Upper-air Stations Research Articles

Water vapor transport over China and its relationship with drought and flood in Yangtze River Basin

The characteristics of water vapor transport (WVT) over China and its relationship with precipitation anomalies in the Yangtze River Basin (YRB) are analyzed by using the upper-air station data in China and ECMWF reanalysis data in summer from 1981 to 2002. The results indicate that the first mode of the vertically integrated WVT is significant whose spatial distribution presents water vapor convergence or divergence in the YRB. When the Western Pacific Subtropical High (WPSH) is strong and shifts southward and westward, the Indian Monsoon Low Pressure (IMLP) is weak, and the northern part of China stands behind the middle and high latitude trough, a large amount of water vapor from the Bay of Bengal (BOB), the South China Sea (SCS) and the western Pacific forms a strong and steady southwest WVT band and meets the strong cold water vapor from northern China in the YRB, thus it is likely to cause flood in the YRB. When WPSH is weak and shifts northward and eastward, IMLP is strong, and there is nearly straight west wind over the middle and high latitude, it is unfavorable for oceanic vapor extending to China and no steady and strong southwest WVT exists in the region south of the YRB. Meanwhile, the cold air from northern China is weak and can hardly be transported to the YRB. This brings on no obvious water vapor convergence, and then less precipitation in the YRB.

An Evaluation of a Diagnostic Wind Model (CALMET)

Abstract A U.S. Environmental Protection Agency (EPA)-approved diagnostic wind model [California Meteorological Model (CALMET)] was evaluated during a typical lake-breeze event under fair weather conditions in the Chicago region. The authors focused on the performance of CALMET in terms of simulating winds that were highly variable in space and time. The reference winds were generated by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) assimilating system, with which CALMET results were compared. Statistical evaluations were conducted to quantify overall model differences in wind speed and direction over the domain. Below 850 m above the surface, relative differences in (layer averaged) wind speed were about 25%–40% during the simulation period; wind direction differences generally ranged from 6° to 20°. Above 850 m, the differences became larger because of the limited number of upper-air stations near the studied domain. Analyses implied that model differences were dependent on time because of time-dependent spatial variability in winds. Trajectory analyses were made to examine the likely spatial dependence of CALMET deviations from the reference winds within the domain. These analyses suggest that the quality of CALMET winds in local areas depended on their proximity to the lake-breeze front position. Large deviations usually occurred near the front area, where observations cannot resolve the spatial variability of wind, or in the fringe of the domain, where observations are lacking. Results simulated using different datasets and model options were also compared. Differences between CALMET and the reference winds tended to be reduced with data sampled from more stations or from more uniformly distributed stations. Suggestions are offered for further improving or interpreting CALMET results under complex wind conditions in the Chicago region, which may also apply to other regions.

An Extended Pacific–North American Index from Upper-Air Historical Data Back to 1922

Abstract This paper presents a reconstruction of a Pacific–North America (PNA) index from historical upper-level data for the period 1922–47. The data have been compiled from a number of sources and cover the Pacific–North American sector relatively well over this time period. Temperature and geopotential height profiles from aircraft, kite, and radiosonde ascents back to 1922 have been digitized and validated. Wind speed and direction from pilot balloon data back to the early 1920s, provided by NCAR, have also been used. A statistical regression approach is used for the reconstruction and calibrated in the post-1948 period using NCEP–NCAR reanalysis data. Split-sample validation experiments were performed within the NCEP–NCAR period, and sensitivity experiments with different subsets of predictors were performed. Similar reconstructions and validation experiments were carried out using a 540-yr control run from the Community Climate System Model, version 3 (CCSM3). The reconstructed index series together with validation statistics for both the historical and model data are presented. Excellent reconstruction skill is found for the winter months, while the reconstructions are somewhat worse in summer. Compared with a reconstruction based only on surface data, the addition of the newly digitized upper-air stations improves the reconstruction skill in all seasons. The historical reconstruction is presented with respect to its imprint on hemispheric fields of surface air temperature, sea level pressure, and precipitation with a special focus on extreme cases. In addition, the extended PNA index is compared with indices of the North Atlantic Oscillation, the Pacific decadal oscillation, and the El Niño–Southern Oscillation. The relationship to these indices is found to be stationary over the analysis period.

Three MCS Cases Occurring in Different Synoptic Environments in the Sub-Sahelian Wet Zone during the 2002 West African Monsoon

Abstract Three mesoscale convective systems (MCSs) occurring in the sub-Sahelian wet zone of West Africa are examined using observations from the 2002 Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) field campaign, the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, and Meteosat infrared imagery. These datasets enable the analysis of the synoptic-scale environment in which the MCSs were embedded, along with a high-resolution monitoring of surface parameters during the systems’ passages. The available data imply that cases I and II were of a squall-type nature. Case I propagated into a moderately sheared and rather moist lower and middle troposphere over the Upper Ouémé Valley (UOV). In contrast, case II was associated with a well-sheared and dry lower troposphere and a large, moist instability. In either case, behind the convective cluster a westward-propagating cyclonic vorticity maximum that was likely captured by the ECMWF analysis as a result of the special upper-air station at Parakou (Benin). In case I, the fast-moving vorticity signal slowed down over the Guinean Highlands where convection dissipated. Farther downstream, it might have played a role in the consolidation of an African easterly waves (AEW) trough over the West African coast and the eastern Atlantic. Case III proved to be a more stationary pattern of convection associated with a vortex in the monsoon flow. It also exhibited a moist and low shear environment.

Measurement Requirements for Climate Monitoring of Upper-Air Temperature Derived from Reanalysis Data

Abstract Using a reanalysis of the climate of the past half century as a model of temperature variations over the next half century, tests of various data collection protocols are made to develop recommendations for observing system requirements for monitoring upper-air temperature. The analysis focuses on accurately estimating monthly climatic data (specifically, monthly average temperature and its standard deviation) and multidecadal trends in monthly temperatures at specified locations, from the surface to 30 hPa. It does not address upper-air network size or station location issues. The effects of reducing the precision of temperature data, incomplete sampling of the diurnal cycle, incomplete sampling of the days of the month, imperfect long-term stability of the observations, and changes in observation schedule are assessed. To ensure accurate monthly climate statistics, observations with at least 0.5-K precision, made at least twice daily, at least once every two or three days are sufficient. Using these same criteria, and maintaining long-term measurement stability to within 0.25 (0.1) K, for periods of 20 to 50 yr, errors in trend estimates can be avoided in at least 90% (95%) of cases. In practical terms, this requires no more than one intervention (e.g., instrument change) over the period of record, and its effect must be to change the measurement bias by no more than 0.25 (0.1) K. The effect of the first intervention dominates the effects of subsequent, uncorrelated interventions. Changes in observation schedule also affect trend estimates. Reducing the number of observations per day, or changing the timing of a single observation per day, has a greater potential to produce errors in trends than reducing the number of days per month on which observations are made. These findings depend on the validity of using reanalysis data to approximate the statistical nature of future climate variations, and on the statistical tests employed. However, the results are based on conservative assumptions, so that adopting observing system requirements based on this analysis should result in a data archive that will meet climate monitoring needs over the next 50 yr.

A mesoscale convective system trapped along the Spanish Mediterranean Coast

Abstract. This paper describes the evolution of a mesoscale convective system (MCS) developed over the Alboran Sea on 7 February 2005, using surface, upper-air stations, radar and satellite observations, and also data from an operational numerical model. The system developed during the night as a small convective storm line in an environment with slight convective instability, low precipitable water and strong low-level vertical wind shear near coast. The linear MCS moved northwards reaching the Spanish coast. Then it remained trapped along the coast for more than twelve hours, following the coast more than five hundred kilometres. The MCS here described had a fundamental orographic character due to: (1) the generation of a low-level storm inflow parallel to the coast, formed by blocking of the onshore flow by coastal mountains and (2) the orientation of both the mesoscale ascent from the sea towards the coastal mountains and the midlevel rear inflow from the coastal mountains to the sea. The main motivation of this work was to obtain a better understanding of the mechanisms relevant to the formation of heavy rainfall episodes occurring at Spanish Mediterranean coast associated with this kind of stationary or slowly moving MCSs.

Total and methyl mercury patterns in Arctic snow during springtime at Resolute, Nunavut, Canada

Patterns of gaseous elemental mercury (GEM) were monitored at 20 and 150 cm above the snowpack near Resolute Bay, Cornwallis Island, Nunavut, Canada near the Upper Air Station of Environment Canada (74°42′N, 94°58′W) from 7 May (day 127) to 12 June (day 163) 2003. At this time of year there was 24 h daylight but still a strong diel change in solar radiation. Daily patterns of GEM-tracked solar radiation with a lag of about 2 h and the GEM gradient between these two heights showed the direction of flux. In addition to the previously established autocatalytic reactions involving halogens where reactive gaseous mercury and fine particulate mercury result in direct deposition to the snow, both diffusion to and volatilization from the snow occurred on a regular basis. Total mercury (THg) in the snowpack increased to near 30 ng L −1 following 8 d of atmospheric mercury depletion then decreased to values near 1 ng L −1. Losses from the snow could not be accounted for in melt water as stream runoff values were also low. In other words, most of the mercury associated with increased levels in snow was volatilized back to the atmosphere either directly from the snow or from the water surfaces. However, using accepted mass transport coefficients, the flux appeared low and other mechanisms are suggested. In contrast to THg, methyl mercury (MeHg) in the snow reached values near 140 pg L −1 but also declined to less than detection limit (10 pg L −1) with the onset of warmer temperatures. MeHg in stream runoff water was similar to maximal values seen in the snow. This observation is consistent with the view that MeHg came in the snowfall or was deposited to the snow pack rather than produced in the snow. In contrast, much of the THg associated with mercury depletion events was volatilized back to the atmosphere.

Meteorological pattern classification and application for forecasting air pollution episode potential in a mountain-valley area

In this study, we developed an automated scheme to classify the synoptic meteorological conditions governing over the northern Thailand during the winter period (November–January) into six-synoptic patterns representing different development stages of the regional pressure system. The meteorological classification scheme is hybrid in nature combining the air mass and the flow pattern based approaches. The classification is done though the principal component analysis of data from six regional surface meteorological stations and one upper-air station at 01:00 Thailand Local Standard Time daily for 5-year winter periods, which is followed by a two-stage clustering technique. Examination of the SO2 pollution levels in the complicated terrain of the Mae Moh valley in the northern Thailand, where a coal-fired power plant is located, shows that the scheme successfully identifies the patterns that are conducive to high pollutant and those that are conducive to low pollutant build up in the valley. Levels and spatial distributions of the daily highest 1-h SO2 concentrations in the valley exhibited similar characteristics within each pattern, and distinct differences between the six-synoptic patterns. The scheme is recommended for the early warning of the episode potentials in the valley. The stepwise regression was performed to develop prediction models for the two most polluted synoptic patterns, which have high frequencies of exceeding the National Ambient Air Quality Standards. The regression equations have R2 values above 60% and produce the daily highest 1-h SO2 concentrations with average relative errors of around 33%.

Designing meteorological networks for Switzerland according to user requirements

AbstractWith growing international collaboration (e.g. the European Composite Observing System programme –EUCOS), changing demands of data users, technical progress with new measuring systems and national budget cuts, the design of meteorological networks has become increasingly important. By analysing future user requirements and taking into account cost‐benefit considerations, MeteoSwiss has elaborated a network configuration for ground‐based and upper air stations. It is evident that for ground‐based stations no separate networks should be distinguished –instead, the needs of climatological modellers and forecasters should all be integrated into one network. The future upper air network will be complemented with additional windprofilers, a mini radar and additional radar information supplied by neighbouring countries. Copyright © 2003 Royal Meteorological Society.

Tornadoes in Lithuania in the period of 1950–2002 including analysis of the strongest tornado of 29 May 1981

Lithuania is a small European country situated on the eastern coast of the Baltic Sea. The main “weather factory” in these middle latitudes is the Atlantic Ocean. On the other hand, being close to continental weather conditions in the east, Lithuania encounters a variety of weather patterns. According to the data of Lithuanian Hydrometeorological Service, there were 23 tornadoes recorded in Lithuania in the period of 1950–2002. It has been established that tornadoes occur mostly in the warm period of the year—in spring and summer. The afternoon, between 12 and 15 UTC, is the most favorable part of the day for tornadoes to occur. Synoptic situation of the very strongest tornado (F2 on the Fujita Scale) of 29 May 1981 in the eastern part of the country was typical for most of the tornadoes there. A cold front was moving from the southwest of Lithuania to the northeast alongside a low, passing in parallel to the upper level flow. A closed surface low developed along the front. Convergence at the surface level, associated with divergence at higher levels, was the main cause of upward vertical motion. The powerful storm formed over the Eastern Lithuania at the point of coincidence of the wet bulb maximum potential temperature and the strongest convergence. Unfortunately, there was no good proximity sounding to the tornado. The Kaunas upper air station's data did not fully represent the convection process, as Kaunas was already in the cold air mass when the tornado developed in the warm air, so the Minsk (Belarus) data was analyzed in addition and compared to the Kaunas soundings. The analysis of tornadoes in Lithuania shows their initiation conditions to be similar to those in the rest of Europe and the USA.

Circulation characteristics of a monsoon depression during BOBMEX-99 using high-resolution analysis

The skill and efficiency of a numerical model mostly varies with the quality of initial values, accuracy on parameterization of physical processes and horizontal and vertical resolution of the model. Commonly used low-resolution reanalyses are hardly able to capture the prominent features associated with organized convective processes in a monsoon depression. The objective is to prepare improved high-resolution analysis by the use of MM5 modelling system developed by the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR). It requires the objective comparison of high and low-resolution analysis datasets in assessing the specific convective features of a monsoon depression. For this purpose, reanalysis datasets of NCAR/NCEP (National Center for Atmospheric Research/National Centers for Environmental Prediction) at a horizontal resolution of 2.5‡ (latitude/longitude) have been used as first guess in the objective analysis scheme. The additional asynoptic datasets obtained during BOBMEX-99 are utilized within the assimilation process. Cloud Motion Wind (CMW) data of METEOSAT satellite and SSM/I surface wind data are included for the improvement of derived analysis. The multiquadric (MQD) interpolation technique is selected and applied for meteorological objective analysis at a horizontal resolution of 30 km. After a successful inclusion of additional data, the resulting reanalysis is able to produce the structure of convective organization as well as prominent synoptic features associated with monsoon depression. Comparison and error verifications have been done with the help of available upper-air station data. The objective verification reveals the efficiency of the analysis scheme.

Modeling air-quality in complex terrain using mesoscale and dispersion models – part II: Evaluation of a dispersion model

Air-quality in a complex terrain (Colorado-River-Valley/Grand-Canyon Area, Southwest U.S.) is modeled using a higher-order closure mesoscale model and a higher-order closure dispersion model. Non-reactive tracers have been released in the Colorado-River valley, during winter and summer 1992, to study the dispersion of pollutants from a coal-fired power plant. The main objectives of the extensive field program MOHAVE (";Measurements Of Haze And Visibility Experiment";) were to investigate and identify the possible short- and long-term impacts of atmospheric pollutants from major urban areas and industrial sources on the Grand Canyon and its vicinity. In part I, the mesoscale model (MIUU model) is described. The model results are compared with data from the meteorological network of surface and upper-air stations within MOHAVE. The model results are also compared with those from another mesoscale model (MM5). In part II, the dispersion model is described. It is an Eulerian diffusion model. The model simulations of air-quality in the MOHAVE complex terrain during the program are compared with the available data. Keywords: Mesoscale modeling, Dispersion, Air-quality in complex terrain [Global Jnl Pure & Appl. Sci. Vol.9(3) 2003: 397-402]

Modeling Air-Quality in Complex Terrain Using Mesoscale and Dispersion models – Part I: Evaluation of a mesoscale model

Air-quality in a complex terrain (Colorado-River-Valley/Grand-Canyon Area, Southwest U.S.) is modeled using a higher-order closure mesoscale model and a higher-order closure dispersion model. Non-reactive tracers have been released in the Colorado-River valley, during winter and summer 1992, to study the dispersion of pollutants from a coal-fired power plant. The main objectives of the extensive field program MOHAVE (";Measurements Of Haze And Visibility Experiment";) were to investigate and identify the possible short- and long-term impacts of atmospheric pollutants from major urban areas and industrial sources on the Grand Canyon and its vicinity. In part I, the mesoscale model (MIUU model) is described. The model results are compared with data from the meteorological network of surface and upper-air stations within MOHAVE. The model results are also compared with those from another mesoscale model (MM5). In part II, the dispersion model is described. It is an Eulerian diffusion model. The model simulations of air-quality in the MOHAVE complex terrain during the program are compared with the available data. Keywords: Mesoscale modeling, Dispersion, Air-quality in complex terrain [Global Jnl Pure & Appl. Sci. Vol.9(3) 2003: 375-380]

Sensitivity of Continental-Scale Climate Trend Estimates to the Distribution of Radiosondes over North America

Abstract As part of its mandate to oversee the design of measurement networks for future weather and climate observing needs, the North American Atmospheric Observing System (NAOS) program hypothesized that replacing some of the existing radiosonde stations in the continental United States (CONUS) with another observing system would have little impact on weather forecast accuracy. The consequences of this hypothesis for climate monitoring over North America (NA) are considered here by comparing estimates of multidecadal trends in seasonal mean 500-mb temperature (T) integrated regionally over CONUS or NA, made with and without the 14 upper-air stations initially targeted for replacement. The trend estimates are obtained by subsampling gridded reanalysis fields at points nearest the 78 (126) existing CONUS (NA) radiosonde stations and at these points less the 14 stations. Trends in T for CONUS and NA during each season are also estimated based on the full reanalysis grid, but regardless of the sampling str...

A Simple Model to Determine In Situ Mixing Height Growth from Surface Measurements

A generic In Situ Mixing Height Growth (IMG) model, capable of predicting the real-time growth of the mixed layer and its diurnal evolution from routinely observed simple surface meteorological is developed. The algorithm for the determination of temporally growing daytime mixing height includes both convective and mechanical turbulence contributions. It accounts for neutral as well as height varying potential temperature gradients above the mixed layer. For thermally stable and mechanically dominated unstable night time Atmospheric Boundary Layer (ABL) the module uses similarity formulae based on the wind velocity [1]), the Monin—Obukhov length [2], and the Coriolis parameter. In the convective case simple slab model is integrated, based on initial lapse rate and the surface heat flux. The lapse rate is evaluated on the basis of vertical atmospheric stability, surface type and surface temperature. This differentiates the IMG model from other existing mixing height models that need initial measured lapse rate for calculation. IMG model is site specific as it calculates the radiative incoming heat flux depending on the solar declination estimates based on-site latitude and longitude. The IMG model is applied to calculate mixing height for India by using surface data (viz. wind speed, surface temperature, surface type) from 152 surface meteorological stations. Results have been evaluated with radiosonde mixing height data procured from 18 upper air stations. Sensitivity analysis of the model with respect to various parameters is performed. The model is formulated after reviewing presently available radiosonde mixing height data in India and can satisfactorily provide an alternative means of estimating mixing height for air pollution dispersion models.

High-Resolution Weather Forecasting over Complex Orography: Sensitivity to the Assimilation of Conventional Data

Abstract Weather forecasting for regions with complex orography, as the Alps, presents several challenges and the task becomes even more difficult when high resolution is required. Moreover, for the Alpine region, some of the problems are due to the lack of observations especially over the Mediterranean Sea. A possibility for improving forecasts is to reuse assimilation techniques locally. In this paper, results obtained through data assimilation are presented: objective analysis (OA) of observations and data analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) are used together to generate a new set of mesoscale initial (ICs) and boundary conditions (BCs). In particular, OA is applied to surface data and radiosoundings using two methods: Cressman and multiquadric. The sensitivity of the weather forecast to the number of upper-air stations assimilated by OA is tested using data from the Piedmont flood (4–6 November 1994). At first, a comparison is made between ICs, obtained through...

Use of adjoint sensitivity analysis to diagnose the CMC global analysis performance: A case study

The sensitivity of forecast errors to initial conditions obtained from the adjoint of a numerical weather prediction model provides new insights into the analysis errors responsible for poor short‐range to mediumrange forecasts. In recent years, we have developed a sensitivity analysis system based on the tangent linear and adjoint of the Global Environmental Multiscale model, in which an iterative procedure minimizing the shortrange forecast errors leads to the so‐called key analysis errors. These errors are dominated by a small number of atmospheric structures, those growing the most rapidly. The algorithm has proven very useful in understanding improvements to the three‐dimensional variational data assimilation (3D‐Var) system implemented in the Canadian Meteorological Centre operational suite in December 2001. The main difference between the old and the new 3D‐Var systems is the assimilation of temperature and surface pressure from surface and upper air stations as opposed to geopotential heights, additional Tiros Operational Vertical Sounder channels, new sources of observations such as temperature observations from aircraft, and wind and temperature from dropsondes. In this paper, we examine key analysis errors of the old 3D‐Var analysis, which led to a very poor 3‐day forecast of a severe winter storm that struck eastern Canada on 10 February 2001. In this case, the same 3‐day forecast from the new 3D‐Var analysis is much better. We compare the difference between the two 3D‐Var analyses and the key analysis errors. We find that the main key analysis errors, in terms of potential vorticity, is located along the west shore of southern California and is characterized by a strong baroclinic structure that has its maximum amplitude in the upper part of the troposphere. The difference between the two analyses is three times more energetic than the key analysis errors and its structure is much more barotropic in the troposphere. However, we show that the large improvement in the new 3D‐Var analysis stems mainly from the reduction of the analysis errors that project onto the key analysis structures.

Numerical study of atmospheric dispersion of a substance released from an industrial complex in the southern coast of Korea

Diurnal variations of wind field and pollutant dispersion in a complex terrain with a shoreline were investigated under the insolation conditions of summer and winter. The area is located in the south of the Korean Peninsula and includes a large petrochemical industrial complex. The Regional Atmospheric Modeling System (RAMS) was used in the simulation study. Initially, horizontally homogeneous wind fields were assumed on the basis of sounding data at the nearby upper-air station for days with morning wind speeds below 2 m s −1. On these days, the sea breeze prevailed in summer while the land breeze lasted for a few hours in the morning; the effect of synoptic winds was strong in winter with some inclusion of wind variations owing to the interaction between sea and land. The predicted wind direction at the location of the weather station captured an important change of the sea/land breeze of the observed one. In the morning, both in summer and winter, complicated wind fields with low wind speeds resulted in high pollutant concentrations almost all over the area. On the other hand, in the afternoon, the wind field was rather uniform and the terrain effects were not significant even in the mountainous area with the development of a mixing layer.

Wind and Diffusion Modeling for Complex Terrain

Abstract Atmospheric transport and dispersion over complex terrain were investigated. Meteorological and sulfur hexafluoride (SF6) concentration data were collected and used to evaluate the performance of a transport and diffusion model coupled with a mass consistency wind field model. Meteorological data were collected throughout April 1995. Both meteorological and plume location and concentration data were measured in December 1995. The meteorological data included measurements taken at 11–15 surface stations, one to three upper-air stations, and one mobile profiler. A range of conditions was encountered, including inversion and postinversion breakup, light to strong winds, and a broad distribution of wind directions. The models used were the MINERVE mass consistency wind model and the SCIPUFF (Second-Order Closure Integrated Puff) transport and diffusion model. These models were expected to provide and use high-resolution three-dimensional wind fields. An objective of the experiment was to determine if...

A scheme for advance prediction of northeast monsoon rainfall of Tamil Nadu

Monthly means of winds. contour height and temperature of seven standard isobaric levels of ten well distributed Indian upper air stations for the months of April, June. July, August and September were subjected to correlation analysis to detect parameters that have predictive value to forecast in advance the northeast monsoon rainfall of Tamil Nadu. The period 1965-87 was taken as developmental period and 1988-94 as test period. Six predictors, out of which three were completely new, were identified. The final forecast of rainfall was obtained as the weighted average of the individual forecasts based on the six predictors by employing a screening technique different from the conventional ones. The system explained between 65-77% variation of the predict and with standard error of 13-18% and provided reasonably correct forecasts during the test period. The physical significance of the predictors has been explained based on the intensity of the subtropical anticyclone over India. The possibility of extending the study to include global parameters in the context of proven negative and significant relationship between Southern Oscillation Index (SOl) and the Indian northeast monsoon has been discussed. Scope for further studies on the topic has been spelt out.