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1.
Abstract

This paper deals with a vicious winter storm which struck southern Ontario on the morning of January 26, 1978. This storm had the most intense circulation of any on record in this location. In Section 2 the synoptic situation up to and during this storm is described. Section 3 provides climatological pressure and wind data with which to compare the storm. In Section 4 the impact of the storm on Southern Ontario is analysed.  相似文献   

2.
Abstract

The dependence of ozone formation on the mixing ratios of volatile organic compounds (VOCs) and nitrogen oxides (NOx) has been widely studied. In addition to the atmospheric levels of VOCs and NOx, the extent of photochemical processing of VOCs has a strong impact on ozone levels. Although methods for measuring atmospheric mixing ratios of VOCs and NOx are well established and results of those measurements are widely available, determination of the extent of photochemical processing of VOCs, known as photochemical age (PCA), is difficult. In this article a recently developed methodology for the determination of PCA for individual compounds based on the change in their stable carbon isotope composition is used to investigate the dependence between ozone and VOC or NOx mixing ratios at a rural site in Ontario, Canada, during fall and winter. The results show that under these conditions the variability in VOC mixing ratios is predominantly a result of the varying impact of local emissions and not a result of changes in the extent of atmospheric processing. This explains why the mixing ratio of ozone shows no systematic dependence on the mixing ratios of VOCs or NOx in this environment and at this time of the year.  相似文献   

3.
Abstract

Past research has unveiled important variations in total precipitation, often related to large‐scale shifts in atmospheric circulation, and consistent with projected responses to enhanced greenhouse warming. More recently, however, it has been realized that important and influential changes in the variability of daily precipitation events have also occurred in the past, often unrelated to changes in total accumulation.

This study aims to uncover variations in daily precipitation intensity over Canada and to compare the observed variations with those in total accumulation and two dominant modes of atmospheric variability, namely the North Atlantic Oscillation (NAO) and the Pacific/North America teleconnection pattern (PNA). Results are examined on both annual and seasonal bases, and with regions defined by similarities in monthly variability.

Seasonally increasing trends in total precipitation that result from increases in all levels of event intensity during the 20th century are found in southern areas of Canada. During the latter half of the century increases are concentrated in heavy and intermediate events, with the largest changes occurring in Arctic areas. Variations in precipitation intensity can, however, be unrelated to variations in the total accumulation. Consistent with these differences, the precipitation responses to the NAO and PNA are often found to occur only at specific levels of event intensity. Precipitation responses to the NAO occur in northeastern regions in summer and winter with the intensity affected in both seasons. The PNA strongly influences precipitation in many regions of the country during autumn and winter. In particular, it strongly influences variations in southern British Columbia and the Prairies, affecting the intensity in only some areas. However, it only influences the frequency of heavier events in autumn and winter in Ontario and southern Quebec, where this response is actually more robust than the response in total accumulation. During these seasons a negative PNA generally leads to more extreme precipitation events.  相似文献   

4.
Abstract

The changes in normal precipitation amounts from 1931–60 to 1951–80 are examined for stations in Eastern Canada. The area covered comprises the Maritime Provinces and those parts of Ontario and Quebec south of a line approximately 200 km north of Lakes Erie and Ontario and the St Lawrence River. Changes are computed for each of the four seasons and for the entire year. On the whole, precipitation has increased, especially in winter and summer. However, there is considerable variation throughout the area, with precipitation decreasing in some regions. General circulation models that simulate the effect of doubling atmospheric CO2 also show an increase in precipitation.  相似文献   

5.
Abstract

Climatological characteristics of the low‐level tropospheric temperature inversion in the Canadian Arctic are examined using 10–40 year records of upper‐air meteorological data. Inversions at the northern sites are primarily surface‐based in winter, and elevated from mid‐spring through summer. At the southern sites, a bimodal pattern is observed with surface‐based inversions occurring during late summer, as well as during winter. From comparisons of our results with other published climatologies, it appears that this bimodal pattern reflects interactions between short‐ and long‐wave radiation, synoptic activity and snowmelt. Maxima in inversion depth and temperature difference across the inversion layer occur in February and March; minima occur in August and September. The annual progression of inversion characteristics closely follows the annual pattern of clear‐sky percentages, reflecting the controlling influence of cloud and clear‐sky radiative forcings on the inversion layer.  相似文献   

6.
The results from a one-dimensional photochemical model of the troposphere representative of summertime conditions at Northern Hemisphere mid-latitudes are presented. A parameterization of mixing processes within the planetary boundary layer (PBL) has been incorporated into the model for both the daytime convective PBL and the formation of the nocturnal PBL. One result of the parameterized PBL is that the concentrations of some trace species in the free troposphere are 20–30% higher than when mixing processes are described by a vertical eddy diffusion coefficient which is held constant with respect to height and time.The calculations indicate that the lifetime of the oxides of nitrogen (NO x =NO+NO2) against photochemical conversion to nitric acid (HNO3) during summertime conditions is on the order of 6 h. This lifetime is short enough to deplete most of the NO x in the PBL, resulting in the finding that other reactive nitrogen species (HNO3 and peroxyacetyl nitrate) are more abundant than NO x throughout the free troposphere, even though NO x is the most abundant reactive nitrogen species at the surface. The effects of the inclusion of anthropogenic nonmethane hydrocarbon (NMHC) chemistry are also discussed. The inclusion of NMHC chemistry has a pronounced effect on the photochemistry of tropospheric oxone and increases thein situ column production by more than 30%.  相似文献   

7.
Abstract

The annual, monthly and diurnal frequencies and durations of wave clouds are summarized from twelve years of observations at Pincher Creek, Alberta. Wave clouds occur on an average of 141 days annually. There is a distinct winter maximum anda summer minimum of occurrence. Wave clouds occur on 50% or more of the days during October and February, the most active months. Wave clouds persist for longer periods in the cooler half of the year. There appears to be a marked diurnal variation in wave cloudfrequency with maxima in the afternoon, however, this may be due to decreased visibility at sunrise and sunset.  相似文献   

8.
Abstract

It has been proposed that large savings in electrical energy are possible if cool, deep water from Lake Ontario is used, as an alternative to electrically operated chillers, to provide the cooling requirements of large buildings in down‐town Toronto. In order to estimate the capital expenditure involved, it is necessary to determine how deep (and how far offshore) the cold water intake must be placed to obtain a reliable supply of cool water throughout the air‐conditioning season. Temperature profile data, collected during ship surveys over the past twenty years in Lake Ontario off Toronto, plus time‐series data from a 21‐level thermistor array installed on the 100‐m contour near the site of the proposed intake during the summer of 1980, have been used to answer this specific question. The two sets are examined to determine the relative importance of thermal variability for inter‐annual, seasonal, and shorter time‐scales. Intake design turns out to be an important consideration for the air‐conditioning scheme. Unless azimuthal swirling velocities are restrained at the intake, drawdown of warmer thermocline water into the intake may occur.  相似文献   

9.
This study analyzes and compares aerosol properties and meteorological conditions during two air pollution episodes in 19–22 (E1) and 25–26 (E2) December 2016 in Northeast China. The visibility, particulate matter (PM) mass concentration, and surface meteorological observations were examined, together with the planetary boundary layer (PBL) properties and vertical profiles of aerosol extinction coefficient and volume depolarization ratio that were measured by a ground-based lidar in Shenyang of Liaoning Province, China during December 2016–January 2017. Results suggest that the low PBL height led to poor pollution dilution in E1, while the high PBL accompanied by low visibility in E2 might have been due to cross-regional and vertical air transmission. The PM mass concentration decreased as the PBL height increased in E1 while these two variables were positively correlated in E2. The enhanced winds in E2 diffused the pollutants and contributed largely to the aerosol transport. Strong temperature inversion in E1 resulted in increased PM2.5 and PM10 concentrations, and the winds in E2 favoured the southwesterly transport of aerosols from the North China Plain into the region surrounding Shenyang. The large extinction coefficient was partially attributed to the local pollution under the low PBL with high ground-surface PM mass concentrations in E1, whereas the cross-regional transport of aerosols within a high PBL and the low PM mass concentration near the ground in E2 were associated with severe aerosol extinction at high altitudes. These results may facilitate better understanding of the vertical distribution of aerosol properties during winter pollution events in Northeast China.  相似文献   

10.
Summary This paper investigates the influence of the planetary boundary-layer (PBL) parameterization and the vertical distribution of model layers on simulations of an Alpine foehn case that was observed during the Mesoscale Alpine Programme (MAP) in autumn 1999. The study is based on the PSU/NCAR MM5 modelling system and combines five different PBL schemes with three model layer settings, which mainly differ in the height above ground of the lowest model level (z 1). Specifically, z 1 takes values of about 7 m, 22 m and 36 m, and the experiments with z 1 = 7 m are set up such that the second model level is located at z = 36 m. To assess if the different model setups have a systematic impact on the model performance, the simulation results are compared against wind lidar, radiosonde and surface measurements gathered along the Austrian Wipp Valley. Moreover, the dependence of the simulated wind and temperature fields at a given height (36 m above ground) on z 1 is examined for several different regions. Our validation results show that at least over the Wipp Valley, the dependence of the model skill on z 1 tends to be larger and more systematic than the impact of the PBL scheme. The agreement of the simulated wind field with observations tends to benefit from moving the lowest model layer closer to the ground, which appears to be related to the dependence of lee-side flow separation on z 1. However, the simulated 2 m-temperatures are closest to observations for the intermediate z 1 of 22 m. This is mainly related to the fact that the simulated low-level temperatures decrease systematically with decreasing z 1 for all PBL schemes, turning a positive bias at z 1 = 36 m into a negative bias at z 1 = 7 m. The systematic z 1-dependence is also observed for the temperatures at a fixed height of 36 m, indicating a deficiency in the self-consistency of the model results that is not related to a specific PBL formulation. Possible reasons for this deficiency are discussed in the paper. On the other hand, a systematic z 1-dependence of the 36-m wind speed is encountered only for one out of the five PBL schemes. This turns out to be related to an unrealistic profile of the vertical mixing coefficient. Correspondence: Günther Z?ngl, Meteorologisches Institut der Universitat München, 80333 München, Germany  相似文献   

11.
Abstract

The Purple Crow Lidar (PCL) is a large power-aperture product monostatic laser radar which until 2010 was located at the Delaware Observatory (42°52′N, 81°23′W, 225 m elevation above sea level) near the campus of the University of Western Ontario. It is capable of measuring temperature from 10 to 110 km altitude, as well as water vapour in the troposphere and stratosphere. We use upper tropospheric and stratospheric vibrational Raman N2 backscatter-derived temperatures to form a climatology for the years 1999 to 2007 from 10 to 40 km attitude. The lidar temperatures are validated using nearby radiosonde measurements from Detroit and Buffalo. The measured temperatures show good agreement with the radiosonde soundings. An agreement of ±1 K is found during the summer months and ±2.5 K during the winter months, validating the calibration of the lidar to within the geophysical variability of the measurements. Comparison between the PCL measurements and the Committee on Space Research International Reference Atmosphere, 1986 (CIRA-86) and the Mass Spectrometer Incoherent Scatter-90 (MSIS-90) models show the models are as much as 5 K warmer below 25 km and 2 to 4 K colder above 25 km during the summer months, in large part because the measured tropopause height is consistently lower than in the models.  相似文献   

12.
Abstract

Fluxes of temperature, water vapour, O3, SO2 and CO2 were estimated from the measurement of their variances, taken over a wetland region in northern Ontario (Canada) during the summer of 1990 and over a deciduous forest when it was fully leafed during the summer of 1988 and when it was leafless during the winter of 1990. A set of flux‐variance relations was employed, including empirical forms of universal functions that could be adjusted with some constants. Results from the present study show that these constants needed to be adjusted with site‐specific data in order to achieve a closer agreement between estimated and observed fluxes. Best estimates were obtained for the fluxes of temperature and water vapour and it was found that the flux estimates of O3, SO2 and CO2 correlated better with water vapour than with temperature. For these trace gases the flux‐variance method yielded estimates of dry deposition velocities that were either comparable with or larger than those obtained from a resistance analogue model. Both methods yielded values that overestimated the observed dry deposition velocities. The employment of the flux‐variance method in an operational network would require the use of fast‐response sensors and a practical method for reducing the noise level of the measured variances.  相似文献   

13.
The effects of two soil datasets on planetary boundary layer (PBL) height are analyzed, using model simulations. Simulations are performed with the MM5 weather prediction system over the Carpathian Basin, with 6?km horizontal resolution, investigating three summer days, two autumn, and one winter day of similar synoptic conditions. Two soil datasets include that of the United States Department of Agriculture, which is globally used, and a regional Hungarian called Hungarian unsaturated soil database. It is shown that some hydraulic parameter values between the two datasets can differ up to 5–50%. These differences resulted in 10% deviations in the space–time-averaged PBL height (averaged over Hungary and over 12?h in the daytime period). Over smaller areas, these relative deviations could reach 25%. Daytime course changes in the PBL height for reference run conditions were significant (p?<?0.01) in ≈70% of the grid points covering Hungary. Ensemble runs using different atmospheric parameterizations and soil moisture initialization setups are also performed to analyze the sensitivity under changed conditions. In these cases, the sensitivity test showed that irrespective of the radiation and PBL scheme, the effect of different soil datasets on PBL height is roughly the same. PBL height is also sensitive to field capacity (Θf) and wilting point (Θw) changes. Θf changes seem to be more important for loamy sand, while Θw changes for the clay soil textural class.  相似文献   

14.
Observations and numerical modelling of Lake Ontario breezes   总被引：1，自引：0，他引：1  
Abstract

Analysis of two years of land‐based data shows that the Lake Ontario breeze develops on 30% of the days during summer. It typically develops in mid‐morning and persists until the late evening when it is replaced by a well developed land‐breeze regime. Simulations of 4 cases with the Colorado State University mesoscale model show good agreement with observations and suggest that local lake breezes are strongly influenced by adjacent water bodies (e.g. Lake Erie), the elongated shape of the lake, and the large‐scale wind direction. With gradient flows across the long axis of the lake, a broad band of along‐lake flow develops during the afternoon (easterly winds during southerly gradient flows and westerly winds during northerly gradient flows). Furthermore, during west‐to‐northwesterly gradient flow a nocturnal cyclonic eddy is predicted at the western end of the lake. These results imply that wind‐field models applied in the vicinity of Lake Ontario should incorporate the entire lake in their modelling domain.  相似文献   

15.
《大气与海洋》2013,51(3):153-162
Abstract

Winter rainfall is a non‐negligible issue for urban drainage in Canada as it can generate significant flooding, especially when it occurs at the same time as high air temperature and in the presence of an appreciable snow cover. According to climate change scenarios, it is expected that the occurrence of these events will increase in a future climate. The purpose of this paper is to perform a trend analysis on six indices related to winter rainfall (January–February) at 60 weather stations located in southern Québec and New Brunswick (Canada) in order to detect possible trends in the frequency or intensity of winter rainfall events during the twentieth century. Datasets were provided by Environment Canada and come from the Canadian Daily Rehabilitated Precipitation Database. The bootstrap‐based Mann‐Kendall test is used to detect possible non‐stationarities in the dataset, while Sen's slope estimator is used to quantify the magnitude of the slope. Results show that 19 stations out of 60 present a significant trend (18 of them being positive) at a 5% level for winter (January–February) total rainfall. In most cases where a trend was detected for winter rainfall there was also an increase in the number of days with rainfall (42% of the stations). These results suggest that globally, for the region under study, rainfall during January and February was more likely to occur, often resulting in a significant increase in the total rainfall during these months. Increasing trends in maximum daily rainfall during January and February were also observed for 9 stations (15% of the stations). The spatial distribution of stations where significant trends were detected is consistent with the hypothesis that trends in winter rainfall are more likely to be observed for stations located in the southern part of the region under study.  相似文献   

16.
Abstract

A 30‐year record (1951–1980) of surface heat fluxes at Ocean Weather Station P in the northeastern Pacific Ocean (50°N, 145°W) was examined for differences in the interdecadal variation between fail and winter. During the latter part of the 1950s and the early 1960s, the winter surface heat flux from the ocean to the atmosphere diminished significantly whereas the fall heat flux increased slightly This difference in the modulation of the winter heat flux from the fall heat flux during this period appears to be caused by the presence of an atmospheric circulation anomaly resembling that of the Pacific/North America (PNA) low‐frequency variability mode during the winter season.  相似文献   

17.
We examine trends in climate variables and their interrelationships over the Tibetan Plateau using global climate model simulations to elucidate the mechanisms for the pattern of warming observed over the plateau during the latter half of the twentieth century and to investigate the warming trend during the twenty-first century under the SRES A1B scenario. Our analysis suggests a 4°C warming over the plateau between 1950 and 2100. The largest warming rates occur during winter and spring. For the 1961–2000 period, the simulated warming is similar to the observed trend over the plateau. Moreover, the largest warming occurs at the highest elevation sites between 1950 and 2100. We find that increases in (1) downward longwave radiation (DLR) influenced by increases in surface specific humidity (q), and (2) absorbed solar radiation (ASR) influenced by decreases in snow cover extent are, in part, the reason for a large warming trend over the plateau, particularly during winter and spring. Furthermore, elevation-based increases in DLR (influenced by q) and ASR (influenced by snow cover and atmospheric aerosols) appear to affect the elevation dependent warming trend simulated in the model.  相似文献   

18.
Summary k-day extreme precipitation depths (k=1,2,3, … 30) for the climatological network of Belgium (165 stations) are analysed to detect a possible evolution in the occurrence of extreme rainfall events during the 1951–1995 reference period. The calendar year and the hydrological summer and winter are considered separately. Spearman’s rank correlation coefficient shows a strong spatial correlation between extreme k-day precipitation events, depending on the time of the year (lower during summer than during winter) and increasing with k. In some cases the distances of de-correlation exceed 200 km which is comparable to the size of the country. Due to this correlation, tests for trends have been carried out on the leading principal components (PC) derived from the covariance matrix. Various PC selection rules have been applied to identify the number of components to analyse. The number of components needed to reproduce a given proportion of the total variance varies, with larger values for summer than for winter and a decrease with growing k. The Fisher test is used as a global test. It combines the individual Mann-Kendall trend tests carried out on the selected PC scores. Significant trends have been found in extreme winter k-day precipitation for all the values of k and none in extreme summer precipitation. The results for the annual k-day precipitation depths are between those for the two seasons: no trend for small k because summer events dominate and a significant trend for k larger than 7 due to the winter events. Analysis of a few stations with long-term series shows no significant trend for the period 1910–1995, these series also reproduce almost the same trends as those found for the shorter 1951–1995 period. Received April 23, 1999 Revised December 6, 1999  相似文献   

19.
利用COSMIC掩星资料研究青藏高原地区大气边界层高度   总被引：5，自引：1，他引：4  
周文  杨胜朋  蒋熹  郭启云 《气象学报》2018,76(1):117-133
以往关于青藏高原边界层的研究都是基于个别站点的常规观测,对青藏高原边界层的整体性认识受限。GPS掩星资料具有测量精度高和垂直分辨率高的特性,其廓线中含有大量有价值的边界层信息。利用2007—2013年COSMIC掩星资料,通过计算大气折射率最小梯度来确定边界层高度,并用无线电探空资料对结果进行了检验。在此基础上,对青藏高原地区边界层高度的特征及其形成机制展开了研究,比较了COSMIC掩星确定的边界层高度和ERA-Int的差别,讨论了最小梯度法用于边界层研究的不确定性。结果表明:青藏高原上COSMIC掩星和无线电探空数据检测的边界层高度相关系数为0.786,平均值偏差为0.049 km,均方根误差为0.363 km,COSMIC掩星数据检测的边界层高度和无线电探空的结果非常接近。青藏高原上边界层高度呈现西高东低的分布特征,高原中西部边界层高度主要为1.8—2.3 km,而高原东部边界层为1.4—1.8 km,最大值在高原西南部。青藏高原地区边界层有明显的季节差异,冬季高原上大部分地区边界层高度超过2.0 km;春季大部分地区高度降低,但在受印度季风影响的高原南部有明显的抬升,最大值可超过3.0 km;夏季高原上边界层高度开始升高,大部分地区超过1.8 km;秋季又开始回落。青藏高原以北塔克拉玛干沙漠和高原以南印度季风活动区是两个高值区,北部的沙漠地区边界层高度在夏季最高,南部印度季风活动区在季风爆发前（4月）达到全年最大值。青藏高原中西部地区有水平风辐合以及广泛的上升运动,为边界层的发展提供了动力条件,而东部的下沉运动对边界层的发展有抑制作用。青藏高原边界层各个季节的空间分布与地表感热通量分布一致。COSMIC掩星资料确定的边界层高度和ERA-Int相比,空间分布基本一致但ERA-Int边界层高度明显偏低。当有系统性强逆温存在的时候,或者云中液态水或冰水含量较大时,用最小梯度法检测的边界层高度不确定性增加。   相似文献   

20.
The logarithmic + polynomial approximation is suggested for vertical profiles of velocity components in a planetary boundary layer (PBL) at neutral and stable stratification. The resistance law functions A and B are determined on the basis of this approximation, using integral relations derived from the momentum equations, the Monin-Obukhov asymptotic formula for the wind profile in a stably stratified near-surface layer and the known expressions for the PBL depth. This result gives a realistic and convenient method for calculating the surface friction velocity and direction and the total dissipation rate of mean flow kinetic energy in terms of geostrophic velocity, buoyancy flux at the surface, the roughness parameter and the Coriolis parameter. In the course of these derivations a review is given of current views on the main problems of the neutral and stable PBL.  相似文献   

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