Optical Properties Of Cirrus Clouds Research Articles

Characteristics and seasonal variations of cirrus clouds from ground-based lidar and satellite observations over Shouxian Area, China

This study investigates the macroscopic and optical properties of cirrus clouds in the 32N region from July 2016 to May 2017, leveraging data from ground-based lidar observations and CALIOP to overcome the inconsistencies in detected cirrus cloud samples. Through extensive data analysis, statistical characteristics of cirrus clouds were discerned, revealing lidar ratio values of 28.5 ± 10.8 from ground-based lidar and 27.4 ± 11.2 from CALIOP. Validation with a decade of CALIOP data (2008-2018) confirmed these findings, presenting a consistent lidar ratio of 27.4 ± 12.0. A significant outcome of the analysis was the identification of a positive correlation between the lidar ratio and cloud centroid temperature, indicating a gradual decrease in the lidar ratio as temperatures dropped. The study established a fundamental consistency in their macroscopic properties, including cloud base height, cloud top height, cloud thickness, cloud centroid height, and cloud centroid temperature. The results for ground-based lidar (CALIOP) are: 10.0 ± 2.1 km (10.0 ± 2.2 km), 11.8 ± 2.1 km (11.5 ± 2.3 km), 1.87 ± 0.83 km (1.52 ± 0.71 km), and 10.5 ± 2.2 km, -46.9 ± 9.7°C (-47.1 ± 10.0°C).These properties exhibited seasonal variations, with cirrus clouds reaching higher altitudes in summer and lower in winter, influenced by the height of the tropopause. The optical properties of cirrus clouds were also analyzed, showing an annual average optical depth of 0.31 ± 0.35 for ground-based lidar and 0.32 ± 0.44 for CALIOP. The study highlighted the distribution of subvisible, thin, and thick cirrus clouds, with a notable prevalence of subvisible clouds during summer, suggesting their frequent formation above 14 km. Furthermore, the study observed linear growth in geometric thickness and optical depth up to 2.5 km from CALIOP and 2.9 km from ground-based lidar. Maximum optical depth was observed at cloud centroid temperatures of -35°C for CALIOP and -40°C for ground-based lidar, with optical depth decreasing as temperatures fell. This suggests that fully glaciated cirrus clouds exhibit the highest optical depth at warmer temperatures, within the complete glaciation temperature range of -35°C to -40°C.

Geometrical and optical properties of cirrus clouds in Barcelona, Spain: analysis with the two-way transmittance method of 4 years of lidar measurements

Abstract. In this paper a statistical study of cirrus geometrical and optical properties based on 4 years of continuous ground-based lidar measurements with the Barcelona (Spain) Micro Pulse Lidar (MPL) is analysed. First, a review of the literature on the two-way transmittance method is presented. This method is a well-known lidar inversion method used to retrieve the optical properties of an aerosol–cloud layer between two molecular (i.e. aerosol and cloud-free) regions below and above, without the need to make any a priori assumptions about their optical and/or microphysical properties. Second, a simple mathematical expression of the two-way transmittance method is proposed for both ground-based and spaceborne lidar systems. This approach of the method allows the retrieval of the cloud optical depth, the cloud column lidar ratio and the vertical profile of the cloud backscatter coefficient. The method is illustrated for a cirrus cloud using measurements from the ground-based MPL and from the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Third, the database is then filtered with a cirrus identification criterion based on (and compared to) the literature using only lidar and radiosonde data. During the period from November 2018 to September 2022, 367 high-altitude cirrus clouds were identified at 00:00 and 12:00 UTC, of which 203 were successfully inverted with the two-way transmittance method. The statistical results of these 203 high-altitude cirrus clouds show that the cloud thickness is 1.8 ± 1.1 km, the mid-cloud temperature is −51 ± 8 ∘C and the linear cloud depolarization ratio is 0.32 ± 0.13. The application of the transmittance method yields an average cloud optical depth (COD) of 0.36 ± 0.45 and a mean effective column lidar ratio of 30 ± 19 sr. Statistical results of the errors associated with the two-way transmittance method retrievals are also provided. The highest occurrence of cirrus is observed in spring and the majority of cirrus clouds (48 %) are visible (0.03 < COD < 0.3), followed by opaque (COD > 0.3) with a percentage of 38 %. Together with results from other sites, possible latitudinal dependencies have been analysed together with correlations between cirrus cloud properties. For example, we noted that in Barcelona the COD correlates positively with the cloud base temperature, effective column lidar ratio and linear cloud depolarization ratio and negatively with the cloud base height.

Backscattering properties of randomly oriented hexagonal hollow columns for lidar application.

The study of the optical properties of cirrus clouds is necessary to improve the accuracy of interpreting data from space lidars and ground-based lidar networks. Existing databases of backscattering properties do not include data on hollow columns. In this paper, the backscattering properties of randomly oriented hollow column ice crystal particles in cirrus at wavelengths of 355 nm, 532 nm, and 1064 nm have been investigated. The backscattering cross section (M11), depolarization ratio (δ), lidar ratio (S), and color ratio (χ) of randomly oriented hollow columns with sizes ranging from 10-316.23 µm are calculated within the framework of the physical optical approximation (PO). For the first time, we introduce the concept of modal hollow columns (MHC) suitable for mid-latitude regions, which simplifies the description of hollow ice crystals in nature. It is found that when the mixing proportion of MHC and solid columns (SC) reaches 50%, the mixing ratio can be distinguished by the lidar ratio for a wavelength of 1064 nm and by the χ(1064,532)-δ(532) relation.

Lidar Observed Optical Properties of Tropical Cirrus Clouds Over Gadanki Region

The presence of cirrus cloud has its impact on the earth’s radiation budget. In order to study the effect of cirrus clouds in the tropical regions, it is essential to understand and characterize their optical properties. The optical properties of high altitude cirrus clouds are obtained using the polarization diversity ground based Mie lidar instrument at a tropical latitude station in the Indian subcontinent. Lidar measurements are taken for one year (2013) at National Atmospheric Research Laboratory (NARL), located at Gadanki (13.5° North, 79.2° East; 375m AMSL), India and are used for the present investigation. Altitude variations of optical depth and depolarization ratio are discussed. In the altitude range of 10–17 km, the range of the optical depth and depolarization ratio of cirrus cloud was found to be 0.01–0.4 and 0.1–0.4 respectively. The interdependence of optical depth as a function of depolarization ratio is analyzed and a positive correlation is observed (0.3950). From the measured optical depth values, it is categorized that 8%, 77% and 14% of the cirrus clouds are sub-visual, thin and thick clouds. The monthly and seasonal variations of optical properties of cirrus clouds were analyzed. Summary of cirrus cloud layer statistics and the statistical variation (seasonal) of the optical properties of cirrus clouds is presented for the period of study.

Variability in cirrus cloud properties using a Polly<sup>XT</sup> Raman lidar over high and tropical latitudes

Abstract. Measurements of geometrical and optical properties of cirrus clouds, performed with a multi-wavelength PollyXT Raman lidar during the period 2008 to 2016, are analysed. The measurements were performed with the same instrument, during sequential periods, in three places at different latitudes, Gwal Pahari (28.43∘ N, 77.15∘ E; 243 m a.s.l.) in India, Elandsfontein (26.25∘ S, 29.43∘ E; 1745 m a.s.l.) in South Africa and Kuopio (62.74∘ N, 27.54∘ E; 190 m a.s.l.) in Finland. The lidar dataset was processed by an automatic cirrus cloud masking algorithm, developed in the frame of this work. In the following, we present a statistical analysis of the lidar-retrieved geometrical characteristics (cloud boundaries, geometrical thickness) and optical properties of cirrus clouds (cloud optical depth, lidar ratio, ice crystal depolarisation ratio) measured over the three areas that correspond to subtropical and subarctic regions as well as their seasonal variability. The effect of multiple scattering from ice particles to the derived optical products is also considered and corrected in this study. Our results show that cirrus layers, which have a noticeable monthly variability, were observed between 6.5 and 13 km, with temperatures ranging from −72 to −27 ∘C. The observed differences on cirrus clouds' geometrical and optical properties over the three regions are discussed in terms of latitudinal and temperature dependence. The latitudinal dependence of the geometrical properties is consistent with satellite observations, following the pattern observed with CloudSat, with decreasing values towards the poles. The geometrical boundaries have their highest values in the subtropical regions, and overall, our results seem to demonstrate that subarctic cirrus clouds are colder, lower and optically thinner than subtropical cirrus clouds. The dependence of cirrus cloud geometrical thickness and optical properties on mid-cirrus temperatures shows a quite similar tendency for the three sites but less variability for the subarctic dataset. Cirrus clouds are geometrically and optically thicker at temperatures between −45 and −35 ∘C, and a second peak is observed at lower temperatures ∼-70 ∘C for the subarctic site. Lidar ratio values also exhibit a pattern, showing higher values moving toward the poles, with higher mean values observed over the subarctic site. The dependency of the mid-cirrus temperatures on the lidar ratio values and the particle depolarisation values is further examined. Our study shows that the highest values of the cirrus lidar ratio correspond to higher values of cirrus depolarisation and warmer cirrus. The kind of information presented here can be rather useful in the cirrus parameterisations required as input to radiative transfer models and can be a complementary tool for satellite products that cannot provide cloud vertical structure. In addition, ground-based statistics of the cirrus properties could be useful in the validation and improvement of the corresponding derived products from satellite retrievals.

The Depolarization Properties of Cirrus Cloud by Lidar: Observation and Model Match

The Polarization Lidar has been developed for cirrus cloud optical properties measurement. The Nd-YAG laser is employed as emitting linear source and two channels are used for detecting back-scattering signals from cirrus cloud including two 532 nm Orthogonal Polarization channels. Measurements were carried out from December 2010, to February 2013. In this work, we consider the only linear depolarization ratio measured at a wavelength of 532 nm. The dependences of depolarization ratios on both the size of ice crystals and the distribution of their distortion angles were also calculated. These results were compared with each other and used for retrieving the microphysical parameters of cirrus clouds observed in Hefei.

Optical and geometrical properties of cirrus clouds from lidar measurements above Hanoi, Vietnam

We report the results of our long-term monitoring study of cirrus clouds using our locally developed lidar sytem located in Hanoi, Vietnam (21.03°N, 105.85°E). Our data, spanning the period from 2011 to 2017, show that the cirrus clouds above Hanoi are generally found between 6.7 and 18.0 km with a mean height of 14.0 km. The cloud thickness is estimated to vary between 0.2 and 7.0 km with an average thickness of 2.3 ± 1.2 km. We also derive an average optical depth of 0.32 ± 0.22 and an average lidar ratio 30 ± 18. We also compare the geometrical and optical properties of cirrus clouds above Hanoi, where until now lidar study is still lacking, with that obtained for other locations in Asia and around the world.

A Statistical Method for Determining Optical and Geometrical Characteristics of Cirrus Clouds

Altitude occurrences of cirrus clouds, their base, mid, and top heights are identified by using a powerful statistical approach called the Variance Centroid Method (VCM). This method is effective in determining the geometrical properties of cirrus clouds at a height range of 8 km to 20 km. This statistical method provides in-depth information on Generation Circulation Models (GCM’s) because of its significant role in the radiative balance of the earth’s atmosphere. The output related to geometrical and optical properties of cirrus clouds obtained from this statistical method for the years 2014 and 2015 are studied. Altitude distributions of base, mid, top, and thickness of tropical cirrus clouds are analyzed in terms of percentage occurrences. For the year 2014, it was found that 14.8%, 75.2%, and 9.9% of cirrus clouds were sub-visual, thin, and thick clouds and for the year 2015, 71.2% and 28.7% of cirrus clouds were found to be thin and thick clouds, and sub-visual types of clouds were not observed. The interdependence of optical depth with the depolarization ratio is discussed and it is observed that the correlation is negative (−0.0303) for the year 2014 and positive (0.1311) for the year 2015. High optical depths with values from 0.502 to 0.849 are observed in the height regions from 9 km to 15 km (for 2014) and for the year 2015, the observed value ranges from 0.514 to 0.822 for the height regions from 9 km to 11 km. Statistical variation of characteristics of tropical cirrus clouds is presented for the period of study. The characterization of these cirrus clouds is highly useful for climate modeling studies, and their impact plays a vital role in the Earth’s radiation budget at the top of the atmosphere.

Additional global climate cooling by clouds due to ice crystal complexity

Abstract. Ice crystal submicron structures have a large impact on the optical properties of cirrus clouds and consequently on their radiative effect. Although there is growing evidence that atmospheric ice crystals are rarely pristine, direct in situ observations of the degree of ice crystal complexity are largely missing. Here we show a comprehensive in situ data set of ice crystal complexity coupled with measurements of the cloud angular scattering functions collected during a number of observational airborne campaigns at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61 % and 81 %) of atmospheric ice crystals sampled in the different regions contain mesoscopic deformations and, as a consequence, a similar flat and featureless angular scattering function is observed. A comparison between the measurements and a database of optical particle properties showed that severely roughened hexagonal aggregates optimally represent the measurements in the observed angular range. Based on this optical model, a new parameterization of the cloud bulk asymmetry factor was introduced and its effects were tested in a global climate model. The modelling results suggest that, due to ice crystal complexity, ice-containing clouds can induce an additional short-wave cooling effect of −1.12 W m2 on the top-of-the-atmosphere radiative budget that has not yet been considered.

Lidar Studies on The Optical Characteristics of High Altitude Cirrus Clouds at A Low Latitiude Station, Gadanki (13.5°N , 79.2°E ) India

Abstract. Light Detection and Ranging (LIDAR) which is analogous to Radio Detection And Ranging (RADAR), has become an important and unique technology for atmospheric research and applications. The technology is widely used for the remote sensing of the Earth’s atmosphere, oceans, vegetation and the characteristics of Earth’s topography. Remote sensing of atmosphere, for its structure, composition and dynamics, is one of the proven applications of the lidar systems. More importantly the lidar technique is applied for the study of clouds, aerosols and minor constituents in the atmosphere. It provides the information on the above with good spatial and temporal resolutions and accuracy. The high altitude cirrus clouds which play an important role in the Earth’s radiative budget and global climate can be studied by using the LIDAR system. These clouds absorb long-wave outgoing radiation from Earth’s surface while reflecting part of the incoming short-wave solar radiation. Lidar measurements are useful in deriving the altitude, top height, bottom height and the optical properties of cirrus clouds, which are essential in understanding the cloud-radiation effects. The optical depth, the effective lidar ratio and the depolarization of the clouds are also derived by inverting the lidar signals from the cirrus clouds. In this paper we present the results on the lidar derived optical and microphysical properties of the cirrus clouds at a tropical station Gadanki (13.5°N, 79.2°E) India during two year period from 2009 to210. The seasonal variations of the cloud properties during the observation period are presented and discussed with reference to earlier period.

Geometric and optical properties of cirrus clouds inferred from three-year ground-based lidar and CALIOP measurements over Seoul, Korea

This study examines cirrus cloud top and bottom heights (CTH and CBH, respectively) and the associated optical properties revealed by ground-based lidar in Seoul (SNU-L), Korea, and space-borne Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP), which were obtained during a three-year measurement period between July 2006 and June 2009. From two selected cases, we determined good agreement in CTH and CBH with cirrus cloud optical depth (COD) between ground-based lidar and space-borne CALIOP. In particular, CODs at a wavelength of 532nm calculated from the three years of SNU-L and CALIOP measurements were 0.417±0.394 and 0.425±0.479, respectively. The fraction of COD lower than 0.1 was approximately 17% and 25% of the total SNU-L and CALIOP profiles, respectively, and approximately 50% of both lidar profiles were classified as sub-visual or optically thin such that COD was <0.3. The mean depolarization ratio was estimated to be 0.30±0.06 for SNU-L and 0.34±0.08 for CALIOP. The monthly variation of CODs from SNU-L and CALIOP measurements was not distinct, whereas cirrus altitudes from both SNU-L and CALIOP showed distinct monthly variation. CALIOP observations showed that cirrus clouds reached the tropopause level in all months, whereas the up-looking SNU-L did not detect cirrus clouds near the tropopause in summer due to signal attenuation by underlying optically thick clouds. The cloud layer thickness (CLT) and COD showed a distinct linear relationship up to approximately 2km of the CLT; however, the COD did not increase, but remained constant when the CLT was greater than 2.0km. The ice crystal content, lidar signal attenuation, and the presence of multi-layered cirrus clouds may have contributed to this tendency.

Deformations of frozen droplets formed at −40°C

The optical properties of cirrus clouds are frequently computed with respect to various ice‐crystal size distributions and shapes; small ice crystals are generally considered to be quasi‐spherical. This approximation can lead to significant errors in the analysis of the in‐situ microphysical data and also in the theoretical study of the radiative properties of cirrus clouds. Effective size and aspect ratio of the particles are the key parameters required to determine the optical parameters that are included in modeling and prediction of climate; their parameterizations need to be as accurate as possible. In this work, laboratory experiments were conducted to study the ice crystal shape formed from the freezing of water droplets at −40°C. Liquid and ice clouds were separately formed and sampled in a cloud chamber at −30 and −40°C, respectively. The effective water droplet diameter ranged from 8 to 20μm, while the effective ice particle diameter ranged from 8 to 30 μm. The average effective diameters were 11 μm for water drops and 14 μm for frozen droplets. The deformation of the frozen droplets was evident during inspection under microscope; bulges and spikes protuberances were found in many of the observed ice particles. The results show that the aspect ratio of the frozen droplets is 1.2.

Tropical cirrus cloud contamination in sun photometer data

Abstract Cirrus clouds are endemic to Southeast Asia and are a source of potential bias in regional passive aerosol remote sensing datasets. Here, performance of the cloud-screening algorithm for the ground-based Aerosol Robotic Network (AERONET) sun photometer data is evaluated for cirrus cloud contamination at Singapore (1.30° N, 103.77° E). Using twelve months of concurrent AERONET Level 1.5 and 2.0 cloud-screened aerosol optical depth (AOD) data, and collocated Level 1.0 Micro-Pulse Lidar Network (MPLNET) measurements, we investigate the baseline AOD bias due to cirrus cloud presence. Observations are considered for a primary sample of all data and a secondary sample where AERONET data are restricted to a zenith viewing angle ≤ 45°. Cirrus clouds are present in zenith-viewing MPL profiles for 34% and 23% of these samples respectively. Based on approximations of cirrus cloud optical properties necessary to estimate cloud optical depth from the single-channel lidar signal, and assuming partial forward scattering of diffuse light by cirrus clouds into the sun photometer’s field of view, we estimate a range in AOD bias due to unscreened cloud presence of 0.034 to 0.060 and 0.031 to 0.055 ± 0.01 for the primary and secondary sample respectively. From the analysis of AERONET AOD for the angle-limited subset alone, we also derive a positive AOD bias of 0.034, which is comparable to the lower bounds for the estimated cloud bias reported for our datasets. These findings, which we attribute to the prevalence of cirrus clouds present from regional convection, are higher than previous reports of global AOD bias in the Moderate Resolution Infrared Spectroradiometer (MODIS) satellite-borne measurements due to residual cirrus cloud presence.

Investigations on the physical and optical properties of cirrus clouds and their relationship with ice nuclei concentration using LIDAR at Gadanki, India (13.5°N, 79.2°E)

Cirrus cloud measurements over the tropics are receiving much attention recently due to their role in the Earth's radiation budget. The interaction of water vapor and aerosols plays a major role in phase formation of cirrus clouds. Many factors control the ice supersaturation and microphysical properties in cirrus clouds and, as such, investigations on these properties of cirrus clouds are critical for proper understanding and simulating the climate. In this paper we report on the evolution, microphysical, and optical properties of cirrus clouds using the Mie LIDAR operation at the National Atmospheric Research Laboratory, Gadanki, India (13.5°N, 79.2°E), an inland tropical station. The occurrence statistics, height, optical depth, depolarization ratio of the cirrus clouds, and their relationship with ice nuclei concentration were investigated over 29 days of observation during the year 2002. Cirrus clouds with a base altitude as low as 8.4 km are observed during the month of January and clouds with a maximum top height of 17.1 km are observed during the month of May. The cirrus has a mean thickness of 2 km during the period of study. The LIDAR ratio varies from 30 to 36 sr during the summer days of observation and 25 to 31 sr during the winter days of observation. Depolarization values range from 0.1 to 0.58 during the period of observation. The ice nuclei concentration has been calculated using the De Motts equation. It is observed that during the monsoon months of June, July, and August, there appears to be an increase in the ice nuclei number concentration. From the depolarization data an attempt is made to derive the ice crystal orientation and their structure of the cirrus. Crystal structures such as thin plates, thick plates, regular hexagons, and hexagonal columns are observed in the study. From the observed crystal structure and ice nuclei concentration, the possible nucleation mechanism is suggested.

Optical properties of cirrus clouds at a tropical Indian station Gadanki, Tirupati (13.5°N, 79.2°E)

The optical properties of the cirrus clouds over a tropical inland station Gadanki, Tirupati were studied using a dual polarization lidar. The extinction coefficient, backscatter coefficient, optical depth and linear depolarization of the cirrus clouds are derived using the range dependent lidar ratio. This work reports the results obtained during the period of December 2006 to July 2007 which covers the three prominent seasons of the year in the Indian subcontinent. A variety of ice crystals like hexagonal thin plate, thick plate, columns, dendrites and aggregates were observed within the cloud. The geometrical and optical thicknesses of the clouds show strong seasonal variations. The occurrence frequency of thin cirrus clouds was found to be relatively high as compared to sub-visible and dense clouds. In almost all the cases, the cloud contains smaller ice crystals in the top part, larger crystals in the middle portion and mixed phase in the bottom portion. Compared to the winter and summer seasons the horizontally oriented ice crystals were observed more in monsoon period. The lidar ratio and linear depolarization ratio of the cirrus clouds were in the range of 3-40 sr and 0.1-1.5 respectively. The maximum linear depolarization ratio was observed for the clouds containing randomly oriented ice crystal with temperature below -80°C. The lidar ratio was found to be maximum for the thin plate crystals and minimum for thick clouds with horizontally oriented ice crystals. The extinction and backscattering coefficients of the clouds were in the range of 0.3x10-4 to 6 x10-4 m-1 and 0.12x10-4 to 3x10-4 m-1 sr-1 respectively during the observation period.

Orographic cirrus in a future climate

Abstract. A cloud resolving model (CRM) is used to investigate the formation of orographic cirrus clouds in the current and future climate. The formation of cirrus clouds depends on a variety of dynamical and thermodynamical processes, which act on different scales. First, the capability of the CRM in realistically simulating orographic cirrus clouds has been tested by comparing the simulated results to aircraft measurements of an orographic cirrus cloud. The influence of a warmer climate on the microphysical and optical properties of cirrus clouds has been investigated by initializing the CRM with vertical profiles of horizontal wind, potential temperature and equivalent potential temperature, respectively. The vertical profiles are extracted from IPCC A1B simulations for the current climate and for the period 2090–2099 for two regions representative for North and South America. The influence of additional moisture in a future climate on the propagation of gravity waves and the formation of orographic cirrus could be estimated. In a future climate, the increase in moisture dampens the vertical propagation of gravity waves and the occurring vertical velocities in the moist simulations. Together with higher temperatures fewer ice crystals nucleate homogeneously. Assuming that the relative humidity does not change in a warmer climate the specific humidity in the model is increased. This increase in specific humidity in a warmer climate results in a higher ice water content. The net effect of a reduced ice crystal number concentration and a higher ice water content is an increased optical depth. However, in some moist simulations dynamical changes contribute to changes in the ice water content, ice crystal number concentration and optical depth. For the corresponding dry simulations dynamical changes are more pronounced leading to a decreased optical depth in a future climate in some cases.

Evaluation of Cirrus Parameterizations for Radiative Flux Computations in Climate Models Using TOVS–ScaRaB Satellite Observations

Abstract Combined simultaneous satellite observations are used to evaluate the performance of parameterizations of the microphysical and optical properties of cirrus clouds used for radiative flux computations in climate models. Atmospheric and cirrus properties retrieved from Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) observations are given as input to the radiative transfer model developed for the Met Office climate model to simulate radiative fluxes at the top of the atmosphere (TOA). Simulated cirrus shortwave (SW) albedos are then compared to those retrieved from collocated Scanner for Radiation Budget (ScaRaB) observations. For the retrieval, special care has been given to angular direction models. Three parameterizations of cirrus ice crystal optical properties are represented in the Met Office radiative transfer model. These parameterizations are based on different physical approximations and different hypotheses on crystal habit. One parameterization assumes pristine ice crystals and two ice crystal aggregates. By relating the cirrus ice water path (IWP) retrieved from the effective infrared emissivity to the cirrus SW albedo, differences between the parameterizations are amplified. This study shows that pristine crystals seem to be plausible only for cirrus with IWP less than 30 g m−2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should allow the cirrus effective ice crystal diameter (De) to increase with IWP, especially in the range up to 30 g m−2. For cirrus with IWP less than 20 g m−2, this would lead to SW albedos that are about 0.02 higher than the ones of a constant De of 55 μm.

Microphysical and optical properties of midlatitude cirrus clouds observed in the southern hemisphere during INCA

AbstractAirborne microphysical and optical properties of cirrus clouds, jet‐stream and wave clouds were measured at temperatures ranging from −25°C to −62°C in the southern hemisphere from Punta Arenas (53°S) in March and April 2000 during the INCA experiment (INterhemispheric differences in Cirrus properties from Anthropogenic emissions). The observations related to cirrus clouds show general decreases of the ice water content (18 mg m−3 to 0.05 mg m−3), extinction coefficient (0.70 km−1 to 0.08 km−1), ice particle concentration (2.2 cm−3 to 0.5 cm−3), and the effective diameter (80 µm to 17 µm) linked to the variation of ambient temperature (−25°C to −60°C). The lowest temperature at which supercooled water droplets were detected was −33°C. The asymmetry parameter shows relatively small variations, with the smallest values (0.758) observed at the lowest temperatures. High‐altitude clouds which form with a rapid vertical transport, i.e. jet‐stream cirrus and orographic‐wave ice‐cloud, are characterized by very high values of ice particle concentrations (up to 100 cm−3) compared to mean values as a whole (1.45 cm−3). These two kinds of high clouds are the subjects of detailed case‐studies. Although formed at a similar range of temperatures and with similar aerosol properties, the strong differences in cloud properties and humidity fields highlight the key role of the dynamical structure in controlling the formation, evolution and subsequent radiative properties of such high‐altitude clouds. Copyright © 2006 Royal Meteorological Society

Cirrus induced polarization in 122 GHz aura Microwave Limb Sounder radiances

Previous simulation studies have outlined the possibility of significant polarization signals in microwave limb sounding due to horizontally aligned ice crystals in cirrus clouds. From the recently launched Aura MLS instrument, we present the first polarized microwave limb sounding observations of cirrus clouds. We also present polarized radiative transfer simulations, which show qualitative agreement with these observations, and indicate the limits to which aligned non‐spherical particles are influencing bulk optical properties of cirrus clouds at microwave wavelengths. Although 122 GHz is not ideal for cloud measurements due to strong O2 absorption, data and simulations suggest that preferential crystal orientation is causing small, but noticeable, partial vertical polarization, which can be replicated in simulations by considering all particles as horizontally aligned oblate spheroids with aspect ratios of around 1.2 ± 0.15.

Improved retrievals of the optical properties of cirrus clouds by a combination of lidar methods

We focus on improvement of the retrieval of optical properties of cirrus clouds by combining two lidar methods. We retrieve the cloud's optical depth by using independently the molecular backscattering profile below and above the cloud [molecular integration (MI) method] and the backscattering profile inside the cloud with an a priori effective lidar ratio [particle integration (PI) method]. When the MI method is reliable, the combined MI-PI method allows us to retrieve the optimal effective lidar ratio. We compare these results with Raman lidar retrievals. We then use the derived optimal effective lidar ratio for retrieval with the PI method for situations in which the MI method cannot be applied.