Cloud-Aerosol Lidar And Infrared Pathfinder Satellite Observations Data Research Articles

Comparisons of PBL heights derived from CALIPSO and ECMWF reanalysis data over China

Planetary boundary layer (PBL) height was estimated using the maximum standard deviation method for Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) attenuated backscatter observations. It was only retrieved under conditions where the clouds accounted for less than 5% in a profile, where it could be compared with ground lidar results at SACOL. The correlation between CALIPSO and the ground lidar was 0.73. We present the seasonal mean patterns of 4-year mid-day PBL heights over China and use them to evaluate the European Centre for Medium-Range Weather Forecasts (ECMWF) PBL depth retrievals, inform boundary layer studies, and improve our understanding of how PBL height mediates exchanges of energy and pollutants between the surface and the atmosphere. We found that the largest PBL heights occurred over the Tibetan Plateau and coastal areas. The smallest PBL heights appeared in the Tarim Basin and northeast of China during local winter. A comparison of CALIPSO and ECMWF PBL under different land-cover conditions showed that the PBL depth estimated by the CALIPSO backscatter climatology is larger over ocean and forest surface than that estimated from ECMWF data. However, the PBL heights of ECMWF that were larger than those of CALIPSO were mainly concentrated over grassland and bare land surface in spring and summer.

Dispersion of the Nabro volcanic plume and its relation to the Asian summer monsoon

Abstract. We use nighttime measurements from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, together with a Lagrangian trajectory model, to study the initial dispersion of volcanic aerosol from the eruption of Mt. Nabro (Ethiopia/Eritrea) in June 2011. The Nabro eruption reached the upper troposphere and lower stratosphere (UTLS) directly, and the plume was initially entrained by the flow surrounding the Asian anticyclone, which prevails in the UTLS from the Mediterranean Sea to East Asia during boreal summer. CALIPSO detected aerosol layers, with optical properties consistent with sulfate, in the lower stratosphere above the monsoon convective region in South and Southeast Asia within 10 days of the eruption. We show that quasi-isentropic differential advection in the vertically sheared flow surrounding the Asian anticyclone explains many of these stratospheric aerosol layers. We use Meteosat-7 data to examine the possible role of deep convection in the Asian monsoon in transporting volcanic material to the lower stratosphere during this time, but find no evidence that convection played a direct role, in contrast with claims made in earlier studies. On longer timescales, we use CALIPSO data to illustrate diabatic ascent of the Nabro aerosol in the lower stratosphere at rates of ~ 10 K per month for the first two months after the eruption, falling to ~ 3 K per month after the Asian anticyclone dissipates. Maps of stratospheric aerosol optical depth (AOD) show local peaks of ~ 0.04–0.06 in July in the region of the Asian anticyclone; we find associated estimates of radiative forcing small, ~ 5–10% of those reported for the eruption of Mt. Pinatubo in 1991. Additionally, we find no clear response in outgoing shortwave (SW) flux due to the presence of Nabro aerosol viewed in the context of SW flux variability as measured by CERES (Clouds and Earth Radiant Energy System).

Discrimination and validation of clouds and dust aerosol layers over the Sahara desert with combined CALIOP and IIR measurements

This study validates a method for discriminating between daytime clouds and dust aerosol layers over the Sahara Desert that uses a combination of active CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) and passive IIR (Infrared Imaging Radiometer) measurements; hereafter, the CLIM method. The CLIM method reduces misclassification of dense dust aerosol layers in the Sahara region relative to other techniques. When evaluated against a suite of simultaneous measurements from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), CloudSat, and the MODIS (Moderate-resolution Imaging Spectroradiometer), the misclassification rate for dust using the CLIM technique is 1.16% during boreal spring 2007. This rate is lower than the misclassification rates for dust using the cloud aerosol discriminations performed for version 2 (V2-CAD; 16.39%) or version 3 (V3-CAD; 2.01%) of the CALIPSO data processing algorithm. The total identification errors for data from in spring 2007 are 13.46% for V2-CAD, 3.39% for V3-CAD, and 1.99% for CLIM. These results indicate that CLIM and V3-CAD are both significantly better than V2-CAD for discriminating between clouds and dust aerosol layers. Misclassifications by CLIM in this region are mainly limited to mixed cloud-dust aerosol layers. V3-CAD sometimes misidentifies low-level aerosol layers adjacent to the surface as thin clouds, and sometimes fails to detect thin clouds entirely. The CLIM method is both simple and fast, and may be useful as a reference for testing or validating other discrimination techniques and methods.

Origins of the Solar Radiation Biases over the Southern Ocean in CFMIP2 Models*

AbstractCurrent climate models generally reflect too little solar radiation over the Southern Ocean, which may be the leading cause of the prevalent sea surface temperature biases in climate models. The authors study the role of clouds on the radiation biases in atmosphere-only simulations of the Cloud Feedback Model Intercomparison Project phase 2 (CFMIP2), as clouds have a leading role in controlling the solar radiation absorbed at those latitudes. The authors composite daily data around cyclone centers in the latitude band between 40° and 70°S during the summer. They use cloud property estimates from satellite to classify clouds into different regimes, which allow them to relate the cloud regimes and their associated radiative biases to the meteorological conditions in which they occur. The cloud regimes are defined using cloud properties retrieved using passive sensors and may suffer from the errors associated with this type of retrievals. The authors use information from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar to investigate in more detail the properties of the “midlevel” cloud regime. Most of the model biases occur in the cold-air side of the cyclone composite, and the cyclone composite accounts for most of the climatological error in that latitudinal band. The midlevel regime is the main contributor to reflected shortwave radiation biases. CALIPSO data show that the midlevel cloud regime is dominated by two main cloud types: cloud with tops actually at midlevel and low-level cloud. Improving the simulation of these cloud types should help reduce the biases in the simulation of the solar radiation budget in the Southern Ocean in climate models.

Evaluating CALIPSO's 532 nm lidar ratio selection algorithm using AERONET sun photometers in Brazil

Abstract. Since the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite first began probing the Earth's atmosphere on 13 June 2006, several research groups dedicated to investigating the atmosphere's optical properties have conducted measurement campaigns to validate the CALIPSO data products. Recently, in order to address the lack of CALIPSO validation studies in the Southern Hemisphere, and especially the South American continent, the Lasers Environmental Applications Research Group at Brazil's Nuclear and Energy Research Institute (IPEN) initiated efforts to assess CALIPSO's aerosol lidar ratio estimates using the AERONET sun photometers installed at five different locations in Brazil. In this study we develop a validation methodology to evaluate the accuracy of the modeled values of the lidar ratios used by the CALIPSO extinction algorithms. We recognize that the quality of any comparisons between satellite and ground-based measurements depends on the degree to which the instruments are collocated, and that even selecting the best spatial and temporal matches does not provide an unequivocal guarantee that both instruments are measuring the same air mass. The validation methodology presented in this study therefore applies backward and forward air mass trajectories in order to obtain the best possible match between the air masses sampled by the satellite and the ground-based instruments, and thus reduces the uncertainties associated with aerosol air mass variations. Quantitative comparisons of lidar ratios determined from the combination of AERONET optical depth measurements and CALIOP integrated attenuated backscatter measurements show good agreement with the model values assigned by the CALIOP algorithm. These comparisons yield a mean percentage difference of −1.5% ± 24%. This result confirms the accuracy in the lidar ratio estimates provided by the CALIOP algorithms over Brazil to within an uncertainty range of no more than 30%.

A new approach to retrieve cloud base height of marine boundary layer clouds

AbstractA novel approach for estimating marine boundary layer cloud base height (CBH) is proposed based on calculated boundary layer lapse rates, collocated cloud top height (CTH), cloud top, and ocean surface temperatures from the A‐Train satellite constellation. The method takes advantage of the assumption that decreases of temperature within and below water clouds may follow the different constant apparent lapse rates in the same region, respectively. The CBHs derived from the new method compare well with the coincident CBH product from the active sensors of Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat. The correlation coefficient, the mean difference, and the standard deviation are 0.79 (0.54), 0.02 km (0.03 km), and ±0.35 km (±0.54 km), respectively, when CTH is derived from CALIPSO data (or Moderate Resolution Imaging Spectroradiometer retrieval). Besides the relatively small bias, the most important advantage of this method compared to previous CBH retrieval techniques is that it is independent of boundary layer cloud types, optical thickness, and illumination.

Macrophysical properties of tropical cirrus clouds from the CALIPSO satellite and from ground‐based micropulse and Raman lidars

Lidar observations of cirrus cloud macrophysical properties over the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program Darwin, Australia, site are compared from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, the ground‐based ARM micropulse lidar (MPL), and the ARM Raman lidar (RL). Comparisons are made using the subset of profiles where the lidar beam is not fully attenuated. Daytime measurements using the RL are shown to be relatively unaffected by the solar background and are therefore suited for checking the validity of diurnal cycles. RL and CALIPSO cloud fraction profiles show good agreement while the MPL detects significantly less cirrus, particularly during the daytime. Both MPL and CALIPSO observations show that cirrus clouds occur less frequently during the day than at night at all altitudes. In contrast, the RL diurnal cycle is significantly different from zero only below about 11 km; where it is of opposite sign (i.e., more clouds during the daytime). For cirrus geometrical thickness, the MPL and CALIPSO observations agree well and both data sets have significantly thinner clouds during the daytime than the RL. From the examination of hourly MPL and RL cirrus cloud thickness and through the application of daytime detection limits to all CALIPSO data, we find that the decreased MPL and CALIPSO cloud thickness during the daytime is very likely a result of increased daytime noise. This study highlights the significant improvement the RL provides (compared to the MPL) in the ARM program's ability to observe tropical cirrus clouds and will help improve our understanding of these clouds. The RL also provides a valuable ground‐based lidar data set for the evaluation of CALIPSO observations.

Seasonal and diurnal variations of aerosol extinction profile and type distribution from CALIPSO 5‐year observations

AbstractThe new Level 3 aerosol profile data derived from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) provide a multiyear global aerosol distribution with high vertical resolution. We analyzed seasonal and diurnal variations of the vertical distributions of aerosol properties represented by 5‐year CALIPSO data. Results show that dust, smoke, and polluted dust are the most frequently detected aerosol types during all seasons. Dust is the dominant type, especially in the middle to upper troposphere, over most areas during boreal spring and summer, while smoke and polluted dust tend to dominate during biomass burning seasons. The seasonal variations of dust layer top height and dust contribution to all‐aerosol extinction are positively correlated with the seasonal variation of the dust occurrence frequency. The seasonal cycle of aerosol properties over west Australia is similar to that over biomass burning regime areas, despite its desert regime. In general, smoke is detected more frequently from the lower to middle troposphere; clean marine and polluted continental aerosols are detected more frequently, while polluted dust is detected less frequently, in the lower troposphere during nighttime than daytime. The all‐aerosol extinction is generally larger, and the aerosol layer top is detected at high altitudes more frequently during nighttime than daytime. The diurnal changes of aerosol properties are similar within the same aerosol regime. Dust extinction shows little diurnal variation except when dust is the dominant aerosol type. The results contribute to an initial global 3‐D aerosol climatology which will likely be extended and improved in the future.

Detection of Optically Thin Mineral Dust Aerosol Layers over the Ocean Using MODIS

Abstract Analyses show that several existing Moderate Resolution Imaging Spectroradiometer (MODIS) dust detection techniques, including an approach based on simple brightness temperature difference thresholds, the D-parameter method, and the multichannel image (MCI) algorithm, may be more effective for detection of highly concentrated dust plumes than for thin dust layers. Using the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) cloud and aerosol classification as a reference, the sensitivities of six MODIS radiative parameters (including brightness temperature differences, and standard deviation and ratios of reflectances) to cloud, clear sky, and dust layers are examined in this paper. Reflectance ratios and the standard deviation of reflectances were confirmed to be useful in the discrimination of dust from cloud and underlying ocean surface, while brightness temperature differences alone were not sufficient to separate dust from cloud and clear sky over the ocean surface. Using a collocated MODIS and CALIPSO training dataset from 2008, visible and infrared MODIS radiative parameters from six latitude bands and four seasons were combined using linear and quadratic discriminant analyses to develop a new algorithm for the detection of optically thin dust over the ocean. The validation using collocated MODIS and CALIPSO data from 2009 shows that the present algorithm is effective in detecting thin dust layers having optical thicknesses between 0.1 and 2.0, but that it tends to misclassify optically thicker dust layers as clouds.

Evaluation of Clouds and Precipitation in the ECHAM5 General Circulation Model Using CALIPSO and CloudSat Satellite Data

Abstract Observations from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat satellites are used to evaluate clouds and precipitation in the ECHAM5 general circulation model. Active lidar and radar instruments on board CALIPSO and CloudSat allow the vertical distribution of clouds and their optical properties to be studied on a global scale. To evaluate the clouds modeled by ECHAM5 with CALIPSO and CloudSat, the lidar and radar satellite simulators of the Cloud Feedback Model Intercomparison Project’s Observation Simulator Package are used. Comparison of ECHAM5 with CALIPSO and CloudSat found large-scale features resolved by the model, such as the Hadley circulation, are captured well. The lidar simulator demonstrated ECHAM5 overestimates the amount of high-level clouds, particularly optically thin clouds. High-altitude clouds in ECHAM5 consistently produced greater lidar scattering ratios compared with CALIPSO. Consequently, the lidar signal in ECHAM5 frequently attenuated high in the atmosphere. The large scattering ratios were due to an underestimation of effective ice crystal radii in ECHAM5. Doubling the effective ice crystal radii improved the scattering ratios and frequency of attenuation. Additionally, doubling the effective ice crystal radii improved the detection of ECHAM5’s highest-level clouds by the radar simulator, in better agreement with CloudSat. ECHAM5 was also shown to significantly underestimate midlevel clouds and (sub)tropical low-level clouds. The low-level clouds produced were consistently perceived by the lidar simulator as too optically thick. The radar simulator demonstrated ECHAM5 overestimates the frequency of precipitation, yet underestimates its intensity compared with CloudSat observations. These findings imply compensating mechanisms in ECHAM5 balance out the radiative imbalance caused by incorrect optical properties of clouds and consistently large hydrometeors in the atmosphere.

The MJO Transition from Shallow to Deep Convection inCloudSat/CALIPSO Data and GISS GCM Simulations

The relationship between convective penetration depth and tropospheric humidity is central to recent theories of the Madden–Julian oscillation (MJO). It has been suggested that general circulation models (GCMs) poorly simulate the MJO because they fail to gradually moisten the troposphere by shallow convection and simulate a slow transition to deep convection. CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed to document the variability of convection depth and its relation to water vapor during the MJO transition from shallow to deep convection and to constrain GCM cumulus parameterizations. Composites of cloud occurrence for 10 MJO events show the following anticipated MJO cloud structure: shallow and congestus clouds in advance of the peak, deep clouds near the peak, and upper-level anvils after the peak. Cirrus clouds are also frequent in advance of the peak. The Advanced Microwave Scanning Radiometer for Earth Observing System (EOS) (AMSR-E) column water vapor (CWV) increases by ~5 mm during the shallow–deep transition phase, consistent with the idea of moisture preconditioning. Echo-top height of clouds rooted in the boundary layer increases sharply with CWV, with large variability in depth when CWV is between ~46 and 68 mm. International Satellite Cloud Climatology Project cloud classifications reproduce these climatological relationships but correctly identify congestus-dominated scenes only about half the time. A version of the Goddard Institute for Space Studies Model E2 (GISS-E2) GCM with strengthened entrainment and rain evaporation that produces MJO-like variability also reproduces the shallow–deep convection transition, including the large variability of cloud-top height at intermediate CWV values. The variability is due to small grid-scale relative humidity and lapse rate anomalies for similar values of CWV.

Cirrus optical depth and lidar ratio retrieval from combined CALIPSO‐CloudSat observations using ocean surface echo

Ocean surface observations from the CloudSat radar and the spaceborne lidar aboard the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) platform are combined in the Synergized Optical Depth of Aerosol (SODA) algorithm and used to retrieve the optical depth of semitransparent single‐layered cirrus clouds. In the operational CALIPSO data analysis, lidar‐derived optical depths are typically estimated using a correction factor for multiple scattering effects and a single global mean lidar ratio. By combining the SODA approach with observations from the CALIPSO Imaging Infrared Radiometer, accurate values for both of these parameters can be derived directly from the measurements. Application of this approach yields a multiple scattering factor of 0.61 ± 0.15 sr, which is essentially identical to the value used operationally. However, the standard lidar ratio used in the CALIPSO daytime operational analysis is found to be biased low by around 25%. As a consequence, the lidar‐derived optical depths retrieved from the daytime operational analyses are more than 30% smaller than those derived using SODA. The lidar ratio for semitransparent cirrus is found to be rather stable over ocean (33 ± 5 sr) with slight variations as a function of temperature and latitude. The geographic distribution shows a moderate decrease of average lidar ratio values over Indonesia during daytime, which may be attributed to a larger occurrence of high‐altitude cirrus layers in this convectively active area.

Evidence of Atmospheric Brown Clouds Over India During the 2009 Drought Year

Using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite derived vertical pro- files (Total Attenuated Backscatter at 532 nm, Depolarization Ratios at 532 nm, Vertical Feature Mask, monthly mean vertical aerosol and extinction variability), monthly precipitation anomalies from National Centers for Environmental Prediction-Climate Prediction Center (NCEP-CPC) and NCEP wind anomalies; we report the presence of Atmospheric Brown Clouds (ABCs) over India (65°E-95°E; 8°N-35°N) during the summer monsoon of 2009. CALIPSO data revealed the persistent presence of a massive aerosol plume till 4 Km above sea level; this plume persisted for over 7 months (March through September 2009) over India and was loaded with anthropogenic aerosols like sulphates, black carbon products from biomass burning and fossil fuel combustion. The CALIPSO vertical profiles, NCEP-CPC monthly precipitation anomalies and NCEP monthly wind anomalies corroborate the presence of aerosol plume.

A study of subvisual clouds and their radiation effect with a synergy of CERES, MODIS, CALIPSO, and AIRS data

[1] Subvisual cirrus clouds that are defined as those whose optical thickness is less than ∼0.3 are found in ∼50% of global observations. Passive remote-sensing instruments, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), generally fail to detect these optically thin clouds. The launch of NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite provides an unprecedented ability to detect thin cloud layers globally. Also, the Clouds and the Earth's Radiant Energy System (CERES) provides accurate measurements of top-of-atmosphere radiation. By using CERES, MODIS, and CALIPSO measurements in a synergistic manner, a quantitative assessment of the influence of subvisual clouds on the Earth's shortwave (SW) radiation is accomplished. The difference between clear-sky radiation flux and the flux obtained with the presence of subvisual clouds clearly shows the cooling effect of subvisual clouds in the SW. The subvisual clouds increase the diurnal mean reflected SW flux by ∼2.5 W m−2. The subvisual clouds' effect on outgoing longwave radiation is also studied using a radiative-transfer model. The model results show that a layer of subvisual clouds having optical thickness of 0.1 can have a warming effect of ∼15 W m−2. These clouds can also affect the polarization of the reflected SW radiation and the accuracy of aerosol retrieval with satellite measurements. This work demonstrates that the study of subvisual clouds is necessary for an accurate and detailed understanding of Earth-atmosphere radiation.

Wintertime pollution over the Eastern Indo-Gangetic Plains as observed from MOPITT, CALIPSO and tropospheric ozone residual data

Abstract. A large wintertime increase in pollutants has been observed over the eastern parts of the Indo Gangetic Plains. We use improved version 4 carbon monoxide (CO) retrievals from the Measurements of Pollution in the Troposphere (MOPITT) along with latest version 3 aerosol data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar instrument and the tropospheric ozone residual products to characterize this pollution pool. The feature is seen primarily in the lower troposphere from about November to February with strong concomitant increases in CO and aerosol optical depth (AOD). The signature of the feature is also observed in tropospheric ozone column data. The height resolved aerosol data from CALIPSO confirm the trapping of the pollution pool at the lowest altitudes. The observations indicate that MOPITT can capture this low altitude phenomenon even in winter conditions as indicated by the averaging kernels.

Global analysis of ice microphysics from CloudSat and CALIPSO: Incorporation of specular reflection in lidar signals

We developed a new radar‐lidar algorithm that can be applied to CloudSat and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data to retrieve ice microphysics. The algorithm analyzes the specular reflection of lidar signals often observed by CALIPSO with large backscattering coefficients and small depolarization ratios. Analyses of CloudSat and CALIPSO data by our former radar‐lidar algorithm showed problems retrieving ice cloud microphysics when specular reflection was present. We implemented additional look‐up tables for horizontally oriented plates. A specular reflection mode in the radar‐lidar algorithm could drastically improve retrieval results. The new radar‐lidar algorithm requires depolarization ratios measured by CALIPSO, in addition to the radar reflectivity factor and backscattering coefficient at 532 nm. We performed several sensitivity studies to retrieval results. Nonsphericity turned out to be the largest source of uncertainties. Global analyses of ice microphysics for CloudSat‐CALIPSO overlap regions were performed. The effective radius decreased as the altitude increased. The effective radius in the specular reflection ranged from 100 to 300 μm. The ice water content (IWC) ranged from 10−4 to several tenths of a gram per cubic meter. Both effective radius and IWC increased as the altitude (temperature) decreased (increased). The largest mixing ratio of oriented particles occurred between −20 and −5°C. The IWC had two maxima in the tropics above 15 km and around 5 km. We also examined the differences in ice microphysics over land and ocean. The effective radius was similar over land and ocean, but the IWC tended to be larger over land.

Radiative effect differences between multi-layered and single-layer clouds derived from CERES, CALIPSO, and CloudSat data

Clouds alter general circulation through modification of the radiative heating profile within the atmosphere. Their effects are complex and depend on height, vertical structure, and phase. The instantaneous cloud radiative effect (CRE) induced by multi-layered (ML) and single-layer (SL) clouds is estimated by analyzing data collected by the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), CloudSat, and Clouds and Earth’s Radiation Energy Budget System (CERES) missions from March 2007 through February 2008. The CRE differences between ML and SL clouds at the top of the atmosphere (TOA) and at the surface were also examined. The zonal mean shortwave (SW) CRE differences between the ML and SL clouds at the TOA and surface were positive at most latitudes, peaking at 120 W m −2 in the tropics and dropping to −30 W m −2 at higher latitudes. This indicated that the ML clouds usually reflected less sunlight at the TOA and transmitted more to the surface than the SL clouds, due to their higher cloud top heights. The zonal mean longwave (LW) CRE differences between ML and SL clouds at the TOA and surface were relatively small, ranging from −30 to 30 W m −2. This showed that the ML clouds only increased the amount of thermal radiation at the TOA relative to the SL clouds in the tropics, decreasing it elsewhere. In other words, ML clouds tended to cool the atmosphere in the tropics and warm it elsewhere when compared to SL clouds. The zonal mean net CRE differences were positive at most latitudes and dominated by the SW CRE differences.

Cloud Vertical Distribution across Warm and Cold Fronts in CloudSat–CALIPSO Data and a General Circulation Model

Abstract Cloud vertical distributions across extratropical warm and cold fronts are obtained using two consecutive winters of CloudSat–Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations and National Centers for Environmental Prediction reanalysis atmospheric state parameters over the Northern and Southern Hemisphere oceans (30°–70°N/S) between November 2006 and September 2008. These distributions generally resemble those from the original model introduced by the Bergen School in the 1920s, with the following exceptions: 1) substantial low cloudiness, which is present behind and ahead of the warm and cold fronts; 2) ubiquitous high cloudiness, some of it very thin, throughout the warm-frontal region; and 3) upright convective cloudiness near and behind some warm fronts. One winter of GISS general circulation model simulations of Northern and Southern Hemisphere warm and cold fronts at 2° × 2.5° × 32 levels resolution gives similar cloud distributions but with much lower cloud fraction, a shallower depth of cloudiness, and a shorter extent of tilted warm-frontal cloud cover on the cold air side of the surface frontal position. A close examination of the relationship between the cloudiness and relative humidity fields indicates that water vapor is not lifted enough in modeled midlatitude cyclones and this is related to weak vertical velocities in the model. The model also produces too little cloudiness for a given value of vertical velocity or relative humidity. For global climate models run at scales coarser than tens of kilometers, the authors suggest that the current underestimate of modeled cloud cover in the storm track regions, and in particular the 50°–60°S band of the Southern Oceans, could be reduced with the implementation of a slantwise convection parameterization.

CALIPSO polar stratospheric cloud observations: second-generation detection algorithm and composition discrimination

Abstract. This paper focuses on polar stratospheric cloud (PSC) measurements by the CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization) lidar system onboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) spacecraft, which has been operating since June 2006. We describe a second-generation PSC detection algorithm that utilizes both the CALIOP 532-nm scattering ratio (ratio of total-to-molecular backscatter coefficients) and 532-nm perpendicular backscatter coefficient measurements for cloud detection. The inclusion of the perpendicular backscatter measurements enhances the detection of tenuous PSC mixtures containing low number densities of solid (likely nitric acid trihydrate, NAT) particles and leads to about a 15% increase in PSC areal coverage compared with our original algorithm. Although these low number density NAT mixtures would have a minimal impact on chlorine activation due to their relatively small particle surface area, these particles may play a significant role in denitrification and therefore are an important component of our PSC detection. In addition, the new algorithm allows discrimination of PSCs by composition in terms of their ensemble backscatter and depolarization in a manner analogous to that used in previous ground-based and airborne lidar PSC studies. Based on theoretical optical calculations, we define four CALIPSO-based composition classes which we call supercooled ternary solution (STS), ice, and Mix1 and Mix2, denoting mixtures of STS with NAT particles in lower or higher number densities/volumes, respectively. We examine the evolution of PSCs for three Antarctic and two Arctic seasons and illustrate the unique attributes of the CALIPSO PSC database. These analyses show substantial interannual variability in PSC areal coverage and also the well-known contrast between the Antarctic and Arctic. The CALIPSO data also reveal seasonal and altitudinal variations in Antarctic PSC composition, which are related to changes in HNO3 and H2O observed by the Microwave Limb Sounder on the Aura satellite.

One year of CNR-IMAA multi-wavelength Raman lidar measurements in coincidence with CALIPSO overpasses: Level 1 products comparison

Abstract. At CNR-IMAA, an aerosol lidar system has operated since May 2000 in the framework of EARLINET (European Aerosol Research Lidar Network), the first lidar network for tropospheric aerosol study on a continental scale. High quality multi-wavelength measurements make this system a reference point for the validation of data products provided by CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), the first satellite-borne lidar specifically designed for aerosol and cloud study. Since 14 June 2006, dedicated measurements have been performed at CNR-IMAA in coincidence with CALIPSO overpasses. For the first time, results on 1-year comparisons between ground-based multi-wavelength Raman lidar measurements and corresponding CALIPSO lidar Level 1 profiles are presented. A methodology for the comparison is presented and discussed in detail. Night-time cases are considered to take advantage from Raman capability of the ground based lidar. Cases with the detection of cirrus clouds in CALIPSO data are separately analysed for taking into account multiple scattering effects. For cirrus cloud cases, few cases are available to draw any conclusions. For clear sky conditions, the comparison shows good performances of the CALIPSO on-board lidar: the mean relative difference between the ground-based and CALIPSO Level 1 measurements is always within its standard deviation at all altitudes, with a mean difference in the 3–8 km altitude range of (−2±12)%. At altitude ranges corresponding to the typical PBL height observed at CNR-IMAA, a mean difference of (−24±20)% is observed in CALIPSO data, probably due to the difference in the aerosol content at the location of PEARL and CALIPSO ground-track location. Finally, the mean differences are on average lower at all altitude ranges for the closest overpasses (at about 40 km) respect to the 80-km overpasses.