The spatial distribution of the winter (December to February) precipitation over Greece was related to the eastern North Atlantic-European scale mid-tropospheric circulation fields by means of empirical orthogonal functions (EOFs) and canonical correlation analysis (CCA). The data used in this study are winter precipitation totals, of 23 stations, equally distributed over Greece, and winter mean 500 hPa geopotential heights (30 to 70°N, 30°W to 50°E) for the period 1958 to 1994. The Greek precipitation data were found to be homogeneous (Alexandersson test). A decrease of winter precipitation over the whole country was found, although significant (Mann-Kendall trend test) only over the northern and eastern parts and in the western mountainous regions. Three CCA patterns represent links that are very reasonable from a physical point of view. It is supposed that stronger westerlies over the eastern North Atlantic and the raising of the 500 hPa geopotential height (and also the sea level pressure) over continental Europe during the last few decades were connected with enhanced atmospheric stabilization and anomalous advection of cold and dry air from northerly directions. This led to the winter dryness over the eastern Mediterranean. The probable mechanisms and processes in the complex atmosphere-ocean system, leading to the regional anomalous climatic conditions, are discussed.

Abstract:The deposition of fog to a wind-exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind-induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface-normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain-free conditions (0Ð11 š 0Ð05 mm h 1 ) and rainy conditions (0Ð24 š 0Ð13 mm h 1 ) were about three to six times the eddy-covariance-based estimate (0Ð04 š 0Ð002 mm h 1 ). Under rain-free conditions, water-budget-based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ¾40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ¾770 mm year 1 for the summit cloud forest just below the ridge top (according to the water budget method) and ¾785 mm year 1 for the cloud forest on the lower windward slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence).

The Luquillo Mountains of northeastern Puerto Rico harbours important fractions of tropical montane cloud forests. Although it is well known that the frequent occurrence of dense fog is a common climatic characteristic of cloud forests around the world, it is poorly understood how fog processes shape and influence these ecosystems. Our study focuses on the physical characteristics of fog and quantifies the fogwater input to elfin cloud forest using direct eddy covariance net flux measurements during a 43-day period in 2002. We used an ultrasonic anemometerthermometer in combination with a size-resolving cloud droplet spectrometer capable of providing number counts in 40 droplet size classes at a rate of 12.5 times per second. Fog occurred during 85% of the time, and dense fog with a visibility <200 m persisted during 74% of the period. Fog droplet size depended linearly on liquid water content (r 2 =0.89) with a volumeweighted mean diameter of 13.8 µm. Due to the high frequency of occurrence of fog the total fogwater deposition measured with the eddy covariance method and corrected for condensation and advection effects in the persistent up-slope air flow, averaged 4.36 mm d −1 , rainfall during the same period was 28 mm d −1 . Thus, our estimates of the contribution of fogwater to the hydrological budget of elfin cloud forests is considerable and higher than in any other location for which comparable data exist but still not a very large component in the hydrological budget. For estimating fogwater fluxes for locations without detailed information about fog droplet distributions we provide simple empirical relationships using visibility data.

A land surface model including cloud (fog) water deposition on vegetation was developed to better predict the heat and water exchanges between the biosphere and atmosphere. A new scheme to calculate cloud water deposition on vegetation was implemented in this model. High performance of the model was confirmed by comparison of calculated heat and cloud water flux over a forest with measurements. The new model provided a better prediction of measured turbulent and gravitational fluxes of cloud water over the canopy than the commonly used cloud water deposition model. In addition, simple linear relationships between wind speed over the canopy ( | U | ) and deposition velocity of cloud water (V dep ) were found both in measurements and in the calculations. Numerical experiments using the model were performed to study the influences of two types of leaves (needle and broad leaves) and canopy structure parameters (total leaf area index and canopy height) on V dep . When the size of broad leaves is small, they can capture larger amounts of cloud water than needle leaves with the same canopy structure. The relationship between aerodynamic and canopy conductances for cloud water at a given total leaf area density (LAD) strongly influenced V dep . From this, it was found that trees whose LAD Ϸ 0.1 m 2 m Ϫ3 are the most efficient structures for cloud water deposition. A simple expression for the slope of V dep plotted against LAD obtained from the experiments can be useful for predicting total cloud water deposition to forests on large spatial scales.

Understanding the hydrology of tropical montane cloud forests (TMCF) has become essential as deforestation of mountain areas proceeds at an increased rate worldwide. Passive and active cloud water collectors, throughfall and stemflow collectors, visibility or droplet size measurements, and micrometeorological sensors are typically used to measure fog water inputs to ecosystems. In addition, stable isotopes may be used as a natural tracer for fog and rain.Previous studies have shown that the isotopic signature of fog tends to be more enriched in the heavier isotopes 2 H and 18 O than that of rain, due to differences in condensation temperature and history. Differences between fog and rain isotopes are largest for synoptic-scale rain storms vs.local fogs or orographic clouds. Isotopic differences have also been observed between locally generated rain and fog on mountains with orographic clouds, but only a few studies have been 1 conducted. Quantifying fog deposition using isotope methods is more difficult in forests receiving mixed precipitation, due to limitations in the ability of sampling equipment to separate fog from rain, and because fog and rain may, under some conditions, have similar isotopic composition.This paper describes the various types of fog most relevant to montane cloud forests and the importance of fog water deposition in the hydrologic budget. A brief overview of isotope hydrology provides the background needed to understand isotope applications in cloud forests.A summary of previous work explains isotopic differences between rain and fog in different environments, and how monitoring the isotopic signature of surface water, soil water and tree sap can yield estimates of the contribution of fog water to streamflow, recharge and transpiration.Next, instrumentation to measure fog and rain, and methods to determine isotopic concentrations in plant-and soil water are discussed. The paper concludes with the identification of some of the more pressing research questions in this field and offers various suggestions for future research.