A11C-0059 0800h
Observational evidence of African desert dust intensification of easterly waves
This paper presents indirect observational evidence that desert dust can modulate the amplitude of easterly waves in the Atlantic Ocean. Twenty two years of NCEP/NCAR reanalysis and dust from a global transport model are used to characterize the evolution of enhanced easterly waves. The results show that approximately 20% of the dust entrainment into the atmosphere over a broad region of North Africa is associated with easterly wave activity, suggesting that easterly waves may regulate desert dust entrainment into the atmosphere. About 10%-20% of the seasonal variability of desert dust concentrations across the North Atlantic is related to easterly waves, which suggests that easterly waves modulate the transport of desert dust. Lag composites of analysis increments (analysis minus first−guess) of geopotential height (700−hPa) anomalies indicate larger amplitudes in the analysis than in the first-guess fields. The results indicate that the temperature structure in the lower troposphere associated with easterly waves is warmer in the analysis than in first−guess fields by about 0.25K per day. We hypothesize that the differences in the amplitudes are due to radiative effects associated with African dust, which have been incorporated in the reanalysis by data assimilation but absent in the model first−guess.
A11C-0060 0800h
Results from the Portable Infrared Aerosol Transmission Experiment (PIRATE) - Caribbean: An examination of the column integrated infrared extinction of Saharan dust and comparisons with data commonly used in models
Infrared optical depth of Saharan dust from field measurements made in Puerto Rico are presented and compared with frequently-used dust models. The Portable Infrared Aerosol Transmission Experiment (PIRATE) - Caribbean was a ground-based experiment that measured the infrared transmission of transportted dust from the Saharan Desert. A Fourier Transform Infrared (FTIR) spectrometer was used in Boqueron, Puerto Rico from June 23 through June 30, 2004 as a high-resolution infrared sun photometer. The visible aerosol optical depth (AOD) around the time of each FTIR measurement was taken from a nearby AERONET sensor at La Parguera, Puerto Rico, for reference. The FTIR recorded the direct solar and scattered radiances from 3 to 14 microns. By collecting the solar radiance for several days, some for which the AOD was either very low ($<$0.1) or high ($>$0.5), the infrared AOD of the dust was determined as a function of wavelength. The measured infrared AOD of the dust is compared with frequently-used dust models, i.e. Volz and Sokolik, for various effective radii and assumed dust compositions. Since Saharan dust is often pervasive over large regions of the globe, these results are potentially important in models and satellite measurements attempting to determine the regional forcing from dust.
A11C-0061 0800h
Understanding the long-term variability of African dust transport across the Atlantic as recorded in both Barbados surface concentrations and large-scale TOMS optical thickness
Mineral dust is by far the most variable aerosol in the troposphere because its generation processes are highly sensitive to climate through both wind speed and rainfall. Assessing the radiative impact of mineral dust in numerical models is thus unrealistic as long as the causes of this variability are not fully understand and accounted for. The inter-annual variability of African dust transport over the north tropical Atlantic is monitored using in-situ surface concentrations measurements performed at Barbados since 1966, along with the TOMS and METEOSAT dust optical thickness (DOT) records covering the last two decades. Despite their differences in spatial coverage, the two dust records are in good agreement at both monthly and annual time scales over the 22 years of common operation. This demonstrates that the Barbados dust record is representative of the year-to-year variability of dust export over the north tropical Atlantic during both winter and summer. The satellite DOT are then used to assess the characteristics of the impact of climate factors, i.e., North Atlantic Oscillation (NAO) and Sahel drought, on dust emission and export as a function of season, and in terms of spatial extend of their influence. The analysis shows a large regional impact of Sahel drought on dust emissions and transport both in winter and in summer, whereas the influence of the NAO dominates the winter export and is more geographically limited to the eastern Atlantic north of 15°N, and possibly some localized source-regions (south Mauritania and Bodele depression). Overall the combination of the 35 years of Barbados measurements of African dust with 22 years of satellite dust survey over the Atlantic highlights the very high dust loads in the mid 1980s related to the severe Sahel drought (maximum impact in 1983), and the persistently high dusty conditions in the 1990s, most probably due to the continuation of relatively dry conditions in Sahel in the recent years.
A11C-0062 0800h
Asian Dust Storm Monitoring Using EOS Measurements
Dust storm is a severe environmental problem in eastern Asia, especially in Mongolia and northern China. Strong dust storms often occur during spring and earlier summer in this region because of desertification, windy conditions, and drought. To study the dust source, mechanism of dust transport, and effect to weather and environment, efficient approaches for dust storm detection and monitoring are critical, many efforts have been taken with ground measurements and satellite remote sensing. Moderate Resolution Imaging Spectroradiometer (MODIS) and Atmospheric Infrared Sounder (AIRS) are two of the key instruments for Earth observation of NASA EOS Aqua (EOS PM) satellite, MODIS covers the entire earth surface every 1 to 2 days and acquiring data in 36 spectral bands with spectral coverage from visible bands to thermal infra-red bands and spatial resolution of 250m, 500m, and 1km, it's very suitable for dust storm detection and monitoring while AIRS provide high spectral thermal measurements. We already investigated the spectral features of Asian dust storms, identified MODIS sensitive bands to dust and designed spectral indices with infra-red band and thermal band measurements from MODIS and AIRS in our previous studies. With four overpasses (two daytime and two nighttime) of MODIS (Terra and Aqua MODIS) each day and two overpasses of AIRS, we can detect Asian dust storm, and monitor the movement and transport of dust. We demonstrated our approaches with comprehensive analysis of dust storms in Mongolia and eastern China. Key words: Dust storm monitoring, remote sensing, MODIS and AIRS
A11C-0063 0800h
The Effect of Dust Storm on Deep Convection over the Tropical North Atlantic
The effect of the Saharan mineral dust on deep convection over the tropical North Atlantic were investigated by anlayzing Moderate Resolution Imaging Spectroradiometer (MODIS) level-2 retrievals. Brightness temperature at 11 $\mu$m were used to indicate the occurrence of deep convection and the tropospheric aerosol loading was represented by the aerosol optical thickness (AOT) at 0.55 $\mu$m. Probability distribution functions (PDFs) of the brightness temperature were constructed for a region offshore the African west coast for August-September 2002, when dust was frequently blown off the Saharan desert and transported across the North Atlantic. For conditions of high aerosol loading (AOT $>$ 0.8), the frequency of brightness temperatures less than 230 K was reduced by a factor of 5-6 from the value associated with conditions of low aerosol loading (AOT $<$ 0.5). The atmospheric effect of the aerosols lasts at least one day after the passage of the aerosols.
A11C-0064 0800h
Aerosol Direct Radiative Forcing Climatology Based on AERONET Measurements
AEosol RObotic NETwork (AERONET) is a sunphotometer network covering virtually all aerosol regimes around the world. It measures and derives spectral aerosol optical depth, single-scattering albedo, phase function/asymmetry factor for both fine-mode and coarse-mode aerosols and for up to 10 years. These high quality data can be used to derive measurement-based aerosol direct forcing climatology that will serve as a baseline for evaluating the satellite-based and model-based forcing assessment. In this study, the aerosol climatology, in conjunction with surface albedo and cloud products from MODIS, is used to calculate the aerosol direct forcing/forcing efficiency (forcing per unit optical depth at 550nm) under cloud-free and cloudy conditions, for total and fine-mode aerosols, and at the top-of-atmosphere (TOA) and the surface. For biomass burning aerosols, we find that the average forcing efficiency over South America is smaller by ~30% at the TOA but larger by ~35% at the surface than that over South Africa, because of stronger absorption by the South Africa smoke. For mineral dust, the surface albedo is another important factor that determines aerosol forcing. We find that, over Saharan deserts, Arabian Peninsula, and their surrounding oceans, the surface albedo ranges from ~0.1 to ~0.35. The dust forcing efficiency substantially decreases at the TOA from -44 to -17 W/m2 and at the surface from -80 to -48 W/m2 with increasing surface albedo. We also find that the forcing efficiency of fine-mode aerosol is larger at the TOA while smaller at the surface than that of total aerosol, which is mainly determined by a larger single-scattering albedo and smaller asymmetry factor of fine-mode aerosol. Cloudy-sky aerosol direct forcing is usually not negligible for the observed cloud optical thickness and is sensitive to the relative location of aerosol and cloud layer.
A11C-0065 0800h
Global shortwave aerosol direct radiative forcing from MODIS measurements for mineral dust, marine aerosol, biomass-burning and industrial pollution.
The shortwave aerosol direct radiative effect results from the extinction of incoming solar radiation by atmospheric particles, through scattering and absorption. This effect has been studied globally using general circulation models, satellite instruments, and locally using in-situ ground-based or airborne measurements as well as sunphotometry. Here, we present a method to identify the aerosol type using the aerosol optical thickness and the ratio of accumulation-mode to total optical thickness retrieved by the MODerate resolution Imaging Spectroradiometer (MODIS) instrument, the aerosol index estimated from the Total Ozone Mapping Spectrometer (TOMS) instrument, and the surface wind speeds derived from the Special Sensor Microwave Imager (SSM/I) instrument. Estimates of the direct radiative forcing are then performed from this aerosol classification and single scattering properties from AERONET using appropriate radiative transfer and surface albedo. We will present the aerosol direct radiative forcing for mineral dust, marine aerosol, biomass-burning aerosol and industrial pollution for the period September 2000 to August 2001, over both oceanic and land surfaces, with the exception of bright surfaces such as deserts and snow-covered areas.
A11C-0066 0800h
Blending MODIS and AERONET Measurements for Monthly and Globally Varying Aerosol Size Distributions
The incorrect specification of atmospheric aerosol size can result in errors in the calculation of the aerosol optical thickness with subsequent errors in the direct radiative forcing. Furthermore model assessments of aerosol emissions using optical thickness for validation are difficult to perform if much of the error in optical thickness is caused by improper assumptions about the aerosol size distribution. To address these concerns the aerosol size distributions retrieved from 100 stations in the AERONET array of surface-based sun photometers are combined with the retrieved fine mode aerosol optical thickness fraction from MODIS-Terra to create a monthly climatology of a geographically varying vertically averaged aerosol size distribution. In this way the more detailed AERONET size distribution is blended with the excellent spatial and temporal coverage provided by MODIS for the four year period from March 2000 to February 2004. Various procedures for the specification of the aerosol size distribution are tested using the ECHAM4 climate model with the result that the global and annual averaged aerosol optical thickness at 500 nm is 0.17. However this value can vary from 0.11 to 0.20 depending on the method and assumptions used which translates into a direct aerosol TOA radiative forcing range of -1.6 to -3.9 W/m^2.
A11C-0067 0800h
TOMS observations of increases in Asian aerosol in winter from 1979 to 2000
Emission inventories indicate that the largest increases in SO2 emissions have occurred in Asia during the last 20 years. By inference, largest increases in aerosol, produced primarily by the conversion of SO2 to sulfate, should have occurred in Asia during the same time period. Decadal changes in regional aerosol optical depths are calculated by analyzing Total Ozone Mapping Spectrometer (TOMS) vertical aerosol optical depths (converted to 550 nm) from 1979 to 2000 on a 1 degree by 1 degree global grid. Aerosol trends are calculated on a regional basis during winter (November - February) to maximize the anthropogenic component of the aerosol record. Largest increases in aerosol optical depths between 1979 and 2000 are present over the China coastal plain and the Ganges river basin in India.
A11C-0068 0800h
Validation of MODIS Fine Mode Retrievals Over Ocean
The MODIS (MODerate Resolution Imaging Spectroradiometer) algorithm can accurately partition aerosol into fine and coarse modes for aerosol optical depths greater than 0.1. The algorithm is validated against both AERONET sky and sun measurements using a co-located data set encompassing four years of data from 30 sites. MODIS slightly overestimates fine fraction in dust dominated regimes and may underestimate in pollution and smoke dominated conditions for individual measurements. The accuracy of monthly mean retrievals improves significantly showing very little offset and almost completely eliminating the trend seen in the individual measurements. These results show that MODIS is an excellent tool for determining aerosol types and sources especially for climate studies.
A11C-0069 0800h
The Role of Convective Plumes and Vortices on the Global Aerosol Budget
The significance of dust aerosol is evident based on the large surface area of arid and semi-arid regions on most continents. Suspended desert dust is a significant contributor to atmospheric aerosol loading, with strong absorption bands in the atmospheric window. Ginoux et al. (2001) estimates the global contribution of suspended aerosols by desert dust at several thousand millions of tons per year, supporting previous estimates by Hidy and Brock (1971), Schütz (1980), and d'Almeida (1986a). In the US Southwest, convective plumes and vortices produce an efficient vertical transport of dust that supplies enough aerosols to the atmospheric boundary layer to significantly scatter and absorb incoming solar radiation (DeLuisi et al., 1976). Evidence of this is given by Gillette and Sinclair's (1990) aircraft measurements of particle concentration and vertical velocities in dust devils. They estimated that at approximately 150 meters above the surface, dust devils over the U.S. Southwest have a mean dust flux of about 0.1 kg m-2 per year. This value represents almost 75% of the total dust flux caused by wind action. In the spring of 2002 our group led an NSF-sponsored pilot field experiment to quantify the intensity and variability of the surface flux of heat and mineral dust under convectively unstable conditions. Data collected in this pilot field campaign show that coherent (organized over their vertical extension) convective plumes and vortices produce heat and dust fluxes in excess of 10 kW m-2 and 1 g m-2 s-1 (Renno et al., 2004). Thus, a single dust devil with a 10 meter radius can pump approximately one half of a ton of dust into the atmosphere in a typical 30 minute life span. Observational data for the US Southwest shows that nearly 2 percent of the observed area is covered by updrafts due to dust devils during their active times from 10:30 to 18:00 (Sinclair, 1966; Ryan and Carroll, 1970). Based on this data and our measurements, a 100 by 100 km area in Arizona can pump nearly 200 tons of dust per second into the atmosphere during convectively active periods (from 10 am to 5 pm). We conclude that dust sources due to convective plumes and vortices, such as that over the US Southwest, may have a large impact on the global aerosol budget. Our presentation will focus on the role of coherent convective plumes and vortices on the global aerosol budget.
A11C-0070 0800h
Smoke Over Haze: Comparative Analysis of Satellite, Surface Radiometer and Airborne In-Situ Measurements of Aerosol Optical Properties and Radiative Forcing Over the Eastern US
In July 2002 Canadian forest fires produced a major smoke episode that blanketed the U.S. East Coast. Properties of the smoke aerosol were measured in-situ from aircraft, complementing operational AERONET and MODIS remote sensed aerosol retrievals. This study compares single scattering albedo and phase function derived from the in-situ measurements and AERONET retrievals in order to evaluate their consistency for application to satellite retrievals of optical depth and radiative forcing. These optical properties were combined with MODIS reflectance observations to calculate optical depth. The use of AERONET optical properties yielded optical depths 2% to 16% lower than those directly measured by AERONET. The use of in-situ derived optical properties resulted in optical depths 22% to 43% higher than AERONET measurements. These higher optical depths are attributed primarily to the higher absorption measured in-situ, which is roughly twice that retrieved by AERONET. The resulting satellite retrieved optical depths were in turn used to calculate integrated radiative forcing at both the surface and TOA. Comparisons to surface (SurfRad and ISIS) and to satellite (CERES) broadband radiometer measurements demonstrate that the use of optical properties derived from the aircraft measurements provided a better broadband forcing estimate (21% error) than those derived from AERONET (33% error). Thus AERONET derived optical properties produced better fits to optical depth measurements, while in-situ properties resulted in better fits to forcing measurements. These apparent inconsistencies underline the significant challenges facing the aerosol community in achieving column closure between narrow and broadband measurements and calculations.
A11C-0071 0800h
A Study on the Optical Properties of Aerosols above the Forest by Remote Sensing
Aerosol retrieval by remote sensing technique is one of the promising method in understanding the chemical and optical properties, column load, and spatial distribution of aerosols. However, though the current technique in use is quite successful about aerosols over ocean with small water-leaving radiances, quantitative retrieval of aerosols over land mass is not yet satisfactory. We try to develop a new method to make the aerosol retrieval over land more accurate than ever before. A sensitivity analysis of reflectance shows that wrong selection of spectral reflectance model results in quite a large difference in retrieved aerosol characteristics. Therefore, a well-suited surface reflectance model is needed to be created. We conducted aerosol and radiation measurements coupled with in situ forest reflectance measurements in sync with satellite radiance measurements by EOS Terra and Aqua from the top of the atmosphere. The experimental site is located in a forest with an extensive and uniform area covered with deciduous trees commonly existing in Japan. The ground-based measurements include Andersen impactor samplings, radiometric measurements with OPC, a sunphotometer and a telephotometer. Forest reflectance was measured with a spectral radiometer covering visible and near infrared above the forest canopy level from a tower standing in the forest. Reflectance was measured directionally, and was found to show no major bi-directional dependency, assuring us that Lambert reflectance model is sufficient for calculation in this particular type of forest. The sampled spectral reflectances were averaged to be 0.0414 at 0.55 $\mu$m. For satellite aerosol retrieval, visible and near infrared bands in MODIS sensors were employed. MODTRAN code was used in radiative transfer in the aerosol-laden atmosphere. Several different types of aerosol were examined, and a rural aerosol model with similar size distribution and composition to the aerosols, which are estimated from OPC measurements and Andersen samplings, was used as an input for the radiative transfer calculation. Absorption coefficients were calculated from the data obtained from a separate carbon sampling. The column aerosol optical depth was obtained to be 0.309 at 0.50 $\mu$m, which is similar to the value derived from sunphotometer.
A11C-0072 0800h
Estimation of fire emissions from satellite-based measurements
Biomass burning is a worldwide phenomenon affecting many vegetated parts of the globe regularly. Fires emit large quantities of aerosol and trace gases into the atmosphere, thus influencing the atmospheric chemistry and climate. Traditional methods of fire emissions estimation achieved only limited success, because they were based on peripheral information such as rainfall patterns, vegetation types and changes, agricultural practices, and surface ozone concentrations. During the last several years, rapid developments in satellite remote sensing has allowed more direct estimation of smoke emissions using remotely-sensed fire data. However, current methods use fire pixel counts or burned areas, thereby depending on the accuracy of independent estimations of the biomass fuel loadings, combustion efficiency, and emission factors. With the enhanced radiometric range of its 4-micron fire channel, the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, which flies aboard both of the Earth Observing System (EOS) Terra and Aqua Satellites, is able to measure the rate of release of fire radiative energy (FRE) in MJ/s (something that older sensors could not do). MODIS also measures aerosol distribution. Taking advantage of these new resources, we have developed a procedure combining MODIS fire and aerosol products to derive FRE-based smoke emission coefficients (Ce in kg/MJ) for different regions of the globe. These coefficients are simply used to multiply FRE from MODIS to derive the emitted smoke aerosol mass. Results from this novel methodology are very encouraging. For instance, it was found that the smoke total particulate mass emission coefficient for the Brazilian Cerrado ecosystem (approximately 0.022 kg/MJ) is about twice the value for North America or Australia, but about 50 percent lower than the value for Zambia in southern Africa.
A11C-0073 0800h
Using the Altair UAV to Address Radiative Balance and Chemistry in the Troposphere: Instrumentation Designed to Tackle Climate Change, Regional Scale Pollution Plumes, and Photochemical Mechanisms
Understanding the feedback mechanisms relevant to climate change requires a concerted effort interleaving validated satellite measurements, fine-scale in situ measurements, and adaptive modeling. NASA's Altair UAV platform, having peak altitudes of 52,000 feet and flight times of 32 hours, provides a unique opportunity for monitoring pollution plumes, validating AURA measurements, and dissecting fine-scale mechanistic questions. We present a payload design capable of addressing many key scientific questions and fully realizing the capabilities of this new aircraft. By sharing common subsystems, utilizing miniaturized lasers and implementing miniature detectors, an unprecedented number of critical measurements can be acquired from a single platform, providing a superior context for modeling and validation. These measurements will serve to validate and extend key Aura measurements, providing fine-resolution measurements over long time scales that can be used for validation, model initialization, and correlation. Coupling measurements of water vapor, total water, cloud structure, and ozone with CO, CO2, N2O, and CH4 tracer fields, this payload will offer new insight into the evolution of convective systems, water transport, and photochemical mechanisms. This Altair instrument package can also image small-scale transport phenomena by monitoring water isotope ratios and carbon monoxide in conjunction with cloud profiles. Ozone and halogen measurements will provide a definitive appraisal of the recovery of the stratosphere while monitoring species (e.g., BrO) suggested as major contributors to mid-latitude ozone loss. As local and regional phenomena begin to have marked impacts on a global scale, NASA is uniquely positioned with Altair UAV to support instrumentation to provide the critical links between satellite measurements and in situ measurements. Bridging this gap with long observation periods and fine spatial resolution allows measurements to be performed systematically both for validation and to identify episodic phenomena that are not well resolved by satellite data. Linking these in situ data to satellite data is a critical step in understanding the evolution of the climate system.
A11C-0074 0800h
Aerosol Transport to the Greenland Summit Site, June, 2003 to August 2004
With the resumption of year-round staffing of the Summit Greenland Environmental Observatory (GEOSummit) in 2003, we were able to sample aerosols year round by size (8 size modes), time (3 hr to 24 hr), and composition (mass, optical attenuation, and elements H, Na to Mo, plus lead) for association with particulate layers in snow, firn and ice. Sampling was accomplished using a 10 L/min slotted 8-stage rotating drum impactor (DELTA 8 DRUM, http://delta.ucdavis.edu)in the clean sector 0.5 km upwind from the main camp pollution sources. The air intake was approximately 2m above the snow surface. The rotation rate of the DRUM was slowed to 0.5 mm/day, allowing continuous sampling for 48 weeks with 12-hr time resolution on a single set of lightly greased 480 ?g/cm2 Mylar substrates. Early results show transport of relatively coarse (12 to 5 ?m aerodynamic diameter) soil aerosols to the site in spring, 2003, in well -defined plumes of 1- to 2-day duration. Trajectory analysis shows potential Asian sources. Sulfur-containing aerosols, also seen in plumes of short duration, occur in two size modes, a typical accumulation mode aerosol (0.75?0.34 ?m) and a very fine aerosol mode ( 0.34?0.09 ?m), the latter likely stratospheric in origin. We wish to acknowledge the excellent on-site support of the GEOSummit staff, including M. Lewis, R. Abbott, B. Torrison, and K. Hess, and T. Wood.
http://delta.ucdavis.edu
A11C-0075 0800h
Ultra-Trace Surface Snow Chemistry at Summit Greenland: July, 2003 to August 2004
From July 2003 to August 2004, replicated surface snow samples were collected each week in the clean air sector at Summit Greenland using ultra-clean sampling techniques. The samples were analyzed for a range of elements from sodium to lead by high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). Comparisons with similar continuous, high-resolution HR-ICP-MS measurements from nearby ice cores indicate that for most elements, the records are very similar. Together with continuous, high resolution air filter sample analyses and 3-D air mass back-trajectory modeling for the same time period, we used the surface snow measurements to evaluate the seasonal timing, source regions and transport pathways, and air-snow transfer characteristics for dust and pollution aerosols. These data provide important calibration for interpretation of ice cores from Greenland as long term historical proxies of atmospheric aerosol concentration and chemistry.
A11C-0076 0800h
Increasing Cloud Droplet Size with Aerosol: a missing piece towards solving aerosol indirect effect puzzles
Since the Twomey effect was proposed in 1977, it has been a common belief that cloud particle size only decreases with aerosol loading. Using NASA's MODIS products, an opposite trend is found, together with a general finding that cloud particle size may increase or decrease with aerosol loading depending on water vapor supply, cloud regime and atmospheric circulation. Convective clouds formed in moist regions show an overwhelmingly positive dependence, as opposed to the negative dependence for stratiform clouds in water-limited regions. The slope of the dependence is driven primarily by water vapor amount that explains 70% of the variance. A new mechanism is proposed to explain this new effect. Consideration of this effect in climate models could lower the estimation of aerosol indirect forcing to agree better with the estimations constrained by the global temperature records. The effect may also help reconcile controversial findings concerning if pollutants suppress or enhance precipitation: suppression in dry regions while enhancement in humid regions. This may offer another explanation for the "south flood and north drought" pattern in Asia.
http://www.atmos.umd.edu/~zli/
A11C-0077 0800h
AEROSOL INDIRECT EFFECT A MEASUREMENT BASED APPROACH
Aerosol introduces one of the largest uncertainties in climate modeling. Among the aerosol impacts on the radiative energy balance it is usually distinguished between the impact directly associated with modifications to aerosol properties (the direct effect) and the impact of changes to other atmospheric properties due to aerosol modifications (indirect effects). Since clouds are the main modulators of our climate, particular important are aerosol associated changes to properties of clouds and to the hydrological cycle. Indirect effects are numerous. Some of them will `warm' (increase the energy to the EAS [Earth-Atmosphere-System]) and some of then will `cool' (decrease the energy to the EAS). The overall sign will vary with aerosol type and environmental conditions. Thus, quantifications on a global basis are anybody's guess. Our current understanding from global modeling tends toward `cooling'. However, it is unclear, if these models properly consider all relevant aerosol-cloud interactions or if effects of local studies can be extended to larger spatial (regional) or temporal scales (seasonal). Currently, uncertainties in aerosol indirect modeling are so large that measurement based approaches have become an option. New capabilities in remote sensing now provide not only simultaneous data on many properties for aerosol and clouds but also on radiative fluxes at the top of the atmosphere capturing changes to the EAS energy balance. Despite data restrictions in terms of spatial coverage and accuracy, now correlations between aerosol and clouds can provide clues to modelling where (region) and when (season) what aerosol indirect effect applies. And the energy balance data can even provide quantitative constraints to modeling on a regional and seasonal basis. Data from the MODIS, MISR and CERES sensors of NASA's EOS Terra platform are analyzed. Global maps of correlations between aerosol and cloud properties are presented and discussed and a measurement based quantification on the aerosol indirect effect will be provided.
A11C-0078 0800h
AVHRR Observations of the Aerosol Indirect Effect for Summertime Stratiform Clouds in the Northeastern Atlantic
Advanced Very High Resolution Radiometer (AVHRR) 4-km imager pixels are collected over the northeastern Atlantic off the coast of the Iberian Peninsula for May-August, 1994-2001. A 3-channel retrieval scheme is used to derive the cloud properties: visible optical depth, droplet effective radius, cloud-top height, and pixel-scale fractional cloud cover. A 2-channel aerosol retrieval scheme is used to determine aerosol optical depth in cloud-free pixels. Aerosol optical depths in one-degree latitude-longitude regions on a given day are compared with the cloud properties in the same and adjacent one-degree regions on the same day. Results are composited for small (e.g. 5 x 5 degree) regions to study the influence of geographically controlled trends. For all years in most regions, there is a statistically significant decrease in droplet effective radius and an increase in cloud visible optical depth as aerosol optical depth increases. Depending on the region, cloud liquid water path can increase, remain constant, or decrease as aerosol optical depth increases. The change in cloud liquid water is probably a function of the relative humidity of the free troposphere above the cloud. Preliminary radiative transfer calculations indicate that the aerosol indirect radiative forcing is about 70 percent larger than the aerosol direct radiative forcing. On the other hand, the indirect forcing calculated from measured cloud optical depths is smaller than that calculated using measured droplet effective radii and assuming constant liquid water. Retrieved aerosol optical depth increases as the percentage of cloudy pixels in a region increases. The increase in aerosol optical depth could result from the aerosol particles swelling in high humidity environments, from undetected sub-pixel scale clouds residing in the pixels used for the aerosol retrievals, or an enhancement in the illumination of the aerosols due to radiation escaping from the sides of nearby clouds. Cloud optical depth also increases as the percentage of cloudy pixels in a region increases. The co-occurrence of increases in aerosol and cloud optical depths with increasing cloud fraction could be misinterpreted as evidence of the aerosol indirect radiative forcing when, in fact, processes unrelated to the indirect forcing may be governing changes in aerosol and cloud optical depths.
A11C-0079 0800h
Statistical Studies on Thin Cirrus Using MODIS Data
The 1.38 um channel on the MODerate resolution Imaging Spectroradiomater (MODIS) is very sensitive to identify and quantify thin cirrus on a global basis. The channel is located in the center of a strong water vapor absorption band, that eliminates illumination from the earth surface and low clouds. This channel is used to produce the MODIS cirrus reflectance product and to mask clouds from the MODIS aerosol products. The aerosol product screens thin cirrus with the 1.38-um channel reflectance above a threshold of 0.01. This threshold can leave residual cirrus contamination that corresponds to aerosol optical thickness of up to 0.10. This contamination, though usually small, may generate significant biases in the seasonal-regional aerosol average optical thickness derived from MODIS. Here we develop a method to estimate this contamination and correct for it by distinguishing between thin cirrus clouds and the dark current noise in the channel. We analyze several years of Terra and Aqua data, over land and ocean to generate a systematic statistics of the presence and opacity of thin cirrus around the globe. The results can be used later on to correct the aerosol optical thickness and radiative forcing of climate.
A11C-0080 0800h
Aerosol-induced radiative flux changes in the Pacific Basin troposphere
In April and May 2001, the Aerosol Characterization Experiment - Asia (ACE-Asia) was conducted to investigate the aerosols of the Pacific Basin troposphere and their potential impact on climate. One of the goals of ACE-Asia was to assess the regional direct aerosol radiative forcing by combining the results from suborbital aerosol measurements with satellite derived aerosol parameters, thereby putting the regional intensive observations into a larger scale context. One methodology to calculate the direct aerosol radiative forcing is to use the suborbital measurements of aerosol properties to devise a parameterization between column aerosol optical depth and direct aerosol radiative forcing, which can then be applied to satellite-derived maps of aerosol optical depth (AOD). The uncertainties in this method stem from the uncertainty in the accuracy and representativeness of the suborbital and spaceborne measurements of aerosol parameters. Using previously published aerosol column closure studies, we have developed a mean model for the vertical structure of aerosol properties in the Asian region during spring of 2001. This model includes the vertical structure of the aerosol single scattering albedo, as well as the wavelength dependence and absolute magnitude of aerosol extinction. The model reflects the predominance of pollution-type particles in the lowest 3 km of the atmosphere and the occurrence of large mineral dust particles at altitudes between 4 and 8 km. At a latitude of 35N, the model yields an instantaneous shortwave aerosol forcing of -30 Wm$^{-2}$ per unit aerosol optical depth at the top of the atmosphere and 65 Wm$^{-2}$ at the surface. In the first part of this paper, we will examine the importance of the various assumption used in this model for the parameterization between aerosol optical depth and aerosol radiative forcing. In the second part of our paper we investigate the consistency between aerosol optical depth fields derived using SeaWiFS, MODIS and MISR. We will review the relevant AOD validation results published to date and we will compare the monthly mean aerosol optical depth derived by these sensors for April 2001 in the latitude region between 25 and 55N and the longitude region between 130W and 130E. A preliminary analysis shows interesting features such as generally lower AOD values in the SeaWiFS retrievals and a stronger longitudinal gradient in MODIS derived AODs across the Pacific Basin when compared to the other two sensors. We will discuss the comparability of these measurements based on the level of coincidence between the three sets of observations, and we will investigate the representativeness of sparse aerosol observation in a generally cloud-dominated environment.
A11C-0081 0800h
An Aerosol Extinction-to-Backscatter Ratio Database Derived from the NASA Micro-Pulse Lidar Network: Applications for Space-based Lidar Observations
Backscatter lidar signals are a function of both backscatter and extinction. Hence, these lidar observations alone cannot separate the two quantities. The aerosol extinction-to-backscatter ratio, S, is the key parameter required to accurately retrieve extinction and optical depth from backscatter lidar observations of aerosol layers. S is commonly defined as 4*pi divided by the product of the single scatter albedo and the phase function at 180-degree scattering angle. Values of S for different aerosol types are not well known, and are even more difficult to determine when aerosols become mixed. Models of S are complicated by having to estimate the index of refraction, and particle size and shape (especially for non-spherical aerosols). Direct measurements of S are difficult and only a few instruments exist that are capable of measuring both extinction and backscatter, at or very near 180 degrees. In addition, direct measurements are not conducted under ambient conditions. However, models and direct measurements have begun to form a better understanding of S for key aerosol species. Another method of determining S is to use backscatter lidar in conjunction with co-located sunphotometer measurements of aerosol optical depth (AOD). The independent measurement of AOD is used as a constraint to solve the lidar equation, a process that calculates an extinction profile and also a layer averaged value for S. The resulting extinction profile may be over or under-estimated at a given altitude because S is a layer average, however this calculation of S does not require any assumption of aerosol composition, size, or shape and is done at ambient conditions. Thus, lidar-sunphotometer S values represent an improvement over models and direct measurements in this regard. Here we present a new lidar-sunphotometer S database derived from observations of the NASA Micro-Pulse Lidar Network (MPLNET). MPLNET is a growing worldwide network of eye-safe backscatter lidars co-located with sunphotometers in the NASA Aerosol Robotic Network (AERONET). Values of S for different aerosol species and geographic regions will be presented. A framework for constructing an S look-up table will be shown. Look-up tables of S are needed to calculate aerosol extinction and optical depth from space-based lidar observations in the absence of co-located AOD data. Applications for using the new S look-up table to reprocess aerosol products from NASA's Geoscience Laser Altimeter System (GLAS) will be discussed.
http://mplnet.gsfc.nasa.gov