CALIPSO Observations Research Articles

Synergistic Use of Remote Sensing and Modeling for Tracing Dust Storms in the Mediterranean

This study focuses on the detection of the dust source region and monitoring of the transport of the dust plume from its primary outflow to final deposition. The application area is the Sahara desert and the eastern Mediterranean, where two dust events occurred during the period 4–6 February 2009, an unusual event for a winter period. The Aqua-MODIS and OMI observations clearly define the spatial distribution of the dust plumes, while the CALIPSO observations of total attenuated backscatter (TAB) at 532 nm, depolarization ratio (DR), and attenuated color ratio (1064/532 nm) on 5 February 2009 provide a clear view and vertical structure of the dust-laden layer. The dust source region is defined to be near the Chad-Niger-Libyan borders, using satellite observations and model (DREAM) output. This dust plume is vertically extended up to 2.5 km and is observed as a mass plume of dust from surface level to that altitude, where the vertical variation of TAB (0.002 to 0.2) and DR (0.2–0.5) implies dust-laden layer with non-spherical particles. CALIPSO profiles show that after the dust plume reached at its highest level, the dust particles start to be deposited over the Mediterranean and the initial dust plume was strongly attenuated, while features of dust were limited below about 1–1.5 km for latitudes northern of ~36° (Greek territory).

The vertical distribution of thin features over the Arctic analysed from CALIPSO observations. Part II: Aerosols

Influx of aerosols from the mid-latitudes has a wide range of impacts on the Arctic atmosphere. In this study, the capability of the CALIPSO-CALIOP instrument to provide accurate observations of aerosol layers is exploited to characterize their vertical distribution, probability density functions (PDFs) of aerosol layer thickness, base and top heights, and optical depths over the Arctic for the 4-yr period from June 2006 to May 2010. It is shown that the bulk of aerosols, from about 65% in winter to 45% in summer, are confined below the lowermost kilometer of the troposphere. In the middle troposphere (3–5 km), spring and autumn seasons show slightly higher aerosol amounts compared to other two seasons. The relative vertical distribution of aerosols shows that clean continental aerosol is the largest contributor in all seasons except in summer, when layers of polluted continental aerosols are almost as large. In winter and spring, polluted continental aerosols are the second largest contributor to the total number of observed aerosol layers, whereas clean marine aerosol is the second largest contributor in summer and autumn. The PDFs of the geometrical thickness of the observed aerosol layers peak about 400–700 m. Polluted continental and smoke aerosols, which are associated with the intrusions from mid-latitudes, have much broader distributions of optical and geometrical thicknesses, suggesting that they appear more often optically thicker and higher up in the troposphere. DOI: 10.1111/j.1600-0889.2010.00517.x

The vertical distribution of thin features over the Arctic analysed from CALIPSO observations. Part I: Optically thin clouds

Clouds play a crucial role in the Arctic climate system. Therefore, it is essential to accurately and reliably quantify and understand cloud properties over the Arctic. It is also important to monitor and attribute changes in Arctic clouds. Here, we exploit the capability of the CALIPSO-CALIOP instrument and provide comprehensive statistics of tropospheric thin clouds, otherwise extremely difficult to monitor from passive satellite sensors. We use 4 yr of data (June 2006–May 2010) over the circumpolar Arctic, here defined as 67–82°N, and characterize probability density functions of cloud base and top heights, geometrical thickness and zonal distribution of such cloud layers, separately for water and ice phases, and discuss seasonal variability of these properties. When computed for the entire study area, probability density functions of cloud base and top heights and geometrical thickness peak at 200–400, 1000–2000 and 400–800 m, respectively, for thin water clouds, while for ice clouds they peak at 6–8, 7–9 and 400–1000 m, respectively. In general, liquid clouds were often identified below 2 km during all seasons, whereas ice clouds were sensed throughout the majority of the upper troposphere and also, but to a smaller extent, below 2 km for all seasons. DOI: 10.1111/j.1600-0889.2010.00516.x

The vertical distribution of thin features over the Arctic analysed from CALIPSO observations

Clouds play a crucial role in the Arctic climate system. Therefore, it is essential to accurately and reliably quantify and understand cloud properties over the Arctic. It is also important to monitor and attribute changes in Arctic clouds. Here, we exploit the capability of the CALIPSO-CALIOP instrument and provide comprehensive statistics of tropospheric thin clouds, otherwise extremely difficult to monitor from passive satellite sensors.We use 4 yr of data (June 2006.May 2010) over the circumpolar Arctic, here defined as 67-82°N, and characterize probability density functions of cloud base and top heights, geometrical thickness and zonal distribution of such cloud layers, separately for water and ice phases, and discuss seasonal variability of these properties. When computed for the entire study area, probability density functions of cloud base and top heights and geometrical thickness peak at 200-400, 1000-2000 and 400-800 m, respectively, for thin water clouds, while for ice clouds they peak at 6-8, 7-9 and 400-1000 m, respectively. In general, liquid clouds were often identified below 2 km during all seasons, whereas ice clouds were sensed throughout the majority of the upper troposphere and also, but to a smaller extent, below 2 km for all seasons.

The vertical distribution of thin features over the Arctic analysed from CALIPSO observations

Influx of aerosols from the mid-latitudes has a wide range of impacts on the Arctic atmosphere. In this study, the capability of the CALIPSO-CALIOP instrument to provide accurate observations of aerosol layers is exploited to characterize their vertical distribution, probability density functions (PDFs) of aerosol layer thickness, base and top heights, and optical depths over the Arctic for the 4-yr period from June 2006 to May 2010. It is shown that the bulk of aerosols, from about 65% in winter to 45% in summer, are confined below the lowermost kilometer of the troposphere. In the middle troposphere (3-5 km), spring and autumn seasons show slightly higher aerosol amounts compared to other two seasons. The relative vertical distribution of aerosols shows that clean continental aerosol is the largest contributor in all seasons except in summer, when layers of polluted continental aerosols are almost as large. In winter and spring, polluted continental aerosols are the second largest contributor to the total number of observed aerosol layers, whereas clean marine aerosol is the second largest contributor in summer and autumn. The PDFs of the geometrical thickness of the observed aerosol layers peak about 400-700 m. Polluted continental and smoke aerosols, which are associated with the intrusions from mid-latitudes, have much broader distributions of optical and geometrical thicknesses, suggesting that they appear more often optically thicker and higher up in the troposphere.

Detecting opaque and nonopaque tropical upper tropospheric ice clouds: A trispectral technique based on the MODIS 8–12 μm window bands

A trispectral technique is developed for detecting tropical upper tropospheric opaque (τ > 6) and nonopaque (τ < 6) ice clouds over ocean based on the brightness temperature differences between the MODIS 8.5 and 11 μm bands and between the 11 and 12 μm bands together with the MODIS detected cloud thermodynamic phase. The brightness temperature differences provide robust information for classifying ice clouds, as illustrated by the observations made by a lidar, a radar, and the MODIS Airborne Simulator over tropical ice anvil systems during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers‐Florida Area Cirrus Experiment. The trispectral technique for detecting tropical upper tropospheric opaque and nonopaque ice clouds is developed based on the analysis of 1 year of data, including MODIS infrared brightness temperatures at 8.5, 11, and 12 μm bands, MODIS‐derived ice cloud optical thicknesses, and cloud top heights from CALIPSO and CloudSat over a region (140°E–180°E, 0°N–20°N) in the Western Pacific Warm Pool. The accuracy of the present trispectral technique is above 80%. A 27 July 2007 MODIS granule over the chosen region is used to verify the trispectral technique. It is found that the classification from the trispectral technique is consistent with a classification based directly on the MODIS ice cloud optical thicknesses. The effects of the variations in the MODIS viewing zenith angle on the detection are found to be negligible. The CALIPSO and CloudSat observations used to develop the classification are more sensitive than MODIS to the height and presence of optically thin cirrus. These differences in cloud heights were found to have a negligible impact on the final detection results.

Frequency and morphology of tropical tropopause layer cirrus from CALIPSO observations: Are isolated cirrus different from those connected to deep convection?

Tropical tropopause layer cirrus (TTLC) profiles identified from CALIPSO lidar measurements are grouped into cloud objects and classified according to whether or not they are directly connected to deep convection. TTLC objects directly connected to deep convection are optically and physically thicker than isolated objects, consistent with what would be expected if convective objects were formed from convective detrainment and isolated objects formed in situ. The frequency of occurrence of these different classifications of TTLC varies depending on height and region. Thirty‐six percent of TTLC profiles contain convective TTLC when TTLC is defined with cloud base height greater than 14 km (±20° Latitude). This estimate of convective TTLC is thought to be a lower limit on the percentage of TTLC formed by convective detrainment. Aged anvil that has persisted longer than the thick regions of the convective cloud would not be classified as convective in our classification. Regions with higher occurrence of deep convection also have higher occurrence of TTLC, and a greater percentage of those TTLC are classified as convective. Definitions of TTLC using higher cloud base height thresholds contain a smaller percentage of TTLC that are classified as convective. Cloud top heights of both isolated and convective TTLC are distributed similarly with respect to the height of the cold point tropopause. Cloud optical depths were found to be higher in TTLC over thick clouds than in TTLC over clear sky during the day but not in night measurements. It is not clear whether this has a physical cause or is due to daytime measurement uncertainties. Both cloud base and top heights are higher over deep convection than over clear sky.

Analysis of Optical Properties of Saharan Dust Derived From Dual-Wavelength Aerosol Retrievals From CALIPSO Observations

Since its first collection of data in June of 2006, the Cloud and Aerosol LIdar with Orthogonal Polarization (CALIOP) lidar instrument aboard the CALIPSO satellite has observed numerous Saharan dust events over West Africa and the Atlantic, predominantly from late May through early September. Due to CALIOP's sensitivity to polarization at 532 nm, the depolarization arising from scattering from the nonspherical dust particles serves as an independent means of discrimination between dust and other aerosol species. In addition to being an important and frequently observed class of aerosol, the Saharan dust provides an excellent basis for the evaluation of elastic-scatter lidar retrieval techniques. The Constrained Ratio Aerosol Model-fit (CRAM) method is one such technique, which applies aerosol modeling constraints in order to limit the ambiguity inherent in aerosol retrievals from elastic-scatter lidar. This letter, in addition to investigating the overall scattering properties of Saharan dust, highlights various retrieval approaches useful in determining the scattering properties of aerosols, as observed by the CALIOP instrument. Herein, we attempt to arrive at an improvement in the dust aerosol model used within CRAM.

CALIPSO observations of wave‐induced PSCs with near‐unity optical depth over Antarctica in 2006–2007

Ground‐based and satellite observations have hinted at the existence of polar stratospheric clouds (PSCs) with relatively high optical depths, even if optical depth values are hard to come by. This study documents a type II PSC observed from spaceborne lidar, with visible optical depths up to 0.8. Comparisons with multiple temperature fields, including reanalyses and results from mesoscale simulations, suggest that intense small‐scale temperature fluctuations due to gravity waves play an important role in its formation, while nearby observations show the presence of a potentially related type Ia PSC farther downstream inside the polar vortex. Following this first case, the geographic distribution and microphysical properties of PSCs with optical depths above 0.3 are explored over Antarctica during the 2006 and 2007 austral winters. These clouds are rare (less than 1% of profiles) and concentrated over areas where strong winds hit steep ground slopes in the Western Hemisphere, especially over the peninsula. Such PSCs are colder than the general PSC population, and their detection is correlated with daily temperature minima across Antarctica. Lidar and depolarization ratios within these clouds suggest they are most likely ice‐based (type II). Similarities between the case study and other PSCs suggest they might share the same formation mechanisms.

Impact of clouds on atmospheric heating based on the R04 CloudSat fluxes and heating rates data set

Among the largest uncertainties in quantifying the radiative impacts of clouds are those that arise from the inherent difficulty in precisely specifying the vertical distribution of cloud optical properties using passive satellite measurements. Motivated by the need to address this problem, CloudSat was launched in April 2006 carrying into orbit the first millimeter wavelength cloud radar to be flown in space. Retrieved profiles of liquid and ice cloud microphysical properties from this Cloud Profiling Radar form the basis of the CloudSat's fluxes and heating rates algorithm, 2B‐FLXHR, a standard product that provides high vertical resolution profiles of radiative fluxes and atmospheric heating rates on the global scale. This paper describes the physical basis of the 2B‐FLXHR algorithm and documents the first year of 2B‐FLXHR data in the context of assessing the radiative impact of clouds on global and regional scales. The analysis confirms that cloud contributions to atmospheric radiative heating are small on the global scale because of a cancelation of the much larger regional heating from high clouds in the tropics and cooling from low clouds at higher latitudes. Preliminary efforts to assess the accuracy of the 2B‐FLXHR product using coincident CERES data demonstrate that outgoing longwave fluxes are better represented than those in the shortwave but both exhibit good agreement with CERES on scales longer than 5 days and larger than 5°. Colocated CALIPSO observations of clouds that are undetected by CloudSat further indicate that while thin cirrus can introduce modest uncertainty in the products, low clouds that are obscured by ground clutter represent a far more important source of error in the current 2B‐FLXHR product that must be addressed in subsequent versions of the algorithm.