Trends in fog frequencies in the Los Angeles Basin

Data from throughout the Los Angeles area were examined to determine the horizontal distribution of dense fog (visibility < 400 m) in the region and trends over time. The relationship between the occurrence of dense fog to the phase of two atmosphere–ocean cycles: the Pacific Decadal Oscillation (PDO) measured by the PDO Index and the Southern Oscillation measured by the Southern Oscillation Index (SOI) was investigated. In addition, the influence of the urban heat island and the amount of suspended particulate matter were assessed. For the three stations that had 22 or more years of data, we examined trends and the relation to atmosphere–ocean cycles. Results show a decrease in the occurrence of very low visibilities (< 400 m) at the stations in close proximity to the Pacific Ocean, Los Angeles International Airport (LAX) and Long Beach International Airport (LGB). Occurrence of the frequency of low visibilities at these two locations was also highly correlated with the phase of the PDO. Only a weak, non-statistically significant relationship was seen with the SOI. At Burbank, a reporting station about 30 km from the ocean, no trends were noted, and there was no evidence of a correlation with either the PDO Index or the SOI. In the Los Angeles Central Business District (CBD) when comparing dense fog occurrence in the early 1960s to a similar period in the early 2000s we saw a decrease in dense fog from a mean of 10 h per year to a mean of 3 h per year. Also contributing to the decrease was decreasing particulate pollution and increased urban warming. A downward trend in particulate concentrations coupled with an upward trend in urban temperatures were associated with a decrease in dense fog occurrence at both LAX and LGB. These trends were evident for the period 1966–1997, but appear to have ended by the late 1990s.     

A Climatological Investigation of the Activity of Summer Subtropical Vortices

By applying a new vortex detection method to the ECMWF 40-yr reanalysis (ERA40) data from 1985 to 2002, the climatology of summer vortices has been investigated in five subtropical regions, i.e., the northwestern Pacific, northeastern Pacific, northwestern Atlantic, northeastern Atlantic, and Australia-South Pacific, followed by validation with NCEP/NCAR reanalysis data. Results are as follows: (1) The spatial distributions of ERA40 vortex activities (VAC) were well consistent with those of NCEP/NCAR reanalysis (NRA) results in all regions, especially in northwestern Pacific. (2) Because of different model resolutions, both the number and intensity of vortices obtained from NRA were significantly weaker thanERA40's. (3) Vortices mainly cruised in coasts and the adjacent seas, from where to the land or the open sea vortex activities were gradually decreased. (4) There were two active centers in the northwestern Pacific:one was located in South China Sea and the other, as the largest center of the five regions, spread from the east side of the Philippines to Japan. (5) Over the northwestern Atlantic, most vortices occurred in Panama and its west-side offshore. (6) The spatial distributions of vortices were alike between the northeastern Pacific and northeastern Atlantic, both spreading from coasts to the west-side sea at 5°-20°N. (7) In the Anstralia-South Pacific, vortices were not as active as those in the other four regions, and mostly took place in the equator-side of near ocean areas. (8) Except the northwestern Pacific and northwestern Atlantic, the VAC interannual variations in the other three regions were different between ERA40 and NRA data. (9)In the northwestern Pacific and northwestern Atlantic, the VAC interannual variation could be separated to several distinct stages. (10) Since the mid 1980s, mean vortex intensity was getting increased in the northwestern Pacific, which was most significant in the subtropical areas on a global basis. In the western North Atlantic, there was a decreasing (increasing) trend of the mean vortex intensity before (after) the mid1990s.     

城市热山波的动力和数值分析

木文采用对数压力坐标系的大气热力-动力学方程组,对城市热岛现象之一的热山波的特征、形成机制、存在条件及其对天气的影响进行了动力学和数值分析。     

地面辐射气候学研究进展——从卫星反演地面辐射能收支的若干问题

地面辐射气候学研究的开展对了解当今地球气候是一重要步骤。为了能获取全球范围具有一定空间，时间分辨率的资料，卫星观测的应用是必不可少的。本文对地气系统辐射能收支的卫星观测研究取得的成果作简要回顾后，重点介绍了地面短波，长波辐射通地面净辐射的地面短波、长波辐射通量及地面净辐射的卫星反演取得的进展，存在问题及今后的发展趋势。     

Preliminary Analysis of the Temporal Patterns of Heavy Rainfall across the Southeastern United States

This paper examines trends in heavy rainfall events across the southeastern United States over the past century. Time series of the number of annual storm events over a 76.2 mm threshold were used for analysis. Significant trends that had some spatial coherence were identified. Increasing trends were detected in a region extending from northeastern Texas to the Appalachians, with a weaker signal of decreasing events along the East Coast. These patterns may be linked to fluctuations in the strength and migration of the Bermuda High.     

Trends in Interdiurnal Temperature Variation for the Central United States, 1945–1985

Temporal variation in the absolute value of interdiurnal variability (AIDV) of January maximum temperatures is examined for the period 1945–1985 in the central United States (90°–105°W). AIDV indicates the magnitude of day-to-day changes in the maximum temperature. Five-year running-mean AIDV values decreased significantly throughout the period. This trend was apparent for the region overall and for the 10 of the 20 study sites analyzed individually. Temporal variation in AIDV values was significantly related to mid-tropospheric flow patterns over the central United States. Meridional circulation was associated with smaller AIDV values in all sites except those in the northwestern and southeastern corners of the study area, whereas zonal flow was linked to larger interdiurnal temperature change. The analysis of selected individual years indicated that mid-tropospheric circulation patterns were more persistent with meridional flow, and surface cyclones and their associated surface fronts tracked through the study area less frequently than in years with zonal flow. Analysis at a daily temporal resolution revealed an added dimension of trends in temperature variability beyond those apparent with a coarser monthly resolution. These results emphasize the importance of finer temporal resolution in the analysis of both past and projected climate change; regionally summarized monthly temperature values mask important variation in temperature trends apparent at shorter temporal and more local spatial scales.     

青藏高原云的气候学特征

利用国际卫星云气候计划（ＩＳＣＣＰ）获取的１９８３年７月～１９９０年６月２．５°×２．５°分辨率的云气候资料以及Ｈａｈｎ等整理的１９７１～１９８１年５°×５°分辨率地面观测云气候资料，综合分析了青藏高原地区冬季和夏季云的水平和垂直分布特征，从而为检验大气环境或气候模式的云模拟能力及进一步研究青藏高原地区云辐射相互作用对气候的影响提供背景依据。     

Thermal and haline fronts in the Yellow/East China Seas: Surface and subsurface seasonality comparison

Seasonal variability of surface and subsurface thermal/haline fronts in the Yellow/East China Seas (YES) has been investigated using three-dimensional monthly-mean temperature and salinity data from U.S. Navy’s Generalized Digital Environmental Model (Version 3.0). The density-compensated Cheju-Yangtze Thermal/Haline Front has (northern and southern) double-tongues. The northern tongue is most evident throughout the depth from December to April. The southern tongue is persistent at the subsurface with conspicuous haline fronts. The thermal (haline) frontal intensity of the northern tongue is controlled mainly by the temperature (salinity) variation on the shoreward (seaward) side of the front. The cold water over the Yangtze Bank is influential in generating the southern tongue and intensifying the Tsushima Thermal Front. The year-round Cheju-Tsushima Thermal Front is evident throughout the depth and intensifies from July to December. The northern arc of the Yangtze Ring Haline Front is manifest in spring and is sustained until summer, whereas the southern one is fully developed in summer because of eastward migration of the Yangtze Diluted Water. The area showing strong frontal intensity in the Chinese Coastal Haline Front shifts seasonally north and south along the Zhejiang-Fujian coast. The Generation and evolution of YES fronts are closely associated with YES circulation (inferred from the linkage of the water masses). Moreover, the subsurface temperature/salinity evolution on the fronts in the Yellow Sea differs from that in the East China Sea owing to local factors such as wintertime vertical mixing and a summertime strong thermocline above the Yellow Sea Bottom Cold Water.     

南海几种海面风应力资料的比较分析

利用卫星遥感（ERS1、ERS2）资料、综合海洋大气资料集（COADS）资料、Hellerman与Rosenstein客观分析资料，提取南海海面风应力因子进行比较分析，发现遥感资料与其他两种资料相比，具有分辨率高、量值合理的特点，建议在研究南海海面风应力气候学特征及季节变化规律，尤其是中、小尺度涡旋时采用遥感资料。     

Storminess and surges in the South-Western Approaches of the eastern North Atlantic: the synoptic climatology of recent extreme coastal storms

The climatology of intense winter cyclone events in the eastern North Atlantic responsible for high magnitude surge generation (top 1% of events) within the region of the South-Western Approaches to northwest France and southwest England is extracted from daily sea-level and 500-hPa level atmospheric pressure analyses. Cluster analysis yields discrete cyclone track regimes linked to upper airflow patterns being responsible for the generation of intense storms (central pressure at sea-level ≤990 hPa) which promote severe surge events ≥60 cm along the French coast of the South-Western Approaches. Fluctuations in storminess are strongly influenced by the southward intrusion and strengthening of the jet stream in mid-Atlantic. These occurrences are often associated with negative sea surface temperature (SST) anomalies near Newfoundland and a strengthening of the thermal gradient across the Atlantic well to the south of its normal position. Resultant cyclogenesis promotes storms displaying a delay in minimum central pressure attainment until well east of 14°W, encouraging enhancement of surge flow. Stepwise multiple regression analysis indicates that the most influential variables in promoting severe surge events in the South-Western Approaches are trans-Atlantic sea surface temperature gradients. The most important influence is the prevailing west–east sea surface temperature gradient during the month of the storm, followed by that for the prior month of the storm and thirdly, the north–south sea surface temperature gradient prevailing during the month of the event. Other influential variables reflect the character of the cyclone, storm duration, mean deepening rate of storm central pressure, the value of the outermost closed isobar around the storm centre, and the longitudinal position of the outermost closed isobar to the right of the cyclone track. In contrast, the North Atlantic Oscillation (NAO) is not very effective as a discriminator of surge activity. This may reflect the greater intra-month volatility of parameters used to derive the NAO index than of other monthly variables considered in the study. The lack of resolvability at the individual storm level probably also arises because of the monthly detailing of the NAO against storms of 2–3 days duration. This behavioural model of extreme storminess in the South-Western Approaches to northwest France provides the basis by which extremes of coastal susceptibility can be calibrated.