Cirriform Clouds Research Articles

Clouds over Hyytiälä, Finland: an algorithm to classify clouds based on solar radiation and cloud base height measurements

Abstract. We developed a simple algorithm to classify clouds based on global radiation and cloud base height measured by pyranometer and ceilometer, respectively. We separated clouds into seven different classes (stratus, stratocumulus, cumulus, nimbostratus, altocumulus + altostratus, cirrus + cirrocumulus + cirrostratus and clear sky + cirrus). We also included classes for cumulus and cirrus clouds causing global radiation enhancement, and we classified multilayered clouds, when captured by the ceilometer, based on their height and characteristics (transmittance, patchiness and uniformity). The overall performance of the algorithm was nearly 70 % when compared with classification by an observer using total-sky images. The performance was best for clouds having well-distinguishable effects on solar radiation: nimbostratus clouds were classified correctly in 100 % of the cases. The worst performance corresponds to cirriform clouds (50 %). Although the overall performance of the algorithm was good, it is likely to miss the occurrences of high and multilayered clouds. This is due to the technical limits of the instrumentation: the vertical detection range of the ceilometer and occultation of the laser pulse by the lowest cloud layer. We examined the use of clearness index, which is defined as a ratio between measured global radiation and modeled radiation at the top of the atmosphere, as an indicator of clear-sky conditions. Our results show that cumulus, altocumulus, altostratus and cirriform clouds can be present when the index indicates clear-sky conditions. Those conditions have previously been associated with enhanced aerosol formation under clear skies. This is an important finding especially in the case of low clouds coupled to the surface, which can influence aerosol population via aerosol–cloud interactions. Overall, caution is required when the clearness index is used in the analysis of processes affected by partitioning of radiation by clouds.

Dust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission

Dust aerosol plays a fundamental role in the behavior and evolution of the martian atmosphere. The first five Mars years of Mars Exploration Rover data provide an unprecedented record of the dust load at two sites. This record is useful for characterization of the atmosphere at the sites and as ground truth for orbital observations. Atmospheric extinction optical depths have been derived from solar images after calibration and correction for time-varying dust that has accumulated on the camera windows. The record includes local, regional, and globally extensive dust storms. Comparison with contemporaneous thermal infrared data suggests significant variation in the size of the dust aerosols, with a 1μm effective radius during northern summer and a 2μm effective radius at the onset of a dust lifting event. The solar longitude (LS) 20–136° period is also characterized by the presence of cirriform clouds at the Opportunity site, especially near LS=50° and 115°. In addition to water ice clouds, a water ice haze may also be present, and carbon dioxide clouds may be present early in the season. Variations in dust opacity are important to the energy balance of each site, and work with seasonal variations in insolation to control dust devil frequency at the Spirit site.

Kelvin‐Helmholtz billows generated at a cirrus cloud base within a tropopause fold/upper‐level frontal system

For the first time, trains of Kelvin‐Helmholtz billows of ∼100–1500 m in depth persisting for about 60 hours at the south extremity of a tropopause fold are reported. The structures were monitored by the VHF Middle and Upper atmosphere radar (Shigaraki, 34.85°N, 136.10°E) in November 2008 at the base of cirriform clouds simultaneously detected by a Ka‐band radar. ECWMF reanalysis data indicate that these clouds were advected and spread out by the sub‐tropical jet‐stream from a tropical storm area while dry and cold air penetrated beneath the upper‐level front. The KH billows were likely continuously generated over horizontal extent of several thousand kilometers for several days without evidence of breaking into three‐dimensional turbulence. Rather than radiative effects, our observations suggest that cooling by sublimation of ice particles precipitating in the frontal layer contributed to their generation. These results should motivate further studies on the interaction between clouds and shear instabilities.

Flying over thin ice

I enjoyed the article “The thinning of Arctic sea ice” by Ron Kwok and Norbert Untersteiner (PHYSICS TODAY, April 2011, page 36). I suggest that the possible cause of the 1 W/m2 surplus heating is from airline traffic in the vicinity. I say this for the following reasons:‣ In the energy balance, Kwok and Untersteiner write, “the solar and atmospheric radiation terms dominate.”‣ The authors also write, “Radiative energy fluxes from the atmosphere and the annual advection . . . are two orders of magnitude larger than 1 W/m2.”‣ People from the US Midwest observed that the sky became much less hazy during the flight moratorium after 9/11.‣ Jet contrails contain water vapor and, more importantly, carbon particles that act as cloud condensation nuclei that enhance the high cloud layer.‣ “High, thin cirriform clouds (composed mostly of ice) tend to promote a net warming effect.”11. C. D. Ahrens, Meteorology Today: An Introduction to Weather, Climate, and the Environment 8th ed., Thomson Brooks/Cole, Belmont, CA (2007), p. 452.9780495011620 ‣ The time frame of the warming coincides with the development of jet air travel.It seems quite possible that a little extra high cloudiness produced by jet flights in the area of the Arctic causes the little extra heating needed for the melting.REFERENCESSection:ChooseTop of pageREFERENCES <<1. C. D. Ahrens, Meteorology Today: An Introduction to Weather, Climate, and the Environment 8th ed., Thomson Brooks/Cole, Belmont, CA (2007), p. 452.9780495011620 , Google Scholar© 2011 American Institute of Physics.

Five years of cloud enhanced surface UV radiation measurements at two sites (in the Northern and Southern Hemispheres)

This is the first paper to report a detailed comparison between ground based measurements of cloud enhanced ultraviolet (UV) radiation at sites located in both hemispheres. The measurements were undertaken at a Northern Hemisphere site (at the campus of the University of Girona in Spain) and a Southern Hemisphere site (campus of the University of Southern Queensland, Toowoomba, Australia). These sites were chosen as they have both simultaneous sky camera and solar radiation (including UV) measurement campaigns operating for at least 5 years over the same time period. Although hemispheric comparisons of UV radiation are not new, this paper focuses on a study of enhancements, including a comparison of the frequency of occurrence and associated cloud types, at the locations with differing climatology. It is found that although there are commonalities between the sites, such as the correlation of the frequency of enhancements with larger solar zenith angle (SZA) both at Girona and Toowoomba, there are differences too, e.g. the frequency of occurrence of the enhancements, and the type of clouds that produce most enhancements (cirrus at Girona, cumulus at Toowoomba). It is suggested that this is due to the differences in latitude coupled with a large amount of Cirriform cloud, especially corresponding to large SZA, at Girona. Possible explanations for this major difference were examined, with the most likely hypothesis relating to multiple scattering due to Cirriform cloud and the subsequent effect on UV radiation on a horizontal surface at large SZA.

Observation of particle fall velocity in cirriform cloud by VHF and millimeter‐wave Doppler radars

In this study, it is demonstrated that a combination of VHF and millimeter‐wave Doppler radars is a key tool for observing particle fall velocity in cirriform clouds. VHF (47‐MHz) and millimeter‐wave (95‐GHz) Doppler radars observed cirriform clouds at West Sumatra, Indonesia (0.2°S, 100.32°E) from 2000 LT 14 to 0800 LT 15 November 2005. Radar reflectivity factor (Ze) observed by the 95‐GHz radar showed that echoes from cloud particles had tops around 12–14 km and bottoms around 8–10 km. Doppler velocity observed by the vertically pointed beam of the 95‐GHz radar (Vair+Z) was compared with vertical air velocity (Vair) observed by the 47‐MHz radar to confirm that Vair+Z, a sum of Vair and reflectivity‐weighted particle fall velocity (VZ), showed consistent changes with Vair and hence VZ is able to be retrieved by subtracting Vair from Vair+Z. The correlation coefficient between VZ and Ze in the middle part of clouds (10.5–12.2 km) was −0.81, which was higher than that (−0.47) in the bottom part (7.2–10.5 km). The change of VZ for Ze in the middle part was larger (Ze = −31.9 VZ − 32.2) than that in the bottom part (Ze = −90.2 VZ − 71.8). These results suggest that particle size was a dominant factor that determined Ze in the middle part. Using VZ, median volume diameter (D0) was estimated to suggest that D0 was larger than ∼70 μm in the bottom part and ranged from ∼40 μm to larger than ∼106 μm in the middle part.

Feature Extraction from Whole-Sky Ground-Based Images for Cloud-Type Recognition

Abstract Several features that can be extracted from digital images of the sky and that can be useful for cloud-type classification of such images are presented. Some features are statistical measurements of image texture, some are based on the Fourier transform of the image and, finally, others are computed from the image where cloudy pixels are distinguished from clear-sky pixels. The use of the most suitable features in an automatic classification algorithm is also shown and discussed. Both the features and the classifier are developed over images taken by two different camera devices, namely, a total sky imager (TSI) and a whole sky imager (WSC), which are placed in two different areas of the world (Toowoomba, Australia; and Girona, Spain, respectively). The performance of the classifier is assessed by comparing its image classification with an a priori classification carried out by visual inspection of more than 200 images from each camera. The index of agreement is 76% when five different sky conditions are considered: clear, low cumuliform clouds, stratiform clouds (overcast), cirriform clouds, and mottled clouds (altocumulus, cirrocumulus). Discussion on the future directions of this research is also presented, regarding both the use of other features and the use of other classification techniques.

High Cloud Properties from Three Years of MODIS Terra and Aqua Collection-4 Data over the Tropics

Abstract This study surveys the optical and microphysical properties of high (ice) clouds over the Tropics (30°S–30°N) over a 3-yr period from September 2002 through August 2005. The analyses are based on the gridded level-3 cloud products derived from the measurements acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard both the NASA Earth Observing System Terra and Aqua platforms. The present analysis is based on the MODIS collection-4 data products. The cloud products provide daily, weekly, and monthly mean cloud fraction, cloud optical thickness, cloud effective radius, cloud-top temperature, cloud-top pressure, and cloud effective emissivity, which is defined as the product of cloud emittance and cloud fraction. This study is focused on high-level ice clouds. The MODIS-derived high clouds are classified as cirriform and deep convective clouds using the International Satellite Cloud Climatology Project (ISCCP) classification scheme. Cirriform clouds make up more than 80% of the total high clouds, whereas deep convective clouds account for less than 20% of the total high clouds. High clouds are prevalent over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), tropical Africa, the Indian Ocean, tropical America, and South America. Moreover, land–ocean, morning–afternoon, and summer–winter variations of high cloud properties are also observed.

Cirrus detrainment‐temperature feedback

In considering the role of cirriform clouds in climate change, it is important to distinguish among the relationships of different high cloud types to large‐scale atmospheric dynamics. While cirrostratus and cirrocumulus (CsCc) have a clear relation to deep convective sources, the ensemble behavior of cirrus is more subtle. An empirical relation is found between cirrus fraction and deep cloud top temperature that points to detrainment temperature as a dominant factor governing tropical and subtropical cirrus. This cirrus‐detrainment‐temperature (CDT) relation provides a target for modelers, and suggests an additional cloud‐climate feedback. As surface temperatures warm, detrainment temperatures cool as deep cloud top height increases. The CDT relation implies that cirrus fraction increases. Because cirrus are optically thinner than CsCc, the competition between longwave feedbacks and cloud albedo feedbacks leads to a hypothesized positive climate feedback by cirrus fraction.

Water Vapor, Condensed Water, and Crystal Concentration in Orographically Influenced Cirrus Clouds

Abstract Results are presented from measurements made with a counterflow virtual impactor (CVI) in cirriform clouds containing crystals with dimensions typically less than 30 μm. Independent measurements of crystal number concentration and cloud water content are presented, and for the first time consistent instrumentation is used to measure both the cloud condensate and water vapor. The experiment was conducted over the Alps where the vertical wind field was very persistent in the stable air, which suggests the presence of standing gravity waves. For the two flights presented in this paper, the relative humidity over ice within the cloud ranged from about 50% to 110%, and the cloud water content ranged from about 1 to 10 mg m−3. The number concentration was typically a few hundred crystals per liter, reaching peak concentrations of several thousand per liter. In general, the cloud water comprised less than 10% of the total water content. Often, the crystals were not detected at all by concurrent PMS-230X...

Small crystals in cirriform clouds: A case study of residue size distribution, cloud water content and related cloud properties

In-situ measurements with the counterflow virtual impactor (CVI) during the ICE-89 campaign are presented. These include measurements of size distribution of ice crystal residues and independent measurements of crystal number concentration and cloud water content (CWC). The investigated clouds appeared to consist predominantly of small ice crystals with a number density weighted radius of mean mass (RMM) of less than 30 μm. This is also supported by a comparison of the CVI and concurrent PMS 2D2-C probe data. Crystals number concentrations and CWC were low and typically less than 301 −1 and 3 mg m −3 respectively. The flight average residual size distributions, as measured by the PMS ASASP-X-SP, showed two very distinct modes around 0.17 μm and 0.35 μm radius. A closer look at the evolution of the residual distribution as a function of time, revealed that the modes usually do not appear together, but rather excluded each other.

Climatological contingency probabilities of clouds

AbstractThe ideas surrounding contingency co‐occurrence probabilities of cloud types are discussed with respect to two types of probability: firstly, that given a lower level cloud (L) there is also an upper level cloud (U), P(L ⇒ U), and secondly the probability that given an upper cloud layer there is a lower cloud layer, P(U ⇒ L). Particular emphasis is placed on the fact that analyses to date have considered the frequency of observation of co‐occurrence, not, the potentially more useful, coexistence probability. From four separate months of global surface observations the P(L ⇒ U) was calculated as a function of lower level cloud amount on a global, regional, and individual station basis for (i) stratiform and altiform and (ii) stratiform and cirriform cloud type combinations. Global patterns of P(L ⇒ U) behaviour are somewhat similar to regional patterns produced by previous work and are, on occasions, reproducible on a regional scale and, in some instances, by individual stations. Values of P(L ⇒ U) involving lower level overcasts are required in the calculation of the P(U ⇒ L) term, thus rendering climatological estimates of P(U ⇒ L) susceptible to biasing if an inappropriate value of P(L ⇒ U) for L = overcast is chosen. To demonstrate this effect, monthly global climatologies of P(U ⇒ L) were generated for several combinations of upper and lower cloud types using (i) an assumed mean value for the overcast case and (ii) a modification to the previously assumed overcast P(L ⇒ U) values, as suggested here. Comparison of these climatologies revealed differences, widely spread across the globe, of mostly within ± 10 per cent according to cloud type, but with a few larger changes in excess of 30 per cent.

Ice Particles Observed in a Cirriform Cloud at −83°C and Implications for Polar Stratospheric Clouds

Mesurements obtained in a thin, cirriform cloud that formed in the temperature range of −83° to −84°C and an altitude range at 16.2 to 16.7 km are presented. Implications of the results for particle generation in polar stratospheric clouds developing with similar conditions are discussed.

Cirrus Clouds. Part I: A Cirrus Cloud Model

Abstract A two-dimensional (x, z), time-dependent, numerical cloud model is developed for the purpose of investigating the role of various physical processes involved in the maintenance of cirriform clouds. In addition to accounting for dynamic and thermodynamic processes including phase changes of water, effects due to microphysical composition and radiative processes are also explicitly incorporated into the model. Diagnostic parameterizations for the local radiative properties of cloudy volumes and the gravity induced relative fall speed of the contained ice water are presented. Results of a simulation of a thin cirrostratus cloud are given. Features of the simulated cloud structure are quite realistic. Quantitative agreement is found between the simulated ice water contents and vertical motions and comparable observations. It is shown that radiative effect may be very significant in the maintenance of cirrus. The effects of the gravity-induced relative fall speed of ice crystals are found to be of cri...

Cloud mapping from the earth's surface using infrared radiance contrasts*

Abstract Radiance criteria for distinguishing low, middle, and high clouds in the 9.5–11.5 μm band of the infrared are developed and used to produce local cloud maps. The performance of this radiance contrast method for mapping clouds from the earth's surface is evaluated with a view to using the technique for objective observation of cloud amount and distribution in the sky hemisphere. Discrimination radiance formulas are developed using a multilayer, wavenumber‐specific infrared radiative transfer model including cloud parameters measured by other workers and atmospheric conditions measured by radiosonde. The clear sky radiance (N0) is the dominant independent variable in the discrimination formulas. The variation of N0 with time (primarily due to changes in atmospheric water vapour content) and zenith angle are found to be important in distinguishing cirriform clouds from clear sky and other clouds. The local cloud maps are produced by applying the discrimination radiances (in voltage form) to the outp...

Effects of the Mount St. Helens Volcanic Cloud on Turbidity at Ann Arbor, Michigan

Measurements of turbidity were made at the University of Michigan irradiance and meteorological measurement facility just prior to, during and after the passage of the volcanic cloud from the 18 May 1980 eruption of Mount St. Helens. They were made with a Volz sunphotometer at wavelengths of 500 and 880 nm. The volcanic cloud gradually moved over the measurement facility on the afternoon of 20 May and remained over it until 2300 GMT 21 May. Its effects on the turbidity coefficient and wavelength exponent in Ångström's turbidity equation are evaluated by comparing results for 20 and 22 May, both of which were cloudless days, with those on 21 May. The volcanic cloud increased the turbidity coefficient by a factor of about 8 and decreased the wavelength exponent by a factor of about 3 compared to average values for 20 and 22 May. Some uncertainty in the results is caused by cirriform clouds in addition to the volcanic cloud on 21 May. These results and effects of the volcanic cloud on recordings of global, direct normal and diffuse solar irradiances are described.

Attenuation of solar energy by high, thin clouds

Estimates of the effects of clouds on solar energy rarely include cirriform clouds. These high ice clouds are usually not included in cloud cover estimates because of the difficulty of direct, human detection. Lidar studies and aircraft observations show that cirrus is frequently present. The attenuation of solar energy by cirriform clouds is calculated and in combination with cirrus frequency data is used to estimate average effects of cirrus on solar energy devices. The marginal cost-effectiveness of large solar installations requires that effects of cirrus be considered in device design.

Cirrus Uncinus Generating Cells and the Evolution of Cirriform Clouds. Part I: Aircraft Observations of the Growth of the Ice Phase

Abstract The growth of the ice phase in cirrus uncinus and cirrostratus clouds was studied through aircraft measurement of cloud particle spectra at different altitudes. Five different cirrus uncinus clouds were studied; one of the cirrus uncinus evolved into cirrostratus. The temperature range of sampling was −19 to −58°C. In cirrus uncinus heads, crystals were determined to be nucleated and grown in the upshear region, before being carried into the trail region of the head downshear as a result of wind shear. The updraft region is upshear, and the downdraft region downshear. A “hole” was found to separate the up-and downshear regions of the head, with a horizontal extent of about 150 m. The concentrations of crystals in the head region were on the order of 0.5 cm −3, with 0.025-0.05 cm −3 longer than 100 µm. Accumulation of particles in the updraft region was noted. The mean length of crystals longer than 100 µm (precipitation size particles) ranged between 0.5 and 1.0 mm, and crystals as long as 2 mm w...

Cirrus Uncinus Generating Cells and the Evolution of Cirriform Clouds. Part III: Numerical Computations of the Growth of the Ice Phase

Equations were developed to calculate the growth of the ice phase in cirrus clouds. Calculations indicated that nucleation of ice crystals in cirrus uncinus heads forming at temperatures lower than −35°C generally should occur near the upwind base of the head, and in cirrostratus clouds at the top of the cloud. The growth of ice crystals and the resulting shape of cirrus uncinus clouds with an updraft velocity of 100 cm s−1 were calculated. With an initial crystal concentration of 0.025 cm −2 and a nucleation temperature of −40°C, crystals of 0.45 mm length, and a maximum ice water content of 0.3 g m −3 were predicted. Latent beat release due to the ice crystal growth increased the initial updraft velocity only slightly. A downdraft velocity comparable in magnitude to the original updraft velocity was calculated to occur in the downshear part of the cirrus uncinus head.

Aircraft measurements of the albedo of the earth's surface and of clouds over India

Measurements of global and reflected solar radiation and of the earth's albedo over extensive land areas in India and over the adjoining seas were made from a Canberra aircraft during July-August 1972 as part of an Indo- U .K. Project Exercise Storm Exchange. Two thermoelectric pyranometers, one facing upwards and the other facing downwards, were used to measure the global and reflected solar radiation, a photographic recorder with suspended coil mirror galvanometers recording the data continuously in flight.&#x0D; Twenty aircraft flights were made from three bases, 7 from Bangalore, 8 from Poona and 5 from Calcutta, over both land and sea, covering areas from 7°N to 240N and 71°E to 89°E. The aircraft flew at heights above ground/ sea from 150 to 15,000m.&#x0D; The values of global solar radiation recorded were of the order of J. 60 to 1.80 cal/cm2/min at heights of 4,600 m to 15,000 m, when there were no clouds; thin cirrus reduced this value by about 16 to 20 per cent and other clouds by larger amounts depending on their type and thickness.&#x0D; The albedo over the sea was found to vary between 0.04 to 0.13, depending on the state of the solar elevation and that over the ground between 0.10 to 0.25. The albedo over active cumulus clouds was about I 0'40 to 0.60 and that over cumulus of smaller extent between 0.25 and 0.40. Stratocumulus clouds have smaller albedo 0.20-0.40 and stratus, 015-0.25. The albedo of altostratus clouds was found to lie between 0.25 and 0.35. The albedo of the cirrus remains of dissipating cumulonimbus clouds was about 0.45 to 0.55. In the growing cumulus cloud the albedo changes from 0.20 to 0.55 depending on the stage of growth. Albedo values change from 0.16-0.27 to 0.30-0.36, when stratus and stratocumulus clouds are associated with cirriform clouds.&#x0D; The observations are compared with satellite and ground observations of surface albedo over India.&#x0D;