bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Indian drought of 2002—a sub‐seasonal phenomenon?
Authors: Bhat G. S. 
Published: January 2007

AbstractA major drought occurred over India in the year 2002 with a seasonal rainfall deficit of 21.5%, a result of 56% below normal rainfall in the month of July. The largest anomalies occurred in the western parts of India, when an Indian monsoon field experiment was in progress there. The present study is based primarily on data collected from a research ship that was deployed 100–250 km off the west coast of India for the experiment. Surface and upper air observations made over the eastern Arabian Sea during July 2002 are presented. Sea‐surface conditions were favourable for supporting deep convection over the study area. Strong atmospheric inversions around 800 hPa prevented the growth of cumulonimbus during the first half and towards the end of July. A second inversion above the melting level was also prominently present. The strength and persistence of the inversions are unusual. Back‐trajectory analysis reveals a major change in the low‐level circulation during July 2002 with frequent advection of dry air from over the deserts around the eastern Arabian Sea instead of marine air from across the equator. Copyright © 2006 Royal Meteorological Society
Microphysical and optical properties of midlatitude cirrus clouds observed in the southern hemisphere during INCA
Authors: Gayet Jean‐Francois , Shcherbakov Valery , Mannstein Hermann , Minikin Andeas , Schumann Ulrich , Ström Johan , Petzold Andreas , Ovarlez Joelle , Immler Franz 
Published: January 2007

AbstractAirborne microphysical and optical properties of cirrus clouds, jet‐stream and wave clouds were measured at temperatures ranging from −25°C to −62°C in the southern hemisphere from Punta Arenas (53°S) in March and April 2000 during the INCA experiment (INterhemispheric differences in Cirrus properties from Anthropogenic emissions). The observations related to cirrus clouds show general decreases of the ice water content (18 mg m−3 to 0.05 mg m−3), extinction coefficient (0.70 km−1 to 0.08 km−1), ice particle concentration (2.2 cm−3 to 0.5 cm−3), and the effective diameter (80 µm to 17 µm) linked to the variation of ambient temperature (−25°C to −60°C). The lowest temperature at which supercooled water droplets were detected was −33°C. The asymmetry parameter shows relatively small variations, with the smallest values (0.758) observed at the lowest temperatures. High‐altitude clouds which form with a rapid vertical transport, i.e. jet‐stream cirrus and orographic‐wave ice‐cloud, are characterized by very high values of ice particle concentrations (up to 100 cm−3) compared to mean values as a whole (1.45 cm−3). These two kinds of high clouds are the subjects of detailed case‐studies. Although formed at a similar range of temperatures and with similar aerosol properties, the strong differences in cloud properties and humidity fields highlight the key role of the dynamical structure in controlling the formation, evolution and subsequent radiative properties of such high‐altitude clouds. Copyright © 2006 Royal Meteorological Society
Spatial and temporal distribution of long‐term short‐wave surface radiation over Greece
Authors: Fotiadi A. , Hatzianastassiou N. , Stackhouse P. W. , Matsoukas C. , Drakakis E. , Pavlakis K. G. , Hatzidimitriou D. , Vardavas I. 
Published: January 2007

AbstractThe short‐wave (SW) surface radiation budget (SRB, downward and absorbed fluxes) over Greece and surrounding areas has been computed for the first time, using a physical deterministic radiative‐transfer model. The model computations were performed on a monthly‐mean basis and at 2.5°×2.5° latitude–longitude resolution over the 17‐year period 1984–2000. Higher spatial resolution of 1°×1° was achieved using the new NASA‐Langley dataset but only for the shorter period 1985–1995. In the first case (2.5°×2.5° resolution), the model input data were taken from global datasets, such as the International Satellite Cloud Climatology Project (ISCCP), the TIROS Operational Vertical Sounder (TOVS) or the Global Aerosol Dataset (GADS), as well as from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) Global Re‐analysis Projects. In the second case (1°×1° resolution), the model input data were taken from the NASA‐Langley dataset supplemented by others (e.g., GADS). It is found that the annual mean values of the downward SW radiation at the surface (DSR) increase from about 150 W m−2 in the north of Greece to 210 W m−2 in the south. (Seasonal means are 50 and 100 W m−2, respectively, in the winter and 270 and 330 W m−2 in summer). The 1°×1° DSR fluxes reveal significant geographical features, indicating an important longitudinal variation, with smaller values along the central mountain axis and in the northern part of the country. The annual mean DSR flux, averaged over the study area, is 181.9±14.4 W m−2, calculated using the ISCCP‐D2 (1984–2000) data or 188.8±14.2 W m−2 using the NASA‐Langley (1985–1995) data. The corresponding averaged mean surface absorbed fluxes are 164.7±13.1 W m−2 and 170.7±13.2 W m−2. Computed time series of annual mean solar radiation arriving at the surface, and a linear fit applied to them, show an increase with time which may possibly be related to climatic change. Time series of annual amplitude of DSR (maximum minus minimum monthly values) indicate a year‐to‐year variation of DSR of about 245 W m−2, whereas a linear fit shows an increase with time but no systematic increase in summer maxima or decrease in winter minima. The model‐computed DSR fluxes are in good agreement with surface measurements, made at eight stations of the Hellenic National Meteorological Service and four other high‐standard stations run by Greek research organizations, with correlation coefficients in the range 0.91–0.99 and standard deviations smaller than 20 W m−2. The ratio of direct to diffuse DSR (an indicator of clear skies) peaks during the summer months at a value of 1.5 in Thessaloniki (northern Greece), at about three in Athens and nine in Crete. Copyright © 2006 Royal Meteorological Society
The surface energy balance and boundary layer over urban street canyons
Authors: Harman I. N. , Belcher S. E. 
Published: January 2007

AbstractA model is developed for the energy balance of an urban area, represented as a sequence of two‐dimensional street canyons. The model incorporates a novel formulation for the sensible‐heat flux, that has previously been validated against wind tunnel models, and a formulation for radiation that includes multiple reflections and shadowing. This energy balance model is coupled to a model for the atmospheric boundary layer. Results are analysed to establish how the physical processes combine to produce the observed features of urban climate, and to establish the roles of building form and fabric on the urban modification to climate.Over a diurnal cycle there are morning and evening transition periods when the net flux of radiation is largely balanced by the flux of heat into the surface. The urban surface has a large surface area in contact with the air, and hence a large active heat capacity, and so the urban area needs to absorb a larger amount of heat than a rural area to change the surface temperature. The morning and evening transitions are therefore prolonged over urban areas, delaying the onset of convective or stable boundary layers after sunrise and sunset. The model shows that the energy balance of the roof behaves very differently from the combined energy balance of the street canyon system of walls and street. The sensible‐heat flux from the street canyon into the boundary layer is increased by the increased surface area, but is decreased by the buildings reducing the local flow speeds. The net result is that, for the two‐dimensional geometry investigated here, the sensible‐heat flux from the canyon is not strongly sensitive to canyon geometry. The sensible‐heat flux from the roof is larger than from the street, and so the total sensible‐heat flux into the boundary layer, and hence also the air temperature, is strongly dependent on the fraction of plan area occupied by roofs. The radiation budget of the street canyon, which largely drives the temperatures of the canyon surfaces, is significantly changed by the limited sky view and multiple reflections caused by the local building form. The canyon surface temperatures thus depend strongly on local building morphology.Finally, two mechanisms are suggested for how urban areas might maintain a positive sensible‐heat flux at night. Firstly, if the roof material has much lower heat capacity than the street canyon surfaces, then the roof can cool the boundary‐layer air faster than the street canyon surfaces cool, leading to a positive heat flux out of the street canyon. Secondly, advection decouples the boundary layer from the local surface energy balance. In this way, cool air, perhaps from a rural area, advected on to an urban surface can lead to a positive sensible‐heat flux which then tends to neutralize any stable stratification in the boundary layer. Copyright © 2006 Royal Meteorological Society
Improving simulations of severe winter storms by initial modification of potential vorticity in sensitive regions
Authors: RØsting B. , KristjÁnsson J. E. 
Published: January 2007

AbstractIn this paper simulations of three selected severe winter storms over the North Atlantic are presented. Modifications of potential vorticity (PV) fields according to features in water vapour (WV) images are combined with information from singular vectors (SVs) in an attempt to improve the initial state over data‐sparse regions. The apparent mismatch between features in the WV images and the upper‐level PV anomalies in the numerical analyses is corrected, mainly at levels indicated as sensitive by the fastest‐growing SVs. The procedure is based on the fact that modification of the analysis in regions where SVs show a pronounced signal may have a large impact on the simulation. Model reruns, based on the inverted corrected PV fields, were then performed. Three cases are presented: the two French Christmas storms of 1999, and the storm affecting UK, the North Sea and southern Norway on 30 October 2000. In these cases a substantial improvement of the simulations of the storms was achieved.The PV modifications were carried out by a digital analysis system, implemented at the Norwegian Meteorological Institute. This system allows the PV modifications to be done interactively within an operational time limit. Copyright © 2006 Royal Meteorological Society
Development of a new wind‐rose for the British Isles using radiosonde data, and application to an atmospheric transport model
Authors: Dore A. J. , Vieno M. , Fournier N. , Weston K. J. , Sutton M. A. 
Published: January 2007

AbstractA six‐hourly dataset of radiosonde ascents spanning a ten‐year period from four stations in the British Isles has been used to generate a set of wind frequency roses and wind speed roses for the pressure level range 950–900 hPa. The wind frequency rose showed close agreement with the long‐term series of the Jenkinson classification scheme. Small but significant inter‐station and interannual variations were observed. Seasonal analysis of the data revealed the higher incidence of north‐easterlies during spring months whilst the stronger wind speeds associated with the winter months were also evident. The use of a harmonic mean was found to be appropriate for calculating a directionally dependent wind speed for use in an atmospheric transport model. A harmonic mean wind speed value of 7.5 m s−1 was generated from the entire dataset, the same as that which has previously been used in other transport models. This is also the same value as the ‘optimized wind speed’ that was generated by Singles et al. The radiosonde wind frequency rose and wind speed rose were input to the FRAME atmospheric transport model. This resulted in an improved correlation with measurements of SO2 concentrations from a national monitoring network when compared to a model simulation using the earlier dataset of Jones. Copyright © 2006 Royal Meteorological Society
Laboratory studies of the effect of cloud conditions on graupel/crystal charge transfer in thunderstorm electrification
Authors: Saunders C. P. R. , Bax‐norman H. , Emersic C. , Avila E. E. , Castellano N. E. 
Published: January 2007

AbstractCollisions between vapour‐grown ice crystals and a riming target, representing a graupel pellet falling in a thunderstorm, were shown by Reynolds, Brook and Gourley to transfer substantial charge, which they showed to be adequate to account for the development of charge centres leading to lightning in thunderstorms. Related experiments by Takahashi and Jayaratne et al. determined that the sign of charge transferred is dependent on the cloud liquid water content and on cloud temperature. There are marked differences between the results of Takahashi and Jayaratne in the details of the dependence they noted of the sign of graupel charging on cloud water and temperature. More recently, Pereyra et al. have shown that results somewhat similar in form to those of Takahashi are obtained by modifying the experimental technique used to prepare the clouds of ice crystals and supercooled water droplets used in the experiments.In order to help resolve the reason for the differences in charge transfer results in various studies, work has continued in the Manchester laboratory with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently. Results indicate a profound effect on the charge sign of the particle growth conditions in the two clouds involved. For example, by suitable adjustments to the water contents of the two clouds, graupel is charged negatively by rebounding ice crystal collisions at higher cloud water contents than have been noted previously. It is suggested that the most important influence on charge sign is the relative diffusional growth rate of the two ice surfaces at the moment of impact and that this is affected by an increase in cloud supersaturation experienced by the ice crystals during the cloud mixing process just prior to collision. A range of cloud conditions is used in the present work in order to help determine the reasons for the various results reported previously.Examination of some thunderstorm observations in the context of the present results points to the importance of mixing on the sign of the charge transferred during particle collisions when two cloud regions of different histories mix together. Copyright © 2006 Royal Meteorological Society
A data assimilation method for log‐normally distributed observational errors
Authors: Fletcher S. J. , Zupanski M. 
Published: January 2007

AbstractIn this paper we change the standard assumption made in the Bayesian framework of variational data assimilation to allow for observational errors that are log‐normally distributed. We address the question of which statistic best describes the distribution for the univariate and multivariate cases to justify our choice of the mode. From this choice we derive the associated cost function, Jacobian and Hessian with a normal background. We also find the solution to the Jacobian equal to zero in both model and observational space. Given the Hessian that we derive, we define a preconditioner to aid in the minimization of the cost function. We extend this to define a general form for the preconditioner, given a certain type of cost function. Copyright © 2006 Royal Meteorological Society
Influence of the Madden–Julian Oscillation on East African rainfall: II. March–May season extremes and interannual variability
Authors: Pohl Benjamin , Camberlin Pierre 
Published: January 2007

AbstractThe Madden–Julian Oscillation (MJO) was shown in Part I to have a significant impact on both East African rainy seasons at pentad time‐scale. The case of the ‘long rains’ (March–May) is further examined by considering both shorter (daily) and longer (interannual) time‐scales. Based on composite analyses, extreme daily rainfall events in the Highland (west) and in the coastal (east) regions of Equatorial East Africa (Kenya and Tanzania) are extracted. Low‐level westerly wind anomalies are seen to accompany wet events in the west and easterly ones in the east. These opposite circulation anomalies preferentially occur at distinct phases of the MJO, which indicates that the latter has a major influence on rainfall at the daily time‐scale.However, this influence undergoes significant year‐to‐year variations. It is found that the common variance between smoothed rainfall time series (5‐day low‐pass filter) and MJO indices varies from 5% to 53% in the Highland region. Significantly lower air temperatures in the upper troposphere are recorded during the MJO cycles that present the highest common variance with East African rainfall. Such a cooling is seen to be related to the Kelvin wave propagation in the upper layers which favours upward atmospheric motion over the region.At the interannual time‐scale, fluctuations in MJO amplitude contribute to the March–May rainfall variability, and 44% of the March–May seasonal rainfall variance in the 1979–95 period in East Africa is explained by this parameter. Years of high MJO amplitude are characterized by earlier onset of the rains, and higher seasonal amounts. Copyright © 2006 Royal Meteorological Society
Accounting for an imperfect model in 4D‐Var
Authors: Tr'emolet Yannick 
Published: January 2007

AbstractIn most operational implementations of four‐dimensional variational data assimilation (4D‐Var), it is assumed that the model used in the data assimilation process is perfect or, at least, that errors in the model can be neglected when compared to other errors in the system. In this paper, we study how model error could be accounted for in 4D‐Var.We present three approaches for the formulation of weak‐constraint 4D‐Var: estimating explicitly a model‐error forcing term, estimating a representation of model bias or, estimating a four‐dimensional model state as the control variable. The consequences of these approaches with respect to the implementation and the properties of 4D‐Var are discussed.We show that 4D‐Var with an additional model‐error representation as part of the control variable is essentially an initial‐value problem and that its characteristics are very similar to that of strong constraint 4D‐Var. Taking the four‐dimensional state as the control variable, however, leads to very different properties. In that case, weak‐constraint 4D‐Var can be interpreted as a coupling between successive strong‐constraint assimilation cycles. A possible extension towards long‐window 4D‐Var and possibilities for evolutions of the data assimilation system are presented. Copyright © 2006 Royal Meteorological Society
Synoptic‐scale weather patterns during Alpine heavy rain events
Authors: Hoinka Klaus P. , Schwierz Cornelia , Martius Olivia 
Published: January 2007

AbstractAutumnal streamer‐type elongated troughs are related to heavy precipitation observed at the southside of the European Alps. In order to study the development of this configuration, the typical structural evolution of the ambient flow in the event of heavy rain is investigated by lagged analyses. The precipitation amounts, both observations at the Alpine southside and from ERA40 forecasts by the ECMWF, are chosen as parameters to be correlated with atmospheric fields provided by the ERA40 data. Indeed, the resulting statistics indicate that the preferable position of elongated streamers is quasi‐north–south aligned and extending southward over the Mediterranean Sea with its southern end located between the Pyrenees and northern Africa. A significant orographic impact on the streamer is revealed in the regression fields of the upper‐level potential vorticity showing an indentation above the western Alps. The lower‐tropospheric regression fields show a zonally elongated pressure signal due to the orographic influence. Up to two days in advance of heavy rain events a regressed meridional humidity flux occurs towards the south‐western Alps, which at the event's peak time is found impinging upon the entire Alpine ridge. The regressed vertical velocity fields exhibit two maxima south of the Alps, a strong one at the western and a weaker one over the eastern Alps. Copyright © 2006 Royal Meteorological Society
Assessment, correction and impact of the dry bias in radiosonde humidity data during the MAP SOP
Authors: Häberli Christian 
Published: January 2007

AbstractThis study reports on the characteristics of humidity measurements in the Mesoscale Alpine Programme (MAP) radiosonde dataset. During the Special Observing Period (SOP) of MAP from 9 September 1999 to 15 November 1999, about 10 100 soundings from 51 sites were collected in the so‐called ‘high‐resolution data domain’. Of these, 39 used Vaisala RS80A radiosondes, which delivered 81% of all soundings. Additionally, five sites providing about 10% of all soundings used either Meteolabor SRS‐400, Graw or VIZ sondes equipped with carbon hygristors for humidity measurements, six sites (8% of all soundings) used Meteolabor temperature sondes without humidity measurement, and one site used Vaisala RS90 sondes. Quality control and careful analysis of the MAP upper‐air dataset revealed several characteristics including the dry bias in Vaisala RS80 radiosondes. Since about two thirds of the stations are part of the routine synoptic network and were operated during the MAP SOP without major procedural changes, this evaluation also allows us to characterize the backbone of the operational upper‐air observations.The dry bias of the Vaisala RS80 radiosonde was corrected using two different algorithms. The first algorithm raised the values of relative humidity of this sonde's profiles by about 4 measurement units (% relative humidity) in the lower troposphere and by 2 in the upper levels. The results of the second algorithm highly depend on the sonde's age. The relative humidity values are raised by 10% in the lower parts of the troposphere and by about 4% in the upper levels. These results are comparable to previous studies which mainly treated soundings from research stations in tropical regions. As in these studies, the increase of humidity in the soundings results in an increase of convective available potential energy and precipitable water, as well as a decrease of convection inhibition. Copyright © Royal Meteorological Society, 2006.
Understanding the variability of clear‐sky outgoing long‐wave radiation based on ship‐based temperature and water vapour measurements
Authors: John V. O. , Buehler S. A. , Engeln A. von , Eriksson P. , Kuhn T. , Brocard E. , Koenig‐langlo G. 
Published: January 2007

AbstractHigh‐resolution radiative transfer model calculations with the Atmospheric Radiative Transfer Simulator (ARTS) were used to simulate the clear‐sky outgoing long‐wave radiative flux (OLR) at the top of the atmosphere. The unique set of radiosonde data collected by the research vessel Polarstern of the Alfred Wegener Institute for Polar and Marine Research during 27 expeditions in the years 1982 to 2003 was used to investigate the sources of clear‐sky OLR variability for ocean areas in different climate zones and seasons. For this dataset, tropospheric temperature variations contribute approximately 33 W m−2 OLR variability, tropospheric relative humidity variations 8.5 W m−2, and vertical structure 2.3–3.4 W m−2. Of these, 0.3–1.0 W m−2 are due to structures on a vertical scale smaller than 4 km, which cannot be resolved by conventional remote‐sensing instruments. It was also found that the poor absolute accuracy of current humidity data in the upper troposphere, approximately 40% relative error in relative humidity, leads to a significant uncertainty in OLR of about 3.8 W m−2 (for a midlatitude summer atmosphere), which should be put in the context of the double CO2 effect of only 2.6 W m−2 (for the same atmosphere).