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Why Study Clouds?
The effect of clouds on climate is one of the principal sources of uncertainty
in our ability to predict future climate change and is thus a top priority
in climate research. The Center for Clouds, Chemistry and Climate (C4) is
dedicated to reducing the uncertain ties involved in cloud-chemistry-climate
interactions, thereby enhancing our understand ing of the climate system,
and improving our ability to predict how it will respond to human and other
influences.
CEPEX: A Major C4 Achievement
C4 seeks to develop a theoretical, observational, and modeling base for
understanding and predicting cloud-chemistry-climate interactions. To this
end, C4 took the lead in designing, implementing, and analyzing the results
of a major field observing program known as CEPEX the Central Equatorial
Pacific Experiment, conducted in March of 1993. The project yielded important
new information about the absorption of solar radiation by clouds and the
relative roles of cirrus cloud radiative effects and surface evaporation
in limiting maximum sea surface temperatures in the equatorial Pacific.
This large, interdisciplinary experiment, involving 15 institutions from
the US and Germany, has created a much-needed observational basis for unraveling
phenomena which were previously understood only theoretically. C4's infrastructure
and frame work for cooperative research were instrumental in making CEPEX
a success.
In the quest to comprehend the processes that regulate climate, why focus
on tropical Pacific sea surface temperature (SST)? The equatorial Pacific
contains the largest pool of the warmest water on Earth, and as such, is
an ideal site to observe how the ocean-atmosphere system might respond to
a global warming scenario. It has been observed that sea surface temperatures
in this "warm pool" do not rise above 303K (30°C),
and that this cap has existed throughout Earth's history. Even 50 million
years ago, when dinosaurs roamed the Earth in the Cretaceous period, paleoclimatic
evidence suggests that equatorial oceans were limited to the same maximum
temperature as now, despite the fact that the climate was much warmer. It
is in this context that C4 helped launch CEPEX the most intensive study
ever undertaken of this region, from land, sea, air, and space, in an attempt
to better understand the processes that act like a thermostat to regulate
sea surface temperature.
CEPEX
Platform Schematic
The Super Greenhouse Effect and the Thermostat Hypothesis
Scientists have long theorized that warming sea surface temperatures create
deep convection in the atmosphere above the ocean and cause that air to
become laden with water vapor. Because water vapor traps heat, this process
causes additional warming, creating a positive feedback loop that results
in a "super greenhouse effect." In CEPEX, this effect was directly
measured and proven to exist for the first time. A high resolution radiometer
showed that the energy radiated by the atmosphere to the ocean's surface
increased so rapidly that the ocean was unable to rid it self of its heat,
resulting in a super greenhouse effect. CEPEX also provided observations
of processes that limit this effect. For example, deep atmospheric convection
causes increased formation of cirrus clouds. These clouds are highly reflective
and thus reduce incoming solar radiation, preventing SSTs from rising further.
In this way, the cirrus clouds act as a thermostat for SSTs.
C4 investigators Ramanathan and Collins first introduced this cirrus thermostat
hypothesis in 1991 to explain two related issues for the present climate:
first, why are maximum SSTs so close to the 300 Kelvin threshold for
deep convection; and second, what forces limit SSTs and deep convection?
Simply put, the thermostat hypothesis states that as SSTs rise above a threshold
of 300K, convective storms develop that shade the surface, substantially
reducing surface insolation. The reduction of solar radiation in conjunction
with other processes cools the ocean and maintains the temperature close
to the 300K threshold. CEPEX provided observational evidence of this
hypothesis by measuring variations in radiation at the interface of the
western Pacific warm pool and the colder eastern Pacific. The investigators
also measured cloud microphysical properties in order to better understand
the radiative effects of convective cirrus anvil clouds. These measurements,
in combination with those collected in the Tropical Ocean Global Atmosphere
- Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE), provide an
extensive data set on the tropical climate system.
Alternative Hypotheses
Other processes thought to limit SSTs were studied in CEPEX, including the
role of evaporation in limiting sea surface temperature. Conventional wisdom
prior to the experiment was that a warmer ocean increases evaporation, and
that evaporation from the surface helps to lower SSTs. But CEPEX analysis
revealed that in fact, there was less evaporation where the ocean was hottest.
Surface evaporation did increase with rising SST up to 301K, but then
decreased with rising SST at higher temperatures, implying that evaporation
is not an effective cooling factor when SST exceeds a threshold value. CEPEX
observations also helped advance a theory as to why this is so: there is
a decrease in surface wind speed with rising SST in a convectively disturbed
atmosphere that may be responsible for the observed decrease in evaporation,
as though a cooling fan were somehow switched to a lower speed. This result
came as a surprise to many who believed that the evaporation effect was
the dominant negative feedback that regulated SST. CEPEX provided the first
actual measurements to help settle the theoretical debate.
Visual
clues
The Ocean's Heat Budget
CEPEX investigations are centered on the heat budget of an ocean column
and its change with SSTs. A comprehensive exploration of thermostat mechanisms
necessarily involves a combination of observations, simple empirical models,
and coupled ocean-atmosphere general circulation models (GCMs). Such GCMs
are able to simulate large time and space ranges that are far beyond the
reach of field experiments. The observations from CEPEX and TOGA-COARE are,
in turn, useful for testing and validating the physics in these coupled
models. These observations come from satellites, aircraft, ships, and the
Tropical Atmosphere-Ocean Buoy system (which provides an important, on-going,
long -term data base). Because of limited resources and the one-month time
period of the field experiment, the CEPEX approach is to "ground truth"
the satellite and buoy data with the aircraft and ship data wherever possible,
and then use the longer and more widely geographically distributed data
for obtaining general conclusions.
CEPEX findings show that convection both heats and cools the ocean surface.
Ongoing research will increase our understanding of the net effect of convection
on SST (which depends on the interaction between atmospheric forcing and
the response of the ocean mixed layer). The CEPEX data show that convection
moistens and therefore warms the atmospheric column locally. The convective
clouds also heat the surface with longwave radiation, but this forcing is
less than 20% of the longwave cloud forcing for the entire column. If the
mixed layer is in thermal equilibrium, the reduction in insolation due to
cloud shading should be 50% larger than predicted by radiative models. The
reduction in solar energy reaching the ground is apparently caused by enhanced
shortwave absorption in the cloudy tropical atmosphere. The net effect of
convection on the local radiation balance is dominated by this cloud shading
effect.
Enhanced Solar Absorption by Clouds
Two notable articles on which C4 scientists collaborated were published
in Science, 27 January 1995. The subject of these articles is a key
area of inquiry for C 4 and one of the cutting edge issues in current climate
research. "Absorption of Solar Radiation by Clouds: Observations Versus
Models," by Cess et al ., reports the results of observing the
same clouds from above and below to assess how much shortwave solar radiation
is actually absorbed by clouds. Co-located satellite and surface measurements
of solar radiation at five geographically diverse locations showed that
clouds actually absorb about 25 watts per square meter more radiation than
predicted by theoretical models. This could result in about 20% less solar
energy reaching the ground than was previously believed. "Warm Pool
Heat Budget and Shortwave Cloud Forcing: A Missing Physics?," by Ramanathan
et al., reports similar findings based on data collected over the
tropical western Pacific Ocean during the CEPEX and TOGA-COARE missions.
The authors conclude that the shortwave cloud forcing in this region was
large, about -100 watts per square meter, and that it exceeded its observed
value at the top of the atmosphere by a factor of 1.5.
The results reported in these two articles have dramatic implications for
the general circulation models used in climate research. None of these models
has accurately reproduced Earth's current climate as yet. But when one of
these models (the National Center for Atmospheric Research's Community Climate
Model 2, known as NCAR CCM2) was recently altered to make its clouds absorb
shortwave radiation in line with the new observations, the results were
promising; it compared better with observations of the present climate than
the previous version of the model. C 4 investigators will continue to work
towards better comprehension of the physics of enhanced absorption by clouds
and will use this knowledge to improve the accuracy of the models that may
offer us a window on future climate.
Model
Evolution
Improving Climate Models
CEPEX data are already helping to modify the general circulation models
that attempt to forecast how the planet will respond to increasing levels
of greenhouse gases. One such data set is CEPEX's first-ever measurements
of a cross section of the water vapor column. This new data is consequential
because water vapor is the most significant green house gas several times
more powerful than carbon dioxide at directly warming the Earth. The data
show that deep convection in the atmosphere (caused by a warming ocean)
causes increases in water vapor over a 1000-kilometer scale, literally saturating
the atmosphere over a wide area. This new information is not consistent
with previous conventional wisdom that convection dries out the atmosphere.
It also alerts us that warmer oceans and increasing convection may moisten
the atmosphere and enhance the greenhouse effect, much as the models predict.
CEPEX data suggest the following necessary conditions for coupled ocean-atmosphere
models to obtain more accurate simulations of the tropical climate:
- The models must reproduce the fact that the frequency of convection
rises with rising SST. This increase has been demonstrated using several
independent, long-term satellite records. (The frequency begins to decrease
with SST above 29.5°C, but SSTs in this range are relatively rare.)
- The long-term average wind speeds and evaporation for the warm pool
in the simula tions should be lower than the corresponding fields in the
eastern Pacific.
- The models must produce a super-greenhouse effect at the surface and
at the tropopause. CEPEX observations confirm the enhanced greenhouse trapping
detected by satellites, and have shown that the net radiative emission from
the ocean decreases with increasing SST.
- The advection of heat out of the modeled mixed layer should be a small
component of the layer heat budget if previous observations are confirmed
by results from TOGA COARE.
- The treatment of cloud radiative transfer should reproduce the enhanced
absorption of shortwave radiation in the cloudy atmosphere.
- Improvement in the coupled GCMs will be critical for continued investigation
of thermostat mechanisms for the tropical climate.
Climate Model Evaluation
Global Climate Models are the most comprehensive tools for studying Earth's
climate system. A climate model includes among other effects the interactions
between cloud radiative and convective processes and their effects on large
scale dynamics. These models also include the capability for clouds to transport
chemical species. Global models must be compared with observational data
on various scales to lend credence to the ability to realistically reproduce
the present climate. A major focal effort within C 4 is to compare the National
Center for Atmospheric Research (NCAR) Community Climate Model (CCM) to
field observations. The CCM is a global climate model that is used by a
large number of universities for research ranging from paleoclimate to climate
predict ability studies. Thus the effort to evaluate and improve this model
benefits both C 4 and the wider university community.
The major focus of this effort has been to compare various versions of the
CCM with CEPEX data. The evaluation includes comparison of model simulated
latent heat flux, boundary layer structure and dynamics, cloud and radiative
properties with observations from CEPEX. Initial comparisons between model
and data indicated that the model surface winds were too strong and this
led to excessive latent heat fluxes, by as much as 80 Wm-2. The next phase
of the project was implemented of a new deep convective parameterization
developed by G. Zhang at C4. Evaluation of the new CCM including this convection
scheme indicated a significant reduction in surface wind speed and an associated
reduction of surface latent heat flux. Wind speed over a wide range of the
Pacific are now within 1 to 2 ms-1 of observed values. The boundary layer
heights are within 50 meters of those observed from ship borne LIDAR. Thus,
there has been a major improvement in the climate simulation of the tropical
Pacific in the CCM. How ever, improvements extended beyond just the tropical
Pacific. The overall dynamical simulation of the model has significantly
improved. For example, the winter northern hemisphere stationary wave pattern
is much improved over CCM2.
The improvement in the CCM have been so encouraging that this improved version
has now become the new standard model, deemed CCM3. Furthermore, this version
of the model is being used in the NCAR Climate System Modeling (CSM) project,
which has coupled CCM3 to a full depth global ocean model. The next phase
of the C 4 Climate Model Evaluation project is to study the effects of enhanced
shortwave absorption on the simulated climate. A study by Kiehl et al. (1995)
investigated this effect in CCM2 and found significant changes to the simulated
climate. A similar study will be carried out in CCM3 to see if the model
sensitivity has changed.
Atmospheric Chemistry and Chemical Transport
As part of the effort to enhance our understanding of climate processes,
data on atmospheric chemistry, particularly ozone concentrations, were also
collected during CEPEX. The data reveal that some of the lowest concentrations
of ozone in Earth's atmosphere were found high in the tropical troposphere,
just below the tropopause. In pristine areas, ozone concentrations are expected
to increase with height, from the surface up into the stratosphere (where
other processes complicate matters considerably). The lowest amounts of
ozone are expected to occur near the surface, where ozone is rapidly destroyed
through chemical reactions. Because these reactions decrease with height,
ozone values are expected to grow with height above the surface. The surprise
in the CEPEX data was that very low ozone values were found quite high in
the troposphere values lower than those seen near the surface.
A recently released paper by C4's Dieter Kley, et al., reports the
lowest ozone concentrations ever mea sured in the troposphere below 10 parts
per billion by volume over an extended section of the tropical Pacific,
an area characterized by intense convection. According to the authors, "The
production of close-to-zero ozone mixing ratios in convective regions over
tropical warm oceans has several potentially important chemical implications.
The very low O3 and NO concentrations and strong reflection of solar radiation
from the top of the clouds may lead to low OH concentrations in the convective
regions compared to elsewhere in the tropical troposphere. This will result
in reduced removal rates of gases that are released into the atmosphere
from the tropical oceans. As a consequence, in combination with vigorous
upward transport of boundary layer air, a substantial fraction of a number
of otherwise relatively short-lived gases released at the Earth's surface
may reach the upper troposphere, and maybe even the lower stratosphere.
Examples are dimethylsulfide, and organic halide gases with potentially
important in fluences on sulfate aerosol formation and lower stratospheric
ozone photochemistry."
C4 investigators are currently exploring theories to explain the low ozone
phenomenon they observed during CEPEX. Paul Crutzen is leading the exploration
of a chemical explanation, meaning that some unknown and unusual chemical
process may be causing the very large ozone loss rates observed in the upper
troposphere. Dieter Kley is looking into a dynamical explanation, which
assumes that the very low ozone amounts seen high in the troposphere result
from the transport of low-ozone air from the boundary layer. The final answer
may lie in a combination of these two explanations.
Chemical Transport Modeling
These and other questions regarding atmospheric chemistry can be investigated
in the context of a chemical transport model. One of C4's major undertakings
in this area is called MATCH (Model of Atmospheric Transport and Chemistry).
MATCH can use either meteorological fields from GCMs or analyzed datasets
to drive the chemical transport, and a variety of convective transport schemes
can be used in the model. The model currently includes a set of gas phase
reactions and researchers are working on adding the reactions important
in modeling aqueous phase chemistry, as well as formulations that will help
account for cloud effects.
The ongoing development of the model is being led by C4 investigator Phil
Rasch, in collaboration with Paul Crutzen, Mark Lawrence and other colleagues.
The team is preparing to further study the low ozone phenomenon using a
comprehensive chemistry model developed by Lawrence and Crutzen which has
been merged with MATCH. In addition, a parameterization for the representation
of cloud water content and cloud microphysics, developed by Rasch for the
NCAR CCM2, is also currently being inte grated with MATCH. Results from
this merger may be helpful in investigating the theory that the low tropospheric
ozone amounts involve chemical reactions taking place in the presence of
condensate. The sum of these modeling efforts provides a valuable tool for
investigating the low ozone phenomenon as well as other questions regarding
atmospheric chemistry and chemical transport.
CIDS: C4's Innovative Data Resource
A unique and exciting outcome of the experiment is the C4 Integrated Data
Set (CIDS), a user-friendly data base developed by Erwin Boer, Adam Susman
and colleagues, which makes accessible all of CEPEX's raw and derived data
sets through a variety of software tools. CIDS makes it simple to access
complex information (from micro-scale ice crystal distribution to hundred-kilometer-scale
water vapor data to cloud morphology measurements from lidars) in a single
system. A computer queried in plain English can co -locate and deliver heterogeneous
data from ships, buoys, satellites, and aircraft. This data base is currently
being used by modelers to test and validate GCMs, as well as by researchers
and students. As part of C4's mission to see that this data is put to the
best possible use, it intends to make CIDS widely available, not only to
the scientific community, but also to interested students at the high school
and college levels. Since March 1995, CIDS is available via the C4's World
Wide Webb site. By last count in early November, the number of accesses
was fast approaching the 10,000 mark with about 18,000 files transfered.
webmanager@fiji.ucsd.edu
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Last Updated : 11/24/97