Alpine cloud climatology using long-term NOAA-AVHRR satellite data

Summary  Three different climates have been identified by our evaluation of AVHRR (Advanced Very High Resolution Radiometer) data using APOLLO (AVHRR Processing scheme Over Land, Clouds and Ocean) for a five-years cloud climatology of the Alpine region. The cloud cover data from four layers were spatially averaged in boxes of 15 km by 14 km. The study area only covers 540 km by 560 km, but contains regions with moderate, Alpine and Mediterranean climate. Data from the period July 1989 until December 1996 have been considered. The temporal resolution is one scene per day, the early afternoon pass, yielding monthly means of satellite derived cloud coverages 5% to 10% above the daily mean compared to conventional surface observation. At non-vegetated sites the cloudiness is sometimes significantly overestimated. Averaging high resolution cloud data seems to be superior to low resolution measurements of cloud properties and averaging is favourable in topographical homogeneous regions only. The annual course of cloud cover reveals typical regional features as foehn or temporal singularities as the so-called Christmas thaw. The cloud cover maps in spatially high resolution show local luff/lee features which outline the orography. Less cloud cover is found over the Alps than over the forelands in winter, an accumulation of thick cirrus is found over the High Alps and an accumulation of thin cirrus north of the Alps. Received December 17, 1999 Revised July 18, 2000     

A Canadian haze climatology

Maps of annual average haze frequencies for Canada show maxima in southwestern Ontario and around Sudbury, with a smaller maximum in the Vancouver area. Small maxima also exist in the Maritimes during summer and over the western Prairies in winter. The principal cause of these maxima appears to be the man-made emissions in and upwind of these areas. There are significant variations from summer to winter, both in time and space. In particular, there has been an increase in summer haziness in Ontario, southern Quebec and the Maritimes, and a general decrease in winter haziness in all regions, during the last twenty years.     

A statistical approach to climatology

Summary After introductory remarks on the manyfold difficulties in climatological work the author attempts a more precise definition of Climatology, which is often called the science of the average weather, as the science of the multivariate distributions of meteorological elements with time and space. A climatological study has to be made in various stemps, each of which has to keep constant a minor number of variables than the preceding one. In the selection of the mathematical models one has to pay attention to two principles, which often oppose each other: simplicity and generality. The goodness of fit of the model has to be proved. In proceeding to higher levels of the climatological analysis. Studies should be undertaken by means of some stations with long series of data with regard to the necessary length of the period of observations. The author is convinced that all these costly investigations would be of utmos value for humanity.

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Zusammenfassung Es werden zunächst Beispiele für die mannigfachen Schwierigkeiten der klimatologischen Arbeit angeführt, wie sie z. B. durch unenheitliche Definitionen und Repräsentationsarten, heterogene Beobachtungsreihen, baobachtungslücken u. ä. m. verursacht werden. Dann wird die Definition der Klimatologie als Wissenschaft vom durchschnittlichen Wetter präzisiert als Wissenschaft, die die multivariablen statistischen Verteilungen der meteorologischen Elemente mit Orts- und Zeitkoordinaten zu studieren hat. Eine klimatologische Studie muß mit Teilstudien beginnen, die zunächst einige der Variablen konstant halten; das Resultat dieser Teilstudien muß Wegweiser sein beim Fortschreiten zu höheren Dimensionen. Bei der Auswahl der mathematischen Modelle in allen Stufen ist auf swei Prinzipien Rücksicht zu nehmen, die einander widersprechen können: das Modell soll einfach sein, soll aber doch zu verallgemeinern sein. Die Güte der Anpassung des Modells andie Daten muß geprüft werden, z. B. durch die Methoden der Konfidenzintervalle oder der Subperioden. Beim Fortschreiten zu höheren Modellen sind die Methoden der Analyse von einfachen oder multiplen Zeitreihen zu berücksichtigen. Für die Frage einer hinreichend langen Bearbeitungsperiode sollten Untersuchungen aller Stufen mit einer Anzahl von Schlüsselstationen durchgeführt werden. Der Autor ist der Ansicht, daß sich alle die angeführten kostspieligen Untersuchungen für die Allgemeinheit bezahlt machen werden.

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Résumé L'auteur présente tout d'abord quelques exemples des nombreuses difficulté inhérentes à tout travail en climatologie. Ces difficulté proviennent soit dufait que les définitions et les formes de représentation des qu'il y ait des lacunes dans les séries d'observations, soit d'autres causes encore. Il définit ensuite la climatologie comme étant la science du temps moyen et plus particulièrement la science chargée d'étudier la répartition statistique multiple des éléments météorologiques et cela auusi bien dans le temps que dans l'espace. Une recherche en climatologie doit débuter par des études partielles c'est à dire ne laissant, au préalable, varier que quelques éléments du climat, les autres restant constants. Les résultats obtenus lors de ces études partielles indiqueront alors dans quelles directions poursuivre. Lors du choix des modèles mathématiques, il faut tenir compte, à tous les niveaux, de deux principes qui peuvent être contradictoires: Le modèle doit être simple, mais il doit permettre des généralisations. Il faut étudier les possibilités d'adaptation du modèle choisi aux séries de chiffres à étudier en utilisant par exemple des périodes secondaires ou l'intervalle situé entre les limites de confiance. En passant à des modèles de degré supérieur, il est nécessaire de tenir compte des méthodes d'analyse de séries chronologiques simples et multiples. Pour déterminer quelle longueur au moins doit avoir une période d'observations, il faudrait étudier à tous les degrés un contrôle avec un certain nombre de stations typiques. L'auteur pense que toutes ces recherches sont rentables pour la collectivité, même si leur coût est élevé.

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Lecture given at the IV. Session of the Commission for Climatology of WMO, August 18, 1965 in Stockholm.     

A climatology of Southern Hemisphere anticyclones

A climatology of anticyclones generated by an objective automatic scheme applied to 15 years of once-daily Australian Bureau of Meteorology hemispheric analyses is presented. Contour maps of the anticyclone system density, positions of formation and dissipation together with other statistics are shown. The distribution of anticyclones through the hemisphere was found to be dominated by a mid latitude belt of high density, located in the band 25–42°S, typically 24° south of the time-mean subtropical ridge. Within this band the anticyclone density displays considerable structure with greater system numbers over the eastern parts of the three subtropical ocean basins in the vicinity of the three subtropical ocean time-mean anticyclones. During winter the system density displays a bifurcation in the New Zealand sector, with the highest density along the 30 and 45°S latitude bands. The movement of systems in the subtropical ocean basins was found to be in a general easterly direction with a weak equatorwards component, the transport of systems closely following the orientation of the belt of highest system density. In the vicinity of the African and South American continents, movement was more complex with east-south-east motion upstream, and east-north-east movement downstream, the net transport being such as to encourage a general steering of systems around the continental land masses more particularly during the warmer seasons. To highlight the dynamic role played by these systems and their cyclonic counterparts, we present a limited investigation of the response of Southern Hemisphere synoptic systems to variations of the broader atmospheric system and compare these findings to those obtained by more traditional analysis techniques.     

A hail climatology in Mongolia

The temporal and spatial characteristics of hail frequency in Mongolia are examined using the hail observation data from 61 meteorological observatories for 1984-2013. The annual number of hail days averaged over all observatories and the entire period is 0.74. It exhibits a decreasing trend, particularly since 1993 with a rate of decrease of 0.214 per decade. Hail occurrence is concentrated in summer, with 72% of the total hail days occurring in June, July, and August. Moreover, hail occurrence is concentrated in the afternoon and early evening, with 89% of the total hail events occurring between 1200 and 2100 local standard time (LST). Spatially, observatories where relatively frequent hail events are observed are concentrated in the north central region where almost all of the land is mountainous or covered by grassland, whereas relatively less frequent hail events are observed in the southern desert region. The relationship between hail frequency and thermodynamic factors including the convective available potential energy (CAPE), the temperature lapse rate between 700 and 500 hPa, the water vapor mixing ratio averaged over the lowest 100 hPa layer, and the freezing-level height is examined using the ERA-Interim reanalysis data. It is found that in summer, CAPE and the low-level water vapor mixing ratio are larger on hail days than on all days, but there is no clear relationship between hail frequency and the 700-500 hPa temperature lapse rate. It is also found that annually, CAPE and the low-level water vapor mixing ratio decrease, while the freezing-level height increases, which seems to be responsible for the annually decreasing trend of hail frequency in Mongolia.     

A new global ocean hydrographic climatology

This report describes the main features of the recently published World Ocean Experiment–Argo Global Hydrographic Climatology. This climatology is based on profile data from ships, Argo floats, and sensors attached to marine mammals. As an important deviation from the widely used climatologies produced previously by the National Oceanographic Data Center, the spatial interpolation was performed on local potential density surfaces, so that no ‘artificial water masses’ were created. In addition to monthly fields of temperature and salinity, gridded maps of the upper mixed layer depth are now provided.     

The simulation of medicanes in a high-resolution regional climate model

Medicanes, strong mesoscale cyclones with tropical-like features, develop occasionally over the Mediterranean Sea. Due to the scarcity of observations over sea and the coarse resolution of the long-term reanalysis datasets, it is difficult to study systematically the multidecadal statistics of sub-synoptic medicanes. Our goal is to assess the long-term variability and trends of medicanes, obtaining a long-term climatology through dynamical downscaling of the NCEP/NCAR reanalysis data. In this paper, we examine the robustness of this method and investigate the value added for the study of medicanes. To do so, we performed several climate mode simulations with a high resolution regional atmospheric model (CCLM) for a number of test cases described in the literature. We find that the medicanes are formed in the simulations, with deeper pressures and stronger winds than in the driving global NCEP reanalysis. The tracks are adequately reproduced. We conclude that our methodology is suitable for constructing multi-decadal statistics and scenarios of current and possible future medicane activities.     

A climatology of Southern Hemisphere extratropical cyclones

A climatology of extratropical cyclones determined by an objective automatic scheme applied to 15 years (1975–89) of once-daily Australian Bureau of Meteorology hemispheric analyses is presented. Contour maps of the positions of formation (cyclogenesis), dissipation (cyclolysis) together with other cyclone statistics are presented. The distribution of cyclones through the hemisphere was found to be dominated by a permanent high latitude core coincident with the circumpolar trough. During the winter and intermediate seasons, two mid latitude branches are evident in the cyclone density originating in the Tasman Sea and South American sectors, both spiraling poleward and merging with the circumpolar core in the Southern Oceans. Systems were observed to move in an castsouth-east direction, away from their location of formation, exhibiting peak speeds of migration in the mid latitudes. Little seasonality was evident in the densitiy distribution of cyclones through the Southern Oceans, but a considerable amount was found in their central pressure. Correspondence to: I Simmonds     

A European pattern climatology 1766–2000

Using monthly independently reconstructed gridded European fields for the 500 hPa geopotential height, temperature, and precipitation covering the last 235 years we investigate the temporal and spatial evolution of these key climate variables and assess the leading combined patterns of climate variability. Seasonal European temperatures show a positive trend mainly over the last 40 years with absolute highest values since 1766. Precipitation indicates no clear trend. Spatial correlation technique reveals that winter, spring, and autumn covariability between European temperature and precipitation is mainly influenced by advective processes, whereas during summer convection plays the dominant role. Empirical Orthogonal Function analysis is applied to the combined fields of pressure, temperature, and precipitation. The dominant patterns of climate variability for winter, spring, and autumn resemble the North Atlantic Oscillation and show a distinct positive trend during the past 40 years for winter and spring. A positive trend is also detected for summer pattern 2, which reflects an increased influence of the Azores High towards central Europe and the Mediterranean coinciding with warm and dry conditions. The question to which extent these recent trends in European climate patterns can be explained by internal variability or are a result of radiative forcing is answered using cross wavelets on an annual basis. Natural radiative forcing (solar and volcanic) has no imprint on annual European climate patterns. Connections to CO₂ forcing are only detected at the margins of the wavelets where edge effects are apparent and hence one has to be cautious in a further interpretation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A climatology of major avalanche winters in Western Canada

Abstract

A nearly continuous record of avalanches on the Canadian Pacific Railway for the 70 years from 1910 is used to identify four major avalanche winters (1919–20, 1932–33, 1934–35 and 1971–72). The selection is based on the frequency and mass of avalanche snow, and the length of rail line affected near Rogers Pass, British Columbia. Daily weather data are compiled for these winters, and surface synoptic charts are examined for the most catastrophic avalanche events. Their climatology is further analysed by applying a weather‐typing scheme and by constructing monthly sea‐level pressure anomaly maps. More than one climatology is needed to explain these winters and the daily sea‐level map types are not very discriminating. Major avalanche winters tend to be cold in December or January or both. Some show large monthly anomalies in the average pressure distribution over the province. Large avalanches occur when there is a change in the atmospheric circulation, with a Pacific frontal system and warm sector replacing an Arctic high. These aspects of synoptic climatology are related to the mechanisms for producing big avalanches.     

A contribution to the radiation climatology of Northern Europe

Theoretical and Applied Climatology - Global radiation data were assembled for twenty-three Northern European stations, in an area bounded approximately by latitudes 51° and 61° N, and...     

Tornado climatology of Austria

After several decades of little work, a revised tornado climatology for Austria is presented. Tornadoes seldom form in the alpine areas, however, near the eastern flanks of the Alps, favourable conditions for tornado genesis are found. Whereas in the alpine regions less than 0.3 tornadoes per 10,000 km² a year touch down (averaged for provinces or major parts of a province), we can count 0.9 in the greater Graz area, 1.0 in the greater Linz area and 1.2 tornadoes per 10,000 km² a year in the greater Vienna area, suggesting the existence of so-called tornado alleys. As these regions are the most populated areas of Austria, there is a possible population bias in the dataset. The overall average for Austria is 0.3 tornadoes per 10,000 km² a year.The database consists of 89 tornadoes, one landspout and six waterspouts, with a total of 96 events. The seasonal peak is in July with a maximum probability of tornadoes in the late afternoon and early evening hours. Every fifth tornado occurs in the hour after 5 p.m. The maximum intensity determined for a tornado in Austria was T7 on the TORRO-Scale (F3 on the Fujita-Scale), the most common intensity is T2 on the TORRO-Scale (F1 on the Fujita-Scale).     

A global monthly climatology of soil moisture and water balance

Global monthly climatology of available soil moisture content is derived on a 4° by 5° grid from observed precipitation and air surface temperature by use of a simple water budget model. The governing equations and methods of calculation for deriving these fields, which follow the formulation of Thornthwaite, are first described and the importance of the various assumptions and simplifications of this approach are discussed. The derived global fields are then presented. A comparison of some of the derived fields with other calculations is also made in order to permit an evaluation of the results: For example, our indirect estimate of the river run-off is generally in good agreement with more direct estimates, except for high latitude regions where the freezing of the soil may play an important role.Yale Mintz died on 27 April 1991. This work was carried out jointly over a number of years preceding his death     

Lightning climatology in Colombia

Theoretical and Applied Climatology - Colombia’s lightning climatology was studied using 16 years of high-resolution data from the Tropical Rainfall Measuring Mission Lightning Imaging Sensor...