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1.
Abstract The detailed characteristics of a CASPII warm frontal passage are presented in this article. This storm, Intensive Operating Period (IOP) 13 (February 26–27, 1992), was observed in detail with an array of diverse instruments. It has the advantage over earlier freezing precipitation studies of having simultaneous, in situ and remote sensing measurements by aircraft and ground‐based Doppler radar. The associated precipitation was in the form of banded structures parallel to the front. Within these bands were embedded precipitation cores, some parallel to the band, some perpendicular. The warm front itself was characterized by major perturbations in its kinematic and thermodynamic features. The cores oriented parallel to the front were the result of embedded convection generated, at least in part, by the irregularities in the frontal surface. The cores oriented perpendicular to the front were closely associated with the 0°C isotherm on the underside of the frontal inversion. Precipitation phase changes played a significant role in the occurrence of wide near 0°C regions, both vertically and horizontally. These regions had a profound influence on the observed precipitation types and led to complex precipitation‐thermodynamic‐dynamic interactions. Instabilities produced by these interactions are seen in wave‐like features observed by the Doppler radar in these regions, both parallel and perpendicular to the frontal zone. 相似文献
2.
Man‐Li C. Wu 《大气与海洋》2013,51(1):150-174
Abstract Global precipitation estimates using satellite data are derived using difference fields of outgoing long‐wave radiation (OLR). The difference fields consist of clear OLR minus cloudy OLR, which is a measure of long‐wave cloud radiative forcing at the top of the earth‐atmosphere system; and clear daytime OLR minus clear night‐time OLR, which is a measure of the diurnal variation of surface heating. All geophysical parameters used to compute OLR are derived from an analysis of the HIRS2/MSU sounding data. The derived global precipitation estimates show good agreement with collocated raingauge data over land. The correlation coefficient between the precipitation estimates derived using difference fields of OLR and raingauge data over land is about 0.65 for the FGGEyear. The correlation coefficient between precipitation estimates derived using difference fields of OLR and the GOES Precipitation Index (GPI) fraction is about 0.914 from 30°S to 30°N for July 1983, and between the precipitation estimates derived using difference fields of OLR and the difference field of atmospheric reflectance is about 0.86. Using one set of coefficients, global precipitation fields are derived for each 10‐day period and each month of the FGGE year (from December 1978 to November 1979). These fields contain rich information on seasonal variations. 相似文献
3.
Abstract The thunderstorms of a June cold‐front passage have been observed by radar, on a constant‐altitude map at 6‐km height as “Larsen areas’ ‘ within which the precipitation intensity level exceeds 30 dBZ, and by two sferics receivers, frequency 100 ± 50 kHz, one at the radar and the other 57 km distant. From the sferics, a distribution of storms with angular extent was obtained. This yielded the distribution with azimuth of the sferics random observational errors: normal with standard deviation 2.1°. Combined with radar‐determined ranges (between 100 and 200 km), it also yielded the distribution with cross‐range linear extent of the sferics sources: log normal with median 9.2 km, standard deviation corresponding to a factor 1.6, for widths 6 to 15 km. No correlation was found between storm width and sferics rate. A power‐law relation has been found between sferics rate and the Larsen area A, sufficiently good that the sferics rate can be predicted within a factor 2 from the Larsen area, but different from that obtained for another day. For both days, the Larsen area A could be replaced by another precipitation parameter, P, in effect a weighted area taking into account the variation within A of the precipitation intensity. This more complex parameter was somewhat better than Aas a predictor of the sferics rate. 相似文献
4.
G.W. Reuter 《大气与海洋》2013,51(2):216-229
Abstract Precipitation production is investigated for 9 intense thunderstorms that developed over the Lowveld in South Africa. A C‐band radar is used to observe the 3‐dimensional reflectivity pattern. Using an empirical relation between reflectivity factor and precipitation content and integrating over the storm volume provides an estimate of the total precipitation content aloft. Likewise, an area integration of the instantaneous rain rate at cloud base yields an estimate of the rate of total outflow. At their maturing stage, the storms had precipitation contents of 0.2 to 5.0 Tg and rainfall rates of about 0.3 to 2.0 Gg s?1. The total accumulation of rain at the ground ranged from 1 to 10 Tg. The characteristic storm updraft, defined as the ratio of the area‐averaged rainfall rate to the volume‐averaged precipitation content, was about 5 ms?1 for all storms. The time evolution of integral storm parameters is also presented and related to the overall storm development. The precipitation production values observed in the Lowveld storms compares well with previous estimates reported for large thunderstorms observed in Alberta and New England. 相似文献
5.
Abstract Fourteen rain‐snow boundaries and freezing precipitation episodes within Canadian East Coast winter storms are examined. Radar, sounding, and surface observations are used to study the characteristics of these features. Rain‐snow boundaries and freezing precipitation episodes are coupled phenomena. The deepest radar echoes and the heaviest precipitation generally occur in the adjacent snow region, and surface wind shifts typically are associated with the transition to rain. 相似文献
6.
Abstract Observations of the changes in mean annual and seasonal precipitation with time in British Columbia and western Alberta between the periods 1931–60 and 1951–80 indicate that more stations show an increase than a decrease. Changes tend to be positive on the coast, negative in south‐central British Columbia, positive on the west slopes of the Rockies, and negative to the east. This suggests an orographic influence and increased flow from the southwest at mountain‐top. The possibility that these changes in precipitation are associated with CO2 warming is considered. Both the surface air temperature and sea surface temperature were raised by 2°C for cyclogenesis events over the northeastern Pacific and the consequences were investigated with the aid of a limited‐area, 8‐level primitive equations model. The lows are deepened by as much as 6 mb and precipitation amounts are increased up to 8 mm owing to the increased mixing ratio and the feedback of latent heat energy. 相似文献
7.
《大气与海洋》2013,51(4):331-345
Abstract Droughts, a major concern to all of Canada, particularly to the Canadian Prairies, typically occur once every three years. Most research addressing such droughts has concentrated on the large‐scale conditions associated with this form of extreme weather whereas little research has been conducted on its smaller scale characteristics. The present study addresses these smaller scale issues. Several datasets (surface observations, drought indices, and precipitation data) were used to identify droughts occurring over the Prairies since 1953 and to compare their characteristics with those years associated with non‐drought conditions. Warm season, May to September, hourly surface observations (temperature, moisture, clouds, and precipitation type) from ten observation sites were compared for four categories of moisture across the Canadian Prairies, extreme drought, drought, non‐drought, and extreme wet conditions. When compared to non‐drought or extreme wet conditions, drought and extreme drought conditions are naturally associated with warmer temperatures and drier conditions; there is also little change in the amount of total cloud cover but cloud bases are higher and the maximum temperature occurs 1–2 h later in the afternoon. Near‐surface energy increases systematically from extreme wet to extreme drought conditions. 相似文献
8.
Abstract Radar reflectivity measurements and sounding data were analyzed to investigate snowfall production in a long‐lasting snowband that formed in advance of a warm surface front moving across Alberta. The sounding data indicated that the band could have been forced by slantwise overturning during the release of moist symmetric instability combined with frontogenesis. The stability analysis presented here is novel in that it includes ice phase thermodynamics, neglected in previous studies of slantwise convection. Radar reflectivity fields were analyzed to determine the total snow content and the mass outflow rate as factors of time. The peak value of total snow content was 17 kilotons per km of snowband, and the peak mass outflow rate was 10 tons s‐1 km‐1. The snowfall rate averaged across the cloud base was about 0.8 cm h‐1, and the average snow content remained close to 0.2 g m‐1. The characteristic time (defined as the ratio of total snow content over mass outflow rate) was about 30 minutes, which is approximately the time needed for the growth of snowflakes by aggregation in the observed temperature range. The precipitation efficiency of the snowband, defined as the ratio of snow mass outflow to water vapour inflow was estimated to be 14%. The precipitation production values observed in the Alberta snowband are compared with previous estimates reported for frontal rainbands and Alberta thunderstorms. 相似文献
9.
《大气与海洋》2013,51(2):93-105
Abstract Global warming due to increased greenhouse gases is believed to result in not only higher surface temperatures but also an acceleration of the hydrological cycle leading to increased precipitation. Although climate models consistently predict increases in global temperatures due to increasing greenhouse gases and the accompanying global warming, observations at the climatic timescales necessary to confirm the models are rare. Multidecadal studies at global and regional scales are necessary to determine whether the presently observed changes in temperature and precipitation are due to short‐term fluctuations or long‐term trends. In this study, we address this issue by examining changes in temperature and precipitation on Long Island, New York over a 74‐year time period (1931 to 2004) using a network of rain gauges and temperature measurements. The mean annual temperature on Long Island has increased at a rate of 0.05°C per decade, which is less than that of observed global values and is most likely due to the urban warming effects of New York City, not large‐scale climate change. The mean total annual precipitation has increased at a rate of 0.71 cm per decade during the study period, which is consistent with global observations. Intra‐annual temperature fluctuations are decreasing at a rate of 0.36% per decade, while precipitation variations are increasing at a rate of 0.91% per decade. Empirical orthogonal function analysis indicates that variations in temperature and precipitation on Long Island are dominated by island‐wide fluctuations that are directly related to the North Atlantic Oscillation, the Arctic Oscillation, and the El Niño Southern Oscillation. 相似文献
10.
Abstract The accuracy of temperature and precipitation forecasts for Toronto was studied for the 20‐year period 1960–1979. Since any archive of official forecasts extends for only a small part of this period, it was necessary to retrieve the forecasts from newspaper records. The possible errors involved in such a data source were examined through a comparison of newspaper reported observations and the official record. On only a few occasions were significant differences observed. For temperature forecasts, the record indicates a significant loss of skill over the 20‐year periodin the prediction of maximum temperature for the first day. This was observed not only for the Bloor Street observing station for which the entire 20‐year record was analysed, but also for observing stations at Toronto Island, Downsview and Malton. The loss of skill over the years is greatest in winter when temperature is consistently predicted too low at all stations. For the entire period under study, precipitation forecasts consisted only of words and no quantitative information (such as probability of precipitation forecasts) was issued. Word choice is intended to carry information on the duration and expected spatial coverage of precipitation, but substantial inconsistencies between word choice and subsequent precipitation occurrence were found. Consequently, the verification procedure for these forecasts was very simple and ignored any differences implied in word choice. With this technique precipitation forecasts were shown to have improved over the 20‐year period. 相似文献
11.
Maurice Danard 《大气与海洋》2013,51(1):52-72
Abstract The influences of surface fluxes and convective precipitation are investigated for two 36‐h periods of cyclogenesis over the northeastern Pacific Ocean. Three methods are tested of specifying the fraction of moisture supply that produces convective precipitation in a modified form of Kuo's (1974) parametrization scheme using an 8‐level primitive equations model. When convection is included, precipitation amounts are greater and the cyclone deepening is better predicted than when convection is not included. Predicted cyclogenesis is very sensitive to sea temperature. As the low moves over warmer water, the effect of sensible heating is to increase the moisture convergence in the atmospheric boundary layer. This increases the precipitation rates and accelerates deepening. It is concluded that the CISK mechanism plays an important role in extratropical cyclogenesis. 相似文献
12.
Abstract In this study, the development of a moderate coastal storm with intense precipitation that occurred during 12–14 February 1993 is examined using a high‐resolution version of the Canadian Regional Finite‐element (RFE) model with more realistic physical representations. It is shown that the improved RFE model predicts well the coastal cyclogenesis events and also the distribution and intensity of heavy mixed precipitation (rain and snow) associated with the storm. It is found that the cyclogenesis takes place in response to the low‐level inshore advection of high‐θe air from the maritime boundary layer, and the approach of a mid‐level shortwave trough with a warm pool above that is previously associated with a decaying cyclone upstream. More rapid deepening of the cyclone ensues as intense precipitation falls along the warm and cold fronts near the cyclone centre. Diagnosis of the control and sensitivity simulations reveals that the low‐level inshore warm advection and the propagation of the stratospheric warm pool contribute more significantly to the surface pressure falls during the incipient stage, whereas the mid‐level shortwave trough plays an important role in the cyclogenesis at later stages. Overall, latent heat release accounts for about 50% of the cyclone's total deepening, in agreement with the presence of a moderate baroclinic environment and the generation of intense precipitation. The diabatic and kinematic structures near the rain‐snow boundary are examined to gain insight into the influence of melting snow on the cyclogenesis. It is shown that the improved RFE model reproduces well the rain‐snow boundary structures as previously observed. Moreover, a thermally indirect circulation (perturbation) can be seen in the vicinity of the rain‐snow boundary. It is found, however, that melting of snow tends to produce a weak negative or negligible impact on the cyclogenesis, as opposed to previous hypotheses. 相似文献
13.
Abstract The Canadian Atlantic Storms Program (CASP) provided an opportunity for comparing two quite different remote‐sensing approaches to the detection of precipitation: radar backscatter and ambient ocean sound. Several of the gales passing the observation area during CASP produced substantial precipitation with the periods of radar backscatter showing close coincidence with simultaneous acoustic signals. The ambient sound record most readily yields an indication of precipitation from shifts in spectral slope. An important result of the experiment is the demonstration that an identifiable precipitation signal occurs even in strong wind conditions. The surface bubble layer formed during strong winds only partially attenuates the higher frequency acoustic components generated by precipitation. During rain‐free periods the attenuation can be interpreted in terms of the bubble size distribution. 相似文献
14.
Lauren M. Candlish Richard L. Raddatz Geoffrey G. Gunn Matthew G. Asplin David G. Barber 《大气与海洋》2013,51(3):249-264
Abstract This study presents a semi‐analytic non‐iterative solution for the Monin‐Obukhov similarity equations under unstable surface conditions. The solution is represented in terms of the non‐dimensional Monin‐Obukhov stability parameter z/L. This parameter is given as a function of the bulk Richardson number and other surface parameters including the heat and momentum roughness lengths which are generally assumed to be different in this formulation. The proposed formulations give results that are both quantitatively and qualitatively consistent with the fully iterated numerical solution for a wide range of surface parameters. 相似文献
15.
Abstract The relationships between monthly anomalies of sea surface temperature (SST) and monthly anomalies of several surface wind parameters are examined using ten years of data from the mid‐latitude North Pacific Ocean. The wind parameters involve both u3 * and curl τ, where u* is the atmospheric friction velocity and τ the surface stress. These quantities are calculated from surface wind components analysed on synoptic (6‐hourly) maps. In order to examine the effect of synoptic disturbances, the time series of surface wind components at each grid point is high‐pass filtered (passing periods less than 10 days) and the above wind parameters are calculated from both filtered and unfiltered wind components. Two statistically significant relationships are found between monthly anomalies of SST and those of the various wind parameters. The first is a large coherent negative correlation between monthly anomalies of u3 * calculated from the high‐pass filtered wind components and month‐to‐month changes in the SST anomalies in the Central Pacific. This relationship is attributed to the production of turbulent vertical mixing in the ocean by synoptic disturbances in the atmosphere. The second relationship is a large positive correlation between curl τ calculated from the unfiltered wind components and SST anomaly changes in the Eastern Pacific. This relationship, which is opposite to that expected from Ekman pumping, is attributed to a negative association between the wind stress curl and the meridional advection of heat by the eastern boundary current system. It is shown that these atmospheric forcing mechanisms explain up to 10 per cent of the variance of monthly SST anomalies in a large part of the mid‐latitude North Pacific Ocean. This amount is in addition to, but certainly less than, that which can be explained by anomalous horizontal advection through statistical relationships with sea‐level pressure anomalies (Davis, 1976). 相似文献
16.
《大气与海洋》2013,51(2):122-131
Abstract We assessed the impacts of some key Pacific ocean‐atmosphere circulation patterns on annual cycles of temperature and precipitation across British Columbia, Yukon, and southeast Alaska. The El Niño‐Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and ENSO conditional on PDO states were considered in composite analyses of 71 long, high‐quality datasets from surface meteorological stations. Month‐by‐month, station‐by‐station Monte Carlo bootstrap tests were employed to assess statistical significance. The results trace precipitation and temperature responses as a function of location, season, and climate mode. In summary, temperature responses were relatively uniform, with higher (lower) temperatures during the warm (cool) phases of these circulation patterns. Nevertheless, strength and seasonal persistence varied considerably with location and climate mode. Impacts were generally most consistent in winter and spring but could extend through most of the year. Overall spatiotemporal patterns in precipitation response were decoupled from those in temperature and were far more heterogeneous. Complexities in precipitation signals included north‐south inverse teleconnectivity along the Pacific coast, with a zero‐response hinge point in the approximate vicinity of northern Vancouver Island; seasonally opposite anomalies in several interior regions, which might conceivably reflect contrasting effects of Pacific climate modes on wintertime frontal storms versus summertime convective storms; and a consistent lack of substantial response in northwestern British Columbia and possibly southwestern Yukon, conjectured to reflect complications associated with the Icefield Ranges. The product is intended primarily as a basic‐level set of climate response maps for hydrologists, biologists, foresters, and others who require empirical assessments of relatively local‐scale, year‐round ENSO and PDO effects across this broad region. 相似文献
17.
Abstract Current understanding of the regional nature of global changes in the climato‐logical regime of the earth is limited. General circulation climate models (GCMs) cannot provide consistent and detailed information on the regional patterns of precipitation, soil moisture and runoff that are required by water resource planners. A case study is presented that couples a limited area model, with high spatial resolution and realistic land‐surface parametrization, to a global climate model. Results for July are presented for the continent of Australia. 相似文献
18.
Abstract In this study, the internal circulation structures of the 14 July 1987 intense mesoscale convective system (MCS) are investigated using an improved high‐resolution version of the Canadian regional finite‐element model. It is found that although the MCS is characterized by a leading convective line followed by a trailing stratiform rainband, the associated circulation structures differ substantially from those in the classical midlatitude squall system. These include the rapid propagation and separation of the leading convection from the trailing rainband, the development of a surface‐based instead of an elevated rear‐to‐front descending flow and a shallow front‐to‐rear ascending flow associated with the stratiform precipitation, the generation of low‐ and mid‐level rather than mid‐ to upper‐level stratiform cloudiness and the development of a strong anticyclonic vorticity band at the back edge of the stratiform region. It is shown that the trailing stratiform rainband is dynamically forced by frontogenetical processes, and aided by the release of conditional symmetric instability and local orographical lifting. The intense leading and trailing circulations result from latent heat released by the convective and explicit cloud schemes, respectively. Sensitivity experiments reveal that the proper coupling of these two cloud schemes is instrumental in obtaining a realistic prediction of the above‐mentioned various mesoscale components. Vorticity budget calculations show that tilting of horizontal vorticity contributes the most to the amplification of the anticyclonic vorticity band, particularly during the squall's incipient stage. The sensitivity of the simulated squall system to other model physical parameters is also examined. 相似文献
19.
Abstract Observations of sea‐surface temperatures and salinities, made by a variety of methods during August and September 1975 in the northeast Pacifie Ocean, are examined to evaluate the quality of surface data. The bucket method is capable of providing sea‐surface temperatures to an accuracy (standard deviation) of ±0.15°C. The thermograph/salinograph when corrected by applying a “field‐calibration” value, gives temperatures with a standard deviation one half that obtained by the bucket method. Expendable bathythermograph temperatures were, on the average, 0.3°C urate as the true values. Were it not for this offset they would have been as accurate as those obtained with bucket thermometers. Engine‐intake temperatures observed by the engine‐room crew were, on the average, 0.3°C larger than the true values, but were characterized by large inaccuracies, with a standard deviation about an order of magnitude greater than those found for other methods. These variations are believed to be due to reading errors. Sea‐surface salinities observed with the bucket could be, with reasonable care, accurate within the limitation of the salinometer method used aboard ships. The quality of data has been found to vary significantly between observers. Results obtained from this cruise and from weathership data (1956–1976) suggest that the surface temperatures and salinities observed during the past, 1956–1962, in the northeast Pacific Ocean have generally been overestimated. 相似文献
20.
Abstract The effects of marine fronts on the local atmospheric surface layer and air‐sea interaction were studied. Several mesoscale fronts were crossed by a research vessel in the Greenland Sea. Air temperature, humidity and stability conditions, and the fluxes of momentum, as well as sensible and latent heat, were investigated. For relatively calm conditions, close air‐sea coupling was observed in the temperature whereas for stronger winds, the air temperature of the surface layer was not markedly modified by the front below. Changes in the moisture content in the frontal area were observed and, in one case, evaporation was observed on the warm water side and condensation on the cold water side of the front. Frontal differences in heating from the sea were assumed to affect the surface‐layer wind field. 相似文献