Altocumulus Research Articles

Evaluating the cloud effect on solar irradiation by three-dimensional cloud information

A detailed analysis of the effect of clouds on irradiance yields insights into the interaction between clouds and radiation and the precise forecasting of short-term solar radiation. Existing research has mainly focused on the impact of the cloud fraction (CF), i.e., the area distribution of clouds, on radiation. However, three-dimensional cloud information, such as the cloud thickness, also significantly affects solar radiation. This study proposes an innovative index based on the luminance of clouds in all-sky images (ASIs) to characterize the three-dimensional information of clouds, namely, CT, which is usually omitted in the binarization of ASIs. By using two-year synchronous ground irradiance and ASI observation data, the relationship between CT and the cloud radiative effect (CRE) is studied for different types of clouds under two situations, the sun being obscured and unobscured by clouds. The comparison with the commonly used CF indicates that CT has the same or higher negative correlation with the CRE of global horizontal irradiance. Moreover, the correlation between CT and CF with the CREs of direct normal irradiance (DNI) and diffuse irradiance (DFI) is explored in detail. The results indicate that CT has a more negative correlation with the CRE of DNI than that of CF when the sun is obscured, particularly for altocumulus clouds. A detailed discussion indicates that cloud radiative enhancement effects mainly occur under cumulus clouds with a minimum distance between the clouds and the sun (CSMD) of less than three solar radii, but cirrus clouds and altocumulus clouds can also induce cloud radiative enhancement effects.

Przestrzenna i czasowa zmienność rodzajów chmur nad Polską w latach 1996-2020

The subject of the study is a comprehensive qualitative and quantitative analysis of the type of cloud cover over Poland in 1996–2020. The analysis was based on the characteristics of particular types of clouds at three levels, based on hourly measurement data from 36 meteorological stations (IMWM-NRI). The variability of the share of cloud types was analysed on three levels in the multi-annual, annual and daily course. Maps of spatial distribution for selected clouds were plotted. In the study of the multi-year period, clouds of the low-level were observed most frequently, and the most numerous cloud type was Stratocumulus. Altocumulus clouds were found to have the highest frequency on the mid-level, while Cirrus clouds of various species were found on the high-level. On the low-level, the highest increase in the share in the analysed multi-year period was recorded for Stratocumulus clouds, and the highest decrease for moderately and strongly uplifted Cumulus clouds. On the mid-level, the largest increase in the share was observed for Altocumulus clouds, and the largest decrease for Nimbostratus clouds. Among the clouds of the high-level, the greatest increase in the proportion of Cirrostratus clouds not stretching the sky was observed, while the greatest decrease in the share was for the Cirrus uncinus clouds. The annual and daily courses on all the levels showed cyclic occurrence, meaning that the different types of clouds have characteristic periods when they are observed more or less frequently, both on an annual and daily basis.

Locations for the best lidar view of mid-level and high clouds

Abstract. Mid-level altocumulus clouds (Ac) and high cirrus clouds (Ci) can be considered natural laboratories for studying cloud glaciation in the atmosphere. While their altitude makes them difficult to access with in situ instruments, they can be conveniently observed from the ground with active remote-sensing instruments such as lidar and radar. However, active remote sensing of Ac and Ci at visible wavelengths with lidar requires a clear line of sight between the instrument and the target cloud. It is therefore advisable to carefully assess potential locations for deploying ground-based lidar instruments in field experiments or for long-term observations that are focused on mid- or high-level clouds. Here, observations of clouds with two spaceborne lidars are used to assess where ground-based lidar measurements of mid- and high-level clouds are least affected by the light-attenuating effect of low-level clouds. It is found that cirrus can be best observed in the tropics, the Tibetan Plateau, the western part of North America, the Atacama region, the southern tip of South America, Greenland, Antarctica, and parts of western Europe. For the observation of altocumulus, a ground-based lidar is best placed at Greenland, Antarctica, the western flank of the Andes and Rocky Mountains, the Amazon, central Asia, Siberia, western Australia, or the southern half of Africa.

Areas of mammatus on the underside of a layer of altocumulus make for a spectacular sunrise

Areas of mammatus on the underside of a layer of altocumulus make for a spectacular sunrise

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.

The structure of turbulence and mixed-phase cloud microphysics in a highly supercooled altocumulus cloud

Abstract. Observations of vertically resolved turbulence and cloud microphysics in a mixed-phase altocumulus cloud are presented using in situ measurements from an instrumented aircraft. The turbulence spectrum is observed to have an increasingly negative skewness with distance below cloud top, suggesting that long-wave radiative cooling from the liquid cloud layer is an important source of turbulence kinetic energy. Turbulence measurements are presented from both the liquid cloud layer and ice virga below. Vertical profiles of both bulk and microphysical liquid and ice cloud properties indicate that ice is produced within the liquid layer cloud at a temperature of −30 ∘C. These high-resolution in situ measurements support previous remotely sensed observations from both ground-based and space-borne instruments and could be used to evaluate numerical model simulations of altocumulus clouds at spatial scales from eddy-resolving models to global numerical weather prediction models and climate simulations.

Trumpet‐Shaped Topography Modulation of the Frequency, Vertical Structures, and Water Path of Cloud Systems in the Summertime Over the Southeastern Tibetan Plateau: A Perspective of Daytime–Nighttime Differences

AbstractThe warm and humid summer monsoon frequently reaches the steep southern slope of the Himalayas, which is conducive to the formation of unique cloud systems. However, few studies have comprehensively evaluated the characteristics and mechanisms of cloud systems over the trumpet‐shaped topography (TST) region in the southeastern Tibetan Plateau. Therefore, we investigated the occurrence frequency, vertical structures, and water path of clouds over the TST regions and their daytime–nighttime differences using the combined measurements of the CloudSat and CALIPSO satellites from 2007 to 2010 during June–August. Results show a marginal difference in occurrence frequency of total clouds between the daytime (95.4%) and nighttime (97%) over the TST region, while different‐typed clouds exhibit various daytime–nighttime differences in frequency, vertical structures, and water path. In particular, deep convection, cirrus, altocumulus, and nimbostratus clouds tend to occur more frequently at nighttime, while stratocumulus and cumulus clouds occur more in the daytime. Multilayered clouds form more easily at nighttime, especially triple‐layered clouds. The cloud top/bottom heights and liquid/ice water paths of clouds are higher at nighttime than in the daytime over the TST region, which is associated with the increase in deep convection, cirrus, and altocumulus clouds at nighttime. In general, the differences in cloud properties are mainly related to the combined effects of the TST‐induced unique mountain‐valley circulation, the large‐scale monsoonal circulation, and the thermal difference between the day and night. These findings provide valuable observational evidence for the further understanding and accurate simulation of cloud systems over the southeastern Tibetan Plateau region.

Observations and Simulation of Elevated Nocturnal Convection Initiation on 24 June 2015 during PECAN

Abstract The environment of elevated nocturnal deep convection initiation (CI) on 24 June 2015 is investigated using radiosonde data from the Plains Elevated Convection at Night (PECAN) field experiment and a convection-allowing simulation. Elevated CI occurs around midnight in ascending westerly flow above the northeastern terminus of the nocturnal low-level jet (LLJ) several hundred kilometers poleward of the leading edge of a surface warm front. This CI originates from within preexisting banded altocumulus clouds that are supported by persistent large-scale ascent within the entrance region of a midtropospheric jet streak. Model trajectories calculated backward from convective updraft cores during CI indicate abrupt lifting at the leading edge of the surface front during the late afternoon to altitudes above that of the subsequent southerly LLJ. This air remains significantly subsaturated during northward movement until after several hours of weaker but persistent ascent within the highly elevated westerly airstream during the evening. Unlike in many previous studies of frontal overrunning by the LLJ, strong local drying occurs within the LLJ core. Nevertheless, vertical displacements from persistent mesoscale ascent were sufficient for trajectory air parcels to reach their LFC and sustain deep convection. The mesoscale upward displacement along trajectories is well explained by isentropic upglide associated with frontal overrunning at horizontal distances greater than 100 km from the CI and subsequent mature convection. However, the significant additional mesoscale vertical displacements needed for deep CI to occur in the westerlies above the horizontally convergent ~100-km-wide LLJ terminus region, were associated with local cooling and are not accounted for by steady isentropic upglide.

ОПРЕДЕЛЕНИЕ ПАРАМЕТРОВ ОБЛАЧНОГО ПОКРОВА СИСТЕМАМИ АВТОМАТИЧЕСКОЙ ОБРАБОТКИ СПУТНИКОВЫХ ДАННЫХ

The paper discusses the results of comparing cloud cover properties determined by using polar orbiting satellite data (AVHRR/NOAA and MSU-MR/Meteor-M No. 2) for the European territory of Russia and Western Siberia. The cloud characteristics were computed by two threshold techniques: Complex Threshold Technique (CTT) (developed at the European Centre of the State Research Center ‗Planeta‘) and Cloud Cover Detection Technique (CCDT) (developed at the Siberian Centre of ‗Planeta‘). Pixel-by-pixel comparison was performed for very close in time satellite observations, and it showed that in spite of technical similarity of the two radiometers and little difference between both techniques used for the classifications, the results were not the same. The quality of the MSU-MR classification is significantly worse than that of the two AVHRR classifications. In fact, the MSU-MR derivation of cloud parameters fails in optically thin cirrus and altocumulus clouds, thus underestimating the cloud top height for multilayered clouds. As a result, the cloud top is found to be lower, warmer and less iced in comparison with both AVHRR estimates, regardless of the region and other conditions; on the contrary, the cloud top of low and middle clouds appears to be colder, higher and more iced according to MSU-MR data. The MSU-MR cloud mask is strongly underestimated at night during the cold period of the year. The CTT and CCDT‘s cloud top height, temperature and water phase retrieved by AVHRR data are quite close for both regions.

Cloud type identification for a landfalling typhoon based on millimeter-wave radar range-height-indicator data

As a basic property of cloud, accurate identification of cloud type is useful in forecasting the evolution of landfalling typhoons. Millimeter-wave cloud radar is an important means of identifying cloud type. Here, we develop a fuzzy logic algorithm that depends on radar range-height-indicator (RHI) data and takes into account the fundamental physical features of different cloud types. The algorithm is applied to a ground-based Ka-band millimeter-wave cloud radar. The input parameters of the algorithm include average reflectivity factor intensity, ellipse long axis orientation, cloud base height, cloud thickness, presence/absence of precipitation, ratio of horizontal extent to vertical extent, maximum echo intensity, and standard variance of intensities. The identified cloud types are stratus (St), stratocumulus (Sc), cumulus (Cu), cumulonimbus (Cb), nimbostratus (Ns), altostratus (As), altocumulus (Ac) and high cloud. The cloud types identified using the algorithm are in good agreement with those identified by a human observer. As a case study, the algorithm was applied to typhoon Khanun (1720), which made landfall in south-eastern China in October 2017. Sequential identification results from the algorithm clearly reflected changes in cloud type and provided indicative information for forecasting of the typhoon.

An exceptionally bright solar corona in thin altocumulus cloud, photographed from Hillier Gardens, south Hampshire

WeatherVolume 74, Issue 7 p. E1-E1 Inside Cover Photograph An exceptionally bright solar corona in thin altocumulus cloud, photographed from Hillier Gardens, south Hampshire First published: 05 July 2019 https://doi.org/10.1002/wea.3586Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume74, Issue7Special Issue: Student Conference 2018July 2019Pages E1-E1 RelatedInformation

A Study of the Characteristics of Vertical Cloud Base Height Distribution over Eastern China

Cloud is an important factor that affects weather and climate, and the vertical distribution of cloud determines its role in the atmospheric radiation transfer process. In this paper, the characteristics of different cloud types and their vertical cloud base height distributions over Eastern China are investigated with a four-year 2B-CLDCLASS-LIDAR product. The intercomparison of cloud base height distribution from ground-based lidar, CloudSat and CALIPSO measurements was studied with observations over the Hefei and Jinhua areas. The 2B-CLDCLASS-LIDAR product has the potential to uncover geographical and seasonal changes in cloud base height distribution over the Hefei area and Jinhua area, which may be beneficial for local climate models, although the CPR on CloudSat suffers from surface clutter or blind-zones. The results show that for non-precipitation cloud over the defined region (Eastern China), the occurrence frequencies of altocumulus, stratocumulus, and cirrus clouds are 29.4%, 21.0%, and 18.9%, respectively. The vertical occurrence frequencies of their cloud base heights are 0.5–8.5 km, below 3.5 km, and 5.5–17.0 km. The precipitation clouds are dominated by nimbostratus (48.4%), cumulus (17.9%), and deep convective clouds (24.2%), and their cloud base heights are all below 3.0 km. The cloud base height distributions have large differences below 3 km between the satellite measurement and ground-based measurement over Hefei site. Between the Hefei site and Jinhua site, the difference in cloud base height distribution measured by ground-based lidar is in good agreement with that measured by satellite over their matched grid boxes. Over the Hefei site, the vertical occurrence frequencies of cloud base height measured by ground-based lidar are higher than the satellite measurement within 0–0.5 km during all the seasons. It is suggested that more cloudy days may result from the sufficient water vapor environment in Hefei. In summer, the occurrence frequency of the cloud base height distribution at a height of 0–2.0 km is lower than other seasons over Jinhua city, which may be associated with the local weather system. Over the Jinhua site, the difference in seasonal cloud base height distribution based on satellite is in good agreement with that based on ground-based lidar. However, it does not appear over Hefei site. Thus, a multi-platform observation of cloud base height seems to be one of the essential ways for improvement in the observation of cloud macroscopic properties.

Why is there a tilted cloud vertical structure associated with the northward advance of the East Asian summer monsoon

AbstractThe cloud fraction demonstrates an obvious northward‐tilting structure associated with the northward advance of the East Asian summer monsoon. As a follow‐up study, this study explores the physical explanation of the tilted structure based on ERA‐Interim data. The lower convergence center is located on the northern side of the convective center, and the upper divergence center is located on the southern side of the convective center. This specific atmospheric circulation promotes the formation of the tilted cloud vertical structure. Further study shows that the convective instability layer is thinner, and the convective available potential energy is smaller on the northern side of the convective center, favoring the formation of cumulus and altocumulus clouds in the lower troposphere. Condensation from cumulus convection releases latent heat, enhancing convective activity and generating an abundance of deep convective clouds. As the cloud anvils after deep convective clouds collapse, high clouds accumulate in the upper troposphere and are transported to the southern side of the convective center. Therefore, the observed clouds demonstrate a tilted structure. This study can be used to validate cloud vertical structure simulated by climate and weather models.

A spectacular display of crepuscular rays bursting through a field of altocumulus clouds over Romsey

WeatherVolume 73, Issue 11 p. E4-E4 Inside Cover Photograph A spectacular display of crepuscular rays bursting through a field of altocumulus clouds over Romsey First published: 02 November 2018 https://doi.org/10.1002/wea.3413Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume73, Issue11November 2018Pages E4-E4 RelatedInformation

The last rays of the sun (which set 15min before this image was captured) highlight a layer of altocumulus cloud on 23 June 2018

WeatherVolume 73, Issue 9 p. E4-E4 Inside Cover Photographs The last rays of the sun (which set 15min before this image was captured) highlight a layer of altocumulus cloud on 23 June 2018 First published: 04 September 2018 https://doi.org/10.1002/wea.3396Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume73, Issue9September 2018Pages E4-E4 RelatedInformation

Altocumulus stratiformis at sunrise, as viewed from Fovant, near Salisbury

Altocumulus stratiformis at sunrise, as viewed from Fovant, near Salisbury

Precipitation rate climatology related to different cloud types using satellite imagery over Iran

Cloud types have a substantial influence on precipitation. This paper presents a study of the monthly variations of daytime different cloud types over Iran using data collected from Moderate-resolution Imaging Spectroradiometer (MODIS) aboard Terra during 2001–2015, MODIS aboard Aqua during 2002–2015, International Satellite Cloud Climatology Project (ISCCP) H-series cloud type data during 2001–2009 and precipitation rate associated with different cloud types using Tropical Rainfall Measuring Mission (TRMM) satellite products during 2001–2009. Different cloud types were determined using MODIS cloud optical thickness and cloud top pressure data based on ISCCP algorithm. The results showed that stratocumulus and cumulus clouds have maximum occurrence frequency over marine areas especially southern seas. The maximum frequency of nimbostratus and deep convective occurrence occurred over mountainous regions particularly at the time of Aqua overpass and cirrus and cirrostratus are observed over southeast of Iran during warm months due to monsoon system. Altostratus cloud is extended in each month except January, at the time of Terra overpass while nimbostratus is seen at the time of Aqua overpass during warm months in the study area. Cumulus and altocumulus clouds have shown remarkable frequency in all months especially over marine regions during warm and fall months. The higher value of precipitation rate is related to altostratus with a rate approximately 7 mm/h at the time of Terra overpass during April. Altostratus has the maximum recorded precipitation rate except in Nov., Dec., Sep., and Jan. at the time of Terra overpass, whereas the maximum precipitation rate is linked to nimbostratus cloud activity (up to 5 mm/h) except for March, April, and Sep. at the time of Aqua overpass. Deep convective (up to 1.32 mm/h), cirrostratus (up to 1.11 mm/h), and cirrus (0.02 mm/h) are observed only in warm months. The results were compared with ISCCP cloud types so that precipitation rate classified from low to high and Spearman rank correlation was calculated. The results showed good agreement between these two cloud type data; however, there were few notable difference between them.

Why are mixed‐phase altocumulus clouds poorly predicted by large‐scale models? Part 2. Vertical resolution sensitivity and parameterization

AbstractSingle‐column model simulations of mixed‐phase altocumulus clouds were shown to have a strong vertical resolution sensitivity in Part 1 of this paper. Coarse‐resolution models were unable to simulate the long‐lived supercooled liquid layer at cloud top, typically only 200 m thick. In this paper, the sensitivity to vertical resolution is investigated using idealized simulations. Vertical gradients of ice water mixing ratio and temperature near cloud top are found to be inadequately represented at coarse resolution. The vertical discretization using grid box mean values, rather than the full vertical profile, leads to biased calculations of mixed‐phase microphysical process rates and affects the diagnosis of thin liquid water layers. As a result, the liquid water layer becomes quickly glaciated and altocumulus cloud lifetime is underestimated. Similar impacts are expected for mixed‐phase boundary layer clouds commonly observed at high latitudes. A novel parameterization is introduced that accounts for the vertical gradients of ice water mixing ratio and temperature in the microphysics calculations and the diagnosis of liquid near cloud top. It substantially improves the representation of altocumulus layers in coarse vertical resolution single‐column model simulations and reduces the bias identified in Part 1. The new parameterization removes the large underestimate in supercooled water content caused by the resolution sensitivity for temperatures warmer than −30°C. Given the radiative importance of mixed‐phase altocumulus clouds, their underestimation by numerical weather prediction models, and their potential to act as a negative climate feedback, there is a need to reevaluate the global climate sensitivity by implementing the findings in these two papers in a climate model.

Why are mixed‐phase altocumulus clouds poorly predicted by large‐scale models? Part 1. Physical processes

AbstractMixed‐phase layer clouds are radiatively important and their correct representation in numerical models of the atmosphere is needed for both weather forecasts and climate prediction. In particular, midlevel mixed‐phase layer clouds (altocumulus) are often poorly predicted. Here the representation of altocumulus cloud in five operational models and the ERA‐Interim reanalysis is evaluated using ground‐based remote sensors. All models are found to underestimate the supercooled liquid water content by at least a factor of 2. The models with the most sophisticated microphysics (separate prognostic variables for liquid and ice) had least supercooled liquid of all models, though they could simulate the correct liquid‐over‐ice structure of individual clouds. To investigate the reasons for the lack of predicted supercooled liquid water, a single‐column model (EMPIRE) was developed incorporating the relevant physical processes for altocumulus cloud. The supercooled liquid water was found to be the most sensitive to factors that significantly affect the glaciation rate, including aspects of the ice microphysics formulation, as well as the model vertical resolution. Using observations to improve the ice particle size distribution formulation and the parametrization of ice cloud fraction also lead to a significant increase in supercooled liquid water in the simulated clouds. The study highlights the main parameterized processes that need careful attention in large‐scale models in order to adequately represent the liquid phase in mixed‐phase layer clouds. In Part 2, the reason for the sensitivity to vertical resolution is investigated and a new parameterization for models with coarse vertical resolution is proposed.

Altocumulus species, varieties and features

Altocumulus species, varieties and features