Diurnal variation of surface wind over central eastern China

Hourly wind observations from 452 meteorological stations are used to document the diurnal cycle of the surface wind over the central eastern China (100°–122°E, 20°–42.5°N). Both the surface wind speed and the wind direction show large diurnal variation with pronounced topographic effects. At most stations, the surface wind speed reaches the maximum in the afternoon and the minimum in early-morning. This diurnal phase shows small seasonal variation, whereas the diurnal amplitude varies significantly in different seasons. The diurnal amplitude of the surface wind speed reaches maximum in spring over the northern and southwestern China and in summer over the southern China. The diurnal cycle of the wind direction is more complicated. Over the coastal (mountain) regions, the diurnal wind direction is greatly influenced by the land–sea (mountain–valley) breezes with large (small) seasonal variation. Over the northern plain region, the wind direction exhibits small diurnal variation but with remarkable seasonal rotation. The surface wind over the stations located on the top of mountains shows distinct diurnal variation, which represents the diurnal cycle of the tropospheric low-level wind. The wind speed over these stations is highest in pre-dawn and lowest in the afternoon. The wind anomaly rotates clockwise from late night to late afternoon, and shows significant seasonal variation as influenced by the annual cycle of the monsoon system. The contribution of the diurnal surface wind to the diurnal feature of precipitation is briefly discussed.     

Diurnal and seasonal variations of wind farm impacts on land surface temperature over western Texas

This paper analyzes seasonal and diurnal variations of MODerate resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data at ~1.1 km for the period of 2003–2011 over a region in West-Central Texas, where four of the world’s largest wind farms are located. Seasonal anomalies are created from MODIS Terra (~10:30 a.m. and 10:30 p.m. local solar time) and Aqua (~1:30 a.m. and 1:30 p.m. local solar time) LSTs, and their spatiotemporal variability is analyzed by comparing the LST changes between wind farm pixels (WFPs) and nearby non wind farm pixels (NNWFPs) using different methods under different quality controls. Our analyses show consistently that there is a warming effect of 0.31–0.70 °C at nighttime for the nine-year period during which data was collected over WFPs relative to NNWFPs, in all seasons for both Terra and Aqua measurements, while the changes at daytime are much noisier. The nighttime warming effect is much larger in summer than winter and at ~10:30 p.m. than ~1:30 a.m. and hence the largest warming effect is observed at ~10:30 p.m. in summer. The spatial pattern and magnitude of this warming effect couple very well with the geographic distribution of wind turbines and such coupling is stronger at nighttime than daytime and in summer than winter. Together, these results suggest that the warming effect observed in MODIS over wind farms are very likely attributable to the development of wind farms. This inference is consistent with the increasing number of operational wind turbines with time during the study period, the diurnal and seasonal variations in the frequency of wind speed and direction distribution, and the changes in near-surface atmospheric boundary layer (ABL) conditions due to wind farm operations. The nocturnal ABL is typically stable and much thinner than the daytime ABL and hence the turbine enhanced vertical mixing produces a stronger nighttime effect. The stronger wind speed and the higher frequency of the wind speed within the optimal power generation range in summer than winter and at nighttime than daytime likely drives wind turbines to generate more electricity and turbulence and consequently results in the strongest warming effect at nighttime in summer. Similarly, the stronger wind speed and the higher frequency of optimal wind speed at ~10:30 p.m. than that at ~1:30 a.m. might help explain, to some extent, why the nighttime LST warming effect is slightly larger at ~10:30 p.m. than ~1:30 a.m. The nighttime warming effect seen in spring and fall are smaller than that in summer and can be explained similarly.     

Diurnal variation of horizontal wind direction fluctuations in complex terrain at Geysers,Cal.

Horizontal diffusion in the surface layer is dependent on the standard deviation of wind direction fluctuations . Diurnal variation of this parameter in complex terrain was studied for the July 1979 Geysers, Cal., experiment using data from a network of 11 short meteorological towers in the 25 km² Anderson Creek watershed Valley side slopes are roughly 20 ° and maximum terrain difference is about 1 km.Values of for wind directions sampled for one hour at a height of 10 m are about 35 ° during the daytime. They slowly decrease to about 20 ° by 8 to 10 p.m. as stability increases but wind speeds are still relatively high. After 10 p.m. the drainage flow sets in at most stations, with speeds of 1 to 2 m s^-1, and average increases to about 30° during the period 11 p.m. to 6 a.m. In general, highest values of at night are associated with lowest values of wind speed and greatest static stability. This enhancement of by the terrain suggests that horizontal diffusion at night always conforms to that expected during nearly neutral stabilities. That is, Pasquill class D diffusion applies to the horizontal component all night in complex terrain.     

The orthogonal structure of Monsoon rainfall variation over Sri Lanka

Summary The spatial organization of Monsoon rainfall over Sri Lanka is examined using Orthogonal Factor Analysis (OFA) on long-term mean monthly rainfall data. Three types of orthogonal structure of Monsoon regime in Sri Lanka have been identified. Interpretation of orthogonal factor scores revealed that a large amount of rainfall occurs from March to October in the southwestern parts of Sri Lanka, from December to February in the eastern parts, and in November in the northern and mid-western parts which are all represented by high positive factor scores. Orthogonal factor scores for the first three factors account for 93.6% of the total variance of mean monthly rainfall and clearly indicate that the southeast and northwest parts of the country with lowest rainfall, resulting from lack of Monsoons, are represented by negative factor scores. The three orthogonal factors identified different rainfall maxima in different time periods and, additionally, significant spatial differences between regions. Seasonal changes in the Monsoon wind system, ITCZ weather phenomena, and topography were the main factors which influence the spatial structure of Monsoon rainfall over Sri Lanka.With 4 Figures     

Estimating missing daily temperature extremes in Jaffna,Sri Lanka

The accuracy of reconstructing missing daily temperature extremes in the Jaffna climatological station, situated in the northern part of the dry zone of Sri Lanka, is presented. The adopted method utilizes standard departures of daily maximum and minimum temperature values at four neighbouring stations, Mannar, Anuradhapura, Puttalam and Trincomalee to estimate the standard departures of daily maximum and minimum temperatures at the target station, Jaffna. The daily maximum and minimum temperatures from 1966 to 1980 (15 years) were used to test the validity of the method. The accuracy of the estimation is higher for daily maximum temperature compared to daily minimum temperature. About 95% of the estimated daily maximum temperatures are within ±1.5 °C of the observed values. For daily minimum temperature, the percentage is about 92. By calculating the standard deviation of the difference in estimated and observed values, we have shown that the error in estimating the daily maximum and minimum temperatures is ±0.7 and ±0.9 °C, respectively. To obtain the best accuracy when estimating the missing daily temperature extremes, it is important to include Mannar which is the nearest station to the target station, Jaffna. We conclude from the analysis that the method can be applied successfully to reconstruct the missing daily temperature extremes in Jaffna where no data is available due to frequent disruptions caused by civil unrests and hostilities in the region during the period, 1984 to 2000.     

Diurnal wind variation in the planetary boundary layer (PBL) over Sriharikota,India

Pibal ascents were taken every three hours at a coastal station, Sriharikota (India) on the east coast in four different campaigns each representing a season in India. A diurnal pattern of winds in the PBL winds was found in all seasons but the pattern varies from season to season. The details are described and discussed.     

Controls on precipitation distributions in Sri Lanka

Summary The rainfall patterns and regions of the monsoonal island of Sri Lanka have been investigated. On the basis of quality-controlled monthly rainfall data from 110 gauges for the period 1931–60, three major homogeneous rainfall regions were derived based on a variety of rainfall parameters using cluster and discriminant analysis. For each of these regions, identified as the South-west Monsoon (SWM), the Northeast Monsoon (NEM) and the Inter-monsoon II (IMII) regions, multiple regression methods were used to determine the geographical factors influencing the spatial patterns of rainfall. For the NEM and IMII regions, the level of explanation as indicated by the coefficients of multiple determination were reasonable with values generally above 0.60. For the SWM region, the level of explanation was much lower at about 0.20. Examination of the residuals of the regression equations demonstrated the significance of local factors in influencing the patterns of rainfall distribution in Sri Lanka.With 6 Figures     

Rainfall Variations of Sri Lanka, Part 2: regional gluctuations

Summary Based on the regional division of another paper [39], the rainfall variations of Sri Lanka have been investigated for the respective regions by power spectrum analysis and filtering methods, making use of data for the period from 1881 to 1980. The 3–4 year periodicity was observed over the entire island, but other cycles differ from region to region. The 13–16 months oscillation arises in Regions A, D and E, which roughly correspond to the Wet Zone and Dry Zone. The 10 and 2 year oscillations emerge in Regions A, B and C, where the southwest monsoon dominates the fluctuation patterns. In particular, it was confirmed that the quasi-biennial oscillation is not only in Sri Lanka, but also in other low latitude countries. The quasi-five year oscillation is noticed in Regions D and E, where the northeast monsoon influences on the fluctuation patterns. Irregularities in amplitude and in phase changes were noticed in their longer period fluctuation.

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Variationen des Niederschlags in Sri Lanka. Teil 2: Regionale Fluktuationen

Zusammenfassung Aufgrund einer früher vorgenommenen regionalen Einteilung [39] wurden die Niederschlagsvariationen in Sri Lanka für die einzelnen Regionen mit Spektralanalyse und Filterungsmethoden unter Verwendung von Beobachtungsdaten aus der Periode 1881–1980 untersucht. Die 3- bis 4jährige Periodizität wurde auf der ganzen Insel festgestellt; aber andere Zyklen unterscheiden sich von Region zu Region. Eine 13- bis 16monatige Oszillation zeigt sich in den Regionen A, D und E, die ungefähr der Feuchtzone und der Trockenzone entsprechen. 10- bis 2jährige Oszillationen treten in den Regionen A, B und C auf, wo der Südwestmonsun die Fluktuationsformen beherrscht. Im besonderen wurde festgestellt, daß die quasi-zweijährige Oszillation nicht nur in Sri Lanka, sondern auch in anderen Ländern niedriger Breiten vorkommt. Eine quasifünfjährige Oszillation wurde in den Regionen D und E festgestellt, wo der Nordostmonsun die Fluktuationsformen beeinflußt. Unregelmäßigkeiten in der Amplitude und in Phasenänderungen wurden in ihrer längerperiodischen Fluktuation festgestellt.

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With 4 Figures     

Diurnal Variation of Southwest Monsoon Rainfall at Indian Stations

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An objective classification of homogeneous rainfall regimes in Sri Lanka

Summary Homogeneous rainfall regimes in Sri Lanka have been devised through multivariate statistical techniques. The nature of previous subjective classifications are reviewed and the need for an objective classification is stressed. Each of the major classifications of rainfall regimes, micro, meso and macro, throw new light on our understanding of the climates of Sri Lanka. New boundaries between dry/wet and dry/arid zones have emerged and are compared with previous examples. Three rainfall dominance regimes are also identified. Finally a brief account of the micro scale rainfall regimes is given.With 5 Figures     

Diurnal and seasonal variations of radioactivity and electrical conductivity near the surface for a continental location Mysore, India

Simultaneous measurements of the activity concentrations of radon and its progeny, and conductivity of both polarities i.e., positive and negative conductivities, were made at a height of 1 m above the ground at Mysore (12°N, 76°E, 767 m above mean sea level), India. Diurnal and seasonal variations of activities of radon and its progeny show their peak values in the early morning hours throughout the year. Observations show an increase in the concentration of radon and its progeny during nighttime compared to the daytime values, and are higher in winter than in other seasons. The electrical conductivity of the atmosphere that depends on the ionization rate also exhibits similar trends. These results are discussed in terms of ionization rate due to radioactivity and the influence of meteorological parameters on radioactivity. The results also show that the concentrations of radon, its progeny, and the electrical conductivity of both polarities exhibit a positive correlation with the relative humidity and are negatively correlated with the temperature of the atmosphere.     

青藏高原不同季节地表温度变化特征分析

基于欧洲中尺度气象预报中心(ECMWF)提供的ERA-Interim地表温度,利用经验正交函数(EOF)等方法,分析了青藏高原四季地表温度的时空变化特征.结果发现:青藏高原春、夏、冬季地表温度变化以整体型为主,并且大部地区地表温度呈现升高的趋势;秋季地表温度略有下降趋势,并且以东部和西部地表温度的反向型异常变化最为显著.此外还发现,青藏高原不同季节地表温度的异常变化具有一定的联系,其中整体型变化可以持续3个季节.     

斯里兰卡穹顶区的形成和演变机制分析

通过SODA再分析资料和AVISO观测资料研究了斯里兰卡穹顶区（SLD）的迁移和消散机制.斯里兰卡穹顶区是孟加拉湾西南部的一个气旋涡旋,主要出现在西南季风（5-9月）期间,与西南季风海流侵入孟加拉湾同时存在.正风应力旋度引起的Ekman抽吸是形成SLD的主要原因.回归分析结果表明SLD区域的风应力旋度与Ekman抽吸存在较强的正相关（r²=0.93,p>0.5）.此外,结果表明SLD在发展过程中的移动主要受正风应力旋度移动的影响,SLD的消减与该正风应力旋度减弱和西传的暖Rossby波有关,而冷Rossby波的传播有益于SLD的发展.在SLD消减时期,孟加拉湾涡旋（BBD）独立发展并进一步与SLD融合,回归分析发现BBD区域的Ekman抽吸与当地风应力旋度的关系密切（r²=0.76,p>0.5）,这表明了BBD在形成阶段由局地的风应力主导.9月之后,风应力旋度减弱,BBD和SLD开始了合并过程.动力方面,EKE分析显示SLD衰退的同时,BBD的EKE大幅增加;热力方面,10-11月时,由Ekman抽吸引起的SLD和BBD次表层冷水汇合,清晰地表明了二者之间的热动力学联系.     

天津市夏季降水日变化特征

利用1954－2007年天津市夏季逐时自记降水资料,分析了天津市夏季降水（包括逐小时降水量、降水频次、降水强度以及不同持续时间降水）日变化规律。结果表明：天津市一日内不同时次的多年累积降水量具有显著的日变化特征,呈明显的双峰型,高值分别出现在午后17时和午夜02时。逐小时降水强度与降水量的变化特征非常一致,而多年累积降水频次在凌晨02时至08时较高,之后至11时逐步降低,11时至24时变化不大。降水量与降水频次及降水强度的关系均达到显著性水平（P < 0.001）,但逐小时降水强度与降水量相关性明显高于降水频次,表明降水量变化与降水强度有直接的关系,而降水频次对累积降水量的贡献占较小的权重。持续不同时间降水事件的发生次数在一日内的变化特征明显不同,长时性降水峰值集中在清晨,而短时性降水尤其是1-3 h降水主要以午后为主。     

The diurnal variation of wind direction at Calgary

The winds at Calgary airport show a diurnal variation. Night winds tend to be northwest, with afternoon winds from the Southeast. Other data show that this variation is not associated with local topography, but seems to be an effect of the Rocky Mountains to the west.     

Annual variation of surface roughness obtained from wind profile measurements

Summary Profile measurements of wind and temperature have been performed at the Agricultural University of Norway on a routine basis since 1997. 10-min. averages are stored in a database together with other relevant meteorological parameters. The database can be used to determine the seasonal variation of surface aerodynamic roughness, showing the growth of grass between cutting during the growing season, the effect of snowfall and the melting of snow etc. However, careful screening of the data must be conducted before reliable estimates can be made. The main objective of this study is to establish simple practical rules for filtering out unreliable datasets for the evaluation of the surface roughness parameter z₀, and to present its annual variation. The resulting values for the summer period agree well with values found in the literature for homogenous grass covered surfaces. In the transition periods during autumn and spring, and during wintertime in mild weather conditions, the surface is generally non-homogenous with a mixture of snow patches, ponds of melting water and shrubs of withered grass. The results show that the mechanical interaction between a non-homogeneous land surface and the boundary layer flow can be described by one roughness parameter, with a numeric value somewhere in between the ideal values for the different surface characteristics. Another use of the database is to investigate drainage flow and the relationship between drainage flow, prevailing wind direction and the mean vertical velocity of the air. Most micrometeorological studies of the fluxes of heat and water vapour in the surface layer, assume the mean vertical velocity to be zero, focusing on eddy fluxes and thereby excluding any transport in the mean flow. In certain situations, this may lead to serious errors. This work shows that convergence of horizontal flow leads to an upward movement of air, which is enhanced if the prevailing direction of the wind opposes the outflow of the cold drainage winds from the area.