study. This study assumes clear sky conditions for this site in the months of March, April, November and December. The agreement between the ground based measurements and satellite derived estimate points to the fact that this is a reasonable assumption for most of the times. From Figures 1 (b) to 1 (e) which compare the incoming / outgoing longwave radiation in the day and night time, it is seen that the satellite estimates match well with the ground based measurements. It is also emphasized that while the ground based observations refer to a point measurement, the satellite data are areally averaged retrievals. Several other sources of uncertainty in the satellite retrievals are assumptions used in the retrieval algorithm, non-uniform field of view of the sensor etc. The validity of the empirical equations used for the determination of net radiation also adds to the total uncertainty.
The purpose of this paper was to introduce a new cloud detec- tion algorithm for the total sky partly cloudy images. Tradi- tionally, the 2-D R/B methods were widely used and accepted as standard algorithms to detect clouds. The 3-D RGB meth- ods were developed by some researchers in order to improve the accuracy of cloud detection. However, Yang et al. (2015) suggested using the 1-D green channel of the RGB image in- stead of the 2-D R/B and the 3-D RGB methods for cloud detection methods by analyzing the imaging principle of the color camera; so, in this paper, the proposed CSBD algorithm was based on the green channel of the images. The database of CSBL was built by rotating the original TCI images for clear sky conditions. The center of the sun was always on the perpendicular bisector of the image in the CSBL. For any single cloudy TCI image, the CSB image with the same so- lar elevation angle as the TCI image was retrieved from the CSBL. The histogram adjustment was performed to ensure the two images had similar brightness distribution. Finally, the differencing and binarization processing were applied to obtain the final cloud identification result. The test results showed that the proposed CSBD algorithm outperforms the traditional methods, especially in the circumsolar and near- horizon regions and for thin clouds when the sun is visible in the image. Additionally, some bright noises, due to the re- fraction of light, can also be correctly classified as non-cloud pixels.
11 Read more
In central India, monsoon clouds begin to appear in June, therefore starting week of the this month some days are clear sky and after two to three week monsoon clouds begin to appear so that few days are cloudy and few days are hazy. This test is performed on the bases of three different climatic condition for three different days of the month of June these different climate condition was Clear sky day, Hazy day & cloudy day at three different dates was i.e 11 th , 20 th , 25 th of june. One by one we are evaluate the Energy
19 Read more
It is a well-known phenomenon that distant objects near the horizon (e.g. forests or mountains) appear blueish in colour because of Rayleigh scattering of light between the observer and these distant objects (Können, 1985; Coulson, 1988). The same phenomenon occurs in the air column underneath clouds. If part of this column is lit directly by the sun, the distribution of the angle of polarization of scattered light originating from the sunlit part of the column is the same as that of the clear sky. It is less well known that the scattering of direct sunlight on the cloud particles results in the same e-vector pattern as that of the blue sky (Können, 1985). As a result of these scattering phenomena, the angle of polarization of the sky (the most important optical cue for the animal polarization compass if the sun is not visible) underneath certain clouds approximates that of the clear sky. The celestial e-vector pattern continues underneath clouds under certain atmospheric conditions, such as when the air columns beneath clouds or parts of clouds are lit by direct sunlight: (i) obliquely from above (for smaller solar zenith angles), (ii) from the side (as with white cumuli) or (iii) from below (as sometimes occurs at dawn and dusk). The implication here is that the earth’s surface has to be in sunlight, but not at the position of the observer. Below, we refer to these illumination conditions simply as ‘directly lit by the sun’. Apart from heavy overcast skies with multiple cloud layers, such conditions occur frequently if the sky is partly cloudy.
10 Read more
converter was used to take four different polarized photographs of each of the two clear-sky cases at the United States Naval Academy in Annapolis, Maryland. We extract relative radiance data from these photographs by adapting existing algorithms for radiometric calibration of digital images . To prevent flare within the fisheye lens during the day, a small shield was used to occult the Sun and the immediate circumsolar region [11, 12]. In figure 5(b), this occulted area appears as a gap in the dashed lines of observed γ(ζ). Although the clear sky in each fisheye photograph occupies an area ∼ 690 pixels on a side, in figure 5 we sample γ (ζ) only at every 15th row and column.
14 Read more
Hlradia, the simplest of the three schemes, considers one SW and one LW spectral band. Clear-sky transmittance, re- flectance and absorptance of SW flux are taken into account at each model level to obtain the radiative heating (vertical divergence of the net SW flux) and net SW fluxes. The ra- diative transfer is parameterised rather than solved explic- itly, in order to make the scheme very fast for NWP use (Savijärvi, 1990). The impact of ozone, oxygen and carbon dioxide on SW irradiance is assumed to be constant over time and space. In older versions of the scheme, aerosols were accounted for using constant coefficients. However, the scheme has recently been modified to include parameterisa- tions of the direct and semi-direct effects of aerosols, cal- culated using the two-stream approximation equations for anisotropic non-conservative scattering described by Thomas and Stamnes (2002).
17 Read more
CERES views the same footprint from different viewing angles when operating in along-track mode. We choose not to directly compare fluxes inverted from different CERES an- gles, as the shape and size of the CERES footprints change with viewing zenith angle. Instead, we take advantage of the collocated MODIS pixels within a CERES footprint. The MODIS imager observes the same area as CERES within ap- proximately 2 min, but from viewing zenith angles close to nadir. The MODIS pixel-level data are spatially and tempo- rally matched with the CERES footprints, and are averaged over the CERES footprints by accounting for the CERES PSF. These CERES footprints are classified into 55 cate- gories of cloud types, which are functions of cloud layer, cloud fraction, cloud optical depth, and cloud effective pres- sure (Table 4). Among them type 0 is for clear sky, types 1 to 27 are for single-layer cloud types, and types 28 to 54 are for multi-layer cloud types.
17 Read more
The LW CRE shows no diurnal or meridional dependency. It varies between − 40 W m −2 and 100 W m −2 . The nega- tive values are assumed to originate from measurement and parameterisation errors. The SW CRE is dominated by the diurnal solar cycle and varies between − 900 W m −2 and 100 W m −2 . Highest negative effects are caused by opti- cally thick clouds and high solar elevations. At nighttime or under clear sky conditions, the SW CRE is zero. Positive SW CREs result from radiative enhancements during broken cloud events. However, measurement and parameterisation errors may contribute. Due to the meridional dependency of the solar zenith angle, the SW CRE varies with the latitude in general. However, due to the small amount of data at higher latitudes, the effect is not significant.
11 Read more
series of in situ incident and reflected illuminance values that are used to calculate an accurate proxy of the albedo values called the albedo index. The illuminance is the total luminous flux incident on a surface per unit area. It is a measure of how much the incident light illuminates the surface, wavelength- weighted by the luminosity function to correlate with the human perception of brightness. In Sect. 2, we present the characteristics of and uncertainties in the LCA measurements along with a comparison with the theoretical values for 10 different ice and snow states and for two different incident ir- radiance spectra (cloudy or clear sky). Then, Sect. 3 presents two experiments carried out on a high-altitude tropical moun- tain site in Bolivia (Zongo glacierized catchment). A first application for punctual in situ measurements validates the LCA in the field via a comparison with traditional radiome- ters for two contrasting surfaces: snow and ice on the glacier or snow and bare soil on the moraine. After that, a sec- ond application on the same glacier documents the snow–ice changes on the surface of the glacier during the period that extends from September 2015 to June 2016.
10 Read more
Abstract. The Tropospheric Monitoring Instrument (TROPOMI) spectrometer is the single payload of the Copernicus Sentinel 5 Precursor (S5P) mission. It measures Earth radiance spectra in the shortwave infrared spectral range around 2.3 µm with a dedicated instrument module. These measurements provide carbon monoxide (CO) total column densities over land, which for clear sky conditions are highly sensitive to the tropospheric boundary layer. For cloudy atmospheres over land and ocean, the column sensitivity changes according to the light path through the atmosphere. In this study, we present the physics-based operational S5P algorithm to infer atmospheric CO columns satisfying the envisaged accuracy (< 15 %) and precision (< 10 %) both for clear sky and cloudy observations with low cloud height. Here, methane absorption in the 2.3 µm range is combined with methane abundances from a global chemical transport model to infer information on atmo- spheric scattering. For efficient processing, we deploy a linearized two-stream radiative transfer model as forward model and a profile scaling approach to adjust the CO abundance in the inversion. Based on generic measurement ensembles, including clear sky and cloudy observations, we estimated the CO retrieval precision to be ≤ 11 % for surface albedo ≥ 0.03 and solar zenith angle ≤ 70 ◦ . CO biases of ≤ 3 % are introduced by inaccuracies in the methane a priori knowledge. For strongly enhanced CO concentrations in the tropospheric boundary layer and for cloudy conditions, CO errors in the order of 8 % can be introduced by the retrieval of cloud parameters of our algorithm. Moreover,
21 Read more
Abstract—The reliable design of a satellite communications network, operating at Ku band and above, requires the exact evaluation of the interference eﬀects on the availability and performance of both the uplink and downlink. In this paper, the case of Uplink Adjacent Satellite Network Interference is examined. We accurately calculate the deterioration of the uplink clear sky nominal adjacent satellite network Carrier-to-Interference threshold, due to spatial inhomogeneity of the propagation medium. At these frequency bands, rain attenuation is the dominant fading mechanism. Here we present an analytical physical model for the calculation of Interference Statistical Distribution between adjacent Broadband Satellite Networks operating at distances up to 500 km. We employ the unconditional bivariate lognormal distribution for the correlated rain fading satellite channels. Useful numerical results are presented for satellite networks located in diﬀerent climatic regions and with various quality of service (QoS) assumptions.
13 Read more
is a critical index used to characterize the performance of an FTS and has an important influence on the accuracy of trace gases retrieval. The ILS of B3M-IR is determined by measuring the transmission spectra of a 1000 K blackbody through a low-pressure gas cell full of CO and by retrieving the phase error and modulation efficiency of the B3M-IR from CO absorption features with the software LINEFIT . The results of B3M-IR ILS measurements are presented in Figure 1. Another important factor that influences the accuracy of trace gas retrieval is the instrumental signal-to-nose ratio (SNR). As shown in Figure 2, B3M-IR can achieve an SNR better than 100 in the microwindows usually used for trace gases retrieval. Using our data acquisition system, we obtained the clear sky solar absorption spectraduring the Olympic Games. The observation was started on July 22th, 2008 and ended on September 5th, 2008.
13 Read more
El-shanshoury et al (2017), developed the model with multiple regression equation for Sharm El-Sheikh city. This model differs from other searchers empirical models that predict the monthly clear sky monthly global solar radiation future time ( H ) . The developed multiple linear regression model is based on three predictor parameters as independent variables. These parameters are: monthly average cosine solar zenith angle at mid-time between sunrise and solar noon cos( ZMT ) , monthly average daily mean temperature ( T ) and monthly average day length (S 0 ). Two predictors are calculated
25 Read more
The ice tests dominate the Ci cloud scenes and reaffirm the sensitivity of AIRS to ice clouds. CloudSat-classified clear scenes contain occasional occurrences of AIRS-detected thin cirrus ( + 1 and + 2), consistent with either thin cirrus that is undetected by the CloudSat radar or thicker cirrus within the AIRS footprint but to the side of the CloudSat ground track (e.g., Kahn et al., 2008). A few occurrences of − 1 and − 2 may also arise from spatial mismatches between AIRS and CloudSat scenes, or from stratus below 1 km in alti- tude that is undetected by CloudSat. In the Sc cloud scenes, trade cumulus clouds dominate as previously shown by Yue et al. (2011) and Kahn et al. (2017). A larger proportion of liquid tests, and a smaller proportion of ice tests, is observed in the Sc cloud scenes compared to clear sky, but undeter- mined phase is dominant in both scene types. The Cu and Sc cloud scene histograms are generally similar with more un- determined cases for Cu, but with a slight reduction of liquid and slight increase in ice observed for Cu compared to Sc.
17 Read more
Abstract:- Energy efficiency provided by daylight requires an accurate estimation of the amount of daylight entering a building. The actual daylight illuminance of a room is mainly influenced by the luminance levels and patterns of the sky in the direction of view of the window at that time. The daylight coefficient concept, which considers the changes in the luminance of the sky elements, offers a more effective way of computing indoor daylight illuminances. Recently, Kittler et al. have proposed a new range of 15 standard sky luminance distributions including the CIE (International Commission onIllumination) standard clear sky. Lately, these 15 sky luminance models have been adopted as the CIE Standard General Skies.This paper aims to find out representative CIE (International Commission on Illumination) Standard Clear Sky model(s) for three different seasons-winter solstice, equinox, and summer solstice applicable for prevailing clear sky climatic conditions in India [Roorkee]. Indian measured sky luminance distribution database is available only for Roorkee[29 0 51 ' N; 77 0 53 ' E]. To find out the best match between Indian measured sky luminance distribution and each of five CIE Standard Clear sky models, only sky component of spatial illuminance distribution over the working plane of a room was simulated by MATLABfor three different seasons. Daylight Coefficient method has been applied for the simulation using Indian sky luminance database.The simulation has been done for the room with eight different window orientations ranging from 0 0 to 315 0 with an interval of 45 0 to generate data for the entire sky vault. To find out the best fit CIE model(s)Indian measured sky luminance distribution data is taken as reference. Analysis revealed that CIE Standard General Clear sky type 15 described as “White-blue sky, turbid with a wide solar corona effect”is the best-fit clear sky model for both summer and equinox seasonsand sky type 11 described as “White blue sky with a clear solar corona” is the best-fit clear sky model for winter seasonat Roorkee.
Figure 6. Performance of the Sun-tracking device at the Kanzelhöhe Observatory ARAD site for days with nearly continuous clear sky (7 May 2015, left column) and clear sky interrupted by frontal movement (22 April 2015, right column): (a) radial distance from the zero point of each valid image (asterisks); (b) direct radiation (1 min averages) derived from the Kanzelhöhe Observatory ARAD site (line) and from actual Sun-sensor measurements of the Sun-tracking device (in 10 min intervals, blue dots) on 7 May 2015. (c–d) As panels a–b but for 22 April 2015. Limits of active tracking (i.e., alignment within 0.02 ◦ accuracy, green line) and passive tracking (i.e., alignment within 0.1 ◦ accuracy, red line) are shown in panels a and c. The yellow dashed line in panels b and d indicates the manufacturer-specified minimum of direct radiation (300 W m −2 ) needed for active-tracking mode. The vertical black dot–dashed line indicates the astronomical noon in all panels.
10 Read more
high-energy, ultraviolet (UV) radiation. Some of this UV radiation is especially effective in causing damage to living beings, the largest decreases in ozone during the past 15 years have been observed over Antarctica, especially during each September and October when the ozone hole forms. During the last several years, simultaneous measurements of UV radiation and total ozone have been made at several Antarctic stations. In the late spring, the biologically damaging ultraviolet radiation in parts of the Antarctic continent can exceed that in San Diego, California, where the Sun is much higher above the horizon. In areas where smaller ozone depletion has been observed, increases are more difficult to detect. In particular, detection of trends in UV-radiation associated with ozone decreases can be further complicated by changes in cloudiness, by local pollution, and by difficulties in keeping the detection instrument in precisely the same condition over many years. Prior to the late 1980s, instruments with the necessary accuracy and stability for measurement of small long-term trends in ground-level UV were not available. Therefore, the data from urban locations with older, less-specialized instruments provide much less reliable information, especially since simultaneous measurements of changes in cloudiness or local pollution are not available. When high- quality measurements have been made in other areas far from major cities and their associated air pollution, decreases in ozone have regularly been accompanied by increases in UV. This is shown in the figure below, where clear-sky measurements performed at six different stations demonstrate that ozone decreases lead to increased UV radiation at the surface in amounts that are in good agreement with that expected from calculations (the "model" curve).
Abstract. Broadband short-wave (SW) surface direct and diffuse irradiances are not typically within the set of output variables produced by numerical weather prediction (NWP) models. However, they are frequently requested for solar en- ergy applications. In order to compute them, a detailed rep- resentation of the aerosol optical properties is important. Nonetheless, NWP models typically oversimplify aerosol representation or even neglect their effect. In this work, a flexible method to account for the SW aerosol optical prop- erties in the computation of broadband SW surface direct and diffuse irradiances is presented. It only requires aerosol optical depth at 0.55 µm and knowledge of the type of pre- dominant aerosol. Other parameters needed to consider spec- tral aerosol extinction, namely, Angström exponent, aerosol single-scattering albedo and aerosol asymmetry factor, are parameterized. The parameterization has been tested using the Rapid Radiative Transfer Model for climate and weather models (RRTMG) SW scheme of the Weather Research and Forecasting (WRF) NWP model for data over the continen- tal US. In principle, it can be adapted to any other SW ra- diative transfer band model. It has been verified against a control experiment and using data from five radiometric sta- tions in the contiguous US. The control experiment con- sisted of a clear-sky evaluation of the RRTMG solar radi- ation estimates obtained in WRF when RRTMG is driven with ground-observed aerosol optical properties. Overall, the verification has shown satisfactory results for both broad- band SW surface direct and diffuse irradiances. The param- eterization has proven effective in significantly reducing the
16 Read more
Lines et al. (2018), and described in Section 2.3, we took the ex- isting ‘hot’ HD 209458b cloudy atmosphere, at t = 100 earth days of simulated cloud formation, and continued this simulation under the same conditions for a single hydrodynamical (and radiative) time-step. During this step, a second diagnostic call to the radiative transfer scheme was enabled whereby the transmitted flux is calcu- lated using spectral files that cover λ = 0.2–10,000 μm using 950 bands, a significant improvement over the 32 bands used to obtain the heating rates (see Amundsen et al. 2014 for more information). We simulated, and extracted the transmitted flux from, atmo- spheres corresponding to 18 scenarios that explore the roles of cloud opacity, composition, and gravitational settling. In the first study, we analysed two clear-sky atmospheres with PT profiles cor- responding to an atmosphere prior to and after modification by ra- diatively active clouds, a cloudy simulation with full opacity from our mixed-composition cloud particles, including both scattering and absorption by cloud particles, one without cloud particle ab- sorption and one without cloud particle scattering. In the second study, we added a further four cases in which we scaled the cloud opacity (by way of the scattering and absorption coefficients) by 0.1, 0.01, 0.001, and 0.0001.
12 Read more
full-physics errors over ocean and slightly larger (0.1 ppm) over land (as shown in Fig. 4). The coverage of post-filtering to 30 %, shown in Fig. 7, is spatially dependent because of a preference to remove measurements over regions that per- sistently contain clouds or aerosols (e.g., the Sahara) or have low signal to noise ratios (e.g., high latitudes). The clear-sky and full-physics retrieval mean error spatial patterns are sim- ilar and both relatively small in magnitude, which indicates that they do not contain large regional biases and, more im- portantly, that the clear-sky retrieval does not have signifi- cantly larger biases than the full-physics retrieval. The scat- ter and the RMSEs both show limited regional variability over ocean and modest variability over land. The regional clear-sky retrieval errors are approximately the same magni- tude as the full-physics retrieval errors over ocean and only marginally larger over land. Overall, these simulated results are promising because they demonstrate that the clear-sky X CO 2 retrieval has global and regional error statistics similar
14 Read more