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
- Radiative heating in thin cirrus and water-vapor transport to the lower stratosphere
- Proposal for absolute calibration of VIZ RH sensors used in CEPEX dropsondes
- Validation of GFDL line-by-line model estimates of fluxes and heating rates
- Cloud-scale numerical simulation
- Derivation of cloud physical and microphysical properties using MAS data
- Collocating surface and aircraft short-wave radiometric measurements
- Regional modeling of the CEPEX region
- Interactions between convective systems and radiation—CEPEX studies
- Analysis of AVHRR data in support of CEPEX
- Three-dimensional observations of water-vapor fluxes in the boundary layer
- Vertical transport of liquid water and water vapor in the atmosphere
- An investigation of tropical cirrus physical processes and climatic influence
- Study of convective super-clusters using calibrated GMS data
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.
- 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).
- 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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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:
- If all 120 to-be-flown radiosondes have RH sensors from the same batch then, from a total of
130 RH-sensors, 11 will be randomly selected. A dropsonde will be outfitted with one at a time of the
selected RH sensors and subjected to a computer-controlled chamber run. Pressure, temperature, and
RH will be increased to mimic the descent of the dropsonde through the troposphere, beginning at
initial conditions that are equivalent to 45,000 ft (13.5 km). Absolute measurements of the water-vapor
mixing ratio during the simulation run will be recorded by the LAFI (Kley et al., 1982) and
simultaneously by the Oltmans (1990) frostpoint hygrometer. This procedure will be repeated several
times and for each of the 11 RH sensors.
- If the RH sensors come from different batches, this procedure will be repeated for 11 sensors
from each batch.
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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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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
- dynamics and thermodynamics of maritime deep convective storms, transport of trace
gases and water vapor by this type of storm, and some preliminary studies toward the
improvement of current cloud schemes in GCMs or mesoscale models (C. Wang and P. Rasch)
- microphysics of maritime deep convective storms; shapes and behaviors of ice crystals in
a bulk model (C. Wang and J-P Chen)
- maritime deep convective storm chemistry, uptake of trace gases by ice-phase particles,
and haze and deep convective clouds (C. Wang, J-P Chen, and P. J. Crutzen)
- chemical and climate impacts of ice-phase particles from the anvil region (R. Turco,
J-P Chen, and C. Wang)
- Interaction among adjacent deep convective clouds and between those clouds and the
ambient atmosphere (C. Wang and other scientists)
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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:
- 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.
- 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).
- Intercomparison of collocated AVHRR and MAS narrow-band visible and infrared
measurements. The results will be especially valuable for calibrating the 0.6 µm 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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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 µ is the cosine of the solar zenith angle)
produces the same conclusion; CCM2 underestimates populations of small insolation/µ 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/µ 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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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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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:
- Model the deep convection in the western Pacific and the associated stratiform anvils.
The processes by which these anvils spread into the central Pacific, and their possible effects on
the sea surface energy balance there, are central concerns of the C4-CEPEX experiment.
- Examine the role of solar and infrared radiative transfer in the development and
maintenance of the stratiform anvil clouds. Among the issues to be considered here is the
development of circulations associated with the anvils. These circulations may be important for
chemical transport processes in the CEPEX domain.
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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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 µm) will be included in the analysis.
The parameters to be mapped within each of the grid areas are as follows:
- sun/target/satellite viewing geometry at time of overpass
- means and percentiles of the five-channel radiances
- number of cloud-free pixels, means, and percentiles of the radiances associated with
cloud-free pixels
- number of identifiable cloud layers (up to three)
- number of overcast pixels, means, and standard deviations of radiances associated with
overcast pixels in each of the layers
- total cloud cover
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-µm 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 µm 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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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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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:
- 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.
- 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:
- 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.
- 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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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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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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