Calibration of a three‐component angular distribution model of sky radiance

Abstract

A three‐component (isotropic, circumsolar and horizon‐brightening) model of the angular distribution of sky (short‐wave) radiance has been tested and validated against a data base of measured sky radiance. The data base encompasses cloud cover 0.0 to 1.0 and solar zenith angles 30 to 80°. Empirical constants have been derived for the model enabling the prediction of sky radiances for any sky condition.     

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 output from a narrow‐view infrared radiometer pointed at a sky‐scanning mirror. It is necessary to assume that the radiance from a cloud observed at the surface decreases unambiguously with an increase in cloud base height. Cloud maps for five days in July 1978 indicate the mapping technique shows promise under a wide range of sky conditions. Cloud motions rapidly degrade the maps’ quality over time‐scales that are much less than the current manual cloud sampling period of 1 h.     

A source of hail embryos

Abstract

The sensitivity of the annual cycle of ice cover in Baffin Bay to short‐wave radiation is investigated. The Princeton Ocean Model (POM) is used and is coupled with a multi‐category, dynamic‐thermodynamic sea‐ice model in which the surface energy balance governs the growth rates of ice of varying thickness. During spring and summer the short‐wave radiation flux dominates other surface heat fluxes and thus has the greatest effect on the ice melt. The sensitivity of model results to short‐wave radiation is tested using several, commonly used, shortwave parameterizations under climatological, as well as short‐term, atmospheric forcing. The focus of this paper is short‐term and annual variability. It is shown that simulated ice cover is sensitive to the short‐wave radiation formulation during the melting phase. For the Baffin Bay simulation, the differences in the resulting ice area and volume, integrated from May to November, can be as large as 45% and 70%, respectively. The parameterization of the effect of cloud cover on the short‐wave radiation can result in the sea‐ice area and volume changes reaching 20% and 30%, respectively. The variation of the cloud amount represents cloud data error, and has a relatively small effect (less then ±4%) on the simulated ice conditions. This is due to the fact that the effect of cloud cover on the short‐wave radiation flux is largely compensated for by its effect on the net near‐surface long‐wave radiation flux.     

地基红外测云仪探测能力分析

利用地基红外测云仪(WSIRCMS)在2011年11月北京观象台的连续观测数据,从总云量、云底高和天空类型3个方面初步分析其探测能力。结果表明:1该仪器能够不分昼夜同时实现云高、云量(高、中、低和总云量)和天空类型的连续自动探测;2与参考标准云量的差值在±10%以内的样本数占总样本数的72.5%,有霾存在时,对中高云的观测能力较弱,造成云量观测结果差异较大;3与激光云高仪的天顶方向的无云一致率达94.9%;在中低云情况下,云高观测结果一致性较好,高云时存在较大差异,WSIRCMS观测云高偏高;4与人工分类的天空类型一致的样本数占总样本数的82.63%,对波状云、积状云和混合云的识别能力稍低。     

Tropospheric low‐level temperature inversions in the Canadian Arctic

Abstract

Climatological characteristics of the low‐level tropospheric temperature inversion in the Canadian Arctic are examined using 10–40 year records of upper‐air meteorological data. Inversions at the northern sites are primarily surface‐based in winter, and elevated from mid‐spring through summer. At the southern sites, a bimodal pattern is observed with surface‐based inversions occurring during late summer, as well as during winter. From comparisons of our results with other published climatologies, it appears that this bimodal pattern reflects interactions between short‐ and long‐wave radiation, synoptic activity and snowmelt. Maxima in inversion depth and temperature difference across the inversion layer occur in February and March; minima occur in August and September. The annual progression of inversion characteristics closely follows the annual pattern of clear‐sky percentages, reflecting the controlling influence of cloud and clear‐sky radiative forcings on the inversion layer.     

Performance validation of the Gueymard sky radiance model

Abstract

The Gueymard model of short‐wave sky radiance has been validated against sky radiance date measured in the Rocky Mountain foothills of Alberta. Except for sky points within the circumsolar region in clear skies and for low sun positions in near overcast skies the model performs as well as other current sky radiance models.     

Cloud measurements on day 245 of gate

Abstract

On day 245 of GATE (2 September 1974) two lines of convection ‐ north (N) and south (S) ‐ were studied by aircraft stacked vertically, patrolling a north‐south line at longitude 22.85° W of length about 200 km. These lines were part of a complex of convection related to an easterly wave. Photogrammetry shows that the northern line consisted at first of a dense aggregate of small clouds, of width roughly 50 m at cloud base. There were a few tall clouds. No line organization was apparent from the aircraft at 1208 GMT. Fifteen minutes later there was line organization, readily apparent from the DC‐6 aircraft. From study of Electra and DC‐6 records, it appears that a vigorous cold northeasterly surface current initiated line N, and that this was a downdraft originating at altitude about 2.5 km in a mesoscale cloud feature to the north. It appears further that penetrative convection to 14 km followed after the first rain, which moistened air near the surface, and thus lowered cloud bases from about 500 to 350 m.

In overhangs of cloud, anvils to the north of line S, active cloud towers only about 2 km wide were found. The anvils were roughly 2 km thick. Patterns of clouds corresponded to a profile of A/B‐scale divergence; active towers at low levels corresponded to convergence up to the 60 kPa level, and stratus coincided with A/B‐scale divergence aloft at about 50 and 26 kPa.

Statistical analyses from the aircraft films indicated that the area covered by clouds of dark base ‐ signifying concentrated updrafts ‐ was ～5%, much less than that covered by rain at cloud base, ～ 18%. Cloud cover at altitude 4 km was ～ 10%.     

The delineation of rain areas from visible and IR satellite data for GATE and mid‐latitudes

Abstract

Two‐dimensional pattern matching has been used to delineate raining areas of clouds from GATE and Montreal GOES visible and IR satellite data, with radar as ground truth. For the cases examined, the cloud cover was of the order of 4 times larger than the rain area, requiring skill to separate out low‐thick or high‐thin non‐precipitating clouds from cumulus systems, which is difficult using a single threshold. The more flexible approach described here has allowed useful rain maps to be generated for all the types of weather systems examined. The optimum boundary separating raining from non‐raining areas is relatively insensitive to diurnal and day‐to‐day variations, but is different for the tropical Atlantic and for Montreal.     

A new algorithm to estimate sky condition based on 5 minutes-averaged values of clearness index and relative optical air mass

Summary This work describes a new algorithm to characterize sky condition in intervals of 5 min using four categories of sun exposition: apparent sun with cloud reflection effects; apparent sun without cloud effects; sun partially concealed by clouds; and sun totally concealed by clouds. The algorithm can also be applied to estimate hourly and daily sky condition in terms of the traditional three categories: clear, partially cloudy and cloudy day. It identifies sky conditions within a confidence interval of 95% by minimizing local climate and measurement effects. This is accomplished by using a logistic cumulative probability function to characterize clear sky and Weibull cumulative probability function to represent cloudy sky. Both probability functions are derived from frequency distributions of clearness index, based on 5 minutes-averaged values of global solar irradiance observed at the surface during a period of 6 years in Botucatu, Southeastern of Brazil. The relative sunshine estimated from the new algorithm is statistically comparable to the one derived from Campbell-Stocks sunshine recorder for both daily and monthly values. The new method indicates that the highest frequency of clear sky days occurs in Botucatu during winter (66%) and the lowest during the summer (38%). Partially cloudy condition is the dominant feature during all months of the year.     

Measurement of soil water contents and frozen soil depth during a thaw using time‐domain reflectometry

Abstract

The results of a field test of time‐domain reflectometry (TDR) to measure apparent liquid soil water contents and to locate the unfrozen‐frozen interface during thawing conditions is presented. The apparent liquid water content was observed in the fall and through a late winter thaw on two sand sites, one with a natural snow cover and the other with snow removed throughout the winter. Temperatures were monitored at intervals throughout the profile. The results indicate that TDR provides a method for monitoring apparent liquid water content andfreeze‐thaw processes.     

A new approach to identify the sensitivity and importance of physical parameters combination within numerical models using the Lund–Potsdam–Jena (LPJ) model as an example

Daily global solar irradiation (R _s) is one of the main inputs in environmental modeling. Because of the lack of its measuring facilities, high-quality and long-term data are limited. In this research, R _s values were estimated based on measured sunshine duration and cloud cover of our synoptic meteorological stations in central and southern Iran during 2008, 2009, and 2011. Clear sky solar irradiation was estimated from linear regression using extraterrestrial solar irradiation as the independent variable with normalized root mean square error (NRMSE) of 4.69 %. Daily R _s was calibrated using measured sunshine duration and cloud cover data under different sky conditions during 2008 and 2009. The 2011 data were used for model validation. According to the results, in the presence of clouds, the R _s model using sunshine duration data was more accurate when compared with the model using cloud cover data (NRMSE = 11. 69 %). In both models, with increasing sky cloudiness, the accuracy decreased. In the study region, more than 92 % of sunshine durations were clear or partly cloudy, which received close to 95 % of total solar irradiation. Hence, it was possible to estimate solar irradiation with a good accuracy in most days with the measurements of sunshine duration.     

Baseline cloudiness trends in Canada 1953–2002

Abstract

Baseline cloudiness trends in Canada for the 1953 to 2002 time period were assessed using raw hourly observations of total cloud cover from eighty‐five airport stations with at least forty years of data. Trends in the count of the number of hours when more than half of the sky dome was observed to be covered by clouds (Mainly Cloudy conditions) increased significantly by several hundreds of hours during the fifty‐year period at numerous stations along the Canada‐U.S border and the few stations located in the Mackenzie Basin.

Trends in average annual overall, daytime, night‐time, and seasonal total cloud cover were also computed. The general spatial pattern was similar as in the case of Mainly Cloudy conditions, however, the exact areal extent and significance of the trends would vary from one case to another. In particular, the area of negative trends in British Columbia and the Prairies, gained in significance and extent in the case of daytime cloudiness, while it reversed to weak, mostly positive, trends in the case of night‐time cloudiness. This could correspond to the unprecedented increases in both daily temperature maximum and minimum in these locations during the second half of twentieth century. Significant positive seasonal trends were seen at some stations in southern Canada during spring (coasts) and summer (centre of the country) and the Mackenzie Basin during summer and fall; while significant negative trends were seen at stations in northern Alberta and British Columbia during winter and north‐eastern Canada during summer.

The quality of the observations was given careful consideration and potential issues that could affect the continuity of the records of observed total cloud amount are discussed at length. The complete monitoring system was examined: training, affiliation and workload of the observers; automation of the observations in the 1990s by the introduction of the Automated Weather Observing Systems (AWOS); numerous changes in data archiving procedures especially prevalent in the 1990s; underestimation of night‐time cloudiness on nights with insufficient illumination, etc. It was determined that observations, although to a certain degree subjective and not particularly accurate for cloud amounts near 5/10, should be reasonably consistent in the case of human observers (no systematic biases); underestimation of night‐time cloudiness, although relevant if the actual amount is required for certain purposes, probably is not critical in studies concerning cloud trends. Automation, possibly along with the changes in data processing in the 1990s, unequivocally emerged as the least studied subject, however, it is likely to have a considerable impact on the continuity of cloud observations at some stations; in this study about 25% of the stations use AWOS in some way.     

Spatiotemporal Variations of Cloud Amount over the Yangtze River Delta, China

Based on the NOAA's Advanced Very High Resolution Radiometer(AVHRR) Pathfinder Atmospheres Extended(PATMOS-x) monthly mean cloud amount data, variations of annual and seasonal mean cloud amount over the Yangtze River Delta(YRD), China were examined for the period 1982–2006 by using a linear regression analysis. Both total and high-level cloud amounts peak in June and reach minimum in December, mid-level clouds have a peak during winter months and reach a minimum in summer, and lowlevel clouds vary weakly throughout the year with a weak maximum from August to October. For the annual mean cloud amount, a slightly decreasing tendency(–0.6% sky cover per decade) of total cloud amount is observed during the studying period, which is mainly due to the reduction of annual mean high-level cloud amount(–2.2% sky cover per decade). Mid-level clouds occur least(approximately 15% sky cover) and remain invariant, while the low-level cloud amount shows a significant increase during spring(1.5% sky cover per decade) and summer(3.0% sky cover per decade). Further analysis has revealed that the increased low-level clouds during the summer season are mainly impacted by the local environment. For example,compared to the low-level cloud amounts over the adjacent rural areas(e.g., cropland, large water body, and mountain areas covered by forest), those over and around urban agglomerations rise more dramatically.     

On the relation between cloud cover amount and sunshine duration

Summary The total cloud cover is deduced from measurements of monthly mean averages of the percent of possible sunshine duration at three locations in Egypt, Cairo, Bahtim and Sedi-Barrani stations during the period 1987–1995. This sunshine-derived total cloud cover (C_s) is compared to conventional ground-based observations of total cloud covers (C_g) made by meteorological observers. A linear relationship between the two estimates is calculated, and the difference between the two estimates as a function of C_s and C_g is fitted with a least-squares linear equation. It is found that on the average the sunshine-derived values of total cloud cover are about 7% lower than the corresponding ground-based estimated of total cloud cover. Both of these parameters are mainly used in solar radiation models and the error sources are mainly depending upon the way to describe sky cover.     

地基可见光/红外全天空成像仪数据融合

采用60m基线的双站可见光成像仪和扫描式红外成像仪构建地基双波段全天空测云系统,双站可见光成像仪根据双站测距原理准确地获取云底高,通过全方位立体扫描获取全天空可见光云图,扫描式红外成像仪基于大气辐射传输原理测量云底亮温,反演云底高度,通过全方位立体扫描获取全天空红外云图。结合双站可见光成像仪测得云底高,对大气温度垂直递减率进行实时订正,提高扫描式红外成像仪反演云底高精度,达到双波段云底高数据融合目的;基于小波变换多分辨率分析的图像融合技术,对全天空可见光和红外云图进行图像融合,提高能见度低、以及雾、霾等天气条件下云量计算准确度;最终实现昼夜云高、云量同时观测。     

Surface‐absorbed and top‐of‐atmosphere radiation fluxes for the Mackenzie River basin from satellite observations and a regional climate model and an evaluation of the model

Abstract

Both the earth‐reflected shortwave and outgoing longwave radiation (OLR) fluxes at the top of the atmosphere (TOA) as well as surface‐absorbed solar fluxes from Canadian Regional Climate Model (CRCM) simulations of the Mackenzie River Basin for the period March 2000 to September 2003 are compared with the radiation fluxes deduced from satellite observations. The differences between the model and satellite solar fluxes at the TOA and at the surface, which are used in this paper to evaluate the CRCM performance, have opposite biases under clear skies and overcast conditions, suggesting that the surface albedo is underestimated while cloud albedo is overestimated. The slightly larger differences between the model and satellite fluxes at the surface compared to those at the TOA indicate the existence of a small positive atmospheric absorption bias in the model. The persistent overestimation of TOA reflected solar fluxes and underestimation of the surface‐absorbed solar fluxes by the CRCM under all sky conditions are consistent with the overestimation of cloud fraction by the CRCM. This results in a larger shortwave cloud radiative forcing (CRF) both at the TOA and at the surface in the CRCM simulation. The OLR from the CRCM agrees well with the satellite observations except for persistent negative biases during the winter months under all sky conditions. Under clear skies, the OLR is slightly underestimated by the CRCM during the winter months and overestimated in the other months. Under overcast conditions the OLR is underestimated by the CRCM, suggesting an underestimation of cloud‐top temperature by the CRCM. There is an improvement in differences between model and satellite fluxes compared to previously reported results largely because of changes to the treatment of the surface in the model.     

利用全天空数字图像对北京上空云况分布特征的试验分析

利用最新获取的近两年北京上空全天空数字图像资料对云况分布做统计分析,以获得云的分布特征。工作中将图像分为9个扇区和16个环区分别进行分析,从结果看,无云(云量<1)与全天空有云天气(云量>9)情况明显占优,平均各占总量的36%,46%。除去系统误差及计算所带来的误差发现,两年中北京上空多以晴好天气(包含薄卷云)和阴天为主。上空西北部云的分布略显偏多,头顶上空云的出现概率较其他位置低,并有随天顶角增大概率增大的趋势。     

Seasonal climatology of cloud at resolute (75°N)

Abstract

An analysis of surface observations of cloud characteristics from a selected high‐latitude station, Resolute (N.W.T., Canada), is described. The data set has a high temporal resolution (1 h) and is extensive although here we report only on an analysis of 10 years: 1970–1979, inclusive. The pattern of cloudiness variations is more complex than previous studies of Arctic data have suggested. The predominant reported low‐level cloud type is stratocumulus, not stratus. The generally overcast summer conditions are found to extend over a considerable period when cloud cover is composed of several layers, often of types other than stratus. This observation of multilayering suggests that middle and high cloud layers may occur more frequently than they can be observed. Clearly a more accurate awareness of hidden cloud is important for improving solutions of the planetary radiation budget. The surface radiation budget is found to be strongly affected by the development of the summertime overcast but also exhibits features caused by the mid‐summer break‐up of the lower cloud layer.     

Ground-based measurements of local cloud cover

Clouds are believed to reflect temporal climate changes through variations in their amounts, characteristics, and occurrence. In addition, they reflect both weather and climate in a region. In this work, a methodology to determine the local cloud cover (LCC) is proposed using sky images obtained from a ground-based instrument. Three years of sky images from an urban, tropical site were obtained and analyzed through that methodology. Monthly average LCC varied from 3 to 96 %, while seasonal average values were 68 % for summer, 54 % for spring, 46 % for fall, and 23 % for winter. LCC results show a clear seasonal dependence and a fair agreement (r ² = 0.72) with satellite data, which typically underestimate the cloud cover in relation to LCC. Our analysis also suggests the possibility of a measurable link between LCC and natural events like the El Niño Southern Oscillation.