Abstract: An assessment of the performance of the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask algorithm for Terra and Aqua satellites is presented. The MODIS cloud mask algorithm output is compared with lidar observations from ground [Arctic High-Spectral Resolution Lidar (AHSRL)], aircraft [Cloud Physics Lidar (CPL)], and satellite-borne [Geoscience Laser Altimeter System (GLAS)] platforms. The comparison with 3 yr of coincident observations of MODIS and combined radar and lidar cloud product from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site in Lamont, Oklahoma, indicates that the MODIS algorithm agrees with the lidar about 85% of the time. A comparison with the CPL and AHSRL indicates that the optical depth limitation of the MODIS cloud mask is approximately 0.4. While MODIS algorithm flags scenes with a cloud optical depth of 0.4 as cloudy, approximately 90% of the mislabeled scenes have optical depths less than 0.4. A comparison with the GLAS cloud dataset indicates that cloud detection in polar regions at night remains challenging with the passive infrared imager approach. In anticipation of comparisons with other satellite instruments, the sensitivity of the cloud mask algorithm to instrument characteristics (e.g., instantaneous field of view and viewing geometry) and thresholds is demonstrated. As expected, cloud amount generally increases with scan angle and instantaneous field of view (IFOV). Nadir sampling represents zonal monthly mean cloud amounts but can have large differences for regional studies when compared to full-swath-width analysis.

Abstract: [1] CloudSat supports a 94 GHz cloud profiling radar as part of the innovative A-train formation of satellites studying the Earths clouds and atmosphere. Using the vertical profiles of clouds and precipitation, an algorithm has been developed to determine the type of clouds present. Because cloud type corresponds to specific cloud physical properties, this step is needed to apply other algorithms to derive quantitative cloud content and radiative data. This cloud type algorithm is applied to the initial 1-year of radar data to obtain the global distribution of various cloud types over the land and ocean. These initial results appear consistent with previous global cloud type distributions, but with some differences that provide insights into the limitations of CloudSat measurements.

Abstract: The Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Earth Observing System (EOS) Terra and Aqua platforms provides unique measurements for deriving global and regional cloud properties. MODIS has spectral coverage combined with spatial resolution in key atmospheric bands, which is not available on previous imagers and sounders. This increased spectral coverage/spatial resolution, along with improved onboard calibration, enhances the capability for global cloud property retrievals. MODIS operational cloud products are derived globally at spatial resolutions of 5 km (referred to as level-2 products) and are aggregated to a 1° equal-angle grid (referred to as level-3 product), available for daily, 8-day, and monthly time periods. The MODIS cloud algorithm produces cloud-top pressures that are found to be within 50 hPa of lidar determinations in single-layer cloud situations. In multilayer clouds, where the upper-layer cloud is semitransparent, the MODIS cloud pressure is representa...

Abstract: [1] A global 2-month comparison is presented between the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the Moderate Resolution Imaging Spectroradiometer (MODIS) for both cloud detection and cloud top height (CTH) retrievals. Both CALIOP and MODIS are part of the NASA A-Train constellation of satellites and provide continuous near-coincident measurements that result in over 28 million cloud detection comparisons and over 5 million CTH comparisons for the months of August 2006 and February 2007. To facilitate the comparison, a computationally efficient and accurate collocation methodology is developed. With the collocated MODIS and CALIOP retrievals, nearly instantaneous comparisons are compiled regionally and globally. Globally, it is found that the MODIS 1-km cloud mask and the CALIOP 1-km averaged layer product agreement is 87% for cloudy conditions for both August 2006 and February 2007. For clear-sky conditions the agreement is 85% (86%) for August (February). The best agreement is found for nonpolar daytime and the poorest agreement in the polar regions. Differences in cloud top heights depend strongly on cloud type. Globally, MODIS underestimates the CTH relative to CALIOP by 1.4 ± 2.9 km for both August 2006 and February 2007. This value of 1.4 km is obtained using the CALIOP 1 km layer products. When compared to the CALIOP 5-km products, the differences increase to −2.6 ± 3.9 km as a result of CALIOP's increased sensitivity to optically thin cirrus. When only high clouds above 5 km are considered, the differences are found to be greater than 4 km with individual comparisons having differences larger than 10 km. These large differences (>10 km) represent approximately 16% of the nonpolar high cloud retrievals (>5 km). For high clouds it is found that MODIS retrieves a cloud top height for 90% of the clouds detected by the CALIOP 5-km layer products. The large MODIS underestimates for optically thin cirrus occur for cases when MODIS reverts to a window brightness temperature retrieval instead of CO2 slicing. A systematic bias is found for marine low-level stratus clouds, with MODIS overestimating the CTH by over 1 km in dense marine stratocumulus regions. The cause of the bias was identified in the MODIS Collection 5 algorithm; an application of a modified algorithm reduced this bias.

Abstract: [1] There has been so far no global estimate of snowfall. CloudSat has, for the first time, provided an opportunity for us to conduct such an estimate. The present study seizes this opportunity and attempts to investigate the global snowfall characteristics using its cloud radar observations. The retrieval methodology developed in this study includes two parts: first, determining whether a radar echo corresponds to snowfall (instead of rainfall), and second, converting radar reflectivity to snowfall rate. The first part is a snow-rain threshold based on multiyear land station and shipboard present weather reports, and the second part is based on backscatter computations of nonspherical ice particles and in situ measured particle size distributions. Using the above retrieval method, global CloudSat data over 1 year were analyzed. The results show the following. (1) In the Southern Hemisphere, there is an almost zonally orientated high snowfall zone centered around 60°S, where both snowfall frequency and rate are high. In the Northern Hemisphere, however, heavy/frequent snowfall areas are mostly locked to geographical locations. (2) Zonally and annually averaged snowfall rate has its maximum value around 2 mm d−1, which is about one third of the zonally averaged rainfall values found in the tropics, signifying the importance of snowfall in hydrological cycle. (3) Vertical profiles of snowfall rate have the greatest variability in the lowest levels. While near-surface snowfall rate generally increases with cloud top height, there seems to be two prevailing groups of clouds with very different growth rate of snowfall as cloud top height increases. (4) The characteristics of the vertical distribution of snowfall rate are quite similar for over-ocean and over-land snow clouds, except that over-land snow clouds seem to be somewhat shallower than those over ocean.

Abstract: The vertical evolution of microphysics in trade-wind cumuli (Cu) observed from the NCAR C-130 research aircraft during one flight of the RICO (Rain in Cumulus Over the Ocean) study is analyzed. Conditional sampling of > 200 Cu traversed on this flight is used to chose Cu for which the aircraft penetrated single and growing Cu turrets about 250-m below cloud top where maximum LWC is often found and where radar has often observed initial stages of precipitation. The vertical evolution of the sampled set of Cu was assumed to follow Lagrangian behavior. The entrainment rate, entrained parcel scales, mixing mechanisms, and effects on the droplet size distribution are measured and evaluated. A parcel model is applied over the 1100-m maximum Cu height of the traverses to determine the relationship between the observed large number of small droplets and the fewer ultra-giant sea-salt nuclei (UGN) in order to assess the role of these nuclei in evolving the size spectrum and in causing a growing “drizzle tail”. New insight on these topics is obtained by using the PVM (Particle Volume Monitor) probe to measure incloud microphysics with 10-cm resolution.The results include the following: Entrainment causes primarily dilution of the drops without significant size changes, thus either extreme inhomogeneous mixing or more likely homogeneous mixing resulting from mixing with cool and humid entrained air take place. The entrained parcels are surprisingly small following lognormal behavior and decaying rapidly upon entering the Cu, as a result super-adiabatic drops are not evident. The entrained parcels are consistent with the Bragg-scattering “mantle echo” often observed by radar in small Cu. The FSSP (Forward Scattering Spectrometer Probe) droplet spectra are nearly constant with height. These “self-preserving” spectra are a result of an approximate balance between dilution by entrainment of droplets originating at cloud base, droplet activation on entrained CCN (cloud condensation nuclei), and detrainment and coalescence losses. Sea-salt nuclei follow Woodcock’s wind dependence, and are shown with the parcel model to play an important role in forming the observed drizzle that increases with cloud height. Accretion is the dominant coalescence mechanism near cloud top in these Cu.

Abstract: [1] The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board NASA's EOS-Aura satellite is measuring ozone, NO2, and other trace gases with daily global coverage. To correct these trace gas retrievals for the presence of clouds, there are two OMI cloud products, based on different physical processes, namely, absorption by O2–O2 at 477 nm (OMCLDO2) and rotational Raman scattering (RRS) in the UV (OMCLDRR). Both cloud products use a Lambertian cloud model with albedo 0.8 and contain the effective (i.e., radiometric) cloud fraction and the cloud pressure. First, the theoretical framework for the Lambertian cloud model is given and the concept of effective cloud fraction is discussed. Next, an intercomparison of the effective cloud fractions from both products is presented, as well as a comparison with MODIS cloud data. It is shown that the O2–O2 and RRS effective cloud fractions correlate very well (95%) but that there is an offset of about 0.10. From MODIS geometric cloud fraction and cloud optical thickness data a MODIS effective cloud fraction was calculated. The effective cloud fractions from OMCLDO2 and MODIS show a high correlation of 92% with a very small offset (0.01). In order to guide users, a summary of the validation status of effective cloud fraction and cloud pressure from the OMCLDO2 and OMCLDRR cloud products is presented.

Abstract: In determining aerosol-cloud interactions, the properties of aerosols must be characterized in the vicinity of clouds. Numerous studies based on satellite observations have reported that aerosol optical depths increase with increasing cloud cover. Part of the increase comes from the humidification and consequent growth of aerosol particles in the moist cloud environment, but part comes from 3D cloud-radiative transfer effects on the retrieved aerosol properties. Often, discerning whether the observed increases in aerosol optical depths are artifacts or real proves difficult. The paper provides a simple model that quantifies the enhanced illumination of cloud-free columns in the vicinity of clouds that are used in the aerosol retrievals. This model is based on the assumption that the enhancement in the cloud-free column radiance comes from enhanced Rayleigh scattering that results from the presence of the nearby clouds. The enhancement in Rayleigh scattering is estimated using a stochastic cloud model to obtain the radiative flux reflected by broken clouds and comparing this flux with that obtained with the molecules in the atmosphere causing extinction, but no scattering.

Abstract: We present initial validation results of the space-borne lidar CALIOP onboard CALIPSO satellite using coincidental observations from a ground-based lidar in Seoul National University (SNU), Seoul, Korea (37.46 • N, 126.95 • E). We analyze six selected cases between Septem-ber 2006 and February 2007, including 3 daytime and 3 night-time observations and covering different types of clear and cloudy atmospheric conditions. Apparent scattering ratios calculated from the two lidar measurements of total attenuated backscatter at 532 nm show similar aerosol and cloud layer structures both under cloud-free conditions and in cases of multiple aerosol layers underlying semi-transparent cirrus clouds. Agreement on top and base heights of cloud and aerosol layers is generally within 0.10 km, particularly during night-time. This result confirms that the CALIPSO science team algorithms for the discrimination of cloud and aerosol as well as for the detection of layer top and base altitude provide reliable information in such atmospheric conditions. This accuracy of the planetary boundary layer top height under cirrus cloud appears, however, limited during daytime. Under thick cloud conditions , however, information on the cloud top (bottom) height only is reliable from CALIOP (ground-based lidar) due to strong signal attenuations. However, simultaneous space-borne CALIOP and ground-based SNU lidar (SNU-L) measurements complement each other and can be combined to provide full information on the vertical distribution of aerosols and clouds. An aerosol backscatter-to-extinction ratio (BER) estimated from lidar and sunphotometer synergy at the SNU site during the CALIOP overpass is assessed to be 0.023±0.004 sr −1 (i.e. a lidar ratio of 43.2±6.2 sr) from CALIOP and 0.027±0.006 sr −1 (37.4±7.2 sr) from SNU-L. For aerosols within the planetary boundary layer under cloud-free conditions, the aerosol extinction profiles from both lidars are in agreement within about 0.02 km −1. Under semi-transparent cirrus clouds, such profiles also show good agreement for the night-time CALIOP flight, but large discrepancies are found for the daytime flights due to a small signal-to-noise ratio of the CALIOP data.

Abstract: The interplay between clouds and aerosols and their contribution to the radiation budget is one of the largest uncertainties of climate change. Most work to date has separated cloudy and cloud-free areas in order to evaluate the individual radiative forcing of aerosols, clouds, and aerosol effects on clouds. Here we examine the size distribution and the optical properties of small, sparse cumulus clouds and the associated optical properties of what is considered a cloud-free atmosphere within the cloud field. We show that any separation between clouds and cloud free atmosphere will incur errors in the calculated radiative forcing. The nature of small cumulus cloud size distributions suggests that at any resolution, a significant fraction of the clouds are missed, and their optical properties are relegated to the apparent cloud-free optical properties. At the same time, the cloudy portion incorporates significant contribution from non-cloudy pixels. We show that the largest contribution to the total cloud reflectance comes from the smallest clouds and that the spatial resolution changes the apparent energy flux of a broken cloudy scene. When changing the resolution from 30 m to 1 km (Landsat to MODIS) the average "cloud-free" reflectance at 1.65 μm increases from 0.0095 to 0.0115 (>20%), the cloud reflectance decreases from 0.13 to 0.066 (~50%), and the cloud coverage doubles, resulting in an important impact on climate forcing estimations. The apparent aerosol forcing is on the order of 0.5 to 1 Wm −2 per cloud field.

Abstract: [1] In this paper we examine differences between cloud pressures retrieved from the Ozone Monitoring Instrument (OMI) using the ultraviolet rotational Raman scattering (RRS) algorithm and those from the thermal infrared (IR) Aqua/MODIS. Several cloud data sets are currently being used in OMI trace gas retrieval algorithms including climatologies based on IR measurements and simultaneous cloud parameters derived from OMI. From a validation perspective, it is important to understand the OMI retrieved cloud parameters and how they differ with those derived from the IR. To this end, we perform radiative transfer calculations to simulate the effects of different geophysical conditions on the OMI RRS cloud pressure retrievals. We also quantify errors related to the use of the Mixed Lambert-Equivalent Reflectivity (MLER) concept as currently implemented of the OMI algorithms. Using properties from the Cloudsat radar and MODIS, we show that radiative transfer calculations support the following: (1) The MLER model is adequate for single-layer optically thick, geometrically thin clouds, but can produce significant errors in estimated cloud pressure for optically thin clouds. (2) In a two-layer cloud, the RRS algorithm may retrieve a cloud pressure that is either between the two cloud decks or even beneath the top of the lower cloud deck because of scattering between the cloud layers; the retrieved pressure depends upon the viewing geometry and the optical depth of the upper cloud deck. (3) Absorbing aerosol in and above a cloud can produce significant errors in the retrieved cloud pressure. (4) The retrieved RRS effective pressure for a deep convective cloud will be significantly higher than the physical cloud top pressure derived with thermal IR.

Abstract: Recent large-eddy simulation (LES) studies of the impact of aerosol on the dynamics of nocturnal marine stratocumulus revealed that, depending on the large-scale forcings, an aerosol-induced increase of the droplet concentration can lead to either an increase or a decrease of the liquid water path, hence contrasting with the cloud thickening that is expected from a reduction of the precipitation efficiency. In this study, the aerosol impacts on cloud microphysics are examined in the context of the boundary-layer diurnal cycle using 36-h LES simulations of pristine and polluted clouds. These simulations corroborate previous findings that during nighttime aerosol-induced liquid water path changes are sensitive to the large-scale forcings via enhancement of cloud-top entrainment such that, ultimately, the liquid water path may be reduced when the free-tropospheric-entrained air is drier. During the day, however, enhanced entrainment, inhibition of drizzle evaporation below cloud base, and reduced sensible heat flux from the surface lead to a more pronounced decoupling of the boundary layer, which significantly amplifies the liquid water path reduction of the polluted clouds. At night the sign of the liquid water path difference between pristine and polluted clouds depends upon large-scale forcings, while during the day the liquid water path of polluted clouds is always smaller than the one of the pristine clouds. Suggestions are made on how observational studies could be designed for validation of these simulations.

Abstract: [1] The cloud pressures determined by three different algorithms, operating on reflectances measured by two spaceborne instruments in the “A” train, are compared with each other. The retrieval algorithms are based on absorption in the oxygen A-band near 765 nm, absorption by a collision induced absorption in oxygen near 477 nm, and the filling in of Fraunhofer lines by rotational Raman scattering near 350 nm. A Lambertian reflector as cloud model is assumed in the retrievals. The first algorithm operates on data collected by the POLDER instrument on board PARASOL, while the latter two operate on data from the OMI instrument on board EOS-Aura. The satellites sample the same air mass within about 15 min. We compare the retrieval algorithms using synthetic spectra to give the comparison realistic baseline expectations. It appears that these cloud pressures are not the pressure of the cloud top, but of a level inside the cloud. This is corroborated by comparisons with MODIS and CloudSat data: while the top of the cloud is seen by MODIS using emitted IR radiation, both OMI and PARASOL algorithms retrieve a pressure near the midlevel of the cloud. The three cloud pressure products are compared using 1 month of data. The cloud pressures are found to show a similar behavior, with correlation coefficients larger than 0.85 between the data sets for high effective cloud fractions. The average differences in the cloud pressure are small, between 2 and 45 hPa, for the whole data set, with an RMS difference of 65 to 93 hPa. This falls within the science requirement for the OMI cloud pressure to have an accuracy of 100 hPa. For small to medium effective cloud fractions, the cloud pressure distribution found by PARASOL is very similar to that found by OMI using the O2–O2 absorption. Somewhat larger differences are found for very high effective cloud fractions.

Abstract: [1] A new conceptual model that facilitates the inference of the vigor of severe convective storms, producing tornadoes and large hail, by using satellite-retrieved vertical profiles of cloud top temperature (T)–particle effective radius (re) relations is presented and tested. The driving force of these severe weather phenomena is the high updraft speed, which can sustain the growth of large hailstones and provide the upward motion that is necessary to evacuate the violently converging air of a tornado. Stronger updrafts are revealed by the delayed growth of re to greater heights and lower T, because there is less time for the cloud and raindrops to grow by coalescence. The strong updrafts also delay the development of a mixed phase cloud and its eventual glaciation to colder temperatures. Analysis of case studies making use of these and related criteria show that they can be used to identify clouds that possess a significant risk of large hail and tornadoes. Although the strength and direction of the wind shear are major modulating factors, it appears that they are manifested in the updraft intensity and cloud shapes and hence in the T-re profiles. It is observed that the severe storm T-re signature is an extensive property of the clouds that develop ahead in space and time of the actual hail or tornadic storm, suggesting that the probabilities of large hail and tornadoes can be obtained at substantial lead times. Analysis of geostationary satellite time series indicates lead times of up to 2 h.

Abstract: [1] The difference between cloud-top altitude Ztop and infrared effective radiating height Zeff for optically thick ice clouds is examined using April 2007 data taken by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and the Moderate-Resolution Imaging Spectroradiometer (MODIS). For even days, the difference ΔZ between CALIPSO Ztop and MODIS Zeff is 1.58 ± 1.26 km. The linear fit between Ztop and Zeff, applied to odd-day data, yields a difference of 0.03 ± 1.21 km and can be used to estimate Ztop from any infrared-based Zeff for thick ice clouds. Random errors appear to be due primarily to variations in cloud ice-water content (IWC). Radiative transfer calculations show that ΔZ corresponds to an optical depth of ∼1, which based on observed ice-particle sizes yields an average cloud-top IWC of ∼0.015 gm−3, a value consistent with in situ measurements. The analysis indicates potential for deriving cloud-top IWC using dual-satellite data.

Abstract: Continental fair-weather cumuli exhibit significant diurnal, day-to-day, and year-to-year variability. This study describes the climatology of cloud macroscale properties, over the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site. The diurnal cycle of cloud fraction, cloud-base height, cloud-top height, and cloud thickness were well defined. The cloud fraction reached its maximum value near 1400 central standard time. The average cloud-base height increased throughout the day, while the average cloud thickness decreased with time. In contrast to the other cloud properties, the average cloud-chord length remained nearly constant throughout the day. The sensitivity of the cloud properties to the year-to-year variability of precipitation and day-to-day changes in the height of the lifting condensation level (zLCL) and surface fluxes were compared. The cloud-base height was found to be sensitive to both the year, zLC...

Abstract: [1] Images obtained from the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS)-M instrument onboard Venus Express present visible trains of alternating bands of cloud brightness in two different layers: at the upper cloud tops (∼66 km altitude) observed in the dayside hemisphere using reflected ultraviolet light (380 nm) and in the lower cloud (∼47 km altitude) observed in the nightside hemisphere using thermal radiation (1.74 μm). The waves are nearly zonal (with the bands perpendicular to latitude circles), have wavelengths of 60–150 km, propagate westward with low phase velocities relative to the zonal flow, and are confined in wave packets of 400 to 1800 km in length. The waves in the lower cloud observed in the infrared are widely distributed around the planet, and their appearance varies widely throughout the VIRTIS data set. The locations of both types of waves seem not correlated with latitude, local times, surface topography, or the structure of the wind. In both cases the characteristics of the waves correspond to gravity waves propagating in confined stable layers of the atmosphere. We examine the properties of these waves in terms of a linear model and perform a simple analysis to discuss the vertical stability of the atmosphere within Venus clouds.

Abstract: Details about the atmosphere on Venus are gradually emerging from the once apparently impenetrable global cloud cover. Simultaneous imaging in the ultraviolet and infrared by Venus Express provides a new view of the ultraviolet patterns seen in the cloud tops. The picture that emerges is one of dark low latitudes dominated by convective mixing in the sulphuric acid clouds, bringing unknown ultraviolet absorbers up from the lower atmosphere. The cloud-top morphology revealed in the southern hemisphere by Venus Express resembles that found earlier by Pioneer Venus and Venera-15 in the north, suggesting global symmetry between the two hemispheres. When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds. This paper reports multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing that brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the 'cold collar', which suppresses vertical mixing. In low and middle latitudes, the visible cloud top is located at a constant altitude of 72 ± 1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment. When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds1,2,3 and seem to trace dynamical activity in the middle atmosphere4. It has long been unclear whether the global pattern arises from differences in cloud top altitude (which was earlier3 estimated to be 66–72 km), compositional variations or temperature contrasts. Here we report multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing which brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the ‘cold collar’, an annulus of cold air characterized by ∼30 K lower temperatures with a positive lapse rate, which suppresses vertical mixing and cuts off the supply of ultraviolet absorbers from below. In low and middle latitudes, the visible cloud top is located at a remarkably constant altitude of 72 ± 1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment rather than by elevation changes. The cloud top descends to ∼64 km in the eye of the hemispheric vortex, which appears as a depression in the upper cloud deck. The ultraviolet dark circular streaks enclose the vortex eye and are dynamically connected to it.

Abstract: [1] In this study, a methodology to diagnose a global cloud-resolving model (GCRM) is explored on the basis of a joint analysis with satellite measurements. The Madden-Julian Oscillation experiment carried out with the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) is used as the test bed. The NICAM output is compared with Tropical Rainfall Measuring Mission (TRMM) and CloudSat measurements in terms of composite analysis, contoured frequency by altitude diagrams (CFADs), and the joint histogram of cloud top and precipitation top heights. It is found in the composite diagram that the GCRM reproduces a slow, eastward migration of a convective envelope well comparable to the satellite measurement. The GCRM CFAD qualitatively reproduces TRMM and CloudSat CFADs, except that the GCRM tends to overly produce snow in deep convection. The joint histograms reveal that, while the overproduction of snow is evident, NICAM-simulated snow is incapable of producing 94-GHz radar echoes higher than 5 dBZ. This deficiency can be mitigated by a modification to microphysical parameterization in the way that a proportion of small particles is enhanced in the snow mass spectrum.

Abstract: Aerosol-cloud relationships are derived from 14 warm continental cumuli cases sampled during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Cloud droplet number concentration is clearly proportional to the subcloud accumulation mode aerosol number concentration. An inverse correlation between cloud top effective radius and subcloud aerosol number concentration is observed when cloud depth variations are accounted for. There are no discernable aerosol effects on cloud droplet spectral dispersion; the averaged spectral relative dispersion is 0.30 ± 0.04. Aerosol-cloud relationships are also identified from comparison of two isolated cloud cases that occurred under different degrees of anthropogenic influence. Cloud liquid water content, cloud droplet number concentration, and cloud top effective radius exhibit subadiabaticity resulting from entrainment mixing processes. The degree of LWC subadiabaticity is found to increase with cloud depth. Impacts of subadiabaticity on cloud optical properties are assessed. It is estimated that owing to entrainment mixing, cloud LWP, effective radius, and cloud albedo are decreased by 50–85%, 5–35%, and 2–26%, respectively, relative to adiabatic values of a plane-parallel cloud. The impact of subadiabaticity on cloud albedo is largest for shallow clouds. Results suggest that the effect of entrainment mixing must be accounted for when evaluating the aerosol indirect effect.

Abstract: [1] In order to better understand the general problem of satellite cloud top height retrievals for low clouds, observations made by NOAA research vessels in the stratocumulus region in the southeastern Pacific during cruises in 2001 and 2003 to 2006 were matched with near-coincident retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging SpectroRadiometer (MISR) instruments on the Terra satellite, along with a limited set of ISCCP 30-km (DX) retrievals. The ISCCP cloud top heights, determined from the cloud top pressures, were found to be biased high by between 1400 and 2000 m within the limited comparison data set. Like the International Satellite Cloud Climatology Project (ISCCP) results, the MODIS retrievals were biased high by more than 2000 m, while the MISR retrievals had errors on the order of 230 to 420 m, with the wind corrected heights having almost no bias. The extremely large bias in the ISCCP and MODIS retrievals was traced to their reliance on low-resolution observations or models of the atmospheric temperature structure. Cloud top height retrievals based on satellite cloud top temperatures and a constant atmospheric lapse rate agreed substantially better with the ship-based measurements.

Abstract: [1] A new day and night technique for precipitation process separation and rainfall intensity differentiation using the Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager is proposed. It relies on the conceptual design that convective clouds with higher rainfall intensities are characterized by a larger vertical extension and a higher cloud top. For advective-stratiform precipitation areas, it is assumed that areas with a higher cloud water path (CWP) and more ice particles in the upper parts are characterized by higher rainfall intensities. First, the rain area is separated into areas of convective and advective-stratiform precipitation processes. Next, both areas are divided into subareas of differing rainfall intensities. The classification of the convective area relies on information about the cloud top height gained from water vapor-IR differences and the IR cloud top temperature. The subdivision of the advective-stratiform area is based on information about the CWP and the particle phase in the upper parts. Suitable combinations of temperature differences (ΔT3.9–10.8, ΔT3.9–7.3, ΔT8.7–10.8, ΔT10.8–12.1) are incorporated to infer information about the CWP during nighttime, while a visible and a near-IR channel are considered during the daytime. ΔT8.7–10.8 and ΔT10.8–12.1 are particularly included to supply information about the cloud phase. Intensity differentiation is realized by using pixel-based confidences for each subarea calculated as a function of the respective value combinations of the previously mentioned variables. For the calculation of the confidences, the value combinations are compared with ground-based radar data. The proposed technique is validated against ground-based radar data and shows an encouraging performance (Heidke skill score 0.07–0.2 for 15-min intervals).

Abstract: This paper reports on the relationship between lidar backscatter and the corresponding depolarization ratio for nine types of cloud systems. The data used in this study are the lidar returns measured by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite and the collocated cloud products derived from the observations made by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua satellite. Specifically, the operational MODIS cloud optical thickness and cloud-top pressure products are used to classify cloud types on the basis of the International Satellite Cloud Climatology Project (ISCCP) cloud classification scheme. While the CALIPSO observations provide information for up to 10 cloud layers, in the present study only the uppermost clouds are considered. The layer-averaged attenuated backscatter (γ′) and layer-averaged depolarization ratio (δ) from the CALIPSO measurements show both water- and ice-phase features for global cirrus, cirrostratus, and deep convective cloud classes. Furthermore, we screen both the MODIS and CALIPSO data to eliminate cases in which CALIPSO detected two- or multi-layered clouds. It is shown that low γ′ values corresponding to uppermost thin clouds are largely eliminated in the CALIPSO δ–γ′ relationship for single-layered clouds. For mid-latitude and polar regions corresponding, respectively, to latitude belts 30°–60° and 60°–90° in both the hemispheres, a mixture of water and ice is also observed in the case of the altostratus class. MODIS cloud phase flags are also used to screen ice clouds. The resultant water clouds flagged by the MODIS algorithm show only water phase feature in the δ–γ′ relation observed by CALIOP; however, in the case of the ice clouds flagged by the MODIS algorithm, the co-existence of ice- and water-phase clouds is still observed in the CALIPSO δ–γ′ relationship.

Abstract: A new method for the delineation of precipi- tation during daytime using multispectral satellite data is proposed. The approach is not only applicable to the de- tection of mainly convective precipitation by means of the commonly used relation between infrared cloud top temper- ature and rainfall probability but enables also the detection of stratiform precipitation (e.g. in connection with mid-latitude frontal systems). The presented scheme is based on the con- ceptual model that precipitating clouds are characterized by a combination of particles large enough to fall, an adequate vertical extension (both represented by the cloud water path; cwp), and the existence of ice particles in the upper part of the cloud. The technique considers the VIS0.6 and the NIR1.6 channel to gain information about the cloud water path. Ad- ditionally, the brightness temperature differences 1T8.7 10.8 and 1T10.8 12.1 are considered to supply information about the cloud phase. Rain area delineation is realized by us- ing a minimum threshold of the rainfall confidence. To ob- tain a statistical transfer function between the rainfall confi- dence and the four parameters VIS0.6, NIR1.6, 1T8.7 10.8 and 1T10.8 12.1, the value combinations of these four variables are compared to ground based radar data. The retrieval is validated against independent radar data not used for deriving the transfer function and shows an encouraging performance as well as clear improvements compared to existing optical retrieval techniques using only IR thresholds for cloud top temperature.

Abstract: The microphysical properties of mixed-phase altocumulus clouds are investigated using in situ airborne measurements acquired during the ninth Cloud Layer Experiment (CLEX-9) over a midlatitude location. Approximately ⅔ of the sampled profiles are supercooled liquid–topped altocumulus clouds characterized by mixed-phase conditions. The coexistence of measurable liquid water droplets and ice crystals begins at or within tens of meters of cloud top and extends down to cloud base. Ice virga is found below cloud base. Peak liquid water contents occur at or near cloud top while peak ice water contents occur in the lower half of the cloud or in virga. The estimation of ice water content from particle size data requires that an assumption be made regarding the particle mass–dimensional relation, resulting in potential error on the order of tens of percent. The highest proportion of liquid is typically found in the coldest (top) part of the cloud profile. This feature of the microphysical structure for th...

Abstract: [1] The albedo of marine stratocumuli depends upon cloud liquid water content, droplet effective radius (re), and how these parameters vary with height. Using satellite data and shipborne data from the East Pacific Investigation of Climate (EPIC) Stratocumulus Study, this study investigates the cloud re vertical variation for drizzling and nondrizzling clouds. Visible/near-infrared retrievals from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) are used to estimate the vertical profile of re. MODIS re observations and collocated shipborne scanning C-band precipitation radar data show that re generally increases with height in nondrizzling clouds, consistent with aircraft observations. It is found that in clouds with precipitation rates greater than a few hundredths of a mm h−1 the vertical gradient of re is significantly less than that in nondrizzling clouds and can become negative when the drizzle is heavier than approximately 0.1 mm h−1. High values of re at drizzling cloud base are consistent with estimates of the ratio of liquid water in the drizzle drops to that in the cloud droplets. C-band derived cloud base precipitation rates are found to be better correlated with re at cloud base than with re at cloud top, suggesting that passive remote sensing may be useful for drizzle detection.

Abstract: [1] A new 2-year data set of polar low events over the Nordic Seas is presented. The detection of polar lows is based on the combined use of thermal infrared AVHRR imagery and SSM/I derived wind speeds from the satellite climatology HOAPS (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data). A total of 90 polar lows are found in 2004 and 2005 with a maximum of polar low activity during the winter months. The main polar low genesis regions lie between Iceland and Finnmark in the Norwegian Sea, in the Barents Sea and in the lee region of Cape Farewell. Interannual variability in polar low activity results mostly from more frequent cold air outbreaks in 2004. Statistics for several atmospheric parameters (e.g., wind speed, precipitation, cloud top temperatures) which describe the intensity of the cyclones are retrieved from satellite observations. The data set builds a basis for studies about polar low forcing mechanisms and for the validation of mesoscale numerical models.

Abstract: [1] We studied the wavelength-dependence of the attenuation of incoming solar UV radiation by a homogeneous cloud layer. By systematic analysis of irradiances simulated with a radiative transfer model, we were able to separate the wavelength-dependence of the cloud modification factor into different components, and thus achieve an understanding of the physical processes involved. Our results show that short wavelengths, in general, penetrate the cloud more effectively than longer wavelengths and that there are two important contributors to this wavelength-dependence: (1) that induced by multiple scattering between the cloud top and the atmosphere above and (2) that introduced by the wavelength-dependent radiance distribution at the cloud top (including the direct beam) together with the transmittance of the cloud alone as function of angle of incidence. Furthermore, we found that the former does not depend on solar zenith angle, whereas the latter does and hence also introduces a solar zenith angle dependence in the wavelength-dependence of the cloud modification factor.