Treelines have long been recognized as important ecotones and likely harbingers of climate change. However, over the last century many treelines have been affected not only by global warming, but also by the interactions of climate, forest disturbance and the consequences of abrupt demographic and economic changes. Recent research has increasingly stressed how multiple ecological, biophysical, and human factors interact to shape ecological dynamics. Here we highlight the need to consider interactions among multiple drivers to more completely understand and predict treeline dynamics in Europe.

Abstract. We present an automated method to identify periods of clear skies for a 160 ø field of view using only 1-min measurements of surface downwelling total and diffuse shortwave irradiance. The clear-sky detection method is verified using Whole Sky Imager and lidar data, observer reports, and model comparisons. Identified clear-sky irradiance measurements are then used to empirically fit clear-sky irradiance functions using the cosine of the solar zenith angle as the independent variable. These fitted functions produce continuous estimates of clear-sky total, diffuse, and direct component shortwave irradiances. While this method ignores diurnal changes in such variables as column water vapor and aerosol amounts and changes between clear-sky days, it is shown that the resultant clear-sky irradiance estimates have RMS uncertainty comparable to the uncertainty of the measuring instruments themselves. The estimated clear-sky irradiances are used to estimate the effect of clouds on the downwelling shortwave irradiance as a difference between the measured and clear-sky amounts. We show that the cloud effect calculations from this method appear to decrease the uncertainty due to systematic pyranometer offsets and cosine response errors. Thus any data set that includes downwelling diffuse and total shortwave measurements can be processed to identify clearsky periods and produce estimates of clear-sky irradiance and cloud effects. IntroductionUnderstanding the effects of clouds on the shortwave irradiance at the surface is of critical importance for a wide variety of surface radiative energy budget studies. One approach to investigating the effect of clouds is to compute the difference between expected clear (i.e., cloudless) sky irradiance and measured irradiance or, alternatively, the ratio of cloud to clear irradiance. Typically, we calculate surface clear-sky irradiance using radiative transfer models. These models require aerosol, temperature, and humidity profiles as input, as well as surface albedo. In many instances, this information is not readily available. In addition, even if all the physical properties of the atmospheric state were well known, the computed irradiances may not be in agreement 15,609

No abstract

A discussion is presented of daytime sky imaging and techniques that may be applied to the analysis of full-color sky images to infer cloud macrophysical properties. Descriptions of two different types of skyimaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although some uncertainty exists in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky-imager retrievals. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace, traditional human observations of sky cover and, potentially, cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction further enhance the usefulness of sky imagers.

The inability of regional models and global climate models to reproduce Arctic clouds and the Arctic radiation budget may be due to inadequate parameterizations of ice nuclei.

A surface radiation budget observing network (SURFRAD) has been established for the United States to support satellite retrieval validation, modeling, and climate, hydrology, and weather research. The primary measurements are the downwelling and upwelling components of broadband solar and thermal infrared irradiance. A hallmark of the network is the measurement and computation of ancillary parameters important to the transmission of radiation. SURFRAD commenced operation in 1995. Presently, it is made up of six stations in diverse climates, including the moist subtropical environment of the U.S. southeast, the cool and dry northern plains, and the hot and arid desert southwest. Network operation involves a rigorous regimen of frequent calibration, quality assurance, and data quality control. An efficient supporting infrastructure has been created to gather, check, and disseminate the basic data expeditiously. Quality controlled daily processed data files from each station are usually available via the Internet within a day of real time. Data from SURFRAD have been used to validate measurements from NASA's Earth Observing System series of satellites, satellite-based retrievals of surface erythematogenic radiation, the national ultraviolet index, and real-time National Environmental Satellite, Data, and Information Service (NESDIS) products. It has also been used for carbon sequestration studies, to check radiative transfer codes in various physical models, for basic research and instruction at universities, climate research, and for many other applications. Two stations now have atmospheric energy flux and soil heat flux instrumentation, making them full surface energy balance sites. It is hoped that eventually all SURFRAD stations will have this capability. 1 • Introduction The National Oceanic and Atmospheric Administration's (NOAA's) Surface Radiation budget network (SURFRAD) is the first of its kind to operate across the United States. The network began in 1995 with four stations and expanded to six in 1998 (Fig. 1). Its mission is to provide the climate research, weather forecasting, satellite, and educational communities with continuous, accurate, high quality surface radiation budget measurements for different climates of the United States. Quality assurance in the station design,

This study investigates recent variations in downwelling surface solar radiation inferred from a comprehensive set of ground‐based observational records updated for the period 2000–2005. Surface radiation data beyond the year 2000 are particularly interesting as they provide independent and complementary information to the ambitious satellite programs which became operational with the beginning of the new millennium. The surface records suggest a continuation of the surface solar brightening beyond 2000 at numerous stations in Europe and the United States, as well as parts of east Asia (Korea). Surface solar radiation variations in Europe after 2000 are dominated by a large positive anomaly in the year 2003 with its unprecedented summer heat wave, exceeding 10 Wm−2 on an annual and 20 Wm−2 on a summer mean basis in central Europe. The brightening seen at sites in Antarctica during the 1990s, influenced by a recovery from the low atmospheric transparency after the Mount Pinatubo volcanic eruption in 1991, fades after 2000. The brightening tendency also seems to level off at sites in Japan. In China there is some indication for a renewed dimming, after the stabilization in the 1990s. A continuation of the long‐lasting dimming is also noted at the sites in India. Overall, the available data suggest continuation of the brightening beyond the year 2000 at numerous locations, yet less pronounced and coherent than during the 1990s, with more regions with no clear changes or declines. Therefore, globally, greenhouse warming after 2000 may be less modulated by surface solar variations than in prior decades.

Many clouds important to the Earth's energy balance contain small amounts of liquid water, yet despite many improvements, large differences in retrievals of their liquid water amount and particle size still must be resolved.