Research Achievements

Third Term Renewal Proposal,
July 1, 1997 - June 30, 2002

Research Achievements (1994-1997)

During the first term, the center focused primarily on cloud-climate interactions and exploited the tropical western Pacific as a natural laboratory to understand the interaction between surface warming, deep convection, surface evaporation and cloud forcing. This effort culminated in the CEPEX field experiment through which C4 fostered interdisciplinary research between radiation experts, meteorologists and oceanographers; the collaborating groups under C4's umbrella included a healthy mix of experimentalists, theoreticians and modelers.

During its second term, the center gradually broadened its scope to address central problems in cloud-chemistry-climate interactions. At the same time, the knowledge and data obtained during the first term were used to make significant improvements in the NCAR Clommunity Climate Model 3 (CCM3). Research conducted during the second term was organized under 3 major objectives:

- GCM validation and improvement using CEPEX data base
- Ozone chemistry
- Observational studies to unravel cloud-climate-chemistry interactions

Highlights of the accomplishments include the following:

· Over 40 journal papers on CEPEX related research that resulted from collaborations between C4, NOAA, NASA and DOE laboratories, US universities and German research institutions. These research papers, which was coordinated by C4, deal with the following topics: 1) atmospheric and surface energy budget; 2) vertical water vapor distribution and greenhouse effect; 3) cirrus microphysical properties; 4) assessment of hypotheses for SST regulation, including the cirrus thermostat; 5) near zero ozone concentration in convective regions of the Pacific; 6) GCM development and validation; and 7) excess solar absorption.

These papers have been compiled into one C4 report and is attached with this proposal. The principal achievements related to C4's goals of outreach to the scientific community are:

- Development of the C4 Integrated Data System (CIDS), a system for integrating the measurements from field experiments for climate applications;
- Dissemination of the CEPEX results to the scientific community at meetings in the US and Europe and in refereed publications;
- Validation and improvement of the NCAR Community Climate Model (CCM3) and the European Center/Hamburg (ECHAM) model by application of CEPEX observations and analysis; and
- Extension of the in situ observations to climate scales by calibrating satellites against the CEPEX aircraft observations.

· A significant improvement in the NCAR-CCM3 simulation of the tropical climate including surface evaporation, cloud-radiation interactions and the Indian monsoon. This joint C4 NCAR effort resulted primarily from CEPEX-TOGA/COARE data and related research. Our significant success in improving the CCM fits well with the USGCRP paradigm of observations, leading to analysis and understanding, leading to improved predictive modeling and benefits to society.

· Successful development of the Model of Atmospheric Transport and Chemistry (MATCH) for simulating the tropospheric ozone distribution in the pristine tropical Pacific. This important NCAR-MPI-C4 modeling initiative was motivated by the unexpected discovery from CEPEX that near zero ozone concentrations occur within the upper troposphere of convective regions in the tropical Pacific.

· The finding of excess solar absorption in the warm pool clouds, i.e., clouds enhance significantly atmospheric solar absorption, when compared with the clear skies. This serendipitous CEPEX result has started a large national and international effort for understanding the absorption of solar radiation in cloudy atmospheres.

· Development of the international and US component of the INDOEX field program dealing with interactions between clouds, aerosols, chemistry and climate in the Indian Ocean. C4 is the coordinator for this program, in which over 40 research institutions in the US, Germany, France, India and Netherlands will participate.

In addition, the research described here led to several post-doctoral projects, Ph. D. theses and undergraduate projects. Summary of the major findings are reported below. In what follows, C4 references are listed in Appendix D; non-C4 references are listed at the end of Section III.

1. National Center for Atmospheric Research Community Climate Model (NCAR CCM) Improvements

The knowledge we gained through combined observational and modeling studies using CEPEX, TOGA-COARE and other in-situ and satellite data led to a focused effort for validating and improving the NCAR CCM. The CCM serves as the atmospheric component of a new Climate System Model (CSM) developed with USGCRP funding. Two major findings from the data related studies had a large and decisive impact on the CCM validation process: first is the finding of very low surface evaporation accompanied by a very large negative short-wave cloud forcing over the warm oceans of the western Pacific; the second is the deduction that the low evaporation and the large negative cloud forcing is a result of the interaction between warm SST, frequency of deep convection and the large scale Walker and Hadley circulation. The net effect is the realization that convection-large scale interaction should be a key focus.

Zhang, Kiehl and Rasch implemented a new deep convection parameterization in the CCM. They found that using the Zhang-McFarlane scheme resulted in a more realistic precipitation distribution, in particular the Indian monsoon is well simulated (Figure 1). The excessive wind stress and surface evaporation in the western Pacific which was present in CCM2 was significantly reduced. Examination of the thermodynamic structure indicates that the new scheme produces a warmer and moister upper troposphere, leading to a more stable stratification and correspondingly weaker circulation in the tropical Pacific. Zhang et al. (1996) also carried out experiments using the same initial conditions, but different convection schemes to understand the interaction between convection and large scale processes. The simulated atmosphere using the Zhang and McFarlane scheme exhibits quasi-equilibrium between convection and the large scale processes. This scheme is now the standard deep convection scheme in the latest version of the CCM (version 3). This version of the model is also used in the NCAR CSM.

The marine boundary layer
The properties of the CCM3 marine boundary layer have been compared with observations from CEPEX and TOGA COARE (Wang et al., 1996). The focus is on determining the accuracy of the ocean-atmosphere fluxes calculated by the CCM3. The agreement between the sensible and latent heat fluxes from the model and the observations has been improved considerably in the new version of the model. The improvement in the latent heat flux is due to a reduction in the surface winds caused by non-local effects of a new convective parameterization (Zhang, Kiehl, and Rasch, 1996). The current version of the model overestimates the latent heat flux by 17 Wm-2 (Figure 2) and the sensible heat flux by 3.4 Wm-2 for the CEPEX period. In addition, the mean model value of the boundary layer height is within 13m of the average height derived from a ship-based Raman lidar (Cooper et al., 1996). These studies have identified small deficiencies in the bulk parameterizations of the turbulent fluxes in CCM3 which will be corrected in future version of the model.

The short-wave and long-wave radiation fields
The radiative flux calculations and parameterizations of clouds in the CCM3 have been tested extensively against CEPEX observations. At the surface, the mean difference between model long-wave fluxes for clear-sky conditions and fluxes estimated from a ship-based spectral radiometer is less than 1 Wm-2 (Lubin et al., 1995). The difference between the CCM3 and the observations remains small over the range of sea-surface temperatures (SSTs) characteristic of the central tropical Pacific. Comparison of the short-wave fluxes for clear- sky conditions with observations from the R/V Vickers (Conant et al., 1996) indicates that the model bias is less than 4 Wm-2.

The CEPEX data strongly supports the existence of a super-greenhouse effect when the sea- surface temperature exceeds 300 K (Lubin, 1994; Weaver et al., 1994; Valero et al., 1996). The growth rate of the atmospheric greenhouse absorption with SST in the CCM3 has been tested against aircraft observations from CEPEX (Valero et al., 1996). The results show that the model reproduces the observed rate of approximately 13 Wm-2 K-1 to within 1 Wm-2 K-1.

The short-wave and long-wave cloud forcing simulated by the CCM3 have been validated against radiative fluxes estimated from CEPEX satellite data (Collins et al., 1996).

2. Model for Atmospheric Transport and Chemistry (MATCH)

MATCH, a global chemical transport model, has been developed over the last funding period. The focus of C4 in the MATCH effort was on the following questions:

- How does our uncertainty in convective transport processes effect our picture of short lived species in the tropics?
- How accurate is our picture of tropical circulations from global analyzed datasets produced by operational numerical weather prediction (NWP) centers and general circulation models?
- How do the different meteorological pictures of tropical circulations provided by the above products affect our ability to describe the distributions of short- and long-lived trace species?
- Given our best efforts at representing the above transport processes, and the relevant photochemistry, can we explain the low ozone amounts seen during CEPEX (Kley et al., 1996)?

These problems have been attacked in a variety of ways, and the research effort continues.

Rasch et al. (1994, 1995) described the role of chemistry and resolved transport processes in controlling the distribution of ozone and other tracers in the lower stratosphere and their influence on upper tropospheric ozone. Mahowald et al. (1995) and the thesis of Mahowald (1996) have examined the sensitivity of short and long lived trace species to the choice of some well known convection and boundary layer formulations used in global models, as well as the sensitivity of the trace species distributions to the choice of meteorological data. These studies were extended in Rasch et al. (1996) by: adding the Zhang and McFarlane convection scheme mentioned elsewhere in this document to MATCH; and, by adding the prognostic cloud water scheme described in Rasch and Kristjansson (1996) for use in the scavenging of soluble gases and aerosols to MATCH.

An additional requirement in any description of the processes controlling ozone in the stratosphere or troposphere is the efficient and accurate calculation of photolysis rates in clear and cloudy atmospheres. Landgraf et al. (1996) have introduced a new and computationally very efficient method for these calculations in MATCH.

The Ph.D. thesis by Lawrence (1996) provides a comprehensive chemical scheme for MATCH. Lawrence produced a photochemical module appropriate for the study of ozone photochemistry in the Pacific. A series of papers are in preparation describing the results of this work. The papers identify a picture of the processes controlling ozone in the tropics ,e.g., transport from other parts of the troposphere and stratosphere, photochemical production and loss, and deposition processes. An example is seen in Figure 3, which identifies the role of the major processes in controlling ozone in the tropical troposphere. The thesis also examined the sensitivity of the ozone distribution to uncertainties in the lightning production of NOX, and that to anthropogenic emissions of NOX, CO and CH4 from the industrialized world. Unfortunately, the simulations are still not able to capture the very low ozone amounts seen in the CEPEX measurements. The discrepancies are evident in Figure 4, which shows the measured ozone amounts made during CEPEX, and that from a simulation by MATCH of the corresponding time period. We continue to search for the processes that lead to the near zero concentrations (Kley et al., 1996).

3. Major Central Equatorial Pacific Experiment (CEPEX) Results

The objectives of CEPEX, the design of the field phase and some findings, have been reported in the renewal proposal for the second term and will not be repeated here. The attached special volume gives all of the required details of the impressive scientific achievements of CEPEX. Together these 40 journal papers offer a comprehensive view of the equatorial Pacific ocean-atmosphere system. We simply list the topics below with a very brief description of the findings.

Near zero ozone concentrations
The important role of convective clouds and the equatorial marine boundary layer in the chemistry of the troposphere was demonstrated during the 1993 CEPEX cruise. Profiles of ozone (Fig. 4) showed extremely low concentrations near the sea surface, moderate values (10-30 ppb) in the mid troposphere, and again extremely low mixing ratios (5 ppb or less) in the upper troposphere. A sharp gradient marked the onset of the tropopause region (Kley et al. 1996). We attribute these areas of extremely low ozone to rapid photochemical ozone destruction in the moist boundary layer followed by lifting in convective clouds associated with the ITCZ.

Heat budget, evaporation feedback and excess solar absorption
C4 investigators have studied the heat budget of the western tropical Pacific in order to understand the role of various ocean-atmosphere fluxes in maintaining the warm pool. The main results of the investigation are:

- On climatological time scales, evaporation does not act as a negative feedback on ocean warming, in contradiction to earlier theoretical studies (e.g., Newell et al., 1972; Wallace, 1991).

- Clouds appear to enhance solar absorption by about 25 to 30 Wm-2 of the incident solar radiation on a diurnal mean basis (Cess et al., 1995; Ramanathan et al., 1995; Pilewskie and Valero, 1995). The surface short-wave cloud forcing in the warm pool is approximately 100 Wm-2, much higher than the value predicted by general circulation models.

Surface evaporation over the warm pool was estimated from two years of the high quality buoy data collected as part of the TOGA program. The bulk algorithm used for the flux computation was validated using the CEPEX aircraft data (Zhang and Grossman 1996). Taking into account the perturbations on hourly to daily timescales, the average evaporation over the warm pool was estimated to be about 100 Wm-2 (Zhang and McPhaden, 1995; Ramanathan et al., 1995). It is significantly less than that (> 180 Wm-2) required for evaporation to balance the radiative energy flux into the ocean surface when the cloud sheltering effect is ignored (Newell, 1979; Pierrehumbert, 1995). Furthermore, on climatological timescales, evaporation was found to decrease with increasing SST when SST exceeds about 301 K (Zhang and McPhaden, 1995), in sharp contrast to earlier suggestion that evaporation acts as a major negative feedback to maintain the warm pool SST (Newell, 1979; Wallace, 1992). Analysis of satellite data for convection together with numerical experiments suggests that decrease of evaporation with SST in the high SST regime is a result of the large-scale effect of convection on the surface wind field (Zhang et al., 1995).

The enhanced short-wave absorption by clouds was deduced from measurements in the warm pool region using two techniques. First, the short-wave cloud forcing at the ocean surface was derived as a residual term in the heat budget of the ocean mixed layer (Ramanathan et al., 1995). This calculation is based upon the constraints imposed by long-term thermal equilibrium of the warm pool and the small heat fluxes carried by entrainment and advection in the ocean mixed layer. The estimate of 100 Wm-2 obtained by this technique has been confirmed using solar flux measurements from moorings deployed during TOGA COARE (Waliser et al., 1995). Second, the short-wave cloud forcing was derived directly from long- term measurements in the southern tropical Pacific (Cess et al., 1995). Both studies conclude that clouds absorb significantly more solar radiation than predicted by either general circulation models or by detailed radiative transfer models for individual clouds (Lubin et al., 1996).

In contrast to recent studies suggesting that the absorption may be related to water vapor in clear-sky regions (e.g., Arking, 1996), a detailed comparison of models and observations from the central Pacific shows no evidence of "anomalous" clear-sky absorption exceeding 4 Wm-2 (Conant et al., 1996), at least for the equatorial Pacific ocean.

Preliminary results from calculations with the CCM2 suggests that the absorption could significantly alter the large-scale circulation patterns in the atmosphere (Kiehl et al., 1995). There is intriguing evidence that the absorption could mitigate a number of problems in the current generation of GCMs, including the pervasive cold bias in the upper tropical troposphere, the overestimation of surface winds and evaporation in the Pacific (Kiehl et al., 1995), and the generation of bifurcated cold tongues in coupled ocean atmosphere models.

It must be cautioned, however, that substantial controversy surrounds the excess absorption results (e.g., see Stephens, 1996; Pilewskie and Valero, 1996).

East-West gradients in water vapor
CEPEX has provided some of the first vertical cross sections of water vapor in a zonal plane in the central Pacific (Kley et al., 1996). The measurements were made with laboratory- calibrated Vaisala water vapor sensors and frost-point hygrometers, and yield a far more accurate distribution of water vapor in the upper troposphere than is available from conventional operational sounding instrumentation. The data show a moistening of the atmospheric column from the surface to the upper troposphere in regions of deep convection west of the dateline. The role of the large scale circulation in maintaining the overall water vapor distribution was analyzed as part of a Ph.D. thesis by Sherwood (1995).

Vertical structure of the water vapor greenhouse effect
Aircraft data have been used to construct the first vertical profiles of clear-sky greenhouse absorption from the surface to the tropopause over tropical oceans (Valero et al., 1996). The column greenhouse effect increases at nearly the same rate as the surface downwelling flux. At all altitudes sampled by the three CEPEX aircraft, the greenhouse effect is larger in convectively active regions. This supports the association of enhanced greenhouse absorption in regions where convection has transported water vapor aloft (Ramanathan and Collins, 1991).

Effect of convective cloud systems on radiative fluxes
Satellite, aircraft and surface data together lead to the conclusion that cloud systems associated with deep convection dominate the regulation of solar radiation at the sea surface (Collins et al., 1996a). The surface insolation is closely correlated with a satellite index of convective activity, and decreases by 150 Wm-2 (half the clear-sky insolation) during long- lived convective events. The convective cloud cover increases rapidly with SST to a maximum of over 50% at 302 K, and this results in a 25 Wm-2 reduction in surface insolation for each degree rise in SST. Although the long-wave heating of the surface by clouds also increases with SST, the surface cloud forcing is less than 20% of the long-wave cloud forcing on the surface-atmosphere column (Collins et al., 1996b). The ratio of the surface to column long-wave cloud forcing supports the estimate used to formulate the cirrus thermostat mechanism.

Microphysical factors for high albedo of cirrus layers
CEPEX data have been used to test Heymsfield and Milosevitch's (1991) explanation that the high albedos of tropical cirrus are caused by small spherical ice particles in the upper layers of the anvil clouds. Small particles comprise more than half the mass and contribute most of the optical extinction in the upper layers of the cirrus clouds sampled (Heymsfield and McFarquhar, 1996). However, the integrated mass of larger particles in the lower layers is an order of magnitude larger. Radiative transfer calculations based upon the microphysical parameters suggest that the large particles contribute approximately 50% of the albedo of extensive anvils (Heymsfield et al., 1996). Therefore the CEPEX data show that the high albedo of tropical cirrus is due to a combination of factors, including the presence of small particles near cloud top and the large vertical extent and mass of typical anvil cloud systems.

A 1-D mixed layer model for the warm pool
The relative importance of the various feedback processes on the regulation of the maximum SST was examined in a 1-dimensional model for the warm pool (Kuettner and Ramanathan, 1996). In addition to including the equilibrium processes, the model includes day-to-day variations in the atmospheric forcing due to weather. This 1-D model study demonstrates the importance of the negative feedback effects of clouds on the warm pool SST. However, the convincing verification of the thermostat hypothesis must await a detailed coupled ocean- atmosphere study and CEPEX data has paved the way for the development of such a model.

4. Indian Ocean Experiment (INDOEX): Concept, Design and Proof of Concept

The experimental objectives and the design are described briefly in the next section dealing with future plans. Fundamentally, INDOEX is a focused experiment aimed at understanding the cloud-aerosol-chemistry-climate interactions as it relates to anthropogenic activities. The experiment is based on the concept that, during the north east monsoon (December to March) the Arabian Sea and the equatorial Indian Ocean is an ideal natural laboratory to observe the radiative forcing of anthropogenic aerosols. This concept, in turn, relies on the existence of north-south gradient in aerosol concentrations from the Arabian Sea to the south Indian Ocean. Several pre-INDOEX cruises (conducted in 1995 and 1996) confirmed this concept convincingly. The following are the highlights of our INDOEX activities so far.

· Published a white paper describing the goals and objectives of the experiment. This was authored by C4 PIs, and PIs from France, Germany, Netherlands and India.

· Submission of the US proposal to NSF, DOE, NOAA and NASA. The proposal is attached with this renewal proposal.

· Participated in a pre-INDOEX cruise by the NOAA R/V Baldrige during spring of 1995. In this joint C4-UMD-NOAA Aeronomy Lab-University of Miami venture, chemical measurements were undertaken during this cruise in the equatorial Indian Ocean and the Arabian Sea. Sharp gradients in ozone, NO2 and aerosols were documented, with large values in the Arabian Sea, which is closer to the pollution source.

· Participated in a pre-INDOEX cruise on board the Indian R/V Sagar Kanya during January to February of 1996. In this joint C4-National Physical Lab- Physical Research Laboratory study, we measured the meridional gradients in aerosol mass, optical depths, and influence on the radiative forcing at the sea surface. The measured gradients in optical depths gave strong support to the INDOEX design concept of using the north- south gradient in the Arabian Sea and adjacent Indian ocean to quantify the radiative forcing due to anthropogenic aerosols.

4.1. 1995 NOAA Baldrige Cruise: Gradients in reactive species and aerosols

A commercial ship that regularly sails through the Indian Ocean, the Madame Butterfly, was outfitted (by MPI) with an automated ozone detector. The results showed that in the Indian Monsoon season, when winds blow on shore, ozone mixing ratios were very low, on the order of 10 ppb, but in dry season, when winds blow off the continent, mixing ratios were twice as high (Figure 8, INDOEX White Paper 1995).

During the spring of 1995, C4 and NOAA/AOML scientists participated in an Indian Ocean cruise aboard the R/V Malcolm Baldrige. This highly successful pre-INDOEX experiment confirmed the results of Kley et al. (1996) and verified many of the fundamental hypotheses of INDOEX.

Ozone sondes revealed both unusually high and low concentrations of upper tropospheric ozone that were traced with back trajectories to "Hurricane" Marlene (Figures. 9 and 10 from C4 1995 Annual Report). Preliminary analysis of the results suggest that convective lifting of marine boundary layer air by the feeder bands produced low mixing ratios while downward transport in the eye of the storm produced high mixing ratios (Rhoads et al., 1996).

Measurements from the Baldrige revealed a strong latitudinal gradient in trace gas and aerosol concentrations, and in received solar radiation. As the ship sailed from the remote southern hemisphere across the ITCZ into the northern hemisphere continental emissions heavily impacted the oxidizing capacity and radiative properties of the troposphere (Rhoads et al., 1996; 1996a; Dickerson et al., 1995; 1996; 1996a). These findings helped us to plan more effectively the main INDOEX experiment.

Four distinct meteorological regimes were encountered during the cruise: southern hemisphere extra-tropical, SHxT, southern hemisphere maritime equatorial, SHmE, northern hemisphere maritime equatorial, NHmE and northern hemisphere continental tropical, NHcT. The air masses associated with these regimes were characterized by both synoptic situation and chemical composition. Back trajectories showed distinct origins for the air sampled, and trace gas and aerosol data showed concentrations marching upward with each step northward (Figure 14 from the INDOEX proposal). For example, aerosol nitrate, ammonium, and non- sea-salt sulfate increased by a factor of four across the ITCZ.

4.2. 1996 Sagar Kanya Cruise: Aerosol-radiation interactions
A pre-INDOEX cruise experiment was conducted during 5 January - 4 Februry, 1996, over the Arabian Sea and the Indian Ocean using the Indian research vessel, Sagar Kanya (Cruise #109). Different research institutions from India and US participated in the cruise and parameters such as aerosol optical depth, mass concentration, size index, trace gases' concentrations and global and direct surface flux were measured. One of the significant results of the cruise experiment has come from a joint study made on the aerosol parameters measured by the Physical Research Laboratory (Ahmedabad) group and the radiation flux measurements made by C4. The cruise track is shown in Figure 5. The research vessel left Goa (on the western coast of south India) on 4 January, 1996, sailed in the south west direction in the Arabian Sea and entered into the Indian Ocean down to 5 S of the equator. The vessel returned to the Bombay coast on 4 February. All along the cruise track measurements were made on the aerosol optical depths at five spectral bands in and around the visible region using a hand held sun- photometer, direct solar and global (direct and scattered sky) radiation intensities using calibrated pyrheliometer and pyranometer, photosynthetically active radiation (PAR) using BSI instruments and aerosol mass concentration and size distribution using a QCM (Quartz Crystal Microbalance) particle impactor.

The measured aerosol optical depths show (Figure 6) a large spatial gradient along the cruise track, with values for the mid visible wavelength varying from a high of 0.5 near the coast to a low of 0.05 (a factor of 10 decrease) over the Indian Ocean. A significant reduction in the aerosol mass concentration is also seen, varying between 90 mgm-3 measured near the coast and a few mgm-3 over the Indian Ocean. Significant contribution to the observed mass variation arises from particles of size less than 0.5 mm and particles of size greater than 8 mm.

To address the problem of radiative forcing induced by aerosols, the measured aerosol optical depth values are correlated with the simultaneously measured radiation flux data. Regression analysis made between the aerosol optical depth after correcting for the air mass term and the global flux (in the visible region) corrected for the angle of incidence show a linear relation between the two, with a reduction of solar insolation at the sea surface of about 5 Wm.-2 for an increase of 0.1 in aerosol optical depth. This value however is higher than the estimated global-annually averaged net radiative forcing by aerosols which is in the range of -0.3 to -2.5 W m-2. In the present case, large influx of mineral dust originating from the arid and semi arid regions surrounding the Arabian Sea may be a contributing factor to the observed high aerosol optical depth with a corresponding high negative radiative forcing.

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