Banner Cloud Research Articles

Sensitivity of Banner Cloud Formation to Orography and the Ambient Atmosphere: Transition from Idealized to More Realistic Scenarios

Abstract Banner clouds are clouds in the lee of steep mountains or sharp ridges on otherwise cloud-free days. Previous studies investigated various aspects of banner cloud formation in numerical simulations, most of which were based on idealized orography and a neutrally stratified ambient atmosphere. The present study extends these simulations in two important directions by 1) examining the impact of various types of orography ranging from an idealized pyramid to the realistic orography of Mount Matterhorn and 2) accounting for an ambient atmosphere that turns from neutral to stably stratified below the mountain summit. Not surprisingly, realistic orography introduces asymmetries in the spanwise direction. At the same time, banner cloud occurrence remains associated with a coherent area of strong uplift, although this region does not have to be located exclusively in the lee of the mountain any longer. In the case of Mount Matterhorn with a westerly ambient flow, a large fraction of air parcels rises along the southern face of the mountain, before they reach the lee and are lifted into the banner cloud. The presence of a shallow boundary layer with its top below the mountain summit introduces more complex behavior compared to a neutrally stratified boundary layer; in particular, it introduces a dependence on wind speed, because strong wind is associated with strong turbulence that is able to raise the boundary layer height and, thus, facilitates the formation of a banner cloud.

Prevalence, diagnosis, and impact on clinical outcomes of dural ossification in the thoracic ossification of the ligamentum flavum: a systematic review.

Systematic review. Thoracic ossification of the ligamentum flavum (TOLF) has become the principal cause of thoracic spinal stenosis. Dural ossification (DO) was a common clinical feature accompanying with TOLF. However, on account of the rarity, we know little about the DO in TOLF so far. This study was conducted to elucidate the prevalence, diagnostic measures, and impact on the clinical outcomes of DO in TOLF by integrating the existing evidence. PubMed, Embase, and Cochrane Database were comprehensively searched for studies relevant to the prevalence, diagnostic measures, or impact on the clinical outcomes of DO in TOLF. All retrieved studies meeting the inclusion and criterion were included into this systematic review. The prevalence of DO in TOLF treated surgically was 27% (281/1046), ranging from 11 to 67%. Eight diagnostic measures have been put forward to predict the DO in TOLF using the CT or MRI modalities, including "tram track sign", "comma sign", "bridge sign", "banner cloud sign", "T2 ring sign", TOLF-DO grading system, CSAOR grading system, and CCAR grading system. DO did not affect the neurological recovery of TOLF patients treated with the laminectomy. The rate of dural tear or CSF leakage in TOLF patients with DO was approximately 83% (149/180). The prevalence of DO in TOLF treated surgically was 27%. Eight diagnostic measures have been put forward to predict the DO in TOLF. DO did not affect the neurological recovery of TOLF treated with laminectomy but was associated with high risk of complications.

Banner cloud sign: a novel method for the diagnosis of dural ossification in patients with thoracic ossification of the ligamentum flavum.

Dural ossification (DO) is common in patients with ossification of the ligamentum flavum (OLF) and is the leading cause of dural tears. However, the methods used for DO diagnosis are limited. The purpose of this study was to propose a novel CT-based imaging sign, Banner cloud sign (BCs), and clarify its clinical characteristics and correlations with DO. 57 OLF patients who underwent thoracic spine decompression surgery in our single-center between January- and October-2018 were recruited and divided into two groups based on the presence of DO. Patient demographics and radiographic data were analyzed. Hematoxylin-eosin staining and micro-CT were used to detect the micro-morphological changes of DO. The diagnostic value of BCs for DO was assessed by sensitivity and specificity. 12 patients with a total of 19 segments were diagnosed as DO. The incidence of DO was 21.1% (12/57) in OLF patients and 9.5% (19/200) in OLF segments. Patients with DO had a shorter disease duration and a higher incidence of cerebrospinal fluid leakage than those without DO. Hematoxylin-eosin staining and micro-CT showed that the dura mater was ossified and fused with ossified ligamentum flavum, and diffusion along the dura mater, like a banner cloud flying on the mountain. The sensitivity and specificity of BCs in DO diagnosis were 78.9 and 90.6%, respectively. BCs can vividly and intuitively describe the imaging features of DO and has high diagnostic accuracy. It could be a promising and valuable method for the diagnosis of DO.

The diagnostic accuracy of CT-based "Banner cloud sign" for dural ossification in patients with thoracic ossification of the ligamentum flavum: a prospective, blinded, diagnostic accuracy study protocol.

BackgroundOssification of the ligamentum flavum (OLF) is the most common cause of thoracic spinal stenosis, which responds poorly to conservative treatment. Thus, surgery is the only effective treatment for OLF. The existence of dural ossification (DO) makes surgery challenging and increases the risk of intra-/post-operative complications. To date, several methods have been proposed to identify DO, but either the diagnostic accuracy is low or the feasibility is poor. Therefore, the aim of this study was to propose a new imaging sign (Banner cloud sign, BCs), evaluate the accuracy of BCs in the diagnosis of DO, and provide reliable evidence-based data for its application in clinical practice.MethodsA prospective, blinded, diagnostic accuracy study will be conducted to assess and compare the accuracy of BCs in the diagnosis of DO with other radiological signs [Tram track sign (TTs) and Comma sign (Cs)]. A total of 120 patients diagnosed with OLF who underwent decompression at the Peking University Third Hospital between January 2018 and June 2019 will be enrolled. Patients’ medical records and imaging data will be retrieved from the hospital database server. An observational group consisting of six spine surgeons (with different seniority levels) and two epidemiological researchers will read the patients’ images to identify typical imaging signs and determine the presence of DO. Surgical records will be reviewed to confirm the presence of DO, and the results will serve as the reference standard for estimating accuracy. The primary outcome of the study is to determine the accuracy of BCs for DO diagnosis, and the secondary outcome is to compare the sensitivity, specificity, area under the receiver operating characteristic (ROC) curve and inter-observer reliability of each imaging sign. The time taken and level of confidence in DO diagnosis of each observer will also be compared.DiscussionThis study represents the first large-scale investigation of the diagnostic value of BCs, TTs and Cs in the diagnosis of DO, and will provide convincing evidence about their clinical application.Trial registrationRegistered on 29 February 2020. Trial number is ChiCTR2000030380.

The Role of Wind Speed and Wind Shear for Banner Cloud Formation

Abstract Banner clouds are clouds that appear to be attached to the leeward face of a steep mountain. This paper investigates the role of wind speed and wind shear for the formation of banner clouds. Large-eddy simulations are performed to simulate the flow of dry air past an idealized pyramid-shaped mountain. The potential for cloud formation is diagnosed through the Lagrangian vertical parcel displacement, which in the case of a banner cloud shows a plume of large values in the lee of the mountain. In addition, vortical structures are visualized through streamlines and their curvature. A series of sensitivity experiments indicates that both the flow and the banner cloud occurrence are largely independent of the ambient wind speed U. On the other hand, the shear of the ambient wind has a profound impact on the location of the stagnation point on the windward face as well as on the flow geometry in the lee of the mountain. The relevant measure for shear is H/Hs, where H denotes the height of the mountain and Hs = U/Uz is the scale height of the shear (with Uz denoting the scale of the shear). The simulations are also used to compute the line-of-sight velocity component seen by a hypothetical Doppler wind lidar positioned in the lee of the mountain; the analysis suggests that such sensitivities can potentially be detected using modern wind lidar technology.

Orographic banner cloud streaming off the Matterhorn in Switzerland

Orographic banner cloud streaming off the Matterhorn in Switzerland

What Flow Conditions are Conducive to Banner Cloud Formation?

Abstract Banner clouds are clouds that are attached to the leeward slope of a steep mountain. Their formation is essentially due to strong Lagrangian uplift of air in the lee of the mountain. However, little is known about the flow regime in which banner clouds can be expected to occur. The present study addresses this question through numerical simulations of flow past idealized orography. Systematic sets of simulations are carried out exploring the parameter space spanned by two dimensionless numbers, which represent the aspect ratio of the mountain and the stratification of the flow. The simulations include both two-dimensional flow past two-dimensional orography and three-dimensional flow past three-dimensional orography. Regarding flow separation from the surface, both the two- and the three-dimensional simulations show the characteristic regime behavior that has previously been found in laboratory experiments for two-dimensional orography. Flow separation is observed in two of the three regimes, namely in the “leeside separation regime,” which occurs preferably for steep mountains in weakly stratified flow, and in the “postwave separation regime,” which requires increased stratification. The physical mechanism for the former is boundary layer friction, while the latter may also occur for inviscid flow. However, flow separation is only a necessary, not sufficient condition for banner cloud formation. The vertical uplift and its leeward–windward asymmetry show that banner clouds cannot form in the two-dimensional simulations. In addition, even in the three-dimensional simulations they can only be expected in a small part of the parameter space corresponding to steep three-dimensional orography in weakly stratified flow.

Mechanisms of Banner Cloud Formation

Abstract Banner clouds are clouds in the lee of steep mountains or sharp ridges. Their formation has previously been hypothesized as due to three different mechanisms: (i) vertical uplift in a lee vortex (which has a horizontal axis), (ii) adiabatic expansion along quasi-horizontal trajectories (the so-called Bernoulli effect), and (iii) a mixing cloud (i.e., condensation through mixing of two unsaturated air masses). In the present work, these hypotheses are tested and quantitatively evaluated against each other by means of large-eddy simulation. The model setup is chosen such as to represent idealized but prototypical conditions for banner cloud formation. In this setup the lee-vortex mechanism is clearly the dominant mechanism for banner cloud formation. An essential aspect is the pronounced windward–leeward asymmetry in the Lagrangian vertical uplift with a plume of large positive values in the immediate lee of the mountain; this allows the region in the lee to tap moister air from closer to the surface. By comparison, the horizontal pressure perturbation is more than two orders of magnitude smaller than the pressure drop along a trajectory in the rising branch of the lee vortex; the “Bernoulli mechanism” is, therefore, very unlikely to be a primary mechanism. Banner clouds are unlikely to be “mixing clouds” in the strict sense of their definition. However, turbulent mixing may lead to small but nonnegligible moistening of parcels along time-mean trajectories; although not of primary importance, the latter may be considered as a modifying factor to existing banner clouds.

Banner cloud off the top of the Bitexco Financial Tower in Ho Chi Minh City (Vietnam)

WeatherVolume 68, Issue 7 p. E1-E1 Inside Cover Photograph Banner cloud off the top of the Bitexco Financial Tower in Ho Chi Minh City (Vietnam) First published: 27 June 2013 https://doi.org/10.1002/wea.2122Read 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 No abstract is available for this article. Volume68, Issue7July 2013Pages E1-E1 RelatedInformation

Banner clouds observed at Mount Zugspitze

Abstract. Systematic observations of banner clouds at Mount Zugspitze in the Bavarian Alps are presented and discussed. One set of observations draws on daily time lapse movies, which were taken over several years at this mountain. Identifying banner clouds with the help of these movies and using simultaneous observations of standard variables at the summit of the mountain provides climatological information regarding the banner clouds. In addition, a week-long measurement campaign with an entire suite of instruments was carried through yielding a comprehensive set of data for two specific banner cloud events. The duration of banner cloud events has a long-tailed distribution with a mean of about 40 min. The probability of occurrence has both a distinct diurnal and a distinct seasonal cycle, with a maximum in the afternoon and in the warm season, respectively. These cycles appear to correspond closely to analogous cycles of relative humidity, which maximize in the late afternoon and during the warm season. In addition, the dependence of banner cloud occurrence on wind speed is weak. Both results suggest that moisture conditions are a key factor for banner cloud occurrence. The distribution of wind direction during banner cloud events slightly deviates from climatology, suggesting an influence from the specific Zugspitz orography. The two banner cloud events during the campaign have a number of common features: the windward and the leeward side are characterized by different wind regimes, however, with mean upward flow on both sides; the leeward air is both moister and warmer than the windward air; the background atmosphere has an inversion just above the summit of Mt. Zugspitze; the lifting condensation level increases with altitude. The results are discussed, and it is argued that they are consistent with previous Large Eddy Simulations using idealized orography.

山雲に就いて(雲の調査其の三)

The observational data of the mountain cluds, fog and haze at Mt. Tukuba described in this report were mostly carried out between July, 1929 to. June, 1931 during two years. The outline of these investigations is as follows; (1) the annual or daily variation of the frequency of the appearance of mountain clouds and haze; (2) the diurnal or annual change of the height of mountain clouds and the frequency of their appearance for the height of the every stratum of mountain; (3) the variation of the vertical temperature distribution and relation to the mountai nclouds'change; (4) the classification of the forms of the mountain clouds; (5) the estimation of the size of the mountain clouds or fog particles; (6) the change of their formation with reference to the upper clouds-sheet; (7) the comparative study of the forms between the mountain clouds and the model clouds.The following results noticed in this report may be of special interest. The frequency of the appearance of the mountain clouds is very large in summer and small in winter. (See Fig. 1). The frequency of the mountain haze is large in summer and winter, and the smallest in early spring and autumn; it is large in the forenoon and small in the afternoon. The frequency of the fair weather on the mountain is generally large in winter arid small in summer, large in the afternoon and small in the forenoon. The mean height of the mountain clouds or the frequency of their appearance in every layer shows the daily remarkable variation, in which the highest elevation in the middle of the day and the lowest elevation occurred in the morning and evening, and also shows a high elevation in summer and a low in winter. The distribution of the vertical temperature in the layer of the mountain clouds is shown in Fig. 4. The dense clouds appear at the beginning of the elevation of temperature inversion; and with 100 meters up or down from the inversion height, clouds disappear. The lapse late of the temperature is smaller than about 1°/100m decrease below the stratum of the inversion layer, while the lapse late is larger than about 1°/100m increase in it. The thickness of the mountain clouds is nearly the same order of width of the inversion layer.The classification of the mountain clouds is as follows; (i) Mountain lenticular cloud. (ii) Crest cloud. (iii) Banner cloud. (iv) Mountain cumulus or Mountain cumulo-nimbus. (v) Mountain stratus. (vi) Summit or valley cloud. (See Plate 1 and Plate 2). These types of the mountain clouds noticed in this report, may be of special interest. When the crest cloud changes its form from the stratus into the cumulus fine weather will come; but when its form changes from the cumulus into the stratus, we may expect bad weather will come in the night or the following day. The forms of the mountain cumulus and mountain cumulo-nimbus are maintained the character of the summit type. The formation of the upper cloud-sheet of stratus shows long wave form at dawn and after gradually changes into cumuli form. (See Plate 2).The observed size of the mountain clouds or fog particles is about 30 microns for the mean radius; in which 31.8 microns by the direct observation with the microscope the magnification being from 60 to 1000, and 27.4 microns by the observing rings of the colored light with the Broken-bow or Mountainspectre and the coronas of the lamp. The extreme values of these observed radii of the particles are about 5-150 microns with the microscopic observation and about 10-70 microns with the corona. In the Plate 3, the photographic pictures of the Broken-bow and Mountain-spectre are shown.In the last paragraph, we described concerning the following experiment. By the use of the model of the Mt. Tukuba district (scale 1-50, 000), which installed in the rectangular glass-case, we send a smoke of tabacco from one side to another by a pipe. The result of the experiment is shown in the Plate 3 and Plate 4.