Temperature lapse rate in complex mountain terrain on the southern slope of the central Himalayas

This study presents the first results of monthly, seasonal and annual characteristics of temperature lapse rate on the southern slope of the central Himalayas, based on 20 years record of surface air temperature at 56 stations in Nepal. These stations are located at a range of elevations between 72 and 3,920 m above sea level. It is proven that the lapse rate can be calculated with a linear regression model. The annual cycle of temperature lapse rate exhibits a bi-modal pattern: two maxima in the pre- and post-monsoon seasons respectively separated by two minima in winter and summer, respectively. This pattern is different from the findings from the other mountain regions and suggests different controlling factors in the individual seasons. The highest temperature lapse rate occurs in the pre-monsoon and is associated with strong dry convection (i.e., corresponding to the clear weather season and considerable sensible heat flux). The post-monsoon has the second highest lapse rate, and its cause is similar to the pre-monsoon season but with a relatively small thermal forcing effect after the rainy summer. The lowest lapse rate occurs in winter and is associated with strong radiative cooling and cold air flows over low-elevation areas. The summer lapse rate minimum is due to latent heating over the higher elevations and reduced solar heating over the lower elevations.     

The tropospheric temperature lapse rate

Summary Neumann [1] has shown that the mean lapse rate through the whole troposphere decreases from the Equator to the North Pole. The present note shows also that the mean tropospheric lapse rate above the surface layers has little latitudinal variation either in January or July and has a mean value of 6.4°C/km.

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Zusammenfassung Neumann [1] hat gezeigt, daß der mittlere vertikale Gradient der Temperatur durch die ganze Troposphäre vom Äquator bis zum Nordpol abnimmt. In unserer Untersuchung wird gezeigt, daß in der Troposphäre der mittlere vertikale Temperaturgradient oberhalb der bodennahen Schichten weder im Januar noch im Juli große Veränderungen in den verschiedenen Breitengraden aufweist und einen mittleren Wert von 6.4°C/km hat.

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Résumé Neumann [1] a montré que les valeurs du gradient vertical moyen de température diminuent dans tous les niveaux de la troposphère de l'équateur au pôle Nord. Dans la présente étude on montre que, dans la troposphère, au-dessus des couches atmosphériques proches de la surface du sol, le gradient vertical moyen de température ne présente pas de différences importantes entre le diverses latitudess, ni en janvier, ni en juillet, et que sa valeur moyenne est de 6.4°C/km.

     

Near-surface air temperature lapse rate in a humid mountainous terrain on the southern slopes of the eastern Himalayas

Theoretical and Applied Climatology - Based on climatic data from 18 stations on the southern slopes of the eastern Himalayas in Bhutan for the period from 1996 to 2009, this paper investigates...     

Latitudinal variation of tropospheric temperature lapse rate

Summary The latitudinal variation of tropospheric temperature lapse rate is examined on the basis of recently published data for the northern hemisphere. In January, the lapse rate systematically decreases from 6.4°C/km. for the equator to 3.6°C/km. for 70°N; in July, the lapse rate is nearly constant in respect of latitude between the equator and 50°N, but decreases from the latter poleward. The average January–July lapse rate decreases systematically from 6.3°C/km. for the equator to 4.7°C/km. for 70°N. This variation is closely described by the equation =6.25–2 sin⁴, where is the lapse rate, in °C/km, for latitude . For the north pole, the equation gives a lapse rate of 4.25°C/km.

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Zusammenfassung Die Änderung des vertikalen Gradienten der Troposphärentemperatur mit der geographischen Breite wird auf Grund neuen Zahlenmaterials für die Nordhemisphäre untersucht. Im Januar nimmt der Temperaturgradient allmählich von 6.4°C/km am Äquator bis zu 3.6°C/km in 70°N Breite ab; im Juli ist der Temperaturgradient in den Breiten zwischen dem Äquator und 50°N Breite fast gleich und nimmt von hier aus polwärts ab. Der mittlere Temperaturgradient für Januar–Juli nimmt allmählich von 6.3°C/km am Äquator bis auf 4.7°C/km in 70°N Breite ab. Die Breitenabhängigkeit läßt sich gut durch die Gleichung =6.25–2 sin⁴ darstellen, in der der Temperaturgradient in °C/km in der Breite ist. Für den Nordpol ergibt die Gleichung einen Temperaturgradienten von 4.25°C/km.

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Résumé La variation latitudinale du gradient thermique vertical de la température troposphérique est examinée en se basant sur des données publiées récemment d'une valeur de 6.4°C/km pour l'équateur à 3.6°C/km pour 70°N; en juillet le gradient est presque constant pour les latitudes situées entre l'équateur et 50°N, mais il diminue d'ici vers le pôle. Le gradient moyen janvier/juillet diminue systématiquement de 6.3°C/km pour l'équateur à 4.7°C/km pour 70°N. Cette variation peut être représentée par l'équation =6.25–2 sin⁴ où est le gradient en °C/km pour la latitude . Pour le pôle nord, l'équation donne un gradient de 4.25°C/km.

     

Note on rapid pulsations of temperature lapse rate

Pulsations of the lapse rate between 1 and 5 m were recorded on a sunny day over a field of tall weeds (z ₀ 6 cm) and over a parking lot (z ₀ 0.1 cm). The frequency distributions of values recorded every 2 sec in a 20 min span were nearly identical in the two environments. Values ranged from inversions of 0.75 °C to lapses of 1.75 °C. There was a tendency for a wider range of fluctuations in short time intervals in the weeds than over the smoother parking lot. The data suggest that these very short-periodic fluctuations were dynamically induced.     

Prediction of surface potential temperature over complex terrain

Based on a statistical approach, the surface potential temperature at seven observing stations in complex terrain has been examined. It is shown that the surface potential temperature depends primarily on the rate of change of slope wind and on the geostrophic-level potential temperature.     

A new concept for high resolution temperature analysis over complex terrain

Summary The analysis of high resolution temperature data over complex topography is often problematic due to the specific influence of orography and thus, requires a special methodology. The new concept of Low Level Temperature (LLT) is defined, and can be obtained when potential temperature observations are reduced to the height of the so-called Minimum Topography, a special low level topography that accentuates basins and valleys but smoothes out single summits and scarped slopes. The Vienna Enhanced Resolution Analysis (VERA) is used to produce a comprehensive set of LLT analyses over the Alpine region by evaluating three-hourly synoptic ECMWF data for the period 1980–2001. LLT fields are then evaluated climatologically in order to gain two-dimensional representations and single grid point time series. Mean LLT fields for different months and times of the day provide highly resolved spatial and temporal information on temperature fields over basins and valleys i.e., the main settlement areas in mountainous terrain.     

Modeling heavy precipitation in complex terrain

Summary A numerical prediction model is described which uses the full set of prognostic equations on a domain roughly the size of the United States with a 96 km horizontal grid resolution and six sigma-coordinate levels. Within this grid resides a nested domain of approximately 1000×1000 km with 24 km horizontal resolution. In this nested grid only modifications to the wind field by the better resolved terrain are considered on the lowest two sigma levels. The terrain effects necessitate adjustments in the location of these two sigma levels. Adjusted wind fields cause modifications in the mass and moisture divergence fields, hence in precipitation. These modifications are averaged into the appropriate meteorological fields on the larger grid.The algorithms used by our model allow continuous interaction between both grids with high computational efficiency.The relative importance of synoptic forcing and terrain is demonstrated for the cases of the Big Thompson, Colorado, flood of 1976 and the Cheyenne, Wyoming, flood of 1985.With 15 Figures     

冬季高海拔复杂地形下GRAPES-Meso要素预报的检验评估

应用覆盖中国东南部的3 km分辨率GRAPES-Meso4.0模式（GRAPES-Meso4.0_3 km）2015年夏季实时预报试验结果、同区域1600多个中国国家地面气象观测站每日08：00-08：00 BT的24 h累积降水量和逐时降水量观测资料,从降水累积量、降水频率、降水强度、日循环特征等多个角度,对千米尺度分辨率下GRAPES-Meso4.0模式的降水预报性能进行细致评估,并与同版本10 km分辨率业务模式（GRAPES-Meso4.0_10 km）同期结果在相同区域进行类同对比分析和讨论。结果表明：（1）GRAPES-Meso4.0_3 km很好地捕捉到了2015年夏季观测的日均降水量和降水频率的大小及地域分布特征,其一般性降水（中雨及以下）频率平均低于实况约3个百分点,强降水（大雨及以上量级）频率与实况近乎吻合,纠正了GRAPES-Meso4.0_10 km在这两方面存在的显著预报正偏差,均方根误差（RMSE）减小了40%-50%;（2）GRAPES-Meso4.0_3 km在降水强度预报上的优势主要表现为：对降水强度的地域分布细致特征和对短时强降水（雨强≥ 10 mm/h）的频数和分布等把握比较准确,但对强降水（一般性降水）的强度预报偏强（偏弱）;（3）GRAPES-Meso4.0_3 km小时降水量和降水频率的日循环预报可反映出研究区域观测的双峰总体特征以及雨量和频率在日循环中的紧密联系,明显优于GRAPES-Meso4.0_10 km的表现,尽管下午（16时,北京时）峰的预报还存在偏弱现象;（4）模式分辨率提高到千米尺度和模式显式描述云和降水过程,是GRAPES-Meso4.0_3 km降水预报性能较GRAPES-Meso4.0_10 km提高的关键原因,模式初值差异也是不可忽视的影响因素。     

基于机器学习的复杂地形下短期数值天气预报误差分析与订正

初步研发了一套基于机器学习方法XGBoost且考虑地形特征影响的数值预报多模式集成技术,并与传统的等权重平均和线性回归方法的集成效果进行了对比分析。利用北京地区快速更新循环数值预报系统每天8次循环预报给出的近地面2m温度、2 m相对湿度、10 m风速、10 m风向数据产品,分别基于机器学习方法XGBoost、等权重平均方法、线性回归方法构建了3种体现地形因子影响的多模式预报时间滞后集成模型。试验对比分析了暖季、冷季每日不同时刻的模式预报集成订正效果。结果表明:分季节试验中,基于XGBoost模型对2m温度、10m风速的集成预报结果相对原始最优预报结果误差明显优于其他两种传统方法。XGBoost对2 m温度集成的误差可降低11.02%—18.09%,10 m风速集成误差可降低31.23%33.22%,10 m风向集成误差可降低4.1%—8.23%。2 m相对湿度的集成预报误差与传统方法接近。基于XGBoost的多模式集成预报模型可以充分"挖掘"不同模式或不同时刻快速更新循环预报优点,有效降低模式的系统性误差,提供准确性更高的多模式集成确定性预报产品。     

Near-surface air temperature lapse rates in Xinjiang,northwestern China

Theoretical and Applied Climatology - Lapse rates of near-surface (2 m) air temperature are important parameters in hydrologic and climate simulations, especially for the mountainous areas...     

一种2 m温度误差订正方法在复杂地形区数值预报中的应用

利用模式三维预报变量,结合地面要素预报产品,采用2 m温度三维插值方法进行地形订正,以确保预报与观测三维空间上的一致性,在地形订正基础上,利用历史月均预报误差作为参考误差,剔除模式系统性误差,获取具备日变化特征的预报产品。基于陕西地区复杂地形条件下的典型观测站点,利用2016年8月28日48 h预报个例进行对比分析发现,三维插值方法有效改善了地形差异引起的评估误导问题,但无法改进模式预报的日变化趋势,进一步采用系统性误差订正后,日变化特征明显改善,特别是前24 h预报效果体现出与实况良好的一致性及更佳的预报技巧。通过2016年夏季统计评估表明,误差订正后的2 m温度预报产品有效改善了周期性误差振荡,均方根误差稳定在2 K左右,显示出明显的改进优势。     

广东高温影响因子和预报方法

采用二维风场差异的显著性检验方法等,研究广东盛夏7-8月高温的同期及前期风场差异特征,探讨广东盛夏高温的风场影响因子和前期的强预测信号.结果表明:南海中北部风场的异常是影响广东盛夏高温日数的重要因子;前期冬季200 hPa我国东北到印度北部、华南到江南、赤道低纬度80～120 °E地区,以及上年12月500 hPa孟加拉湾到南海中北部、我国中部偏西地区的风场异常可能是广东盛夏高温的强预测信号.分析还表明,前期1月和3月的亚洲500 hPa高度场及前期1月的太阳黑子也与盛夏高温日数关系密切.最后,建立了广东盛夏高温的预报工具.     

A method to calculate cloud-free long-wave irradiance at the surface based on radiative transfer modeling and temperature lapse rate estimates

We demonstrate a method to improve the performance of commonly used parameterizations to calculate the cloud-free down-welling long-wave radiation at the surface. The method uses a monthly climatology of the effective radiating temperature of the atmosphere instead of the instantaneous screen-level temperature. The climatology of the effective radiating temperature can be derived from pyrgeometer measurements and was incorporated into two commonly used schemes. We compared the calculated cloud-free down-welling long-wave irradiances to high-quality pyrgeometer measurements from four Swiss sites. The discrepancies between observations and modified schemes can be reduced by up to 35 %, resulting in a model uncertainty close to 5 W m^?2 which corresponds to the measurement uncertainty of pyrgeometers. Furthermore, we introduce a new long-wave model which is based on radiative transfer calculations in the 8–14- $\upmu $ m wavelength range. In the remaining long-wave spectrum, the radiation is calculated using the Planck function with the effective radiating temperature of the atmosphere. The performance of this new model is consistent with the modified parameterizations.     

Surface-layer flow in complex terrain: Comparison of models and full-scale observations

Four models of surface boundary-layer flow in complex terrain are compared with observations made at Blashaval Hill, North Uist, Scotland. The field experiment is described by Mason and King (1985). Three of the models are derived from the two-dimensional theory of Jackson and Hunt (1975) and are described in Mason and King (1985), Walmsley et al. (1986) and Troen and Petersen (1989). The fourth is a mass-consistent code based on Traci et al. (1979). The model results are in good agreement with each other and are generally within the observed range of variation ( ~ ± 16%) in normalized wind speed. For most wind direcions (7 of 11), model results of normalized wind speed at the summit were within 7% of the observed mean values. For some wind directions, calculations using the Guidelines of Walmsley et al. (1989) suggested that variations in surface roughness were important. This led us to apply one of our models incorporating nonuniform surface roughness. The lack of significant improvement for cases when water lay upstream of Blashaval Hill is attributed to compensating changes at summit and reference sites and to very local effects on the wind data. Sensitivity to topography lying to the west and northwest of Blashaval was also investigated. Results suggested an influence from those distant topographic features for some wind directions. When those features were incorporated, maximum errors in normalized wind speed at the summit were reduced from 18 to 13%.     

Transport and diffusion in complex terrain (review)

This paper reviews activities over the past nine years involving the evaluation, verification and development of atmospheric transport and diffusion models applied to air pollution assessments in complex/ mountainous terrain settings. Results from experiments performed at different complex terrain settings under stably stratified flow conditions have been emphasized. Comparisons of model predictions to observations are made. Physical modeling laboratory tests simulating flow conditions for full-scale experiments complement the data bases and provide information on systematic variations of modeling parameters and input conditions for mathematical model development purposes. Principal findings from these and other experiments on specific technical issues are summarized.     

GRAPES千米尺度模式在西南复杂地形区降水预报偏差与成因初探

利用2019年夏季（6—8月）西南复杂地形区地面观测站逐时和逐日降水量观测数据,从降水量和降水频率入手,对同期GRAPES-Meso 3 km业务模式短期（36 h以内）降水预报性能,特别是在不同典型地貌区—四川盆地子区、云贵高原北部子区和南部子区、青藏高原东缘山地子区的预报偏差进行细致评估与分析。结果表明:（1）GRAPES-Meso 3 km模式能合理地刻画出西南复杂地形区夏季日降水和日内尺度降水的主要特征,以及小时降水频次-强度的基本关系。（2）在各子区,模式日降水量（频率）预报表现为清晰的正偏差,正偏差在盆地子区最显著,为观测值的1.1倍（0.3倍）;日降水量正偏差主要由强降水日降水量预报偏大引起,但频率正偏差在云贵高原南、北子区与其他两个子区不同,主要是中小雨日数预报偏多的贡献;强降水（中小雨）落区预报存在明显（轻微）偏大倾向,强降水预报落区偏大频率在青藏高原东缘山地子区最高,达82.8%,在云贵高原南部子区最低,为53.6%。（3）日循环上,各时次小时降水量（频率）预报整体偏大,且主要正偏差出现在观测的夜雨峰值时段,其中海拔1200 m以下区域的降水频率正偏差从夜间峰值区延续到中午,模式偏强的日降水量预报往往表现为日内偏长的降水时长或小时降水空报。（4）诊断分析显示,模式在四川盆地区突出的夏季日降水预报正偏差是模式对流层低层在云贵高原南-东南侧偏强的西南风预报与西南地区特殊地形结合的产物。      

A wind tunnel boundary-layer simulation of wind flow over complex terrain: Effect of terrain and model construction

The report presents the results of a wind-tunnel study of the flow of the natural wind over complex terrain. A 1:4000 undistorted scale model of Gebbies Pass in the South Island of New Zealand was prepared and tested in the boundary-layer wind tunnel in the Department of Mechanical Engineering at the University of Canterbury.Three forms of construction, viz., terraced, contoured and roughness-added, were compared. Velocity and turbulence profiles, Reynolds stresses and spectra were measured, and correlation of results between different types of construction was calculated. The terraced form was much simpler to construct but was found to be unsatisfactory. The correlation between the contoured and roughness-added models was as high as 0.94, although the roughness-added model made a significant difference to the results in the lower 20%; of the boundary layer. The results of these tests will be compared with results from the field in a future report.