Abstract: . Using 2B-CLDCLASS-LIDAR (radar–lidar) cloud classification and 2B-FLXHR-LIDAR radiation products from CloudSat over 4 years, this study evaluates the co-occurrence frequencies of different cloud types, analyzes their along-track horizontal scales and cloud radiative effects (CREs), and utilizes the vertical distributions of cloud types to evaluate cloud-overlap assumptions. The statistical results show that high clouds, altostratus (As), altocumulus (Ac) and cumulus (Cu) tend to coexist with other cloud types. However, stratus (St) (or stratocumulus, Sc), nimbostratus (Ns) and convective clouds are much more likely to exhibit individual features than other cloud types. On average, altostratus-over-stratus/stratocumulus cloud systems have a maximum horizontal scale of 17.4 km, with a standard deviation of 23.5 km. Altocumulus-over-cumulus cloud types have a minimum scale of 2.8 km, with a standard deviation of 3.1 km. By considering the weight of each multilayered cloud type, we find that the global mean instantaneous net CREs of multilayered cloud systems during the daytime are approximately −41.3 and −50.2 W m−2, which account for 40.1 and 42.3% of the global mean total net CREs at the top of the atmosphere (TOA) and at the surface, respectively. The radiative contributions of high-over-altocumulus and high-over-stratus/stratocumulus (or cumulus) in the all multilayered cloud systems are dominant due to their frequency. Considering the overlap of cloud types, the cloud fraction based on the random overlap assumption is underestimated over vast oceans, except in the west-central Pacific Ocean warm pool. Obvious overestimations mainly occur over tropical and subtropical land masses. In view of a lower degree of overlap than that predicted by the random overlap assumption to occur over the vast ocean, particularly poleward of 40° S, the study therefore suggests that a linear combination of minimum and random overlap assumptions may further improve the predictions of actual cloud fractions for multilayered cloud types (e.g., As + St/Sc and Ac + St/Sc) over the Southern Ocean. The establishment of a statistical relationship between multilayered cloud types and the environmental conditions (e.g., atmospheric vertical motion, convective stability and wind shear) would be useful for parameterization design of cloud overlap in numerical models.

Abstract: Moderate Resolution Imaging Spectroradiometer (MODIS) retrieves cloud droplet effective radius (re) and optical thickness (τ) by projecting observed cloud reflectances onto a precomputed look-up table (LUT). When observations fall outside of the LUT, the retrieval is considered “failed” because no combination of τ and re within the LUT can explain the observed cloud reflectances. In this study, the frequency and potential causes of failed MODIS retrievals for marine liquid phase (MLP) clouds are analyzed based on 1 year of Aqua MODIS Collection 6 products and collocated CALIOP and CloudSat observations. The retrieval based on the 0.86 µm and 2.1 µm MODIS channel combination has an overall failure rate of about 16% (10% for the 0.86 µm and 3.7 µm combination). The failure rates are lower over stratocumulus regimes and higher over the broken trade wind cumulus regimes. The leading type of failure is the “re too large” failure accounting for 60%–85% of all failed retrievals. The rest is mostly due to the “re too small” or τ retrieval failures. Enhanced retrieval failure rates are found when MLP cloud pixels are partially cloudy or have high subpixel inhomogeneity, are located at special Sun-satellite viewing geometries such as sunglint, large viewing or solar zenith angles, or cloudbow and glory angles, or are subject to cloud masking, cloud overlapping, and/or cloud phase retrieval issues. The majority (more than 84%) of failed retrievals along the CALIPSO track can be attributed to at least one or more of these potential reasons. The collocated CloudSat radar reflectivity observations reveal that the remaining failed retrievals are often precipitating. It remains an open question whether the extremely large re values observed in these clouds are the consequence of true cloud microphysics or still due to artifacts not included in this study.

Abstract: Cloud phase and microphysical properties control the radiative effects of clouds in the climate system and are therefore crucial to characterize in a variety of conditions and locations. An Arctic-specific, ground-based, multisensor cloud retrieval system is described here and applied to 2 yr of observations from Barrow, Alaska. Over these 2 yr, clouds occurred 75% of the time, with cloud ice and liquid each occurring nearly 60% of the time. Liquid water occurred at least 25% of the time, even in winter, and existed up to heights of 8 km. The vertically integrated mass of liquid was typically larger than that of ice. While it is generally difficult to evaluate the overall uncertainty of a comprehensive cloud retrieval system of this type, radiative flux closure analyses were performed in which flux calculations using the derived microphysical properties were compared with measurements at the surface and the top of the atmosphere. Radiative closure biases were generally smaller for cloudy scenes re...

Abstract: We present observations of total cloud cover and cloud type classification results from a sky camera network comprising four stations in Switzerland. In a comprehensive intercomparison study, records of total cloud cover from the sky camera, long-wave radiation observations, Meteosat, ceilometer, and visual observations were compared. Total cloud cover from the sky camera was in 65–85% of cases within ±1 okta with respect to the other methods. The sky camera overestimates cloudiness with respect to the other automatic techniques on average by up to 1.1 ± 2.8 oktas but underestimates it by 0.8 ± 1.9 oktas compared to the human observer. However, the bias depends on the cloudiness and therefore needs to be considered when records from various observational techniques are being homogenized. Cloud type classification was conducted using the k-Nearest Neighbor classifier in combination with a set of color and textural features. In addition, a radiative feature was introduced which improved the discrimination by up to 10%. The performance of the algorithm mainly depends on the atmospheric conditions, site-specific characteristics, the randomness of the selected images, and possible visual misclassifications: The mean success rate was 80–90% when the image only contained a single cloud class but dropped to 50–70% if the test images were completely randomly selected and multiple cloud classes occurred in the images.

Abstract: . 2012 Level-2 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite-based cloud data sets are investigated for thresholds that distinguish the presence of cirrus clouds in autonomous lidar measurements, based on temperatures, heights, optical depth and phase. A thermal threshold, proposed by Sassen and Campbell (2001) for cloud top temperature Ttop ≤ −37 °C, is evaluated versus CALIOP algorithms that identify ice-phase cloud layers using polarized backscatter measurements. Derived global mean cloud top heights (11.15 vs. 10.07 km above mean sea level; a.m.s.l.), base heights (8.76 km a.m.s.l. vs. 7.95 km a.m.s.l.), temperatures (−58.48 °C vs. −52.18 °C and −42.40 °C vs. −38.13 °C, respectively, for tops and bases) and optical depths (1.18 vs. 1.23) reflect the sensitivity to this constraint. Over 99 % of all Ttop ≤ −37 °C clouds are classified as ice by CALIOP Level-2 algorithms. Over 81 % of all ice clouds correspond with Ttop ≤ −37 °C. For instruments lacking polarized measurements, and thus practical estimates of phase, Ttop ≤ −37 °C provides sufficient justification for distinguishing cirrus, as opposed to the risks of glaciated liquid-water cloud contamination occurring in a given sample from clouds identified at relatively "warm" (Ttop > −37 °C) temperatures. Although accounting for uncertainties in temperatures collocated with lidar data (i.e., model reanalyses/sondes) may justifiably relax the threshold to include warmer cases, the ambiguity of "warm" ice clouds cannot be fully reconciled with available measurements, conspicuously including phase. Cloud top heights and optical depths are investigated, and global distributions and frequencies derived, as functions of CALIOP-retrieved phase. These data provide little additional information, compared with temperature alone, and may exacerbate classification uncertainties overall.

Abstract: Cloud microphysical data obtained from G-1 aircraft flights over the southeastern Pacific during the Variability of the American Monsoon Systems Ocean-Cloud-Atmosphere-Land Study Regional Experiment field campaign were analyzed for evidence of entrainment mixing of dry air from above cloud top. Mixing diagram analysis was made for the horizontal flight data recorded at 1 Hz and 40 Hz. The dominant observed feature, a positive relationship between cloud droplet mean volume (V) and liquid water content (L), suggested occurrence of homogeneous mixing. On the other hand, estimation of the relevant scale parameters (i.e., transition length scale and transition scale number) consistently indicated inhomogeneous mixing. Importantly, the flight altitudes of the measurements were significantly below cloud top. We speculate that mixing of the entrained air near the cloud top may have indeed been inhomogeneous; but due to vertical circulation mixing, the correlation between V and L became positive at the measurement altitudes in midlevel of clouds, because during their descent, cloud droplets evaporate, faster in more diluted cloud parcels, leading to a positive correlation between V and L regardless of the mixing mechanism near the cloud top.

Abstract: Intensive field experiment is an important approach to obtain microphysical information about clouds and precipitation. From 1 July to 31 August 2014, the third Tibetan Plateau Atmospheric Science Experiment was carried out and comprehensive measurements of water vapor, clouds, and precipitation were conducted at Naqu. The most advanced radars in China, such as Ka-band millimeter-wave cloud radar, Ku-band micro-rain radar, C-band continuous-wave radar and lidar, and microwave radiometer and disdrometer were deployed to observe high spatial-temporal vertical structures of clouds and precipitation. The C-band duallinear polarization radar was coordinated with the China new generation weather radar to constitute a dual- Doppler radar system for the measurements of three-dimensional wind fields within convective precipitations and the structure and evolution of hydrometeors related to precipitation process. Based on the radar measurements in this experiment, the diurnal variations of several important cloud properties were analyzed, including cloud top and base, cloud depth, cloud cover, number of cloud layers, and their vertical structures during summertime over Naqu. The features of reflectivity, velocity, and depolarization ratio for different types of clouds observed by cloud radar are discussed. The results indicate that the cloud properties were successfully measured by using various radars in this field experiment. During the summertime over Naqu, most of the clouds were located above 6 km and below 4 km above ground level. Statistical analysis shows that total amounts of clouds, the top of high-level clouds, and cloud depth, all demonstrated a distinct diurnal variation. Few clouds formed at 1000 LST (local standard time), whereas large amounts of clouds formed at 2000 LST. Newly formed cumulus and stratus clouds were often found at 3-km height, where there existed significant updrafts. Deep convection reached up to 16.5 km (21 km above the mean sea level), and updrafts and downdrafts coexisted in the convective system. Supercooled water might exist in such kinds of deep convective system. The above measurements and preliminary analysis provide a basis for further study of cloud physics and precipitation process over the Tibetan Plateau. These observations are also valuable for modeling studies of cloud and precipitation physics as well as in the development of parameterization schemes in numerical prediction models.

Abstract: Variability and trends in total cloud cover for 1982–2009 across the contiguous United States from the International Satellite Cloud Climatology Project (ISCCP), AVHRR Pathfinder Atmospheres–Extended (PATMOS-x), and EUMETSAT Satellite Application Facility on Climate Monitoring Clouds, Albedo and Radiation from AVHRR Data Edition 1 (CLARA-A1) satellite datasets are assessed using homogeneity-adjusted weather station data. The station data, considered as “ground truth” in the evaluation, are generally well correlated with the ISCCP and PATMOS-x data and with the physically related variables diurnal temperature range, precipitation, and surface solar radiation. Among the satellite products, overall, the PATMOS-x data have the highest interannual correlations with the weather station cloud data and those other physically related variables. The CLARA-A1 daytime dataset generally shows the lowest correlations, even after trends are removed. For the U.S. mean, the station dataset shows a negative but not...

Abstract: . Active remote sensing of marine boundary-layer clouds is challenging as drizzle drops often dominate the observed radar reflectivity. We present a new method to simultaneously retrieve cloud and drizzle vertical profiles in drizzling boundary-layer clouds using surface-based observations of radar reflectivity, lidar attenuated backscatter, and zenith radiances under conditions when precipitation does not reach the surface. Specifically, the vertical structure of droplet size and water content of both cloud and drizzle is characterised throughout the cloud. An ensemble optimal estimation approach provides full error statistics given the uncertainty in the observations. To evaluate the new method, we first perform retrievals using synthetic measurements from large-eddy simulation snapshots of cumulus under stratocumulus, where cloud water path is retrieved with an error of 31 g m−2. The method also performs well in non-drizzling clouds where no assumption of the cloud profile is required. We then apply the method to observations of marine stratocumulus obtained during the Atmospheric Radiation Measurement MAGIC deployment in the Northeast Pacific. Here, retrieved cloud water path agrees well with independent three-channel microwave radiometer retrievals, with a root mean square difference of 10–20 g m−2.

Abstract: The mean structure and diurnal cycle of southeast (SE) Atlantic boundary layer clouds are described with satellite observations and multiscale modeling framework (MMF) simulations during austral spring (September–November). Hourly resolution cloud fraction (CF) and cloud-top height (HT) are retrieved from Meteosat-9 radiances using modified Clouds and the Earth’s Radiant Energy System (CERES) Moderate Resolution Imaging Spectroradiometer (MODIS) algorithms, whereas liquid water path (LWP) is from the University of Wisconsin microwave satellite climatology. The MMF simulations use a 2D cloud-resolving model (CRM) that contains an advanced third-order turbulence closure to explicitly simulate cloud physical processes in every grid column of a general circulation model. The model accurately reproduces the marine stratocumulus spatial extent and cloud cover. The mean cloud cover spatial variability in the model is primarily explained by the boundary layer decoupling strength, whereas a boundary layer ...

Abstract: This study investigates the climatology of vertical distributions of cloud liquid water content, ice water content, and cloud fraction (CFR) associated with eight different cloud types, by utilizing the combined CloudSat radar and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations lidar measurements. The geographical and seasonal variations of these cloud properties for each cloud type are also analyzed. The cloud water content (CWC) of each cloud type is sorted by three parameters obtained from colocated satellite observations to investigate the relationships between large-scale conditions and the vertical structure of clouds. Results show that different cloud types have different altitudes of CWC and CFR peaks, and the altitude of CFR peak does not always overlap with that of CWC peak. Each type of cloud shows a clear asymmetric pattern of spatial distribution between Northern Hemisphere (NH) and Southern Hemisphere (SH). Stratocumulus and stratus clouds make the greatest contribution to the liquid water path, while the ice water path is mostly contributed by deep convective cloud over the tropics and nimbostratus over the middle and high latitudes. Over both middle and high latitudes, clouds have larger seasonal variation in the NH than in the SH. Over ocean, large CWCs of deep convective cloud, cirrus, and altostratus are above 7 km, and are associated with high convective available potential energy (>2000 J/kg), warm sea surface temperature (>303 K), and relatively high precipitation (>1 mm/h). Over land, most of the middle and high clouds have similar CWC distributions compared to those over ocean, but altocumulus and low clouds are quite different from those over ocean.

Abstract: The uncertainties of the Atmospheric Infrared Sounder (AIRS) Level 2 version 6 specific humidity (q) and temperature (T) retrievals are quantified as functions of cloud types by comparison against Integrated Global Radiosonde Archive radiosonde measurements. The cloud types contained in an AIRS/Advanced Microwave Sounding Unit footprint are identified by collocated Moderate Resolution Imaging Spectroradiometer retrieved cloud optical depth (COD) and cloud top pressure. We also report results of similar validation of q and T from European Centre for Medium-Range Weather Forecasts (ECMWF) forecasts (EC) and retrievals from the AIRS Neural Network (NNW), which are used as the initial state for AIRS V6 physical retrievals. Differences caused by the variation in the measurement locations and times are estimated using EC, and all the comparisons of data sets against radiosonde measurements are corrected by these estimated differences. We report in detail the validation results for AIRS GOOD quality control, which is used for the AIRS Level 3 climate products. AIRS GOOD quality q reduces the dry biases inherited from the NNW in the middle troposphere under thin clouds but enhances dry biases in thick clouds throughout the troposphere (reaching −30% at 850 hPa near deep convective clouds), likely because the information contained in AIRS retrievals is obtained in cloud-cleared areas or above clouds within the field of regard. EC has small moist biases (~5–10%), which are within the uncertainty of radiosonde measurements, in thin and high clouds. Temperature biases of all data are within ±1 K at altitudes above the 700 hPa level but increase with decreasing altitude. Cloud-cleared retrievals lead to large AIRS cold biases (reaching about −2 K) in the lower troposphere for large COD, enhancing the cold biases inherited from the NNW. Consequently, AIRS GOOD quality T root-mean-squared errors (RMSEs) are slightly smaller than the NNW errors in thin clouds (1.5–2.5 K) but slightly larger than the NNW errors for thick COD (reaching 3.5 K near the surface). The AIRS BEST quality control retains retrievals with uncertainties closer to those of the NNW. The AIRS error estimates reported in the L2 product tend to underestimate the precision (RMSE) implied by comparisons to the radiosonde measurements and do not reflect the observed cloud dependency of uncertainties.

Abstract: The macrophysical and optical properties of clouds over East Asia (18°N–54°N, 73°E–145°E) from 1 March 2007 to 28 February 2015 are investigated using Cloud-Aerosol Lidar with Orthogonal Polarization data. Data analysis determines the macrophysical properties, such as cloud fraction, cloud vertical structure, cloud top height (CTH), cloud base height, and cloud geometrical depth (CGD), as well as the optical properties of clouds. Statistical analysis shows that the annual cloud fractions of single-layer (SL), multilayer (ML), and total clouds over East Asia are 41.4 ± 0.7%, 25.1 ± 0.9%, and 66.5 ± 1.6%, respectively, with a slight interannual variation. The maximum annual cloud fraction that appeared over the Sichuan Basin is mainly attributed to unique occlusive topographic features. Moreover, the annual vertical distribution of cloud occurrence frequency over East Asia presents a multipeak structure. Furthermore, at a height below 2 km, cloud frequency distribution exhibits a large peak over the south, north, northeast, eastern sea, and East Asia, a small peak over the northwest, and the smallest peak over Tibet, which is mainly ascribed to terrain topographies. For the average uppermost CTH and cloud fraction, the same seasonal characteristic is demonstrated; that is, CTH and cloud fraction are highest in summer and lowest in winter, except in the northwest. This seasonal characteristic mainly results from the East Asian summer monsoon circulation. Overall, the annual cloud optical depths (CODs) of SL, ML, and total cloud over East Asia are 0.98 ± 0.02, 0.83 ± 0.09, and 1.81 ± 0.12, respectively. Moreover, the COD of each layer is mainly below 0.5 (52.3%), and the second peak of probability (10.4%) exists from 2.5 to 3.0. The two crests of probability are caused by clouds of different types. Overall, the annual cloud layer over East Asia mainly consists of cirrus (44.4%), which indicates that cirrus clouds play a leading role. Most geometrically thick clouds (CGD > 2 km) are cirrus and deep convective clouds. In general, annual CGD decreases with the increase in the number of ML cloud system layers, and CGD increases with the increase in altitude, whereas the COD of each layer exhibits a reverse trend.

Abstract: To support the MEGHA-Tropiques space mission, cloud mask and cloud type classification are needed at high spatial and time resolutions over the tropical belt for water vapour and precipitation analysis. For this purpose, visible and infrared radiance data from geostationary satellites (GEO) are used with a single algorithm initially developed by SAFNWC (Satellite-Application-Facility-for-Nowcasting) for Meteosat-Second-Generation. This algorithm has been adapted by SAFNWC to the spectral characteristics and field of view of each satellite. Retrieved cloud cover characteristics (cloud mask, classification and top pressure) are evaluated over four months in summer of 2009 against CALIOP lidar observations from the CALIPSO polar-orbiting satellite. To better identify atmospheric and instrumental issues, separate analyses are performed over land and ocean, for 0130 AM and 0130 PM CALIPSO overpasses and for each GEO. Both mean cloud cover occurrence and instantaneous cloud cover statistics are compared. We found that each classification has specific features, which depend on observed cloud regimes and instrument capabilities. Most important, a common behaviour of the GEOs against CALIOP depending on cloud types is observed. We found that GEO cloud occurrence is lower by about 10% than CALIOP, with the largest biases over land during daytime and the smallest over ocean during daytime. Further detailed analysis reveals specific discrepancies in the retrieved cloud types. As expected, high-level clouds are detected more frequently by the lidar. We show that, over ocean when the optical thickness of detected high-level clouds is limited to greater than 0.1 in the comparisons, multi-spectral radiometry performs very similarly. However, the most significant difference is attributed to non-detection of low-level clouds that are often broken, which causes a reduction of up to 20% in low-level cloud fraction and even 30% in some regions. Other significant differences are seen over land, where mid-level clouds are not detected or are misclassified.

Abstract: Using 13 yr of satellite observations for the Tibetan Plateau, the sensitivities (or partial derivatives) of daytime surface downward shortwave and longwave fluxes with respect to changes in cloud cover and cloud optical thickness are investigated and quantified. Coincident cloud and surface flux retrievals from the NASA Moderate Resolution Imaging Spectroradiometer and the Clouds and the Earth’s Radiant Energy System, respectively, as well as ground-based observations at 11 stations across the plateau are used to examine the spatial and seasonal variability of this sensitivity over the entire plateau. The downward shortwave flux is found to be modulated primarily by changes in cloud cover, but changes in optical thickness also have an impact, as revealed by a multiple regression fit. The coefficient of determination of the regression increases by more than 15% when optical thickness is added. There is significant seasonal and regional variability in the cloud radiative impact. On average, at all ...

Abstract: Simultaneous ship track observations from Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) have been compiled to investigate how ship-injected aerosols affect marine warm boundary layer clouds for different cloud types and environmental conditions By taking advantage of the high spatial resolution multiangle observations available from MISR, we utilized the retrieved cloud albedo, cloud top height, and cloud motion vectors to examine cloud property responses in ship-polluted and nearby unpolluted clouds The strength of the cloud albedo response to increased aerosol level is primarily dependent on cloud cell structure, dryness of the free troposphere, and boundary layer depth, corroborating a previous study by Chen et al (2012) where A-Train satellite data were utilized Under open cell cloud structure the cloud properties are more susceptible to aerosol perturbations as compared to closed cells Aerosol plumes caused an increase in liquid water amount (+38%), cloud top height (+13%), and cloud albedo (+49%) for open cell clouds, whereas for closed cell clouds, little change in cloud properties was observed Further capitalizing on MISR's unique capabilities, the MISR cross-track cloud speed was used to derive cloud top divergence Statistically averaging the results from the identified plume segments to reduce random noise, we found evidence of cloud top divergence in the ship-polluted clouds, whereas the nearby unpolluted clouds showed cloud top convergence, providing observational evidence of a change in local mesoscale circulation associated with enhanced aerosols Furthermore, open cell polluted clouds revealed stronger cloud top divergence as compared to closed cell clouds, consistent with different dynamical mechanisms driving their responses These results suggest that detailed cloud responses, classified by cloud type and environmental conditions, must be accounted for in global climate modeling studies to reduce uncertainties in calculations of aerosol indirect forcing

Abstract: In this study, aerosols spatial, seasonal and temporal variations over Sindh, Pakistan were analyzed which can lead to variations in the microphysics of clouds as well. All cloud optical properties were analyzed using Moderate Resolution Imaging Spectroradiometer (MODIS) data for 12 years from 2001 to 2013. We also monitored origin and movements of air masses that bring aerosol particles and may be considered as the natural source of aerosol particles in the region. For this purpose, the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to make trajectories of these air masses from their sources. Aerosol optical depth (AOD) high values were observed in summer during the monsoon period (June-August). The highest AOD values in July were recorded ranges from 0.41 to 1.46. In addition, low AOD values were found in winter season (December-February) particularly in December, ranges from 0.16 to 0.69. We then analyzed the relationship between AOD and Angstrom exponent that is a good indicator of the size of an aerosol particle. We further described the relationships of AOD and four cloud parameters, namely water vapor (WV), cloud fraction (CF), cloud top temperature (CTT) and cloud top pressure (CTP) by producing regional correlation maps of their data values. The analyses showed negative correlation between AOD and Angstrom exponent especially in central and western Sindh. The correlation between AOD and WV was throughout positive with high correlation values ＞ 0.74 in whole Sindh except eastern most arid strip of the Thar Desert in the region. The correlation between AOD and CF was positive in southern Sindh and goes to negative in northern Sindh. AOD showed a positive correlation with CTP and CTT in northern Sindh and a negative correlation in southern Sindh. All these correlations were observed to be dependent on the meteorological conditions for all of the ten sites investigated.

Abstract: . Multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations of trace gases can be strongly influenced by clouds and aerosols. Thus it is important to identify clouds and characterize their properties. In a recent study Wagner et al. (2014) developed a cloud classification scheme based on the MAX-DOAS measurements themselves with which different "sky conditions" (e.g., clear sky, continuous clouds, broken clouds) can be distinguished. Here we apply this scheme to long-term MAX-DOAS measurements from 2011 to 2013 in Wuxi, China (31.57° N, 120.31° E). The original algorithm has been adapted to the characteristics of the Wuxi instrument, and extended towards smaller solar zenith angles (SZA). Moreover, a method for the determination and correction of instrumental degradation is developed to avoid artificial trends of the cloud classification results. We compared the results of the MAX-DOAS cloud classification scheme to several independent measurements: aerosol optical depth from a nearby Aerosol Robotic Network (AERONET) station and from two Moderate Resolution Imaging Spectroradiometer (MODIS) instruments, visibility derived from a visibility meter and various cloud parameters from different satellite instruments (MODIS, the Ozone Monitoring Instrument (OMI) and the Global Ozone Monitoring Experiment (GOME-2)). Here it should be noted that no quantitative comparison between the MAX-DOAS results and the independent data sets is possible, because (a) not exactly the same quantities are measured, and (b) the spatial and temporal sampling is quite different. Thus our comparison is performed in a semi-quantitative way: the MAX-DOAS cloud classification results are studied as a function of the external quantities. The most important findings from these comparisons are as follows: (1) most cases characterized as clear sky with low or high aerosol load were associated with the respective aerosol optical depth (AOD) ranges obtained by AERONET and MODIS; (2) the observed dependences of MAX-DOAS results on cloud optical thickness and effective cloud fraction from satellite confirm that the MAX-DOAS cloud classification scheme is sensitive to cloud (optical) properties; (3) the separation of cloudy scenes by cloud pressure shows that the MAX-DOAS cloud classification scheme is also capable of detecting high clouds; (4) for some cloud-free conditions, especially with high aerosol load, the coincident satellite observations indicated optically thin and low clouds. This finding indicates that the satellite cloud products contain valuable information on aerosols.

Abstract: Vertical sounding measurements within stratocumuli during two aircraft field campaigns, Marine Stratus/stratocumulus Experiment (MASE) and Physics of Stratocumulus Top (POST), are used to validate Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical thickness (COT), cloud liquid water path (LWP), and cloud effective radius (re). In situ COT, LWP, and re were calculated using 5 m vertically averaged droplet probe measurements of complete vertical cloud penetrations. MODIS COT, LWP, and re 1 km pixels were averaged along these penetrations. COT comparisons in POST showed strong correlations and a near 1:1 relationship. In MASE, comparisons showed strong correlations; however, MODIS COT exceeded in situ COT, likely due to larger temporal differences between MODIS and in situ measurements. LWP comparisons between two cloud probes show good agreement for POST but not MASE, giving confidence to POST data. Both projects provided strong LWP correlations but MODIS exceeded in situ by 14-36%. MODIS in situ re correlations were strong, but MODIS 2.1 µm re exceeded in situ re, which contributed to LWP bias; in POST, MODIS re was 20-30% greater than in situ re. Maximum in situ re near cloud top showed comparisons nearer 1:1. Other MODIS re bands (3.7 µm and 1.6 µm) showed similar comparisons. Temporal differences between MODIS and in situ measurements, airplane speed differences, and cloud probe artifacts were likely causes of weaker MASE correlations. POST COT comparison was best for temporal differences under 20 min. POST data validate MODIS COT but it also implies a positive MODIS re bias that propagates to LWP while still capturing variability.

Abstract: Surface icing can cause dramatic consequences on human activities. What is more, numerical weather prediction models are not very accurate in determining freezing drizzle, which creates uncertainty when forecasting this type of weather phenomenon. Therefore, it is essential to improve the forecast accuracy of these models for such phenomena to mitigate risks caused by unforeseen freezing drizzle events. On 5 February 2012, an episode of freezing drizzle took place in the Guadarrama Mountains, at the center of the Iberian Peninsula. This episode was preceded by weak snowfall. After the freezing drizzle, moderate snowfall was recorded in the study area. This event was simulated using the Weather Research and Forecasting model. Through this analysis, we identified the meteorological factors at both synoptic scale and mesoscale that caused this episode. Wind perpendicular to an orographic barrier-generated updrafts and retention of moisture upwind, which caused orographic clouds to appear on the north side of the Guadarrama Mountains. Atmospheric stability prevented cloud formation at midlevels at the time of the freezing drizzle, which maintained cloud top temperatures warmer than −15°C during the episode. The entrance of moisture and instability at midlevels caused cloud top temperatures substantially colder than −15°C, which coincided with snow in the mountain range. Cloud top temperature and thickness control the efficiency of the glaciation process, thereby determining the type of precipitation at the surface. Freezing drizzle risk and in-cloud icing algorithms were developed with the aim of predicting similar events in the study area, which could mitigate impacts on human activities.

Abstract: . Broadband solar and terrestrial irradiance measurements of high temporal resolution are needed to study inhomogeneous clouds or surfaces and to derive vertical profiles of heating/cooling rates at cloud top. An efficient method to enhance the temporal resolution of slow-response measurements of broadband terrestrial irradiance using pyrgeometer is introduced. It is based on the deconvolution theorem of Fourier transform to restore amplitude and phase shift of high frequent fluctuations. It is shown that the quality of reconstruction depends on the instrument noise, the pyrgeometer response time and the frequency of the oscillations. The method is tested in laboratory measurements for synthetic time series including a boxcar function and periodic oscillations using a CGR-4 pyrgeometer with response time of 3 s. The originally slow-response pyrgeometer data were reconstructed to higher resolution and compared to the predefined synthetic time series. The reconstruction of the time series worked up to oscillations of 0.5 Hz frequency and 2 W m−2 amplitude if the sampling frequency of the data acquisition is 16 kHz or higher. For oscillations faster than 2 Hz, the instrument noise exceeded the reduced amplitude of the oscillations in the measurements and the reconstruction failed. The method was applied to airborne measurements of upward terrestrial irradiance from the VERDI (Vertical Distribution of Ice in Arctic Clouds) field campaign. Pyrgeometer data above open leads in sea ice and a broken cloud field were reconstructed and compared to KT19 infrared thermometer data. The reconstruction of amplitude and phase shift of the deconvoluted data improved the agreement with the KT19 data. Cloud top temperatures were improved by up to 1 K above broken clouds of 80–800 m size (1–10 s flight time) while an underestimation of 2.5 W m−2 was found for the upward irradiance over small leads of about 600 m diameter (10 s flight time) when using the slow-response data. The limitations of the method with respect to instrument noise and digitalization of measurements by the data acquisition are discussed.

Abstract: The high resolution (375 m) of the Visible Infrared Imaging Radiometer Suite on board the Suomi National Polar-Orbiting Partnership satellite allows retrieving relatively accurately the vertical evolution of convective cloud drop effective radius (re) with height or temperature. A tight relationship is found over SE Asia and the adjacent seas during summer between the cloud-free aerosol optical depth (AOD) and the cloud thickness required for the initiation of warm rain, as represented by the satellite-retrieved cloud droplet re of 14 µm, for a subset of conditions that minimize measurement artifacts. This cloud depth (ΔT14) is parameterized as the difference between the cloud base temperature and the temperature at the height where re exceeds 14 µm (T14). For a unit increase of AOD, the height of rain initiation is increased by about 5.5 km. The concern of data artifacts due to the increase in AOD near clouds was mitigated by selecting only scenes with cloud fraction (CF) 0.1 and ΔT14 > ~20°C, the increase of ΔT14 gradually levels off with further increase of AOD, possibly because the AOD is enhanced by aerosol upward transport and detrainment through the clouds below the T14 isotherm. The bias in the retrieved re due to the different geometries of solar illumination was also quantified. It was shown that the retrievals are valid only for backscatter views or when avoiding scenes with significant amount of cloud self-shadowing. These artifacts might have contributed to past reported relationships between cloud properties and AOD.

Abstract: The Extreme Universe Space Observatory on the Japanese Experiment Module (JEM-EUSO) on board the International Space Station (ISS) is the first space-based mission worldwide in the field of Ultra High-Energy Cosmic Rays (UHECR). For UHECR experiments, the atmosphere is not only the showering calorimeter for the primary cosmic rays, it is an essential part of the readout system, as well. Moreover, the atmosphere must be calibrated and has to be considered as input for the analysis of the fluorescence signals. Therefore, the JEM-EUSO Space Observatory is implementing an Atmospheric Monitoring System (AMS) that will include an IR-Camera and a LIDAR. The AMS Infrared Camera is an infrared, wide FoV, imaging system designed to provide the cloud coverage along the JEM-EUSO track and the cloud top height to properly achieve the UHECR reconstruction in cloudy conditions. In this paper, an updated preliminary design status, the results from the calibration tests of the first prototype, the simulation of the instrument, and preliminary cloud top height retrieval algorithms are presented.

Abstract: A new methodology is developed to detect the cloud structures at different vertical levels using the dual oxygen absorption bands located near 60 GHz and 118 GHz, respectively. Observations from Microwave Temperature Sounder (MWTS) and Microwave Humidity Sounder (MWHS) on board the recently launched Chinese FengYun-3C satellite are used to prove the concept. It is shown that a paired oxygen MWTS and MWHS sounding channel with the same peak weighting function altitude allows for detecting the vertically integrated cloud water path above that level. A cloud emission and scattering index (CESI) is defined using dual oxygen band measurements to indicate the amounts of cloud liquid and ice water paths. The CESI distributions from three paired channels reveal unique three-dimensional structures of clouds and precipitation within Super Typhoon Neoguri that occurred in July 2014.

Abstract: The ability of six microphysical parameterizations included in the Weather Research and Forecasting (WRF) numerical weather prediction (NWP) model to represent various macroscopic cloud characteristics at multiple spatial and temporal resolutions is investigated. In particular, the model prediction skills of cloud occurrence, cloud base height, and cloud cover are assessed. When it is possible, the results are provided separately for low-, middle-, and high-level clouds. The microphysical parameterizations assessed are WRF single-moment six-class, Thompson, Milbrandt-Yau, Morrison, Stony Brook University, and National Severe Storms Laboratory double moment. The evaluated macroscopic cloud properties are determined based on the model cloud fractions. Two cloud fraction approaches, namely, a binary cloud fraction and a continuous cloud fraction, are investigated. Model cloud cover is determined by overlapping the vertically distributed cloud fractions following three different strategies. The evaluation is conducted based on observations gathered with a ceilometer and a sky camera located in Jaen (southern Spain). The results prove that the reliability of the WRF model mostly depends on the considered cloud parameter, cloud level, and spatiotemporal resolution. In our test bed, it is found that WRF model tends to (i) overpredict the occurrence of high-level clouds irrespectively of the spatial resolution, (ii) underestimate the cloud base height, and (iii) overestimate the cloud cover. Overall, the best cloud estimates are found for finer spatial resolutions (1.3 and 4 km with slight differences between them) and coarser temporal resolutions. The roles of the parameterization choice of the microphysics scheme and the cloud overlapping strategy are, in general, less relevant.

Abstract: Operational nowcasting techniques for sea fog over the Yellow Sea rely on data from weather satellites because ground-based observations are hardly available. While there are several algorithms for detecting low stratus (LST) that are applicable to geostationary weather satellite data, sea fog retrieval is more complicated. These schemes mostly need ancillary data such as Cloud Optical Thickness (COT) and Droplet Effective Radius (DER). To retrieve the necessary parameters for sea fog detection over the Yellow Sea, the Comprehensive Analysis Program for Cloud Optical Measurement (CAPCOM) scheme developed by Kawamoto et al. (2001) was adapted to the Japanese Multifunctional Transport Satellites (MTSAT) system-Japanese Advanced Meteorological Imager (JAMI). COT and DER values were then retrieved for 64 cases over the Yellow Sea (= 85,000 LST pixels) and compared with the COT and DER products from the MYD06/MOD06, CAPCOM-MODIS (Moderate Resolution Imaging Spectroradiometer) and CloudSat (cloud radar). Results showed that the COT and DER values retrieved from JAMI were satisfactory. The MTSAT-2 JAMI data delivered better COT values than the MTSAT-1R JAMI data, due to the re-calibration of MTSAT-2 JAMI’s visible (VIS) band in 2011. Similarly, improvements were seen in DER retrieval, even though the VIS re-calibration primarily affects COT retrieval. By comparing the difference in stratus thickness calculated by MTSAT-1R and MTSAT-2, the COT and DER retrieved from MTSAT-2 JAMI can be used in ground fog retrieval schemes. These values exhibit less bias, especially in cases involving high cloud top and thin cloud thickness. Both the COT and DER retrievals from MTSAT-2 JAMI offer potential as reliable parameters for Yellow Sea fog detection.