospheric Sciences, Oregon State University), and W. D. Collins (Center for Clouds, Chemistry, and Climate, SIO)

Data collected by the MAS instrument (PI: M. D. King) onboard the ER-2 will be used to study the physical and microphysical properties of clouds in the MAS field of view. The data will serve three important functions:
  1. Estimation of cloud parameters for high cirrus above the Learjet cruising altitude of 12 km. Microphysical data for these clouds will not be measured by instruments onboard the Learjet.
  2. Testing and validation of mesoscale cirrus properties from combined AVHRR and TOVS (Tiros operational vertical sounder) data from NOAA satellites. For example, coincident estimates of cloud-top pressure from MAS and TOVS can be compared with lidar profiles of cloud reflectivity, giving cloud-top altitude (PI: J. Spinhirne).
  3. Intercomparison of collocated AVHRR and MAS narrow-band visible and infrared measurements. The results will be especially valuable for calibrating the 0.6 m data from the NOAA-11 and NOAA-12 satellites, as there is no in-flight calibration of the visible radiometers.
The MAS is a downward-pointing scanning multichannel imaging spectrometer and is a prototype of the MODIS instrument being developed for Earth Observing System (EOS) satellites. Scientific applications for the MODIS data include remote sensing of aerosol and cloud properties and estimation of total precipitable water and atmospheric stability (King et al., 1992). The MAS system contains a subset of the 36 spectral bands planned for MODIS. The configuration of MAS for CEPEX and TOGA-COARE, including central wave numbers and bandwidths of each channel (M. D. King, private communication), is shown in Table B.1. The cloud parameters that can be estimated from MAS and the channels required for calculation are indicated in the right-hand portion of the table (King et al., 1992).

Estimates of cloud properties from MAS (e.g., cloud emissivity) will be valuable for interpreting radiative measurements from the RAMS instrument package. The critical cirrus measurements from MAS for testing the thermostat hypothesis are optical thickness, fractional area coverage, emissivity, and effective cirrus-crystal radius. These data will help resolve how cirrus physical and microphysical properties vary from colder to warmer oceans.

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B.6 Collocating Surface and Aircraft Short-Wave Radiometric Measurements: R. D. Cess (State University of New York, Stony Brook)

This project will consist of collocating short-wave radiometric measurements made from ships and high-flying aircraft to provide one means of testing cloud radiative processes in GCMs. An analogous measurement collocation, using satellite and tower measurements, serves to demonstrate this procedure.

For the period from 29 June 29 to 19 July 1987, 202 Geostationary Operational Environmental Satellite (GOES) short-wave pixel measurements have been collocated with near-surface insolation measurements made at the Boulder Atmospheric Observatory tower. Simultaneous unfiltering and calibration of the GOES measurements was achieved through comparison to collocated ERBE pixel measurements. The upper panel in Figure B.1 compares the top-of-the-atmosphere albedo, as determined from GOES, to that from version 2 of the NCAR Community Climate Model (CCM2) at the grid point representing the tower location. Although the histogram distributions are quite similar, CCM2 underpredicts the albedo by consistently underestimating the higher albedo populations, despite the fact that the model predicts substantial cloud cover. Comparison with the tower measurements (middle panel of Figure B.1, where is the cosine of the solar zenith angle) produces the same conclusion; CCM2 underestimates populations of small insolation/ values caused by dense cloud cover. Both data sets thus suggest that CCM2 is underestimating cloud optical depth. However, combining the data sets reveals that CCM2 is likewise probably underestimating atmospheric short-wave absorption when clouds are present.

Albedo versus insolation/ scatter plots are shown in the bottom panel of Figure B.1; the points on the right represent clear skies, and the positive upward trend in progressing to the left is caused by increasing cloudiness. The slope differences can be shown to be statistically significant, and this difference cannot be corrected by simply increasing cloud optical depth within CCM2. Instead, it may be shown that all differences in Figure B.1 can be corrected only by simultaneously increasing cloud backscatter and cloud short-wave absorption. This again raises the issue of so-called anomalous cloud absorption.

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B.7 Regional Modeling of the CEPEX Region: L. Bengtsson (Max-Planck Institute for Meteorology, Hamburg) and P. Crutzen and J. Lelieveld (Max-Planck Institute for Chemistry, Mainz)

In collaboration with the Max-Planck Institute for Meteorology (Hamburg) and Chemistry (Mainz) in Germany, a regional-scale high-resolution limited-area model (HIRHAM)* will be employed to study cloud and radiation processes in the CEPEX region. The HIRHAM model has been derived from an operational weather forecast model in use in several European countries. By using the same descriptions for physical processes—such as cloud formation and radiation—as those in the Hamburg Global Climate Model (Roeckner et al., 1992), these models can be "nested" so that the global model provides the regional boundary conditions. The horizontal grid scale of the HIRHAM model is variable (20—50 km), its domain is 1 to 4 x 106 km, and it has 19 layers in the vertical direction up to 10 mb. The model is capable of simulating cyclogenesis in great detail, whereas shallow and deep convective clouds have been parameterized with a comprehensive mass- flux scheme (Tiedtke, 1989). A new radiative transfer code that accounts for clouds, aerosols (five types), CO2, CH4, N2O, O3, and CFCs will be implemented (Morcrette, 1991).

The project’s aim will be to use the extensive data set obtained within CEPEX—for example, regarding water-vapor, cloud properties, precipitation, and radiation fluxes—to validate the HIRHAM performance and subsequently to use the model to investigate the feedbacks as suggested in the thermostat hypothesis of Ramanathan and Collins (1991). The responses of water vapor, clouds, and liquid- and ice-water abundances and distributions to climate forcings, especially through tropical sea surface temperature changes, are of major interest in this respect. Furthermore, the model will be expanded with an atmospheric chemistry routine to support studies in the C4 Atmospheric Chemistry Program. These studies involve heterogeneous processes in both liquid and ice clouds, particularly in the convectively produced cirrus anvils in the equatorial Pacific region, convective transport of species within the troposphere, and exchange of water vapor and ozone between the troposphere and stratosphere.

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B.8 Interactions Between Convective Systems and Radiation—CEPEX Studies: L. Donner (Geophysical Fluid Dynamics Laboratory, Princeton University)

A limited-area non-hydrostatic (LAN) model (Lipps and Hemler, 1986) developed at GFDL Princeton University will be employed to study deep convection in the western and central Pacific. Through collaborative efforts with V. Ramaswamy, this model has been modified to include a delta-Eddington formulation for radiative transfer. The immediate goals of the CEPEX study will be to: CEPEX observations of cloud microphysical characteristics and radiative fluxes will be used to examine the ability of the LAN model to treat microphysical and radiative-transfer processes in anvil clouds. The model will be forced with thermodynamic and dynamic fields, which coincide with the microphysical and radiation observations. CEPEX observations of cloud microphysics— including characterization of small ice-particles, radiative fluxes above and below cirrus cloud decks, and surface-energy-balance components—are critical for this effort. The LAN model is currently in use to test a newly developed parameterization for cumulus convection for GCMs (Donner, 1993). This cumulus parameterization treats mesoscale processes and provides a basis for estimating some of the radiative properties of cirrus anvils. In using the LAN model as a test bed for advanced parameterizations of this type, it is crucial that the behavior of the LAN model itself be adequately tested against observations. CEPEX will provide a unique data set for several of the physical processes treated in the LAN model and the cumulus parameterization, including anvil spreading, convective heating, convective moistening and drying, and formation of radiatively active constituents in the anvil.

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B.9 Analysis of AVHRR Data in Support of CEPEX: J. A. Coakley (College of Oceanic and Atmospheric Sciences, Oregon State University)

The NOAA-11 and NOAA-12 AVHRR observations collected during CEPEX and the NOAA-9 AVHRR observations extracted from the NCAR archives (1985 and 1989) will be analyzed using a modified version of the spatial coherence method (Coakley and Baldwin, 1984). The analysis will provide mapped fields of surface and cloud properties at the times of the satellite overpasses. The results will be gridded on an equal-area map with 1 longitude spacing at the equator. Results for all five channels of the AVHRR (0.69, 0.89, 3.7, 11, and 12 m) will be included in the analysis. The parameters to be mapped within each of the grid areas are as follows: Radiances for the cloud-free pixels will be used to estimate SSTs, an index of low-level water- vapor burden (the difference in the 11- and 12-m brightness temperatures), and the reflectivity of the cloud-free ocean background at visible wavelengths. The latter, corrected for viewing geometry, will serve as an index for aerosol burden. Radiances for overcast pixels will be used to obtain visible optical depth and, in conjunction with analyzed fields of temperature and pressure, cloud-top pressure. The possibility of extracting an index of hydrometeor size from reflected and emitted radiances at 3.7 m will be explored. The method will be developed based on comparison of retrievals based on reflected visible and near-infrared radiances obtained with the MAS (King et al., 1992).

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B.10 Three-Dimensional Observations of Water-Vapor Fluxes in the Boundary Layer: D. I. Cooper, W. E. Eichinger, C. R. Quick, J. Tiee, C. F. Keller, R. W. Scarlett, and J. W. Ogle (Los Alamos National Laboratory and Institute of Geophysics and Planetary Physics, University of California)

Investigators will measure time-dependent, three-dimensional water-vapor scalars, and possibly fluxes, between the ocean surface and the atmospheric boundary-layer. The experimental team and equipment have acquired considerable experience over land using a UV Raman lidar system to make time-dependent measurements of water vapor (Cooper, 1992). This system offers the opportunity to study relatively large areas at temporal and spatial scales previously unattainable. Recent results confirm some traditional theories and challenge some of the underlying assumptions concerning the homogeneity of the surface-atmosphere interface and the use of point sensors to characterize large areas.

The experiment can be fielded on shipboard in a cubical volume approximately 2.5—3 meters on a side. It would consist of a laser/300-kv power supply and a recording/storage device within a sea container with a telescope and three photomultipliers mounted above. Four to five people are required to operate this device. Using a periscope mirror assembly with a steerable upper mirror, the system is able to scan up to 85º orizontally and up to 90º vertically. The angular step size in scans is generally 0.1º or 1.0º, but can be any user-selected value. This allows a large number of measurements to be made almost simultaneously and allows visualization of the processes. Useful range for these scans is approximately 1 km. An entire three-dimensional scan requires a minute or so, and ship motion must be recorded or accounted for in some fashion. Data storage is by read/write optical disk. In the present operation, data are sent to disk approximately every half hour, a process that requires about 20 minutes. Thus, on the order of one-half hour of data can be collected every hour.

Results are three-dimensional, two-dimensional, or one-dimensional maps of water-vapor mixing ratio (g water/kg air), latent energy flux (Wm-2), etc. These allow further analysis (such as determination of variograms, which are related to the autocorrelation function) and are used to derive characteristic lengths of turbulent eddies. Accuracy of measurements is of the order 3—5%. Quick-look readout is nearly immediate, and so preliminary data can be made available during observing runs. The Los Alamos project offers a means of gathering important, time-dependent, three-dimensional information about water-vapor fluxes in the boundary layer.

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B.11 Vertical Transport of Liquid Water and Water Vapor in the Atmosphere: R. T. Pierrehumbert (University of Chicago)

The main interest of this project in the CEPEX data concerns vertical transport of liquid water and water vapor in the atmosphere, with secondary projects relating to dynamics of tropical convection and cirrus radiative heating. Moisture is, of course, critical to the radiative and thermodynamic driving of the tropical circulation.

With regard to moisture transport, the following two inquiries are planned:
  1. Using flight-level data, the probability distribution functions of water vapor and liquid water and of the gradients of these quantities will be analyzed. Preliminary analyses of these quantities have been made using GOES satellite data, radiosondes, model moisture fields, and FIRE (First ISCCP Regional Experiment) aircraft data. These analyses suggest a log-normal distribution of relative humidity and also show important discrepancies between modeled (GFDL) and observed distributions. Recent theories and laboratory experiments suggest that tracer distributions in turbulent flow should be exponential or stretched-exponential rather than log- normal. Pursuing the reasons for the discrepancy in the atmospheric case will prove illuminating. Dropsonde data will be used to flesh out the details of the vertical distribution of moisture, although it cannot provide the degree of spatial resolution necessary for the detailed studies.
  2. Evaporation can exert a strong influence on sea surface temperature. The boundary- layer moisture fluxes will be examined under various convective regimes to get a fix on the kind of events transporting moisture. Probability distributions not only of moisture but also of moisture fluxes will be examined. Also looked for will be multifractal scaling laws in the transport. Studies of evaporation due to large-scale winds in the tropics do not suggest a close connection between sea surface temperature and evaporation. However, the turbulent winds driving evaporation in convective situations may not be properly resolved in the observations. Observations from CEPEX and TOGA-COARE should help shed light on this matter.
The following auxiliary projects are proposed:
  1. Interaction of tropical convection with the broader-scale tropical circulation. Fast Kelvin waves in the tropics enforce a more or less uniform horizontal atmospheric temperature distribution. However, if SST becomes warm enough to induce convection locally, it will rapidly warm the column of the atmosphere. This heating will be redistributed on a time scale of one to several days. We hope to be able to identify one or more convective events where the initial stages of this relaxation can be detected locally.
  2. Cirrus radiative heating. The very cold blackbody temperature of tropical cirrus suggests that these clouds are not in radiative equilibrium with the rest of the atmosphere. Either they are in the process of warming or the radiative warming is offset by convective or other thermal couplings with the rest of the atmosphere. If CEPEX succeeds in getting aircraft data above, below, and through cirrus, it will be possible to use the data to figure out the fate of cirrus radiative heating. Observations of the fine-scale time fluctuations of long-wave and visible radiation is also planned as an indirect check on theories of the spatial inhomogeneity of scatterers and absorbers in clouds.

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B.12 An Investigation of Tropical Cirrus Physical Processes and Climatic Influence: E. J. Jensen and O. B. Toon (NASA Ames Research Center) and V. Ramanathan (Center for Clouds, Chemistry, and Climate, SIO)

Satellite observations from the ERBE and the International Satellite Cloud and Climatology Project (ISSCP) indicate both strong short-wave radiative cooling and long-wave warming induced by cirrus clouds over the tropical western Pacific (Ramanathan and Collins, 1991). An understanding of the cirrus microphysical and optical properties, which result in the observed forcing terms, is important for determining the current role of clouds in climate and the potential for cloud-climate feedbacks. Observations at mid-latitudes suggest that long-wave forcing by high cirrus typically dominates, resulting in a net cooling effect. Neither the microphysical properties responsible for the large short-wave forcing nor the physical process responsible for the cloud formation are clearly understood for tropical cirrus. A combination of observations, microphysical modeling, and radiative transfer calculations are required to resolve these issues.

The primary objective of this study is to investigate the relationship between tropical cirrus microphysical and radiative properties and the physical processes leading to cirrus formation and evolution. The approach used will be to combine data from the CEPEX with microphysical modeling of cirrus and radiative transfer calculations. These results will be related to satellite observations of the climatic impact of tropical cirrus. A detailed microphysical model developed at NASA Ames will be used to simulate cirrus clouds. First, the evolution of thin cirrus anvils will be simulated with a one-dimensional version of the model. Next, we will use the microphysical model within a convective cloud model to study the dependence of cirrus properties on the lower- tropospheric environment and convection. We will use CEPEX observations of cirrus microphysical properties and radiative fluxes to relate the cirrus radiative forcing to their composition. CEPEX data will also be needed to evaluate and constrain model results.

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B.13 Study of Convective Super-Clusters Using Calibrated GMS Data: J. A. Coakley (College of Oceanic and Atmospheric Sciences, Oregon State University) and W. D. Collins and E. Boer (Center for Clouds, Chemistry, and Climate, SIO)

Hourly GMS data will be used to follow the evolution of convective super-clusters in the CEPEX region. Nakazawa (1988) showed with GMS data that short-lived synoptic-scale convective clusters are often organized into larger super-clusters. These clusters are associated with strong westerly wind bursts counter to the prevailing low-level easterlies. Recent modeling work (Lau et al., 1989) suggests dynamics in these systems involve a wide range of spatial and temporal scales, spanning both synoptic and low-order planetary modes. The satellite information is particularly crucial in determining the characteristic cluster size distributions and the relationship of size to lifetime for the central equatorial Pacific. Williams and Houze (1987) and Mapes and Houze (1992) have estimated the distributions and diurnal variability of the clusters in the warm pool. However, these results may not apply to the central Pacific, where there is a large gradient in SST. The super-clusters in the CEPEX region will be characterized with the hourly GMS data using a combination of the Houze classification and Coakley and Bretherton’s (1982) spatial-coherence algorithm. In addition, the amount of thick cirrus produced by the clusters and the effect of clusters on cloud radiative forcing and evaporation will be investigated using in-situand GMS data.

The effect of the clusters on the solar insolation is a particularly important problem. In order to use the GMS visible measurements quantitatively, the GMS narrow-band visible radiometers will be intercalibrated with coincident AVHRR data. The GMS visible sensors, like those in the AVHRR instrument, are not internally calibrated after launch. Overlapping images will be matched from GMS, NOAA-11, and NOAA-12, and the relationship between near-simultaneous radiance measurements will be calculated. The absolute calibration will involve comparison with measurements from the MAS instrument onboard the ER-2.

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* HIRHAM is the Hamburg version of HIRLAM = high-resolution limited-area model, developed in a collaborative effort by Sweden, Norway, Denmark, Finland, Ireland, and the Netherlands.

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