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[en] This study uses reanalysis data from ECMWF ERA-Interim and GCM output from the CCSM3 to investigate how sea ice and clouds interact locally (within individual grid boxes) and whether similar variability between the two datasets is captured. During autumn (October), the vertically integrated low cloud amount increases over increased sea ice in the reanalysis, but decreases in the GCM output. Closer inspection, however, reveals that both datasets have more low cloud cover over increased sea ice within the lower boundary layer (1000-925 hPa for the reanalysis and 1000-975 hPa for the GCM output), but they differ in their integrated response within the lower troposphere. These results highlight the differences between the datasets and show the importance of understanding where cloud changes occur, because clouds vary in their effect on the radiation budget as a function of height.
[en] In another lecture a general introduction was given to satellite rainfall estimation techniques. The families of techniques which have been devised to accommodate relatively large numbers of images each day fall into two categories, namely the ''Life History'' techniques and the ''Bispectral'' techniques. In the former, potential rainclouds are identified in infrared satellite imagery by temperature thresholding, after which each individual element of rain cloud is monitored through its individual life cycle. Unfortunately, this is demanding computationally, and has only been employed in research campaigns. The objective techniques which will be described hereafter are under development in the Remote Sensing Unit of the University of Bristol for operational application to areas of continental or sub-continental size. 6 refs, 1 fig
[en] We present a catalog of high-velocity clouds in the region of the Magellanic Leading Arm. The catalog is based on neutral hydrogen (H I) observations from the Parkes Galactic All-Sky Survey. Excellent spectral resolution allows clouds with narrow-line components to be resolved. The total number of detected clouds is 419. We describe the method of cataloging and present the basic parameters of the clouds. We discuss the general distribution of the high-velocity clouds and classify the clouds based on their morphological type. The presence of a significant number of head-tail clouds and their distribution in the region is discussed in the context of Magellanic System simulations. We suggest that ram-pressure stripping is a more important factor than tidal forces for the morphology and formation of the Magellanic Leading Arm and that different environmental conditions might explain the morphological difference between the Magellanic Leading Arm and Magellanic Stream. We also discuss a newly identified population of clouds that forms the LA IV and a new diffuse bridge-like feature connecting the LA II and III complexes.
[en] The Sun illuminates both the Moon and the Earth with practically the same luminous fluxes which are in turn reflected by them. The Moon provides a dim light to the Earth whereas the Earth illuminates the Moon with somewhat brighter light which can be seen from the Earth and is called earthshine. As the amount of light reflected from the Earth depends on part of the Earth and the cloud cover, the strength of earthshine varies throughout the year. The measure of the earthshine light is luminance, which is defined in photometry as the total luminous flux of light hitting or passing through a surface. The expression for the earthshine light in terms of the apparent magnitude has been derived for the first time and evaluated for two extreme cases; firstly, when the Sun’s rays are reflected by the water of the oceans and secondly when the reflector is either thick clouds or snow. The corresponding values are and respectively. The earthshine value reported by Jackson lies within these apparent magnitudes. This paper will motivate the students and teachers of physics to look for the illuminated Moon by earthlight during the waning or waxing crescent phase of the Moon and to reproduce the expressions derived here by making use of the inverse-square law of radiation, Planck’s expression for the power in electromagnetic radiation, photopic spectral luminous efficiency function and expression for the apparent magnitude of a body in terms of luminous fluxes. (paper)
[en] Presented here is the first analysis of hourly solar ultraviolet irradiance (290–385 nm) and broadband global irradiance data, registered in a radiometric station located in the outskirts of Granada (37.18°N, 3.58°W, 660 m a.m.s.l.), an inland location in south-eastern Spain, during a 2-year period. According to the prevailing cloudless conditions, the results show that the highest UV radiation levels are received in June or July and the lowest in December. Hourly monthly means of the ratio UV to broadband solar radiation covers a range from a minimum of ca. 3% to a maximum of 5%. The higher values of this ratio are associated to cloudy situations. In this way for 1994, the lowest values of this ratio are encountered in December, and the highest value appears in May, associated with a higher frequency of cloudy days. The study of the ratio UV to global radiation hourly monthly means reveals a clear effect of the optical air mass and of the cloud cover. It is found that, the UV to broadband global radiation ratio increases with decreasing optical air mass and increasing cloud cover. (author)
[en] We present virial models for the global evolution of giant molecular clouds (GMCs). Focusing on the presence of an accretion flow and accounting for the amount of mass, momentum, and energy supplied by accretion and star formation feedback, we are able to follow the growth, evolution, and dispersal of individual GMCs. Our model clouds reproduce the scaling relations observed in both galactic and extragalactic clouds. We find that accretion and star formation contribute roughly equal amounts of turbulent kinetic energy over the lifetime of the cloud. Clouds attain virial equilibrium and grow in such a way as to maintain roughly constant surface densities, with typical surface densities of order 50-200 Msun pc-2, in good agreement with observations of GMCs in the Milky Way and nearby external galaxies. We find that as clouds grow, their velocity dispersion and radius must also increase, implying that the linewidth-size relation constitutes an age sequence. Lastly, we compare our models to observations of GMCs and associated young star clusters in the Large Magellanic Cloud and find good agreement between our model clouds and the observed relationship between H II regions, young star clusters, and GMCs.
[en] Clouds play a major role in the radiative balance of our planet and largely control dynamic and thermal atmospheric processes that affect the climatic system. In addition to cloud cover (CC), different types of clouds are of special interest, as they contribute in different ways to the energy budget between the earth and the atmosphere. The study analyzes historical sub-daily reports from cloudiness observations conducted at the National Observatory of Athens, dating since the late nineteenth century. Long-term trends and diurnal, intra-annual, and inter-annual variability of both CC and frequency of prevailing cloud types were estimated. The analysis revealed statistically significant positive trends (p < 0.001) in CC over the entire study period, more pronounced in spring and summer. Important changes in the prevalence frequency (PF) of certain cloud types over the last 60 years were also detected. Low, middle, and high clouds exhibited significant long-term trends of opposite sign. A marked increase in the PF of low and high clouds was observed in the recent decades, accompanied with simultaneous decrease in the middle cloud occurrence. Convective clouds prevailed increasingly more frequently over time during winter, while low stratiform clouds declined, in agreement with other observations worldwide. Significant correlation was also found between the PF of stratiform clouds and precipitation frequency.
[en] We establish with certainty that the Magellanic Stream (MS) is some 400 longer than previously known and that the entire MS and Leading Arm system is thus at least 2000 long. With the Green Bank Telescope (GBT), we conducted a ∼200 deg2, 21 cm survey at the tip of the MS to substantiate the continuity of the MS between the Hulsbosch and Wakker data and the MS-like emission reported by Braun and Thilker. Our survey, in combination with the Arecibo survey by Stanimirovic et al., shows that the MS gas is continuous in this region and that the MS is at least ∼1400 long. The MS tip is composed of a multitude of forks and filaments. We identify a new filament on the eastern side of the MS that significantly deviates from the equator of the MS coordinate system for more than ∼450. Additionally, we find a previously unknown velocity inflection in the MS tip near MS longitude LMS ∼-1200 at which the velocity reaches a minimum and then starts to increase. We find that five compact high-velocity clouds cataloged by de Heij et al. as well as Wright's Cloud are plausibly associated with the MS because they match the MS in position and velocity. The mass of the newly confirmed ∼400 extension of the MS tip is ∼2 x 107 Msun (d/120 kpc)2 (including Wright's Cloud increases this by ∼50%) and increases the total mass of the MS by ∼4%. However, projected model distances of the MS at the tip are generally quite large and, if true, indicate that the mass of the extension might be as large as ∼108 Msun. From our combined map of the entire MS, we find that the total column density (integrated transverse to the MS) drops markedly along the MS and follows an exponential decline with LMS of NHi = 5.9 x 1021exp(LMS/19.03) cm-2. Under the assumption that the observed sinusoidal velocity pattern of the LMC filament of the MS is due to the origin of the MS from a rotating LMC, we estimate that the age of the ∼1400 long MS is ∼2.5 Gyr. This coincides with bursts of star formation in the Magellanic Clouds and a possible close encounter of these two galaxies with each other that could have triggered the formation of the MS. These newly observed characteristics of the MS offer additional constraints for MS simulations. In the Appendix, we describe a previously little discussed problem with a standing wave pattern in GBT H I data and detail a method for removing it.
[en] The sampling interval is an important parameter which must be chosen carefully, if measurements of the direct, global, and diffuse irradiance or illuminance are carried out to determine their averages over a given period. Using measurements from a day with rapidly moving clouds, we investigated the influence of the sampling interval on the uncertainly of the calculated 15-min averages. We conclude, for this averaging period, that the sampling interval should not exceed 60 s and 10 s for measurement of the diffuse and global components respectively, to reduce the influence of the sampling interval below 2%. For the direct component, even a 5 s sampling interval is too long to reach this influence level for days with extremely quickly changing insolation conditions. (author)