Search

Although ozone appears in the Earth's atmosphere in a small abundance, it plays a key role in the energy balance of the planet through its involvement in radiative processes. Its absorption of solar radiation leads to the temperature increase with height defining the tropopause and the stratosphere. Moreover, excluding water vapor, O is the third most important contributor (after CO and CH) to the greenhouse radiative forcing. Thus, the total removal of O content in an Earth-like atmosphere may cause interesting response of the climate system that deserves further investigation. The present paper addresses this issue by means of a global climate model where the atmosphere is coupled with a passive ocean of a given depth. The model, after reaching the statistical equilibrium under present climate conditions, is perturbed by a sudden switch off of the O content. Results obtained for the new equilibrium suggest that the model gets in a colder state mainly because of the water vapor content decrease. Most of the cooling occurs in the Southern Hemisphere while in the Northern Hemisphere the ice cap melts quite consistently. This process appears to be governed by the northward cross-equatorial heat transports induced by changes in the general circulation. [ABSTRACT FROM AUTHOR]

This paper deals with fractal analysis of daily solar irradiances measured with a time step of 10 minutes at Golden and Boulder located in Colorado. The aim is to estimate the fractal dimensions in order to perform classification of daily solar irradiances. The estimated fractal dimensionand the clearness indexKTare used as classification criteria. The results show that these criteria lead to three classes: clear sky, partially covered sky and overcast sky. The results also show that the evaluation of the fractal dimension of the irradiance signal based on a data set with 10 minutes time step is possible. [ABSTRACT FROM AUTHOR]

The three-cell model, including the Hadley, Ferrel, and Polar cells in each of two hemispheres, has been accepted for a long time and the strongest Hadley cell has been used to study the climate change in recent years. However, two questions, why the upper level flow of Ferrel cell does not match observations and how many cells exist in the two polar regions, still exist. Using three different reanalysis datasets for the last 30 years, this paper showed that there might be an additional cell in each of two polar regions. The analyses of meridional-vertical section streamline (MSS), meridional-mass stream function (MSF), and climatic vertical velocity provide some evidences to support the existence of the new Arctic and Antarctic cells located in the troposphere. Thus, an eight-cell model in the global troposphere is proposed in this study. The maximum intensity of the Hadley cell in the boreal winter indicated by MSF in the Northern Hemisphere (NH) is stronger than that of the Ferrel cell for about 4.8 times, so the upper level northeasterly wind of Ferrel cell is too weak to be detected when compared with the stronger southwesterly wind of the Hadley cell. [ABSTRACT FROM AUTHOR]

A regional analysis of soil temperature (ST) is essential for improving our understanding of the soil thermal regime and its link with the atmosphere. This study attempts to assess trends in the Poyang Lake Basin (PLB) ST magnitude during 1960–2016 from station observations at multiple depths. The Mann–Kendall, Thiel-Sen, linear regression, and probability density statistics (PDF) are used for ST trend assessment with a significance level of 95%. The ST seasonal variability shows minimum values in winter (8℃) and maximum in the summer season (32 ℃). On an interannual scale, spring and winter seasons exhibited a significant increase in both land surface temperature (LST) (0.4℃, 0.4℃) and ST (0.3 °C, 0.15℃) magnitude than summer (LST − 0.1℃, ST 0.2℃) and autumn seasons (LST 0.3℃, ST 0.2℃). The northern basin exhibited a significant increase in LST, and ST magnitude, especially during the cold seasons (spring, winter) than the warm seasons. The maximum and minimum temperature trend and their diurnal difference infer an increase in the minimum temperature, especially during the summer, autumn, and winter seasons. The PDF further inferred that extreme cold events' frequency decreased, and a significant increase in extreme warm events is obvious in the recent decade. The increasing trend in soil temperature magnitude is more in the northern basin than the high-altitude southern basin. Large-scale global warming and regional water and energy cycle changes can be the leading factors of such a warming trend. [ABSTRACT FROM AUTHOR]

This paper designs a comprehensive approach based on the engineering machine/ system concept, to model, analyze, and assess the level of carbon dioxide (CO) exchange between the atmosphere and terrestrial ecosystems, which is an important factor in understanding changes in global climate. The focus of this article is on spatial patterns and on the correlation between levels of CO fluxes and a variety of influencing factors in eco-environments. The engineering/machine concept used is a system protocol that includes the sequential activities of design, test, observe, and model. This concept is applied to explicitly include various influencing factors and interactions associated with CO fluxes. To formulate effective models of a large and complex climate system, this article introduces a modeling technique that will be referred to as stochastic filtering analysis of variance (SF-ANOVA). The CO flux data observed from some sites of AmeriFlux are used to illustrate and validate the analysis, prediction, and globalization capabilities of the proposed engineering approach and the SF-ANOVA technique. The SF-ANOVA modeling approach was compared to stepwise regression, ridge regression, and neural networks. The comparison indicated that the proposed approach is a valid and effective tool with similar accuracy and less complexity than the other procedures. [ABSTRACT FROM AUTHOR]

Based on homogenized land surface air temperature (SAT) data (derived from China Homogenized Historical Temperature (CHHT) 1.0), the warming trends over Northeast China are detected in this paper, and the impacts of urban heat islands (UHIs) evaluated. Results show that this region is undergoing rapid warming: the trends of annual mean minimum temperature (MMIT), mean temperature (MT), and mean maximum temperature (MMAT) are 0.40 C decade−1, 0.32 C decade−1, and 0.23 C decade−1, respectively. Regional average temperature series built with these networks including and excluding “typical urban stations” are compared for the periods of 1954–2005. Although impacts of UHIs on the absolute annual and seasonal temperature are identified, UHI contributions to the long-term trends are less than 10% of the regional total warming during the period. The large warming trend during the period is due to a regime shift in around 1988, which accounted for about 51% of the regional warming. [ABSTRACT FROM AUTHOR]

Sufficient site-level observations are needed to resolve the discrepancies between in situ and space-based observations for reliable climate mitigation and adaptation strategies. Resolving such discrepancies from climate-sensitive and understudied Himalaya is the priority for the Indian sub-continent. This study investigates characteristics and dynamics of equivalent black carbon aerosol (eBC) including sources over a glaciated valley at the transitional climate zone between central and western Himalaya. Thermo-topographic factors influencing valley-scale dynamics of eBC were observed by coupled measurements on instantaneous eBC (Aethalometer) and meteorological state parameters at a high altitude (4000 m asl) in the Indian Himalaya covering an annual cycle (October 2015 to August 2016). Results indicate a very large variation (38–5638 ng m−3) in mean daily BC concentration (308 ± 37 ng m−3). Seasonally, the eBC concentration was found highest during the pre-monsoon (1276 ± 115 ng m−3) and lowest during the monsoon season (308 ± 37 ng m−3). In contrast, the magnitude was comparable during winter and post-monsoon seasons (400–500 ng m−3). BC-induced mean annual radiative forcing at the atmosphere was + 10.1 ± 3.0 W m−2. These results indicate a substantial eBC burden over the region even at this high tropospheric altitude in this pristine glacier environment. On the other hand, the diurnal-scale variability in eBC concentration is primarily governed by high-altitude meteorological processes. Further, source apportionment analyses underscore the seasonal scale influence of monsoon and westerly circulation systems. [ABSTRACT FROM AUTHOR]

The surface radiation and energy flux in the source area of the Yellow River are estimated by using the Moderate Resolution Imaging Spectroradiometer products from 2000 to 2019 and verified through the in situ observed data. The changes in land cover type and its effect on water exchange and climate are analyzed. Results show that the latent heat estimated by Priestley-Taylor formula in the energy limited area is close to the measured value, and the root mean square error is 17.8 W m−2. From 2000 to 2019, the daytime land surface temperature (LST) in the source area of the Yellow River mainly shows a negative change trend and decreases significantly in some areas, about − 0.2 °C/a. The nighttime LST shows a positive change trend, and the trend is mainly concentrated at 0–0.15 °C/a. The trend of average land surface temperature calculated from daytime and nighttime LST is mainly concentrated between ± 0.05 °C/a, with a weak warming trend. The LST difference between day and night decreases significantly, and the variation values are mainly concentrated in the range of 0–0.3 °C/a. The albedo in the study area mainly shows a positive trend, and the change value is mainly concentrated between − 0.002 and 0.004/a. The net radiation (Rn) decreases, and the sensible heat (Hs) and latent heat (LE) show a negative trend. The change in land cover type in the source area of the Yellow River mainly occurred in 2016 and 2017. The conversion between grasslands and barren accounts for 70% of the total conversion times. The conversion between grasslands and croplands accounts for 11%. The conversion between water body and barren accounts for 5.9%. The conversion between grasslands and water body accounts for 5.1%, and the other thirty conversions of land cover type account for 8% of the total conversion times. In the dominant land cover conversion type, the mutual conversion between grasslands and barren amplifies the reduction of daytime LST, decelerates the increase in average land surface temperature, and inhibits the water exchange between land and atmosphere because the cooling effect caused by the conversion from barren to grasslands is significantly stronger than the warming effect caused by the conversion from grasslands to barren. Grassland protection is still an important task of ecological protection in the source area of the Yellow River. [ABSTRACT FROM AUTHOR]

In the last decades, anthropogenic drivers have significantly influenced the natural climate variability of Earth's atmosphere. Climate change has become a subject of major interest for different levels of our society, such as national governments, businesses, local administration, or citizens. While national and local policies propose mitigation and adaptation strategies for different sectors, public perception is a key component of any implementation plan. This study investigates the CC perception in Romania, based on a national-scale online survey performed in the spring of 2020, aiming to outline the prominence of environmental and CC issues, level of information and interest, perceived causes, changes perceived in meteorological phenomena at the regional scale, perceived impacts, and the psychological representation of the CC. The study investigates single causal factors of perception. We found that particularly (i) the regional differences on climate change intensity strongly bias the perception of CC causes; (ii) age is very likely to influence the acceptance of CC, the importance of environmental issues, and the levels of information and interest; while (iii) age, gender, and place of residence (rural–urban) are very likely to control the changes perceived in the occurrence of various meteorological phenomena, and their impact. This research is the first statistically relevant analysis (± 4%, statistical significance) developed at national and regional scales and the only study of climate change perception performed during the COVID-19 pandemic in Romania. Its results may represent the baseline for more in-depth research. [ABSTRACT FROM AUTHOR]

Understanding the dynamics and distribution of CO2 in the subsurface atmosphere of carbonate karst massifs provides important insights into dissolution and precipitation processes, the role of karst systems in the global carbon cycle, and the use of speleothems for paleoclimate reconstructions. We discuss long-term microclimatic observations in a passage of Postojna Cave, Slovenia, focusing on high spatial and temporal variations of pCO2. We show (1) that the airflow through the massif is determined by the combined action of the chimney effect and external winds and (2) that the relationship between the direction of the airflow, the geometry of the airflow pathways, and the position of the observation point explains the observed variations of pCO2. Namely, in the terminal chamber of the passage, the pCO2 is low and uniform during updraft, when outside air flows to the site through a system of large open galleries. When the airflow reverses direction to downdraft, the chamber is fed by inlets with diverse flow rates and pCO2, which enter via small conduits and fractures embedded in a CO2-rich vadose zone. If the spatial distribution of inlets and outlets produces minimal mixing between low and high pCO2 inflows, high and persistent gradients in pCO2 are formed. Such is the case in the chamber, where vertical gradients of up to 1000 ppm/m are observed during downdraft. The results presented in this work provide new insights into the dynamics and composition of the subsurface atmosphere and demonstrate the importance of long-term and spatially distributed observations. [ABSTRACT FROM AUTHOR]

Earth is a cloudy planet—at every moment, 62–72% of the planet is covered by cloud. Its presence and properties affect the flow of energy between the sun, Earth and the atmosphere. More comprehensive information about cloudiness is important to improve our understanding of the impact of cloud on Earth's radiation budget. The aim of our work is to assess the accuracy of ground-based high-level cloud detections over Poland compared with satellite data. This project is the very first evaluation of error in visual high-cloud observation. Specifically, we answer a fundamental question: how effective are ground-based observers in detecting high-level cloud? Our unprecedented analysis is based on two datasets: traditional, visual observations from 43 selected synoptic stations and satellite (lidar) data, for the years 2006–2017. Ground-based observations were obtained from the archives of the Polish National Weather Service (IMWM-NRI), while satellite data came from the CALIPSO mission. Our final dataset comprised 7548 ground-based and satellite observations. Accuracy was assessed using the following indicators: probability of detection (Pd), probability of false alarm (Pfa), and overall accuracy (ACC). Overall (day and night observations), in the absence of lower-level cloud, observers correctly detected 80% of high-level cloud. However, when lower-level cloud was present, observers could not detect around 70% of high-level cloud. In sum, ground-based, high-level cloud detection is a function of the presence of lower-level cloud, the time of day, and the size of the area over which cirrus are observed/detected. [ABSTRACT FROM AUTHOR]

It has been confirmed that the karst rocky desertification (KRD) has severely changed the interactions between atmosphere and terrestrial ecosystem which would cause a significant effect on regional climate system. In this study, the Weather and Research Forecasting model was applied to investigate the biogeophysical impact on temperature and precipitation change by land use/land cover change data with different KRD conditions in 1993, 2003, and 2013 in Southwest China. The results showed that an improving trend of KRD has been found in Southwest China, especially in east Sichuan, Chungking, southwest Yunnan, and south Guangxi. The 2-m air temperature decreased by 0~− 1 °C in general with this improving trend. The possible reason was the decreasing KRD accompanied by the decreasing albedo, causing the increasing net shortwave radiation and the increasing net radiation. Meanwhile, the increasing evaporation strengthened the latent heat flux and weakened the sensible heat flux so that decreased temperature was addressed in forest areas. The effect of KRD change transferred to the upper troposphere through atmosphere vertical convection, which made the subtropical high to be strengthened and westerly extended. Therefore, the upward moisture flux at the surface (QFX) was weakened in the central part of Guangxi, southern Guizhou, which led to the decrease of the precipitation. Moreover, the southwest monsoon was strengthened, which caused the increasing water vapor flux and led to the heavy rainfall in the west of Yunnan. [ABSTRACT FROM AUTHOR]

In this study, the Sudan low and the associated Red Sea trough (RST) are objectively identified using the mean sea level pressure (SLP) data from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset covering the period 1955–2015. The Sudan low was detected in approximately 60.6% of the autumn periods, and approximately 83% of the detected low-pressure systems extended into RSTs, with most generated at night and during cold months. The distribution of the RSTs demonstrated that Sudan, South Sudan and Red Sea are the primary development areas of the RSTs, generating 97% of the RSTs in the study period. In addition, the outermost areas affected by RSTs, which include the southern, central and northern Red Sea areas, received approximately 91% of the RSTs originating from the primary generation areas. The synoptic features indicated that a Sudan low developed into an RST when the Sudan low deepened in the atmosphere, while the low pressures over the southern Arabian Peninsula are shallow and the anticyclonic systems are weakened over the northern Red Sea. Moreover, stabile areas over Africa and Arabian Peninsula form a high stability gradient around the Red Sea and the upper maximum winds weaken. The results of the case studies indicate that RSTs extend northward when the upper cyclonic and anticyclonic systems form a high geopotential gradient over Arabian Peninsula. Furthermore, the RST is oriented from the west to the east when the Azores high extends eastward and the Siberian high shrinks eastward or shifts northward. [ABSTRACT FROM AUTHOR]

Črna Jama is the coldest section of cave within the Postojna Cave System. Mean annual air temperatures at the Črna Jama 2 site are 5.6 °C (2015) and 5.7 °C (2016), and at the Črna Jama 3 site 7.1 °C (2015) and 7.2 (2016), whereas the mean external air temperature was 10.3 °C (2015) and 10.0 °C (2016). In Lepe Jame, the passage most heavily visited by tourists, the mean cave-air temperature is 10.7 °C (2014-2017). Črna Jama exhibits winter and summer temperature regimes. During warm periods (Tcave < Tout), it acts as a cold air trap, exchanging no air with the outside atmosphere. Under such conditions the cave-air temperature shows no short-term diurnal temperature oscillations. Cave-air temperature is significantly stable and affected only by elevation of the groundwater table, which is associated with precipitation. During cold periods (Tcave > Tout), ventilation takes place and dense, cold, outside air sinks into Črna Jama because of the favourable cave entrance morphology. Recent Črna Jama air temperature data (2014-2017) indicate a < 0.5 °C higher temperature than that recorded in historical data since 1933. Črna Jama is the most appropriate place within the Postojna Cave System to study long-term climatic changes. There are hardly any tourist visits to the cave, and human impacts on the cave climate are essentially reduced. [ABSTRACT FROM AUTHOR]

In this paper an attempt is made to detect prolonged (of more than 24 hours duration) temperature inversions in the planetary boundary layer over Athens, to study their main characteristics and to find out the synoptic situations with which the inversions are associated. Given the close relationship between the synoptic-scale atmospheric circulation and the occurrence, maintenance and decay of temperature inversions, a simultaneous three category classification of presented inversions and their respective synoptic situations is presented. The classification relies mainly on the similarities and differences in theformation and the maintenance of prolonged temperature inversions. To provide a record of the structure of the lower troposphere and the synoptic conditions favourable to the formation of inversions, mean ascents of temperature and dew-point temperature and mean wind profiles for the years 1980-1994 were calculated for each category into which a total of 297 cases fell. The main element of this structure whichstrongly affects the pollution of the lower troposphere is the prolonged temperature inversion. Also, for each category, mean 500 and 850hPa heights and temperature charts, 500 hPa height anomaly charts, mean sea level (MSL) pressure charts and MSL pressure anomaly charts were drawn. [ABSTRACT FROM AUTHOR]

The goal of the paper is to present and examine the method of classification of daily circulation patterns that allows (i) a fair comparison of groupings among different datasets (typically representing theobserved climate and that simulated by a general circulation model, GCM) and (ii) huge datasets, common in GCM studies based on daily values, to be classified. The circulation classification method is shownto be a useful tool in GCM validation and analysis of climate changeresponse, particularly in comparisons of (i) shapes of the mean typepatterns, (ii) the frequency and persistence of the types, (iii) theprobability of transitions from one type to another, and (iv) conditional surface temperature distributions. It is also shown that a simultaneous examination of multiple classifications is beneficial in eliminating subjectivity of any single classification and allowing a detailed inspection of differences between climates. The classification method is a modification of the T-mode principal component analysis (PCA). The T-mode refers to the input data matrix where gridpoint values are arranged in rows and daily patterns in columns. The classification procedure is applied to observed daily 500 hPa geopotential height patterns and those simulated by the control and $2\times {\rm CO}2$ ECHAM3 GCM runs. [ABSTRACT FROM AUTHOR]

A dispersion model is proposed to predict the continuous vertical variation of the dispersion parameters sigmay, and sigmaz, in case of hot pollutant release to the atmosphere. In sucha case, the plume rises far above the ground and is subject to varying levels of turbulence. The framework in this paper can be divided into three approaches: (1) determination of the eddy diffusivities Ky(z, sigmay) and Kz(z, sigmaz) as functions of height above ground and plume dimensions, (2) determination of both the plume rise and its vertical velocity using a modified version of Brigg's formula, and (3) numerical solution of actual problems with buoyant plumes at each time step. The model resultshave been applied to a case of pollutant release from fire destruction of a chemical storehouse roof. [ABSTRACT FROM AUTHOR]

Radiosonde measurements obtained at the Arctic site Ny-Ålesund (78.9°N, 11.9°E), Svalbard, from 1993 to 2014 have been homogenized accounting for instrumentation discontinuities by correcting known errors in the manufacturer provided profiles. The resulting homogenized radiosonde record is provided as supplementary material at . From the homogenized data record, the first Ny-Ålesund upper-air climatology of wind, temperature and humidity is presented, forming the background for the analysis of changes during the 22-year period. Particularly during the winter season, a strong increase in atmospheric temperature and humidity is observed, with a significant warming of the free troposphere in January and February up to 3 K per decade. This winter warming is even more pronounced in the boundary layer below 1 km, presumably amplified by mesoscale processes including e.g. orographic effects or the boundary layer capping inversion. Though the largest contribution to the increasing atmospheric water vapour column in winter originates from the lowermost 2 km, no increase in the contribution by specific humidity inversions is detected. Instead, we find an increase in the humidity content of the large-scale background humidity profiles. At the same time, the tropospheric flow in winter is found to occur less frequent from northerly directions and to the same amount more frequent from the South. We conclude that changes in the atmospheric circulation lead to an enhanced advection of warm and moist air from lower latitudes to the Svalbard region in the winter season, causing the warming and moistening of the atmospheric column above Ny-Ålesund, and link the observations to changes in the Arctic Oscillation. [ABSTRACT FROM AUTHOR]

ECMWF reanalysis (ERA-interim) data of winds for two solar cycles (1991-2012) are harmonically analyzed to delineate the characteristics and variability of diurnal tide over a tropical site (13.5° N, 79.5° E). The diurnal cycle horizontal winds measured by Gadanki (13.5° N, 79.2° E) mesosphere-stratosphere-troposphere (MST) radar between May 2005 and April 2006 have been used to compute 24 h tidal amplitudes and phases and compared with the corresponding results obtained from ERA winds. The climatological diurnal tidal amplitudes and phases have been estimated from surface to ∼33 km using ERA interim data. The amplitudes and phases obtained in the present study are found to compare reasonably well with Global Scale Wave Model (GSWM-09). Diurnal tides show larger amplitudes in the lower troposphere below 5 km during summer and in the mid-stratosphere mainly during equinoctial months and early winter. Water vapor and convection in the lower troposphere are observed to play major roles in exciting 24-h tide. Correlations between diurnal amplitude and integrated water vapor and between diurnal amplitude and outgoing longwave radiation (OLR) are 0.59 and −0.34, respectively. Ozone mixing ratio correlates ( ρ = 0.66) well with diurnal amplitude and shows annual variation in the troposphere whereas semi-annual variation is observed at stratospheric heights with stronger peaks in equinoctial months. A clear annual variation of diurnal amplitude is displayed in the troposphere and interannual variability becomes prominent in the stratosphere which could be partly due to the influence of equatorial stratospheric QBO. The influence of solar activity on diurnal oscillations is found to be insignificant. [ABSTRACT FROM AUTHOR]

Total ultraviolet (UV) irradiation (290-400 nm) observed at Sanya (SY, 109°28′E, 18°13′N), China during 2005-2012 was used to investigate UV variability and its relationship with global solar radiation under various sky conditions in Southern China for the first time. Monthly mean daily UV irradiation generally increased from 0.585 MJ m in December to 0.965 MJ m in July with annual mean value being about 0.767 MJ m. Monthly mean daily F (ratio of UV to G) changed from 4.19 % in December to 4.68 % in July with annual mean being about 4.45 % at SY, China. By analyzing the dependences of hourly UV irradiation on optical air mass, clearness index, and cosine of solar zenith angle, respectively, under various sky conditions, three different UV estimation equations have been introduced and validated at SY, Inner Mongolia, and Lhasa. The statistical results indicated that estimation Eq. 3 (based on clearness index and cosine of solar zenith angle) produced the most satisfied estimates at Sanya: mean bias error, mean absolute bias error, and root-mean-square error were only −0.72 % (−0.23 %), 4.67 % (4.45 %), and 6.43 % (5.88 %), respectively, at hourly (daily) basis. Hourly and daily UV irradiation during 1992-2012 were then reconstructed, it was discovered that UV irradiation increased slightly during the study period and the most significant increases (0.904 MJ m day decade) were observed in summer. [ABSTRACT FROM AUTHOR]

A seasonal analysis of the atmospheric circulation over the Iberian Peninsula (IP) based on circulation types (CTs) obtained from sea level pressure and 500-hPa geopotential height is presented. The study covers the period of 1958-2008, when a high variability and important changes in winter and spring precipitation and temperature have been reported. Frequency, persistence, and the most probable transitions of the circulation types are analyzed. Among the clustering methods available in the literature, two of the most reliable classification methods have been tested, K-means and simulated annealing and diversified randomization. A comparison of both methods over the IP is presented for winter (DJF). The quality of the circulation types obtained through both methods as well as the better stability achieved by K-means suggest this method as more appropriated for our target area. Twelve CTs were obtained for each season and were analyzed. The patterns obtained were regrouped in five general situations: anticyclonic, cyclonic, zonal, summertime, and hybrid-mixed. The analysis of frequencies of these situations offers a similar characterization of the atmospheric circulation that others previously obtained by subjective methods. The analysis of the trends in frequency and persistence for each CT shows few significant trends, mainly in winter and spring with a general decrease of the cyclonic patterns and an increase of the anticyclonic situations. This can be related to the negative precipitation trends reported by other authors. Regarding the persistence, an interesting result is that there is a high interannual variability of the persistence in autumn and spring, when patterns can persist longer than in other seasons. An analysis of the most probable transitions between the CTs has been performed, revealing the existence of cyclic sequences in all seasons. These sequences are related to the high frequency of certain patterns such as the anticyclonic situations in winter. Finally, a clear seasonal dependence of the transitions between cyclonic situations associated with extratropical disturbances was found. This dependence suggests that the transitions of low-pressure systems towards the south of the IP are more likely in spring and autumn than in winter. [ABSTRACT FROM AUTHOR]

The present study investigates the interdecadal change in the relationship between southern China (SC) summer rainfall and tropical Indo-Pacific sea surface temperature (SST). It is found that the pattern of tropical Indo-Pacific SST anomalies associated with SC summer rainfall variability tends to be opposite between the 1950-1960s and the 1980-1990s. Above-normal SC rainfall corresponds to warmer SST in the tropical southeastern Indian Ocean (SEIO) and cooler SST in the equatorial central Pacific (ECP) during the 1950-1960s but opposite SST anomalies in these regions during the 1980-1990s. A pronounced difference is also found in anomalous atmospheric circulation linking SEIO SST and SC rainfall between the two periods. In the 1950-1960s, two anomalous vertical circulations are present between ascent over SEIO and ascent over SC, with a common branch of descent over the South China Sea that is accompanied by an anomalous low-level anticyclone. In the 1980-1990s, however, a single anomalous vertical circulation directly connects ascent over SC to descent over SEIO. The change in the rainfall-SST relationship is likely related to a change in the magnitude of SEIO SST forcing and a change in the atmospheric response to the SST forcing due to different mean states. A larger SEIO SST forcing coupled with a stronger and more extensive western North Pacific subtropical high in recent decades induce circulation anomalies reaching higher latitudes, influencing SC directly. Present analysis shows that the SEIO and ECP SST anomalies can contribute to SC summer rainfall variability both independently and in concert. In comparison, there are more cases of concerted contributions due to the co-variability between the Indian and Pacific Ocean SSTs. [ABSTRACT FROM AUTHOR]

In this work, an attempt is made to systematically evaluate the effect of convective scale downdrafts on the model-simulated rainfall, in both aqua- and actual-planet frameworks, using the NCAR CAM3. From the aqua-planet simulations, it was found that there is a reduction in the total rainfall (TRF) with increase in the intensity of downdrafts, which is primarily attributed to the reduction in the deep convective component (DRF). However, with stronger downdrafts, the shallow convective and the large-scale components (SRF and LRF, respectively) are found to increase. The reduction in DRF is due to the increased evaporation of convective precipitation within the downdrafts. It is found that, with intense downdrafts, there is an increase in relative humidity throughout the troposphere, due to the combined effect of both moisture and temperature. There is an overall increase in specific humidity of the atmosphere with stronger downdrafts, excepting at around the 900-hPa level. In addition, there is a reduction in temperature throughout the troposphere, primarily due to the reduction in the overall temperature tendency due to moist processes and that due to the radiative processes. The changes in the radiative forcing are found to be primarily due to a significant increase in the low cloud fraction with strong downdrafts. In the actual-planet framework, it is seen that, with strong convective downdrafts, there is a reduction in TRF and DRF and a corresponding increase in SRF and LRF, similar to the results obtained from the aqua-planet simulations. The vertical structures of the thermodynamic variables (RH, q, and T) show similar sensitivity to the downdraft intensity as that seen in the aqua-planet framework. Sensitivity of frequency and intensity of model-simulated rainfall to the downdraft intensity was also analyzed, and it was seen that there were significant differences in the frequency distribution of rainfall. It was seen that, with an increase in downdraft intensity, there is an increase in the frequency of light rain (1-10 mm/day) for TRF with a corresponding reduction in all other rainfall bins. A similar behavior was seen for the DRF as well, while the SRF and LRF components showed an increase in rainfall accumulation in all the bins. In addition, the impact of convective downdrafts on the mean spatial pattern of rainfall is also analyzed, for the DJF and JJA periods (boreal winter and summer, respectively). For the DJF period, with strong downdrafts, it was seen that grossly over the whole domain, there were a reduction in DRF and an increase in SRF and LRF. In contrast, during JJA, although a major part of the domain showed a reduction in DRF, there were regions like western Arabian Sea and the Somali coast with increase in DRF with intense downdrafts. The SRF and LRF components, however, show a spatially homogeneous increase over almost the entire domain with increase in downdraft intensity. [ABSTRACT FROM AUTHOR]

In order to understand the change in oceanic variability associated with the climate shift of the mid-1970s, we analyze the contribution of momentum forcing to the leading baroclinic modes over the tropical Pacific using Simple Ocean Data Assimilation (SODA, version 2.0.2) for the period of 1958-1997. Specifically, we look at the statistical relationship between the wind projection coefficients and climate indices and attempt to provide a physical explanation for the observed changes. It is found that the wind stress projection coefficients according to the oceanic baroclinic modes are different in terms of their magnitude and phase in the tropical Pacific, reflecting a specific forcing associated with each mode before and after the 1976 climate shift. Compared to that before the 1970s, the first baroclinic mode is had a greater effect on the interannual sea surface temperature due to equatorial wave dynamics, and there was an increased delayed response of the second baroclinic mode variability to the interannual atmospheric forcing after the late 1970s. This reflects changes in ENSO feedback processes associated with the climate shift. Our analysis further indicates that, after the late 1970s, there was a decrease in the wind stress forcing projecting onto the Ekman layer, which is associated with increased mixed-layer depth. This result suggests that the changes in the ENSO properties before and after the late 1970s are largely associated with the changes in the way in which the wind stress forcing is dynamically projected onto the surface layer of the tropical Pacific Ocean over interannual timescales. [ABSTRACT FROM AUTHOR]

The bulk heat transfer coefficient ( C) indicates the land-atmosphere thermal coupling strength. We seek to detect its variability and changes on the Tibetan Plateau (TP) during 1981-2006, particularly concerned with its effects on the sensible heat source. C is parameterized by Monin-Obukhov similarity theory combining routine meteorological measurements. The South Asian monsoon period is characterized by weak sensible heat flux ( H), but strong thermal coupling ( C). Winter sees the greatest diurnal C range, roughly 0.001-0.008. On average, C exhibits a notable nighttime increase (10% decade) and a weak daytime decrease (−5% decade). The strengthening thermal coupling at selected sites alleviates the weakening of H, because it enhances nighttime H increase and reduces daytime H decrease. Moreover, we confirm the finding in earlier studies that sensible heating is weakening on the TP, but find its trend less notable than empirically estimated using C = 0.004. [ABSTRACT FROM AUTHOR]

In this study, we used and compared three different statistical clustering methods: an hierarchical, a non-hierarchical (K-means) and an artificial neural network technique (self-organizing maps (SOM)). These classification methods were applied to a 4-year dataset of 5 days kinematic back trajectories of air masses arriving in Athens, Greece at 12.00 UTC, in three different heights, above the ground. The atmospheric back trajectories were simulated with the HYSPLIT Vesion 4.7 model of National Oceanic and Atmospheric Administration (NOAA). The meteorological data used for the computation of trajectories were obtained from NOAA reanalysis database. A comparison of the three statistical clustering methods through statistical indices was attempted. It was found that all three statistical methods seem to depend to the arrival height of the trajectories, but the degree of dependence differs substantially. Hierarchical clustering showed the highest level of dependence for fast-moving trajectories to the arrival height, followed by SOM. K-means was found to be the least depended clustering technique on the arrival height. The air quality management applications of these results in relation to PM concentrations recorded in Athens, Greece, were also discussed. Differences of PM concentrations, during certain clusters, were found statistically different (at 95% confidence level) indicating that these clusters appear to be associated with long-range transportation of particulates. This study can improve the interpretation of modelled atmospheric trajectories, leading to a more reliable analysis of synoptic weather circulation patterns and their impacts on urban air quality. [ABSTRACT FROM AUTHOR]

Outgoing Longwave Radiation (OLR) data, obtained from NOAA polar orbiting satellites for the months March–June, for 31 years is utilized to investigate low-frequency oscillation in pre-monsoon convection over southwest peninsular Indian region. The analysis reveals a characteristic fall in OLR over the region, about 5–9 weeks prior to the onset of Indian summer monsoon in almost all the years under study. Such fall is shown to be related with a characteristic peak in convective activity associated with the movement of Inter-Tropical Convergence Zone over the region. The results suggest an indirect non-linear relationship between the time of occurrence of the pre-monsoon convective peak and the onset date. A regression equation is developed to predict the onset date. The results of estimation of present method and that of an earlier method (based on conventional synoptic observations) are compared. The performance of both these methods is validated for two independent years (2007 and 2008). The study highlights the potential application of the developed method for the prediction of onset of Indian summer monsoon well in advance using the remotely sensed satellite-derived OLR data. [ABSTRACT FROM AUTHOR]

Based on high-resolution numerical simulations, the influence of topography on the one-dimensional (1-D) tower-based measurements of the net ecosystem–atmosphere exchange rate (NEE) of CO2 was analyzed under unstable conditions. Airflow and transport of a passive scalar were simulated over undulating surfaces covered by tall trees. Compared to their flat surface counterparts, the wind and scalar mixing ratio fields are more disturbed over a steeper surface and/or under a weaker background wind condition, resulting in larger errors in the 1-D NEE estimation. The magnitude of the error is generally larger on the windward side than on the lee side when topography-induced circulation (TIC) does not occur. Applying the ensemble streamline coordinate system to estimating NEE could result in larger errors than applying the local earth coordinate system or terrain coordinate system when the atmospheric flow is significantly distorted. This is especially true when TIC occurs. NEE estimated from the 1-D framework under convective conditions with calm or weak background winds may incur significant errors even over gentle topography. Preference in the selection of a flux tower location is given to the crest area. [ABSTRACT FROM AUTHOR]

Interannual fluctuations in rainfall and ocean-atmosphere fields over and around Africa were studied in the satellite era of 1979–2007 using singular value decomposition. The leading modes of rainfall variability in GPCP satellite-gauge merged fields include a leading mode over central Africa, two modes of marine origin in the Gulf of Guinea and Eastern Africa, and two sub-tropical modes over the Sahel and Southern Africa. This differs from earlier gauge-based studies that tend to isolate three leading modes over western, eastern, and southern Africa. In the sea-surface temperature, sea-level pressure and upper wind fields, ENSO signals dominate the leading modes. However, for the low-level wind field, a trough circulation over the southeast Atlantic – Kalahari is the leading mode. It demonstrates predictive potential when cross-correlated with rainfall at 6- to 12-month lead time. Based on continuous filtered data, the value of various indices and the predictability of different zones are examined. The Sahel achieves the highest rank followed by the Congo and southern zones in the next tier. The Guinea and East African rains, which peak in the March-to-May season, appear least predictable. The seasonal rainfall is shown to modulate economic growth rate, and multi-variate predictive algorithms are tested at 6-month lead time. [ABSTRACT FROM AUTHOR]

CO2 in the rural atmosphere is related to respiration–photosynthesis processes, although the evolution of the low atmosphere is also a determinant factor. CO2 concentrations were measured at surface and meteorological variables obtained from a radio acoustic sounding system sodar at a flat rural site during a 3-year campaign. Yearly and daily cycles of CO2 were described. Maxima were observed in spring and autumn during the night. Wind speed and thermal structure of the lower atmosphere were analysed. Low level jets were observed during the night, their core proving lower in summer. Surface inversions observed with low winds reached up to 100 m. The turbulence layer which developed during the day extended up to 300–400 m and was capped by a stable layer. Median vertical wind speed reached 1 m s−1 in super-adiabatic conditions in summer. Determination of decoupled low level jets proved difficult with the device used and corresponding concentrations were slightly higher than medians calculated with all the observations. The bulk Richardson number was calculated in the lower atmosphere and four intervals were considered: drainage, transitional, shear flows and unstable conditions. Median CO2 concentrations were split according to these intervals. Higher values corresponded to drainage flow, which was associated to more stable conditions being less frequent and lower values to shear flow and unstable conditions, revealing a satisfactory link between the bulk Richardson number as a turbulence indicator in the low atmosphere and CO2 surface concentrations. [ABSTRACT FROM AUTHOR]

Northern Hemisphere monthly mean temperature anomalies during the twentieth century exhibit seasonal differences. To expose the most significant intermonthly dissimilarities, here we investigate this issue in more details. We show that the trends significantly differ for the months close to the equinoxes. Using the technique of multidimensional scaling analysis we find that two underlying attributes are sufficient to acceptably describe the structure of the observed intermonthly dissimilarities. Namely, these are the monthly sample percentiles around the 30th and the 70th together with the linear trends. We find that intraannual temperature anomaly dissimilarities statistically depend on the seasonal cycles of the Northern Hemisphere oceanic heat content, sea ice and snow cover and the Arctic sea ice cover as well as on the seasonality of the chaoticity of the Northern Hemisphere atmospheric dynamics. We also speculate that the annual cycles of these ocean and cryosphere characteristics statistically set the pattern for the observed course of linear warming over the calendar months. [ABSTRACT FROM AUTHOR]

A coupled biosphere-atmosphere statistical-dynamical model (SDM) is used to study the climatic effects of Amazonian deforestation. A soil moisture model based on BATS has been incorporated into the SDM in order to study the biogeophysical feedback of change in surface characteristics to regional climate due to the deforestation. In the control experiment, the mean annual and mean seasonal climate is well simulated by the model when compared with NCEP/NCAR reanalysis data. In the deforestation experiment, the evergreen broadleaf trees in the Amazonian region are substituted by short grass. The effects of Amazonian deforestation on regional climate are analysed taking into account the model simulations for the land portion of the latitude belts comprising the tropical region. Amazonian deforestation results in regional climate changes such as a decrease in evaporation, precipitation, available surface net radiation and soil moisture content, and an increase in temperatures and sensible heat flux. The reduction in transpiration was responsible for the most part of the decrease in total evapotranspiration. The reduction in precipitation was larger than the decrease in evapotranspiration so that runoff was reduced. The simulation of the diurnal cycle of the surface temperature shows an increase in temperature during the day and a decrease at night, which is in agreement with observations, whereas earlier GCM experiments showed an increase both during the day and night. In general, the changes in temperature and energy fluxes are in good agreement with GCM experiments, showing that the SDM is able to simulate the characteristics of the tropical climate that are associated with the substitution of forest by pasture areas. [ABSTRACT FROM AUTHOR]

Observations of atmospheric conditions and processes in cities provide the cornerstone for advances in the understanding of urban climates and are crucial to improving the performance of urban atmospheric models. Here, recent progress in the observation and measurement of the urban atmosphere is considered in terms of: first, the research directions of those involved in conducting urban climate observations; second, advances in technology, both direct in terms of the development of new sensors and indirect in terms of computing power and capabilities in data analysis; and third, enhanced understanding of sensor placement. Increasingly, urban based observational programs are collaborative, multi-institutional, multi-national, interdisciplinary initiatives. This has important implications for the research questions addressed and the potential to investigate processes and effects across multiple spatial and temporal scales. Advances in technology have provided urban climatologists with new, improved and often more affordable instrumentation, and the ability to process and analyze data more rapidly. Greater understanding of urban atmospheric processes, and atmospheric sciences more broadly, have resulted in important insights into appropriate placement of meteorological instruments and the interpretation of results. An important issue that the urban climate community must address is how best to archive data so it is not lost. A database of current and past studies with key metadata about sites, instrumentation and data processing is proposed. Well documented and available urban data sets will enable future researchers to continue to extract valuable new insights and verify existing understanding about urban atmospheric processes. Urban field studies can be costly and time-consuming, so continuing to exploit the rich, significantly untapped, historical data sets will allow steady progress in urban atmospheric research to continue, provided funding are available for analysis of the data. [ABSTRACT FROM AUTHOR]

Trends of monthly air temperature extremes were investigated in five meteorological stations of the Grand-Duchy of Luxembourg during the period 1949–1998. The application of an innovative homogenization method based on the concept of relative homogeneity to climatic time series allows identifying multiple break points, as well as correcting data series in an objective and robust statistical way. The rise of maximum temperature ( Tmax) has occurred at a rate of 1.5 times that of the minimum temperature ( Tmin) in winter (+1.4 °C versus +0.9 °C) and summer (+1.4 °C versus +0.8 °C). No trend in temperature extremes was found in autumn, while spring was affected by a small warming (+0.3 °C) of Tmin and no change in Tmax resulting in a decrease of the diurnal temperature range (DTR) (−0.3 °C). In spring, a strong positive linear relationship between Tmin warming and local terrain slope could be found. Comparison to new-gridded large-scale climatologies indicates generally close agreement to temperature trends during the 1949–1998 period, while a lower local warming was observed in summer during the post-1975 period following the changing-point year of atmospheric circulation over North-western Europe. This study shows that the question of data homogeneity is not trivial and should receive careful attention before quantifying historical temperature trends and identifying their spatial patterns at regional scale. [ABSTRACT FROM AUTHOR]

We study the response of the land-surface to prescribed atmospheric forcing for 31 May 1978 at Cabauw, Netherlands, using the land-surface scheme from the Coupled Atmospheric boundary layer-Plant-Soil (CAPS) model. Results from model runs show realistic daytime surface fluxes are produced using a canopy conductance formulation derived from Cabauw data (for 1987, a different year), andun-tunedparameterizations of root density (near-uniform with depth) and soil heat flux (reduced thermal conductivity through vegetation). Sensitivity of model-calculated surface heat fluxes to initial values of soil moisture is also examined. Results of this study provide the land-surface “base state” for a coupled land-atmosphere modeling study. [ABSTRACT FROM AUTHOR]

Reflectances measured by the ‘visible’ channel of Meteosat are converted to global radiation at the ground by the Heliosat-algorithm. This algorithm is based on the inverse relationship between reflectance and transmittance. This relationship is, however, complicated by two factors: 1) the absorptance of the atmosphere is varying, and 2) the reflectivity is different in different directions. These two factors are again depending on the state of the atmosphere (mainly clouds) and the sun-ground-satellite geometry. The performance of the Heliosat-method is here tested against ground data from Bergen (Norway), Geneva (Switzerland) and Lyon (France), and analysed in light of cloud properties as observed from ground, and different sun-ground-satellite geometries. Situations where modelled and measured radiation differ are identified, and possible causes are suggested. Some of the suggestions are supported by calculations with a radiative transfer model. [ABSTRACT FROM AUTHOR]

Summary The greenhouse effect has been investigated predominantly with satellite measurements, but more than 90% of the greenhouse radiative flux affecting Earth’s surface temperature and humidity originates from a 1000 meter layer above the surface. Here we show that substantial improvements on surface longwave radiation measurements and very good agreement with radiative transfer model calculations allow the clear-sky greenhouse effect be determined with measured surface longwave radiation and calculated longwave outgoing radiation at the top of the atmosphere. The cloud radiative forcing is determined by measured net longwave fluxes and added to the clear-sky greenhouse effect to determine the all-sky greenhouse effect. Longwave radiation measurements at different altitudes were used to determine the clear-sky and all-sky annual and seasonal greenhouse effect and altitude gradients over the Alps. Linear altitude gradients are measured for clear-sky situations, whereas the all-sky greenhouse effect is strongly influenced by varying, cloud amounts at different altitudes. Large diurnal and seasonal variations show the importance of surface heating and cooling effects and demonstrate the strong coupling of the greenhouse effect to surface temperature and humidity. [ABSTRACT FROM AUTHOR]

Above orographically structured terrain considerable differences of the regional wind field may be identified during large-scale extreme wind events. So far, these regional differences could not be resolved by climate models. To determine the relationships between large-scale atmospheric conditions, the influence of orography, and the regional wind field, data measured in the upper Rhine valley within the framework of the REKLIP Regional Climate Project were analyzed and calculations were made using the KAMM mesoscale model. In the area of the upper Rhine valley, ratios of the wind velocity in the Rhine valley at 10 m above ground level, x[subval], and the large-scale flow velocity, x[sublar], are between x[subval]/x[sublar], 0.1 and x[subval]/x[sublar], 1. The x[subval]/x[sublar] ratio exhibits a strong dependence on thermal stratification, i, and decreases from x[subval]/x[sublar], 1 at i = 0 K m[supm1] to x[subval]/x[sublar], 0.2 at i = 0.0075 K m[supm1]. In areas, where the lateral mountainous border of the Rhine valley is interrupted, the x[subval]/x[sublar] ratio increases again with increasing stability or decreasing Froude number. This is obviously due to flow around the Black Forest under stable stratification. It is demonstrated by model calculations that a complex wind field develops in the Rhine valley at small Froude numbers (Fr < 1) irrespective of the direction of large-scale flow. The x[subval]/x[sublar] ratio is characterized by small values in the direct lee side (x[subval]/x[sublar], 0.2) and high values on the windward side of the lateral mountainous border of the Rhine valley (x[subval]/x[sublar], 0.8). [ABSTRACT FROM AUTHOR]

Gives an overview on the Lindenberg Inhomogeneous Terrain-Fluxes between Atmosphere and Surface: a Long-term Study (LITFASS) project, on the design and measurement program of the LITFASS-98 experiment, and on the weather conditions during the period of the experiment. Introduction; LITFASS project overview; LITFASS-98 experiment design and measurements; Intensive observation periods; Weather conditions; Spatial distribution of participation and radiation; Summary.

Summary Discharge from a small grassland catchment in Ireland is simulated with the atmospheric land-surface scheme SEWAB. Hydrological processes are parameterized to represent surface runoff and baseflow generation and soil moisture storage changes. Surface ponding and infiltration are explicitly described in order to account for the rapid response of streamflow to precipitation events. The annual discharge, the evapotranspiration and individual flood flows are accurately modelled. The simulation of soil moisture at various depths is close to the observations from time domain reflectometer measurements. An analysis of the significance of individual hydrological processes for discharge simulation from the small catchment found the ponding process to be essential for time periods of less than 12 hours. A depth dependent saturation hydraulic conductivity improves the simulations on all time scales. [ABSTRACT FROM AUTHOR]

The influence of thunderstorms on the atmospheric radio noise field strength (ARNFS) over Kalyani (West Bengal) at 27 kHz has been analyzed over a two-year period. The results show that the median noise level decreases during the winter and reaches a maximum during the monsoon months. A comparison of the seasonal occurrence of thunderstorm -days with ARNFS indicates that both of them have similar trend over time. However, the trend is more pronounced during daytime, indicating relationship between solar activity, thunderstorms and ARNFS. In thispaper, we have attempted to investigate how tropical thunderstorm and lightning activity might be responsible for the solar modulation ofatmospheric electrification. The influence of ionizing radiation on the electrical global circuit of the atmosphere is also critically analyzed. [ABSTRACT FROM AUTHOR]

Based on analysis of ECMWF data (1981--1987) and numerical simulations using a general circulation model (GCM), a quasi-two-week (10--20 day) oscillation in the tropical atmosphere is studied in this paper.It is shown that the kinetic energy of the quasi-two-week oscillation is larger than that of the intraseasonal oscillation, and is another important low-frequency system in the tropical atmosphere. By comparing it with the intraseasonal oscillation, some obvious differences can be found. For example, the zonal scale of the quasi-two-week oscillation is dominated by perturbations with wavenumber 2--4; its vertical structure mainly shows barotropic features; the zonal propagationis basically westward; and its meridional and zonal components the same size. [ABSTRACT FROM AUTHOR]

A measurement program was conducted in Goteborg Sweden, to examine the spatial variations of incoming longwave irradiance on calm, cloudless nights. Both regional and local spatial variations were examined.Incoming longwave irradiance data was obtained from mobile car transects, and at a fixed site on a building roof at the city center. Ancillary data included sky view factor at various transect locations, and balloon soundings of air temperature and humidity on one night. Measurements revealed that on average, incoming longwave irradiance at the fixed urban site was 11 W m-2 higher than at the rural station, with varying differences for intervening sites. Bulk apparent sky emissivity was higher at the most rural station compared to thefixed urban site, by about 0.03 on average. Nighttime balloon measurements and a sensitivity analysis with a radiative transfer model argue that the bulk apparent sky emissivity differences stem mainly fromthe temperature structure of the lower boundary layer which changes markedly from rural to urban areas. A good relationship was found between sky view factor and incoming longwave irradiance for a range of urban and park locations. The relationship applies to both individualnights and average data. Using a simple obstruction model, canyon wall temperatures are derived, and the relationship between sky view factor and wall temperature is examined. [ABSTRACT FROM AUTHOR]

The Regional Atmospheric Modeling System (RAMS) has been widely usedto simulate relatively short-term atmospheric processes. To perform full-year to multi-year model integrations, a climate version of RAMS(ClimRAMS) has been developed, and is used to simulate diurnal, seasonal, and annual cycles of atmospheric and hydrologic variables and interactions within the central United States during 1989. The model simulation uses a 200-km grid covering the conterminous United States,and a nested, 50-km grid covering the Great Plains and Rocky Mountain states of Kansas, Nebraska, South Dakota, Wyoming, and Colorado. The model's lateral boundary conditions are forced by six-hourly NCEP reanalysis products. ClimRAMS includes simplified precipitation and radiation sub-models, and representations that describe the seasonal evolution of vegetation-related parameters. In addition, ClimRAMS can use all of the general RAMS capabilities, like its more complex radiation sub-models, and explicit cloud and precipitation microphysics schemes. Thus, together with its nonhydrostatic and fully-interactive telescoping-grid capabilities, ClimRAMS can be applied to a wide variety of problems. Because of non-linear interactions between the land surface and atmosphere, simulating the observed climate requires simulating the observed diurnal, synoptic, and seasonal cycles. While previous regional climate modeling studies have demonstrated their abilityto simulate the seasonal cycles through comparison with observed monthly-mean temperature and precipitation data sets, this study demonstrates that a regional climate model can also capture observed diurnaland synoptic variability. Observed values of daily precipitation andmaximum and minimum screen-height air temperature are used to demonstrate this ability." [ABSTRACT FROM AUTHOR]

Series of annual and seasonal temperature and precipitation representing respectively northern and southern Italy are compared for trend,interannual variability and periodicity in the period 1866-1995. Temperature and precipitation trends are almost always anticorrelated except in winter in the North where an anomalous behavior is evident till about 1980. The result is that the Italian climate has become warmer and drier especially in the South since about 1930.The interannualvariability does not present significant maxima, but only minima that cannot be related to the start of a trend either for temperature orfor precipitation. The power spectra of the series show broad significant peaks containing the quasi-biennial oscillation and other well known periodicities probably due to solar cycles or to the North Atlantic ocean-atmosphere oscillation (NAO). [ABSTRACT FROM AUTHOR]

Investigation of long-term measurements of winter mesosphere/lower thermosphere (MLT) region zonal winds has shown a connection of northern hemisphere tropospheric parameters and MLT conditions. These are on one hand due to a connection of the stratospheric polar vortex and the North Atlantic Oscillation (NAO) and on the other hand forced by a coupling of the troposphere/lower stratosphere and the MLT region through planetary wave propagation and wave-mean flow interaction. Theconnection between European and Asian winter climatological parameters and the NAO leads to the fact, that the signal of Eurasian winter conditions can be found in the interannual variability of MLT winds. Some of these winter tropospheric parameters influence the strength of the Indian monsoon in the following summer, and this leads to a correlation between winter MLT winds and summer Indian rainfall, so thatthe midlatitude MLT winter dynamics can be used as a precursory signal of the strength of the Indian monsoon. [ABSTRACT FROM AUTHOR]

An investigation of the main features of large-scale synoptic patterns over Europe and the adjacent areas for extreme winter periods during 1980-1995 over the Eastern Mediterranean (EM) is performed. The NASA reanalysis data set is used to investigate composite sea level pressure (SLP), geopotential height of the 500 hPa surface (H-500) and precipitation-wet, normal and dry patterns for each month during the period October-March. It is found that the wet and dry cool seasons inthe EM are associated with distinct SLP and H-500 anomaly patterns over Europe and the adjacent regions. During the dry spells large-scale positive SLP/H-500 anomaly areas prevail over Eastern Europe. A negative SLP anomaly is normally found during these periods over southwestern and Western Europe. During the wet cool seasons in the EM thereare mainly negative SLP/H-500 anomaly areas over Eastern Europe to the north east of the EM. Positive SLP/H-500 anomalies are found over Western Europe. During wet months a trough zone between the Siberian and the Azorean Highs is positioned over the eastern part of the Mediterranean. During dry months the Siberian anticyclone is more intensive and the zone with low surface pressure is displaced to the centralpart of the northern Mediterranean. [ABSTRACT FROM AUTHOR]

The covariation of the mid-tropospheric flow and the sea surface temperature (SST) of the extratropical North Atlantic is studied for theperiod 1962 to 1992. A statistical approach is adopted and the variables selected for examination, in addition to the SST, are the 500 hPa fields of the geopotential height and the dynamic storm track. Singular Value Decomposition (SVD) of the wintertime monthly mean fields for all paired combinations of these three fields suggests thattheir intraseasonal covariability is dominated by types of recurring patterns. The first is an 'North Atlantic Oscillation-like' variation with height and SST patterns in the form of dipolar anomalies and a storm track pattern whose extremes connote an elongation (shortening) sharpening (broadening) and intensification (weakening) of the track. The second is akin to 'strong and weak zonal flow regimes' and is characterized by height and SST anomalies that are latitudinally aligned andstorm track variations that connote a shortening (lengthening) and northeastward (southeastward) reorientation of the track. Lag correlation analysis of the relative phase of the flow and SST variations suggest that the former leads the latter by the order of a month and is indicative of the flow influencing the SST configuration. However evidence adduced from consideration of lagged composite plots of strong SST patterns cautions against excluding the possibility of a nonlinear ocean-to-atmosphere forcing. It is also demonstrated that the two types of SVD-derived patterns represent a significant part of the last30 years' trends in the selected fields, and the coupled SVD patterns also shed light on the nature and degree of coupling of the three selected fields. [ABSTRACT FROM AUTHOR]

Anomalously wet and dry months in the Mediterranean basin were identified during the period 1860-1990 from observations at five stations located along the west-east axis of the Mediterranean basin (Barcelona, Florence, Malta, Athens and Jerusalem), supplemented by data from Madrid and Lisbon. Wet and dry months were characterized by hydric indices (HI) based on values of the standardized precipitation anomalies. Different patterns of anomalously wet and dry months were qualitatively identified on the basis of the spatial distributions of the hydric indices. The standardized sea level pressure values at 56 grid points in the domain 35 deg. N-65 deg. N, 30 deg. W-40 deg. E, for each of the anomalously wet and dry months, were subjected to T-mode Principal Component Analysis. The mean hydric indices associated witheach principal component in each season are arranged in four distinct different spatial distributions for wet months and in three for drymonths as following: (a) Mediterranean wide distribution of positive/negative anomalies; (b1) Strong positive anomalies to the west, but weaker to eastern Mediterranean; (b2) Strong negative anomalies to the west, but weaker or normal to the east; (c1) Strong positive anomalies to the west and to the east and weaker ones to the central Mediterranean; (c2) Negative anomalies to the west and east, but weaker, ornormal, or positive to the central Mediterranean; (d) Relatively strong positive anomalies to the east and weaker ones to the western Mediterranean. Finally, monthly mean charts of standardized anomaly andmean sea level pressure are presented for each principal component in each season. These charts are used to interpret the spatial distribution of the positive and negative precipitation anomalies in terms of mean circulation over the domain. [ABSTRACT FROM AUTHOR]