Increase In Optical Depth Research Articles

Revisiting AVHRR tropospheric aerosol trends using principal component analysis

The advanced very high resolution radiometer (AVHRR) satellite instruments provide a nearly 25 year continuous record of global aerosol properties over the ocean. It offers valuable insights into the long-term change in global aerosol loading. However, the AVHRR data record is heavily influenced by two volcanic eruptions, El Chichon on March 1982 and Mount Pinatubo on June 1991. The gradual decay of volcanic aerosols may last years after the eruption, which potentially masks the estimation of aerosol trends in the lower troposphere, especially those of anthropogenic origin. In this study, we show that a principal component analysis approach effectively captures the bulk of the spatial and temporal variability of volcanic aerosols into a single mode. The spatial pattern and time series of this mode provide a good match to the global distribution and decay of volcanic aerosols. We further reconstruct the data set by removing the volcanic aerosol component and reestimate the global and regional aerosol trends. Globally, the reconstructed data set reveals an increase of aerosol optical depth from 1985 to 1990 and decreasing trend from 1994 to 2006. Regionally, in the 1980s, positive trends are observed over the North Atlantic and North Arabian Sea, while negative tendencies are present off the West African coast and North Pacific. During the 1994 to 2006 period, the Gulf of Mexico, North Atlantic close to Europe, and North Africa exhibit negative trends, while the coastal regions of East and South Asia, the Sahel region, and South America show positive trends.

Effect of major dust storm on optical, physical, and radiative properties of aerosols over coastal and urban environments in Western India

The present study deals with a major summertime dust storm event that occurred over the Arabian Desert, Saudi Arabia (SA), and the United Arab Emirates (UAE) and spread to Pakistan, the Arabian Sea, and Western India (in particular over Alibaug and Pune), using multi-satellite- and ground-based measurements. Analysis of the aerosol parameters retrieved over Alibaug and Pune from the ground-based MICROTOPS-II Sunphotometer, seven Aeronet Robotic Network (AERONET) stations, including Pune, and satellite (Moderate-resolution Imaging Spectroradiometer (MODIS)) and Ozone Monitoring Instrument ((OMI-Aura)) measurements shows significant changes on dusty days as compared with non-dusty days prior to and after the dust storm event. A large increase in aerosol optical depths (AODs) and decrease in Angstrom exponent (AE, α440-870 nm), showing the presence of a larger fraction of coarser particles comparable to those of other intense dust outbreak episodes worldwide, have been found at all these sites during the dust storm event. Higher observed AODs are considered to be the combined effect of desert dust and burning biomass/biofuel-induced aerosols. AERONET-retrieved mean aerosol volume size distributions (AVSDs) on non-dusty and dusty days are bimodal in nature at the stations of Pune, Kanpur, Jaipur, Karachi, and Lahore, while at the stations of KAUST_Campus (SA) and Mezaira (UAE), AVSDs are monomodal. This indicates that at the first five stations, the aerosol system is a complex mixture of fine- and coarse-mode aerosols with coarse mode exerting a strong influence on the system. Dust-induced turbid conditions triggered significant extinction of 30–40%, in shortwave (SW) global solar irradiance, resulting in an increase of 57% and 74% in aerosol direct radiative forcing (ADRF) at Alibaug and Pune, respectively, causing perturbation in the radiation budget. The space-borne Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)-retrieved aerosol vertical profiles reveal the presence of polluted dust (a mixture of fine- and coarse-mode aerosols), corroborating well with the aerosol behaviour captured by the MICROTOPS/AERONET measurements and MODIS retrievals over the study areas. The Dust Regional Atmospheric Model (BSC-DREAM8b) predictions are also found to be consistent with satellite retrievals, implying the ability of the model to monitor dust transport over the study region.

Radiative Impact of Fireworks at a Tropical Indian Location: A Case Study

During Diwali festival, extensive burning of crackers and fireworks is made. Weeklong intensive observational campaign for aerosol study was carried out at a representative urban location in the eastern Indo-Gangetic Plain (IGP), Varanasi (25.3°N, 83.0°E), from October 29 to November 04, 2005 (Diwali on November 01, 2005), to investigate behavioral change of aerosol properties and radiative forcing between firework affected and nonaffected periods. Results show a substantial increase (~27%) in aerosol optical depth, aerosol absorption coefficients, and aerosol scattering coefficients during affected period as compared to non-affected periods. Magnitudes of radiative forcing at top of atmosphere during affected and non-affected periods are found to be +10 ± 1 and +12 ± 1 Wm−2, respectively, which are −31 ± 7 and −17 ± 5 Wm−2, respectively, at surface. It suggests an additional cooling of ~20% at top of atmosphere, ~45% cooling at surface, and additional atmospheric heating of 0.23 Kday−1during fireworks affected period, which is ~30% higher than the non-affected period average.

Structural evolution of monsoon clouds in the Indian CTCZ

AbstractStructural evolution of monsoon clouds in the core monsoon region of India has been examined using multisensor data. Invigoration of warm clouds above 4.5 km (occurring in only 15.4% days of the last 11 monsoon seasons) is associated with a transition from negative to positive normalized rainfall anomaly. Cloud top pressure reduces with an increase in aerosol optical depth at a higher rate of invigoration in drier condition (characterized by large fraction of absorbing aerosols) than wet condition. Cloud effective radius for warm clouds does not show any significant change with an increase in aerosol concentration in the presence of high liquid water path, probably due to strong buffering role of meteorology. The structural evolution of monsoon clouds is influenced by both dynamic and microphysical processes that prolong the cloud lifetime, resulting in infrequent rainfall. Our results call for improved representation of aerosol and cloud vertical structures in the climate models to resolve this issue.

Brown carbon: a significant atmospheric absorber of solar radiation?

Abstract. Several recent observational studies have shown organic carbon aerosols to be a significant source of absorption of solar radiation. The absorbing part of organic aerosols is referred to as "brown" carbon (BrC). Using a global chemical transport model and a radiative transfer model, we estimate for the first time the enhanced absorption of solar radiation due to BrC in a global model. The simulated wavelength dependence of aerosol absorption, as measured by the absorption Ångström exponent (AAE), increases from 0.9 for non-absorbing organic carbon to 1.2 (1.0) for strongly (moderately) absorbing BrC. The calculated AAE for the strongly absorbing BrC agrees with AERONET spectral observations at 440–870 nm over most regions but overpredicts for the biomass burning-dominated South America and southern Africa, in which the inclusion of moderately absorbing BrC has better agreement. The resulting aerosol absorption optical depth increases by 18% (3%) at 550 nm and 56% (38%) at 380 nm for strongly (moderately) absorbing BrC. The global simulations suggest that the strongly absorbing BrC contributes up to +0.25 W m−2 or 19% of the absorption by anthropogenic aerosols, while 72% is attributed to black carbon, and 9% is due to sulfate and non-absorbing organic aerosols coated on black carbon. Like black carbon, the absorption of BrC (moderately to strongly) inserts a warming effect at the top of the atmosphere (TOA) (0.04 to 0.11 W m−2), while the effect at the surface is a reduction (−0.06 to −0.14 W m−2). Inclusion of the strongly absorption of BrC in our model causes the direct radiative forcing (global mean) of organic carbon aerosols at the TOA to change from cooling (−0.08 W m−2) to warming (+0.025 W m−2). Over source regions and above clouds, the absorption of BrC is higher and thus can play an important role in photochemistry and the hydrologic cycle.

Contrail ice particles in aircraft wakes and their climatic importance

Measurements of gaseous (NO, NOy, SO2, HONO) and ice particle concentrations in young contrails in primary and secondary wakes of aircraft of different sizes (B737, A319, A340, A380) are used to investigate ice particle formation behind aircraft. The gas concentrations are largest in the primary wake and decrease with increasing altitude in the secondary wake, as expected for passive trace gases and aircraft‐dependent dilution. In contrast, the measured ice particle concentrations were found larger in the secondary wake than in the primary wake. The contrails contain more ice particles than expected for previous black carbon (soot) estimates. The ice concentrations may result from soot‐induced ice nucleation for a soot number emission index of 1015 kg−1. For a doubled ice particle concentration in young contrails, a contrail cirrus model computes about 60% increases of global radiative forcing by contrail cirrus because of simultaneous increases in optical depth, age, and cover.

Probing the inner boundaries of Saturn’s A ring with the Iapetus −1:0 nodal bending wave

The Iapetus −1:0 nodal bending wave, the first spiral wave ever described in Saturn’s rings, has been seen again for the first time in 29years. We demonstrate that it is in fact the nodal bending wave, not the 1:0 apsidal density wave as previously reported. We use wavelet analysis to determine the wavelength profile, thus deriving the surface density at every point in the region covered by the bending wave. This profile is consistent with surface densities measured from more localized spiral density waves in the outer Cassini Division and the inner and mid-A Ring, varying smoothly from the low values of the former to the higher values of the latter.Most remarkably, our analysis indicates that there is no significant change in surface density across the boundary between the outer Cassini Division and the inner-A ring, despite the very abrupt increase in optical depth and reflected brightness at this location. We consider anew the nature of the classically identified “inner edge of the A ring,” given that it does not appear to be correlated with any abrupt increase in surface density. There is an abrupt increase in surface density at the Pandora 5:4 density wave, ∼300km outward of the A ring’s inner edge. Further study is needed to robustly interpret our findings in terms of particle properties and abundances, much less to explain the origins of the implied structure.

Microphysical simulations of large volcanic eruptions: Pinatubo and Toba

AbstractSimulations of stratospheric clouds from eruptions ranging in size from the 1991 eruption of Mount Pinatubo to that of Toba 74,000 years ago have been completed using a 3D microphysical sectional aerosol model advectively coupled to a general circulation model with prognostic chemistry (Whole Atmosphere Community Climate Model/Community Aerosol and Radiation Model for Atmospheres). For Pinatubo, properties of the aerosol cloud peak within the ranges derived from observations in the Northern Hemisphere, but reduce faster than observed, and a general low bias is found in the Southern Hemisphere. These biases could be reduced by adding aerosol radiative coupling, a quasi‐biennial oscillation, and the Cerro Hudson eruption to the model. Simulations of eruptions 10 times and 100 times larger than Pinatubo suggest burdens and Aerosol Optical Depth increase less than linearly (a 100‐fold injection increase produces a 20‐fold AOD increase) due to particle growth and sedimentation, consistent with previous work that also found the radiative forcings from large eruptions to be self‐limiting. Global‐averaged AOD remains elevated for 1, 2, and 4 years, respectively, for the three simulated eruptions. The inclusion of van der Waals forces in our coagulation scheme increases peak effective radius and reduces peak AOD by about 10–20%, with bigger effects for larger eruptions. Our simulations find peak mode size to vary by up to an order of magnitude and mode width to vary by up to 50%, suggesting that two‐moment modal models may not accurately capture the evolving size distribution. These simulations suggest the value of including van der Waals forces in the coagulation scheme and sectional size distributions in climate models.

The behavior of spokes in Saturn’s B ring

We present the first results from Cassini ISS observations aimed at determining both the short-term and long-term behavior of the spokes in Saturn’s B ring. We have observed multiple spokes which appear between images where there was not a spoke before. The radial termini of these new spokes expand radially at ∼0.5km/s in both the inward and outward directions and both towards and away from corotation. Defining a spoke’s activity as the area-integrated optical depth over the region of the ring it covers, we find that the majority of spokes which are found in multiple images are either increasing or decreasing in activity. In addition, in analyzing the shapes and motions of spokes, we find the azimuthal profiles to be well-fitted by Gaussians for the majority of spokes. We have found that several of the imaged spokes were undergoing an “active” phase during which the spoke’s optical depth increases and the spoke grows both azimuthally and radially. We interpret these motions to be either due to the Lorentz force acting on dust grains charged by a very high temperature plasma or the group velocity of an advancing spoke-forming front. The spokes’ light scattering behavior suggests that the particles comprising them are not spherical but instead are irregularly shaped.We search for periodicities in the longer-term temporal variability of spoke activity, and for correlations between spoke activity and other processes ongoing in Saturn’s magnetosphere and atmosphere. Using the latest results on the periods and locations of the sources of Saturn’s Kilometric Radiation (SKR) obtained by the Cassini Radio and Plasma Wave experiment (RPWS) (Gurnett, D.A., Groene, J.B., Averkamp, J.B., Kurth, W.S., Ye, S.-Y., Fischer, G. [2011]. Planet. Solar Heliosph. Radio Emis. PRE VII, 51–64), we find that spoke activity observed on both sides of Saturn’s rings occurs with a period equal to, within all uncertainties, the period of the SKR emissions arising from the northern SKR source, though a period equal to that of the southern SKR source seems also to be present. The period for of the Saturn Electrostatic Discharges (SEDs) falls within the uncertainty of the spoke activity on the northern side of the rings. However, spokes have been observed during SED-quiescent periods, which strongly suggests that there is no connection between the two phenomena (Fischer, G., Gurnett, D.A., Kurth, W.S., Zarka, P., Moore, L., Dyudina, U.A., 2011. J. Geophys. Res.).Finally, we use the latest SLS4 coordinate systems and locate the peak spoke activity on both sides of the rings at ≈200° in both the northern and southern SLS4 coordinate systems on both sides of the rings. This peak is ≈100° away from the locations expected from Voyager observations in both coordinate systems.

Ground-based in situ measurements of near-surface aerosol mass concentration over Anantapur: Heterogeneity in source impacts

Surface measurements of aerosol physical properties were made at Anantapur (14.62°N, 77.65°E, 331 m a.s.l), a semiarid rural site in India, during August 2008–July 2009. Measurements included the segregated sizes of aerosolsas as well as total mass concentration and size distributions of aerosols measured at low relative humidity (RH 68% to the total aerosol mass concentration. Accumulation aerosol mass fraction, A f (= M a/M t) was highest during winter (mean value of A f ∼ 0.80) and lowest (A f ∼ 0.64) during the monsoon season. The regression analysis shows that both R eff and R m are dependent on coarse-mode aerosols. The relationship between the simultaneous measurements of daily mean aerosol optical depth at 500 nm (AOD500) and PM2.5 mass concentration ([PM2.5]) shows that surface-level aerosol mass concentration increases with the increase in columnar aerosol optical depth over the observation period.

Aerosol optical depth increase in partly cloudy conditions

Remote sensing observations of aerosol from surface and satellite instruments are extensively used for atmospheric and climate research. From passive sensors, the apparent cloud‐free atmosphere in the vicinity of clouds often appears to be brighter than further away from the clouds, leading to an increase in the retrieved aerosol optical depth (τ). Mechanisms contributing to this enhancement or increase, including contamination by undetected clouds, hygroscopic growth of aerosol particles, and meteorological conditions, have been debated in recent literature, but the extent to which each of these factors influence the observed enhancement (Δτ) is poorly known. Here we used 11 years of daily global observations at 10 × 10 km2 resolution from the MODIS on the NASA Terra satellite to quantify τ as a function of cloud fraction (CF). Our analysis reveals that, averaged over the globe, the clear sky τ is enhanced by Δτ = 0.05 in cloudy conditions (CF = 0.8–0.9). This enhancement in Δτ corresponds to relative enhancement of 25% in cloudy conditions (CF = 0.8–0.9) compared with relatively clear conditions (CF = 0.1–0.2). Unlike the absolute enhancement Δτ, the relative increase in τis rather consistent in all seasons and is 25–35% in the subtropics and 15–25% at mid and higher latitudes. Using a simple Gaussian probability density function model to connect cloud cover and the distribution of relative humidity, we argue that much of the enhancement is consistent with aerosol hygroscopic growth in the humid environment surrounding clouds. Consideration of these cloud‐dependentτeffects will facilitate understanding aerosol‐cloud interactions and reduce the uncertainty in estimates of aerosol radiative forcing by global climate models.

Solar global ultraviolet and broadband global radiant fluxes and their relationships with aerosol optical depth at New Delhi

AbstractThe solar global ultraviolet (GUV) and broadband global (G) radiation flux obtained on a horizontal plane in Delhi, during April 2010 to March 2011 have been used to investigate the temporal variability of these radiations and their ratio, fraction of UV (FUV). For the first time the clearness index (KT) has been estimated over Delhi and its variability during different months of the year and season has been studied in detail. The impact of atmospheric aerosols on KT has also been studied. It has been found that for every unit increase in aerosol optical depth (AOD) at 340 nm, KT decreases by 0.06. A strong anti‐correlation with correlation coefficient − 0.75 is observed between AOD and KT. On the basis of our field experience and observations at Delhi it is found that for highly cloudy and overcast conditions 0⩽KT⩽0.15, for partial cloudy or hazy conditions 0.15⩽KT⩽0.21 and KT > 0.21 for clear sky conditions. In addition, during foggy days in winter we have found KT values lying in the range 0.12–0.18 at Delhi. The day‐time daily‐averaged fluxes GUV and G varied in the range 0.15–1.23 MJ m−2 and 3.36–27.02 MJ m−2, respectively. The GUV and G showed similar pattern during the year except for the wet season when the FUV increased possibly due to an increase in water vapour concentration. Copyright © 2012 Royal Meteorological Society

Computing and Partitioning Cloud Feedbacks Using Cloud Property Histograms. Part II: Attribution to Changes in Cloud Amount, Altitude, and Optical Depth

Cloud radiative kernels and histograms of cloud fraction, both as functions of cloud-top pressure and optical depth, are used to quantify cloud amount, altitude, and optical depth feedbacks. The analysis is applied to doubled-CO2 simulations from 11 global climate models in the Cloud Feedback Model Intercomparison Project. Global, annual, and ensemble mean longwave (LW) and shortwave (SW) cloud feedbacks are positive, with the latter nearly twice as large as the former. The robust increase in cloud-top altitude in both the tropics and extratropics is the dominant contributor to the positive LW cloud feedback. The negative impact of reductions in cloud amount offsets more than half of the positive impact of rising clouds on LW cloud feedback, but the magnitude of compensation varies considerably across the models. In contrast, robust reductions in cloud amount make a large and virtually unopposed positive contribution to SW cloud feedback, though the intermodel spread is greater than for any other individual feedback component. Overall reductions in cloud amount have twice as large an impact on SW fluxes as on LW fluxes, such that the net cloud amount feedback is moderately positive, with no models exhibiting a negative value. As a consequence of large but partially offsetting effects of cloud amount reductions on LW and SW feedbacks, both the mean and intermodel spread in net cloud amount feedback are smaller than those of the net cloud altitude feedback. Finally, the study finds that the large negative cloud feedback at high latitudes results from robust increases in cloud optical depth, not from increases in total cloud amount as is commonly assumed.

Comparing results from a physical model with satellite and in situ observations to determine whether biomass burning aerosols over the Amazon brighten or burn off clouds

Biomass burning (BB) aerosol particles affect clouds through competing microphysical and radiative (semi‐direct and cloud absorption) effects, each of which dominates at different degrees of aerosol loading. Here, we analyze the influence of competing aerosol effects on mixed‐phase clouds, precipitation, and radiative fields over the Amazon with a climate‐air pollution‐weather forecast model that treats aerosol‐cloud‐radiative interactions physically. Extensive comparisons with remotely sensed observations and in situ measurements are performed. Both observations and model results suggest an increase in cloud optical depth (COD) with increasing aerosol optical depth (AOD) at low AODs, and a decrease in COD with increasing AOD at higher AODs in accord with previous observational and modeling studies. The increase is attributed to a combination of microphysical and dynamical effects, whereas the decrease is attributed to a dominance of radiative effects that thin and darken clouds. An analogous relationship is shown for other modeled cloud variables as well. The similarity between the remotely sensed observations and model results suggests that these correlations are physically based and are not dominated by satellite retrieval artifacts. Cloud brightening due to BB is found to dominate in the early morning, whereas cloud inhibition is found to dominate in the afternoon and at night. BB decreased the net top of the atmosphere solar+IR irradiance modestly, but with large diurnal variation. We conclude that models that exclude treatment of aerosol radiative effects are likely to over‐predict the microphysical effects of aerosols and underestimate the warming due to aerosols containing black and brown carbon.

Tropical biomass burning smoke plume size, shape, reflectance, and age based on 2001–2009 MISR imagery of Borneo

Abstract. Land clearing for crops, plantations and grazing results in anthropogenic burning of tropical forests and peatlands in Indonesia, where images of fire-generated aerosol plumes have been captured by the Multi-angle Imaging SpectroRadiometer (MISR) since 2001. Here we analyze the size, shape, optical properties, and age of distinct fire-generated plumes in Borneo from 2001–2009. The local MISR overpass at 10:30 a.m. misses the afternoon peak of Borneo fire emissions, and may preferentially sample longer plumes from persistent fires burning overnight. Typically the smoke flows with the prevailing southeasterly surface winds at 3–4 m s−1, and forms ovoid plumes whose mean length, height, and cross-plume width are 41 km, 708 m, and 27% of the plume length, respectively. 50% of these plumes have length between 24 and 50 km, height between 523 and 993 m and width between 18% and 30% of plume length. Length and cross-plume width are lognormally distributed, while height follows a normal distribution. Borneo smoke plume heights are similar to previously reported plume heights, yet Borneo plumes are on average nearly three times longer than previously studied plumes. This could be due to sampling or to more persistent fires and greater fuel loads in peatlands than in other tropical forests. Plume area (median 169 km2, with 25th and 75th percentiles at 99 km2 and 304 km2, respectively) varies exponentially with length, though for most plumes a linear relation provides a good approximation. The MISR-estimated plume optical properties involve greater uncertainties than the geometric properties, and show patterns consistent with smoke aging. Optical depth increases by 15–25% in the down-plume direction, consistent with hygroscopic growth and nucleation overwhelming the effects of particle dispersion. Both particle single-scattering albedo and top-of-atmosphere reflectance peak about halfway down-plume, at values about 3% and 10% greater than at the origin, respectively. The initially oblong plumes become brighter and more circular with time, increasingly resembling smoke clouds. Wind speed does not explain a significant fraction of the variation in plume geometry. We provide a parameterization of plume shape that can help atmospheric models estimate the effects of plumes on weather, climate, and air quality. Plume age, the age of smoke furthest down-plume, is lognormally distributed with a median of 2.8 h (25th and 75th percentiles at 1.3 h and 4.0 h), different from the median ages reported in other studies. Intercomparison of our results with previous studies shows that the shape, height, optical depth, and lifetime characteristics reported for tropical biomass burning plumes on three continents are dissimilar and distinct from the same characteristics of non-tropical wildfire plumes.

Variability and trends of aerosol properties over Kanpur, northern India using AERONET data (2001–10)

Natural and anthropogenic aerosols over northern India play an important role in influencing the regional radiation budget, causing climate implications to the overall hydrological cycle of South Asia. In the context of regional climate change and air quality, we discuss aerosol loading variability and trends at Kanpur AERONET station located in the central part of the Indo-Gangetic plains (IGP), during the last decade (2001–10). Ground-based radiometric measurements show an overall increase in column-integrated aerosol optical depth (AOD) on a yearly basis. This upward trend is mainly due to a sustained increase in the seasonal/monthly averaged AOD during the winter (Dec–Feb) and post-monsoon (Oct–Nov) seasons (dominated by anthropogenic emissions). In contrast, a neutral to weak declining trend is observed during late pre-monsoon (Mar–May) and monsoon (Jun–Sep) months, mainly influenced by inter-annual variations of dust outbreaks. A general decrease in coarse-mode aerosols associated with variable dust activity is observed, whereas the statistically significant increasing post-monsoon/winter AOD is reflected in a shift of the columnar size distribution towards relatively larger particles in the accumulation mode. Overall, the present study provides an insight into the pronounced seasonal behavior in aerosol loading trends and, in general, is in agreement with that associating the findings with those recently reported by satellite observations (MODIS and MISR) over northern India. Our results further suggest that anthropogenic emissions (due mainly to fossil-fuel and biomass combustion) over the IGP have continued to increase in the last decade.

Effect of absorbing aerosols on snow albedo reduction in the Sierra Nevada

This paper investigates snow albedo changes in the Sierra Nevada Mountain area associated with potential deposition of absorbing aerosols in spring by using the snow albedo, aerosol optical depth, land surface temperature, and other relevant parameters available from the Moderate-Resolution Imaging Spectroradiometer (MODIS) onboard the NASA Terra satellite during 2000–2009. Satellite pixels with 100% snow cover have been selected to derive the monthly mean snow albedo value, along with aerosol optical depth, surface temperature, and days after snowfall in March and April to perform multiple regression analysis. We show that aerosol optical depth, which generally includes dust and black carbon over the Sierra Nevada as a result of the transpacific transport from East Asia and local sources, represents a significant parameter affecting snow albedo variation, second only to the land surface temperature change. The regression analysis illustrates that a one standard deviation increase in land surface temperature (2.2 K) and aerosol optical depth (0.044) can lead to decreases in snow albedo by 0.038 and 0.026, respectively. This study also shows that approximately 26% of snow albedo reduction from March to April over the Sierra Nevada is caused by an increase in aerosol optical depth, which has a profound impact on available water resources in California. However, the results show that there are no significant trends for snow albedo, surface temperature, and aerosol optical depth of snow-covered areas over the Sierra Nevada Mountain area in this 10-year period.

Aerosol-cloud-precipitation relationships from satellite observations and global climate model simulations

Substantial uncertainties exist in the current knowledge of aerosol-cloud-precipitation relationships and stem from the complicated interactions among the atmospheric constituents. We use a straightforward statistical method, the regression analysis technique, to examine the aerosol-cloud-precipitation relationships from satellite observational data sets, including the aqua moderate resolution imaging spectroradiometer (MODIS) aerosol and cloud products and the tropical rainfall measuring mission (TRMM) precipitation rate. Furthermore, the conventional MODIS aerosol product is combined with the Deep Blue algorithm product to reconstruct a complete global map of aerosol optical depth. Numerical simulations using the latest version of the community earth system model (CESM) are also carried out. Globally, distinct statistically significant relationships between aerosol optical depth, cloud fraction, and precipitation rate are obtained over both land and ocean. Signals agreeing with the first and second indirect effects of aerosols are detected, but other factors are likely contributors. The modeling results are found to generally agree with satellite observations, but the model usually overestimates the aerosol-cloud-precipitation relationship. An increasing trend in cloud fraction with the increase of aerosol optical depth (AOD) over ocean regions is found in the observations, while the reverse is true in the model simulation. It is mostly consistent that the model and observation both show a negative relationship between AOD and precipitation rate over land and a positive relationship over ocean.

Balloon-borne study of the upper tropospheric and stratospheric aerosols over a tropical station in India

Using sun-scanning/tracking photometers onboard a high-altitude balloon the upper tropospheric and stratospheric aerosol characteristics have been studied over Hyderabad (17.5°N) in April 2001. In the upper troposphere the aerosol extinction coefficients are found to be around 10−2 km−1, while in the stratosphere the values are in the range 10−3–10−4 km−1, which are about two orders of magnitude smaller than those measured after the Pinatubo eruption over the same site. The balloon-derived aerosol extinction coefficients are in good agreement with the results obtained from lidar experiments conducted over Mt. Abu (24.6°N). In the upper troposphere the integrated aerosol extinction coefficients at 500 nm are found to show an average increase of about 11 ± 1% yr−1 during the period 1985–2001. This increase is found to be consistent with the observed increase in the long-term columnar aerosol optical depth data over selected locations in India. The Ångström wavelength exponent is found to be about 1.33 ± 0.12 in the stratosphere, which is comparable to the Junge size parameter values obtained in 1985. The mode radii of stratospheric aerosols are found to be 0.10 ± 0.01 μm in both the 2001 and 1985 experiments, indicating that the background stratospheric aerosol size distribution has not changed between these years, which is also corroborated by the good agreement between the wavelength exponent and size parameter values obtained in 2001 and 1985. The aerosol optical depth at 1 μm in 2001 is about 0.005, which is five times larger than that measured by SAGE in 1979 over the tropics. The stratospheric aerosol optical depth spectra are calculated for both the volcanic and quiescent conditions by taking into account the mode radius and aerosol number density. The estimated and measured aerosol optical depths agree well. We estimate that the sulfur dioxide and aerosol emissions from coal over India have increased by 10% yr−1 during the past two decades. The increase in emissions from high-speed diesel oil and petrol is higher and is in the range 13– 14% yr−1 over India. These increases could possibly be responsible for the observed increase in the upper tropospheric aerosol optical depths over India.

LARGE-SCALE KINEMATICS, ASTROCHEMISTRY, AND MAGNETIC FIELD STUDIES OF MASSIVE STAR-FORMING REGIONS THROUGH HC3N, HNC, AND C2H MAPPINGS

We have mapped 27 massive star-forming regions associated with water masers using three dense gas tracers: HC3N 10-9, HNC 1-0 and C2H 1-0. The FWHM sizes of HNC clumps and C2H clumps are about 1.5 and 1.6 times higher than those of HC3N, respectively, which can be explained by the fact that HC3N traces more dense gas than HNC and C2H. We found evidence for increase in optical depth of C2H with `radius' from center to outer regions in some targets, supporting the chemical model of C2H. The C2H optical depth is found to decline as molecular clouds evolve to later stage, suggesting that C2H might be used as "chemical clock" for molecular clouds. Large-scale kinematic structure of clouds was investigated with three molecular lines. All these sources show significant velocity gradients. The magnitudes of gradient are found to increase towards the inner region, indicating differential rotation of clouds. Both the ratio of rotational to gravitational energy and specific angular momentum seem to decrease toward the inner region, implying obvious angular momentum transfer, which might be caused by magnetic braking. The average magnetic field strength and number density of molecular clouds is derived using the uniformly magnetic sphere model. The derived magnetic field strengths range from 3 to 88 \mu G, with a median value of 13 \mu G. The mass-to-flux ratio of molecular cloud is calculated to be much higher than critical value with derived parameters, which agrees well with numerical simulations.