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Radiosonde Data (radiosonde + data)
Selected AbstractsPre-rainy season moisture build-up and storm precipitation delivery in the West African SahelINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2008J. Bayo Omotosho Abstract The salient differences between the years of above and below normal precipitation, particularly within the long period of 1972,1990 with persistently decreasing Sahelian rainfall, are investigated for Kano, a Nigerian station within the Sahel. Daily rainfall data from 1916 to 2000, storm records from 1951 to 2000 and radiosonde data for three dry and three wet years are used in this study. Results confirm previous findings that the African Easterly Jet (AEJ) located in the 700,600 mb layer is stronger during the dry than in wet years. Significantly, however, it is shown that during the wet years, there is stronger and deeper early season (April,June) build-up of moisture below the AEJ. Furthermore, throughout the period from April to August, the middle troposphere was almost always drier than normal during the dry years and moist than normal in the wet years. Consequent upon these, the storms, which deliver almost all the rainfall in the Sahel, produce at least 150% more precipitation during the wet than in the dry years, though the June to September or annual total number of storms differs by only about 30%. Finally, during the dry years, the onset of rainfall is found to be generally very late compared to the long-term mean, with shorter length of the rainy season. Copyright © 2007 Royal Meteorological Society [source] Flow dependence of background errors and their vertical correlations for radiance-data assimilationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010Reinhold Hess Abstract This article examines the dependence of background-error statistics on synoptic conditions and flow patterns. Error variances and vertical correlations of background temperatures as used for variational radiance-data assimilation are estimated for two different weather regimes over Europe using the NMC method. The results are validated with real observations, i.e. radiosonde data and microwave satellite radiances and generalised with half a year of global data from the ECMWF forecasting system, where weather conditions are distinguished using model fields of wind speed, mean sea level pressure, and relative vorticity. Strong winds, low pressure, and cyclonic flow generally induce larger background errors of 500 hPa temperature than calm winds, high pressure, and anticyclonic flow, and also broader temperature correlations in the vertical with other tropospheric levels. Copyright © 2010 Royal Meteorological Society [source] Föhn as a response to changing upstream and downstream air massesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 635 2008Georg J. Mayr Abstract Observations of föhn from the field phase of the Mesoscale Alpine Programme (MAP) are used to study how differences between the air masses upstream and downstream of the central Alpine crest determine whether the flow can descend to the lee as either shallow föhn, when it passes through passes in the mountains, or deep föhn, when it overflows the Alpine crest. First, the föhn case of 30 October 1999 is examined using ECMWF analyses and radiosonde data at various upstream and downstream locations. Additional measurements from aircraft, dropsondes, an instrumented car and automatic weather stations are then used for a detailed study of the föhn flow across the Brenner Pass. Advection of cold air around the eastern edges of the Alps and warm air around the western edge of the Alps ahead of a synoptic ridge set up a reservoir of colder air on the south side of the Alps and a reservoir of warmer air to the north. The depth to where the air was colder on the southern side was sufficient for a shallow föhn to flow through the pass. After the passage of the ridge axis, synoptic cold air advection provided another source of colder air, this time from the southwest, growing deeper with time and having a synoptically imposed cross-barrier flow component. The maximum depth to where the air upstream was colder than downstream extended just above the peaks of the highest mountains. An analysis of the detailed measurements across the Brenner Pass showed that this depth was also the top of the layer that descended and accelerated down the lee slopes of the Wipp Valley. Upstream, air above the föhn layer had an even stronger cross-barrier component yet did not descend because it did not have lower potential temperatures than the downstream side at that level. Deep föhn never developed. An examination of other well-documented MAP föhn cases confirmed the conclusion from the 30 October event that shallow and deep föhns , at least for the central Alps , are mostly a response to differences in air masses between the upstream and downstream side. A cross-barrier component of the flow was only a modification but in itself not sufficient to cause the flow to both descend and accelerate down the lee slope, unless potential temperatures on the upstream side were lower in this layer than on the downstream side. Copyright © 2008 Royal Meteorological Society [source] Assessment of the impact of key terrestrial observing systems using DMI-HIRLAMTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 633 2008Bjarne Amstrup Abstract Following a EUCOS decision to make an assessment of the impact on NWP forecasts of different components of the current observing systems in various combinations, a number of OSEs (Observing System Experiments) have been made by a number of NWP centres, with some running global models and some limited-area models. The Danish Meteorological Institute (DMI) carried out OSEs with the limited-area model HIRLAM. The following runs were made by DMI (two periods, one month each): (1) A baseline system (BL); (2) BL + all aircraft wind and temperature data; (3) BL + non-GUAN radiosonde winds; (4) BL + non-GUAN radiosonde temperature and wind; (5) BL + wind profiler; (6) as (4) + aircraft wind and temperature; (7) as (4) + non-GUAN radiosonde humidity; (8) as BL + all in situ data (full combined system); and (9) BL + E-AMDAR only (no ACARS and no traditional AIREPs). The lateral boundaries for these OSEs were provided by runs made by ECMWF, which was one of the centres to make OSEs with a global model. The main conclusions are that the radiosonde data are the most important data, closely followed by the aircraft data, and that aircraft data and radiosonde data are complementary and not redundant data. Furthermore the results show that it is important to have both wind and temperature profile data; wind data alone produce much poorer impact. Copyright © 2008 Royal Meteorological Society [source] Development of a new wind-rose for the British Isles using radiosonde data, and application to an atmospheric transport modelTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 621 2006A. J. Dore Abstract A 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 [source] Validation of precipitable water from ECMWF model analyses with GPS and radiosonde data during the MAP SOPTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 612 2005Olivier Bock Abstract Precipitable water vapour contents (PWCs) from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses have been compared with observations from 21 ground-based Global Positioning System receiving stations (GPS) and 14 radiosonde stations (RS), covering central Europe, for the period of the Mesoscale Alpine Programme experiment special observing period (MAP SOP). Two model analyses are considered: one using only conventional data, serving as a control assimilation experiment, and one including additionally most of the non-operational MAP data. Overall, a dry bias of about ,1 kg m,2 (,5.5% of total PWC), with a standard deviation of ,2.6 kg m,2 (13% of total PWC), is diagnosed in both model analyses with respect to GPS. The bias at individual sites is quite variable: from ,4 to ,0 kg m,2. The largest differences are observed at stations located in mountainous areas and/or near the sea, which reveal differences in representativeness. Differences between the two model analyses, and between these analyses and GPS, are investigated in terms of usage and quality of RS data. Biases in RS data are found from comparisons with both model and GPS PWCs. They are confirmed from analysis feedback statistics available at ECMWF. An overall dry bias in RS PWC of 4.5% is found, compared to GPS. The detection of RS biases from comparisons both with the model and GPS indicates that data screening during assimilation was generally effective. However, some RS bias went into the model analyses. Inspection of the time evolution of PWC from the model analyses and GPS occasionally showed differences of up to 5,10 kg m,2. These were associated with severe weather events, with variations in the amount of RS data being assimilated, and with time lags in the PWCs from the two model analyses. Such large differences contribute strongly to the overall observed standard deviations. Good confidence in GPS PWC estimates is gained through this work, even during periods of heavy rain. These results support the future assimilation of GPS data, both for operational weather prediction and for mesoscale simulation studies. Copyright © 2005 Royal Meteorological Society. [source] Observations of downslope winds and rotors in the Falkland IslandsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 605 2005S. D. Mobbs Abstract A field campaign aimed at observing the near-surface flow field across and downwind of a mountain range on the Falkland Islands, South Atlantic, is described. The objective was to understand and eventually predict orographically generated turbulence. The instrumentation was based primarily on an array of automatic weather stations (AWSs), which recorded 30 s mean surface pressure, wind speed and direction (at 2 m), temperature and relative humidity for approximately one year. These measurements were supported by twice-daily radiosonde releases. The densest part of the AWS array was located to the south of the Wickham mountain range, across Mount Pleasant Airfield (MPA). In northerly flow the array provides a detailed study of the flow downwind of the mountain range. The dataset contains several episodes in which the flow downwind of the mountains is accelerated relative to the upwind flow. During some of these episodes short-lived (typically ,1 hour) periods of unsteady flow separation are observed and these are associated with the formation of rotors aloft. Such events present a significant hazard to aviation at MPA. Examination of radiosonde profiles suggests that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for both downwind acceleration and the formation of rotors. The data are used to show that the downwind fractional speed-up is proportional to the non-dimensional mountain height (based on upstream near-surface winds and a depth-averaged Brunt,Väisälä frequency diagnosed from radiosonde data). Similarly, a relationship is established between a quantity that describes the spatial variability of the flow downwind of the mountains and the upstream wind and depth-averaged Brunt,Väisälä frequency. The dependence of the flow behaviour on the Froude number (defined in the usual way for two-layer shallow-water flow) and ratio of mountain height to inversion height is presented in terms of a flow regime diagram. © Royal Meteorological Society, 2005. S. B. Vosper's and P. F. Sheridan's contributions are Crown copyright [source] Quasi-periodic bora gusts related to the structure of the troposphereTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 598 2004Danijel Belu Abstract This study provides a new insight into the behaviour of the bora wind gusts. Wind speed and direction measured with a 1 s sampling interval between 1 December 2001 and 31 January 2002 at Senj (east Adriatic) provided a sufficiently large database for a study of the gusts behaviour. The performed spectral analysis confirmed the occurrence of pulsations in the bora flow. Moreover, a new type of dynamics, involving the onset, cessation and reappearance of the pulsations within a single episode, has been observed in several cases. Suggested mechanisms responsible for generating these phenomena have been determined from comparisons of surface wind data with the upstream tropospheric thermodynamical structure derived from Zagreb radiosonde data. In particular, it has been shown that the appearance of an upper-tropospheric jet stream results in cessation of the pulsations, and the decrease in the jet stream supports quasi-periodic gust behaviour. As the pulsations are generated by wave breaking, the jet stream appearance has been related to the disappearance of the wave-breaking region in the lee of the mountain, which is in accordance with previous studies of downslope windstorms. Copyright © 2004 Royal Meteorological Society. [source] Comparing mass-consistent atmospheric moisture budgets on an irregular grid: An Arctic exampleTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 592 2003M. Göber Abstract We present a method to minimize the effects of different resolution and mass imbalance when comparing atmospheric energy and water budgets from different datasets. Sizeable differences between re-analysis- and radiosonde-based atmospheric budgets had been found in earlier studies and it had been suspected that the different resolutions of the datasets strongly contributes to these discrepancies. Furthermore, most studies so far had used mass-imbalanced wind fields, which can lead to serious errors. We balance the wind field by using a variational modification algorithm combined with a finite-element discretization which allows the use of data on a grid defined by the radiosonde network. This method permits the computation of flux divergences in integral form and gives a consistent numerical method to get a mass-balanced wind field with minimum modifications. Applying this method to Arctic radiosonde and re-analysis data on the same grid leads to a better agreement with respect to the horizontal distribution and the mean annual cycle of the moisture flux convergence. The constraint of mass balance on the wind field leads to a greatly reduced and more realistic variability in space and time. However, a systematic difference of about 20% remains between the estimate based on a re-analysis dataset sampled only on the coarse grid of the radiosonde network and an estimate based on the use of the full, fine grid of the re-analysis. These systematic differences can be significantly reduced by creating a simulated radiosonde dataset from the re-analysis with doubled resolution. We undertake an extensive analysis of the uncertainty of the estimates originating from the choices made in the specification of the algorithm. Based solely on radiosonde data, which are likely to result in a low bias, we estimate the net water gain of the Arctic atmosphere as 164 ± 10 mm yr,1 (0.45 ± 0.03 mm d,1) for 1979,93. Copyright © 2003 Royal Meteorological Society. [source] Retrieval of microphysical and morphological properties of volcanic ash plumes from satellite data: Application to Mt Ruapehu, New ZealandTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 576 2001A. J. Prata Abstract A quantitative analysis of the properties of several Mt Ruapehu, New Zealand, ash plumes has been performed using multispectral satellite data from the AVHRR-2 and ATSR-2 instruments. The analysis includes: identification of the plume from background clouds using the ,reverse' absorption effect in the thermal channels: modelling and retrieval of particle sizes; determination of the plume height from cloud shadows, stereoscopy and meteorological data; and estimates of the mass of fine particles (radii less than 10 ,m). A new spectral technique for identifying opaque, silica-rich ash clouds is demonstrated by utilizing the near-infrared (1.6 ,m) and visible (0.67 ,m) channels of the ATSR-2, and the optical properties of a simple volcanic cloud are presented for use in radiative transfer studies. It is found that the Ruapehu eruption cloud contained silica-rich ash particles with radii generally less than a few micrometres. The distribution of fine particles is monomodal with a dominant mode peak of about 3 ,m radius. Mass loadings of fine particles are found to be in the range ,1 to ,7 mg m,3, and are consistent with estimates of mass loadings of volcanic clouds from eruptions of other volcanoes. The height of the plume top, derived from radiosonde data and plume-top temperatures in the opaque regions, was found to be between 7.5 and 8.5 km, while the plume thickness was estimated to be between 1.5 and 3 km. Cloud height derived from ATSR-2 stereoscopy on a different plume gave heights in the range 5 to 8 km. The results of this study provide important information on the optical properties of nascent volcanic eruption plumes. This information may prove useful in determining the potential effects of volcanic clouds on local climate, and in assessing any hazard to aviation. [source] Use of GPS/MET refraction angles in three-dimensional variational analysisTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 570 2000X. Zou Abstract The Spectral Statistical Interpolation (SSI) analysis system of the National Centers for Environmental Prediction (NCEP) is modified to include GPS/MET data (meteorological data from the Global Positioning Satellite system) using a GPS ray-tracing operator. The new system is tested by incorporating 30 actual GPS/MET observations of refraction angles obtained during the GPS/MET experiment. This is the first time that real radio occupation refraction angles and refractivities have been incorporated into a three-dimensional variational analysis system. We examine the magnitude and the vertical distribution of the analysis adjustments that result from using refraction-angle observations in the NCEP SSI analysis system. The average magnitudes of the adjustments in the temperature and specific-humidity fields are approximately 0.4 degC and 0.6 g kg,1, respectively. Individual changes can be as large as 4 degC and 4g kg,1, respectively. The greatest adjustments to the temperature occur in the middle and upper troposphere and stratosphere, while the major changes in specific humidity occur in the lower troposphere. An assessment of the impact of the GPS/MET observations on the analysis, verified by conventional (mostly radiosonde) data, is difficult because of the small number of GPS/MET data used. Nevertheless, it is found that, even over data-rich regions (regions containing many radiosonde observations), and even when the verification data were the radiosonde data themselves, the use of GPS/MET refraction angles makes a slight improvement, overall, to the analysed temperatures and winds. The impact on the water-vapour analyses, again as measured against radiosonde data, is mixed, with improvements shown in some layers and degradation in others. Compared with the background field, the use of refraction angles from one occultation results in an analysis whose simulated refraction angles are much closer to the withheld GPS/MET refraction angles at the two nearby occultation locations, and whose temperature and moisture profiles are also closer to those resulting from the direct assimilation of the two withheld occultations. Although the forward model used in this study, with the ray tracing being carried out in a two-dimensional plane, is much cheaper than a more accurate three-dimensional forward model, it is still quite expensive. In order to further reduce the computational requirement for the assimilation of GPS/MET data, we test a scheme in which the GPS/MET-retrieved refractivities (instead of refraction angles) are used above a selected height for each occupation. These heights are determined objectively based on the departures from spherical symmetry of the model field. It is shown that the mixed use of GPS/MET refraction angles and refractivities produces an analysis result similar to the one using refraction angles alone, while the computational cost is reduced by more than 30%. [source] | ||||||||||