Modification Of The Standard ∈-Equation For The Stable Abl Through Enforced Consistency With Monin–Obukhov Similarity Theory

A condition is derived for consistency of the standardmg src="/content/m33l3871513v0713/xxlarge8712.gif" alt="isin" align="MIDDLE" BORDER="0">-equation with Monin–Obukhov (MO) similarity theory of thestably-stratified surface layer. The condition is derivedby extending the procedure used to derive the analogous condition forneutral theory to stable stratification. It is shown that consistencywith MO theory requires a function of flux Richardson number, Ri_f, to be absorbed into either of two closure parameters, c_{mg src="/content/m33l3871513v0713/xxlarge8712.gif" alt="isin" align="MIDDLE" BORDER="0"> 1} or c_{mg src="/content/m33l3871513v0713/xxlarge8712.gif" alt="isin" align="MIDDLE" BORDER="0"> 2}.Inconsistency, on the other hand, results if constant values of these are maintained for all Ri_f, as is done in standardapplication of the equation, and the large overpredictions ofturbulence found in such application to the one-dimensionalstable atmospheric boundary layer (1D-SBL) are traced to thisinconsistency. Guided by this, we formulate a MO-consistentmg src="/content/m33l3871513v0713/xxlarge8712.gif" alt="isin" align="MIDDLE" BORDER="0">-equation by absorbing the aforementioned function intoc_{mg src="/content/m33l3871513v0713/xxlarge8712.gif" alt="isin" align="MIDDLE" BORDER="0"> 1}, and combine this with a Level-2.5 second-orderclosure model for vertical eddy viscosity and diffusivities.Numerical predictions of the 1D-SBL by the modified model converge to a quasi-steady state, rectifying the predictive failure of the standardmg src="/content/m33l3871513v0713/xxlarge8712.gif" alt="isin" align="MIDDLE" BORDER="0"> -equation for the case.Quasi-steady predictions of non-dimensional variables agree stronglywith Nieuwstadt's theory. Qualitative accuracy of predictionsis inferred from comparisons to field data, large-eddy simulationresults and Rossby-number similarity relationships.     

冬季黔北境内云贵准静止锋影响下最高气温预报订正方法研究

本文为探索静止锋位于遵义上空不同位置时,各城镇精细化预报考核站点当日高温的规律,通过普查2009至2020年12月到次年2月间14时、17时地面天气图,并结合探空资料、卫星云图等资料,筛选出云贵准静止锋（以下简称为静止锋）位于遵义境内时的所有个例,分型统计不同类型个例对应的白天最高温、EC2m温度预报数据,EC细网格850hPa温度,计算不同温度预报方法下的准确率,并分析是否可继续订正。结果表明：（1）离锋面越远,温度的预报准确率越高,基于客观预报方法的可订正性越强,实际业务中可以参考本文分析得到的各站点数据。（2）当静止锋呈Ⅰ1型时,遵义市西部站点用EC预报当日20时的850hPa温度进行预报（以下简称850hPa温度预报方法）准确率较高,北部、东部站点可用EC2m最高温度进行订正（以下简称2m温度订正方法）。（3）当静止锋呈Ⅰ2型时,可以同时参考850hPa温度预报方法和2m温度订正方法,订正后准确率高。（4）当静止锋为Ⅱ2型时,对于离锋面较远的东部、南部可着重使用2m温度订正方法。     

中国3个典型城市气溶胶光学厚度地基观测及其MODIS气溶胶产品精度分析

利用中国太阳分光观测网的观测资料结合MODIS(中分辨率成像光谱仪)的气溶胶产品分析了北京、兰州、上海3个典型区域城市的气溶胶光学特性。结果表明:北京AOD(气溶胶光学厚度)年平均为0.41±0.35,春夏高,秋冬低,Angstrm波长指数α年平均为1.40±0.85表现为细模态粒子,MODIS的光学厚度为0.52±0.39与地面观测相关系数为0.91,存在系统性高估;兰州AOD年平均为0.55±0.21,夏季最低,秋冬较高,α年平均为0.95±0.20表现为粗模态粒子,MODIS光学厚度为0.43±0.21与地面观测相关系数仅为0.07,存在系统性低估;上海AOD年平均为0.55±0.21,无明显季节变化,α平均为1.03±0.25,MODIS光学厚度为0.74±0.30与地面观测相关系数为0.75,存在系统性高估。城市地理位置和复杂地表等原因造成反照率的不确定,MODIS气溶胶产品在这3个城市的反演效果仍有很大提升空间。     

四川地区多模式2m温度预报性能分析及集成方法研究

本文通过分析2017年9~12月四川地区ECMWF (European Centre for Medium-Range Weather Forecasting)细网格模式、GRAPES_GFS (Global and Regional Assimilation and Prediction System)全球模式和西南区域模式(South West Center-WRFADAS Real-time Modeling System,SWCWARMS)2m温度168h预报时效内的系统性偏差特征,采用滑动双权重平均法分别对三种模式温度预报产品进行偏差订正,并集成得到各时效2m温度的订正场,结果表明:(1)三种模式的预报存在明显的日变化,整体上EC模式的预报最优。(2)三种模式对于低温和高温的预报,在全省均大致呈现负的系统性误差,特别在高原及过渡区表现的尤为明显。(3)订正后三种模式的预报准确率显著提高,均方根误差减小1.4~2.5℃,大部分地区平均误差维持在± 0.5℃之间,在系统性偏差较大的地区,订正效果更好。(4)两种集成方案预报结果接近,且均优于三种模式的订正预报。     

WRF中土壤图及参数表的更新对华北夏季预报的影响研究

土壤质地及其物理性质的参数化对陆面过程模拟具有明显的影响。研究了土壤质地和土壤水文参数表的更新对WRF（Weather Research and Forecasting）模拟性能的影响。使用北京师范大学土壤属性数据集和修正后的土壤水文参数表替换WRF默认数据,对2017年6—8月华北地区开展数值模拟试验和评估验证。结果表明,模拟结果对土壤类型数据集和水文参数表的更新较为敏感,对地面要素预报有正效果。WRF默认土壤数据集中,中国东部以粘壤土为主,而在北京师范大学土壤数据集里则以壤土为主;修正后的土壤水文参数在Noah陆面过程中增强了裸土潜热蒸发能力。数值模拟试验表明,土壤输入数据和土壤水文参数的更新能够增强陆面向大气的潜热同时减弱感热输送,致使大气底层温度降低而湿度增大。利用华北区域748个地面气象观测站的2 m温度和2 m湿度对2017年夏季的模拟结果进行验证,结果显示更新试验对地面要素的预报偏差有较好的修正作用,能够将2 m温、湿度的预报技巧分别提高3.4%和2.9%。      

GRAPES_RAFS系统2 m温度偏差订正方法研究

本文通过对2013年6月20日至7月20日GRAPES(Global and Regional Assimilation and Prediction System)_RAFS(Rapid Analysis and Forecast System)系统每天8个时次每3h的2 m温度预报进行分析,发现各时次的预报均能较好地表征2 m温度日变化特征,但预报与实况存在一定的偏差,其中西藏东部川西高原、云贵高原、江南武夷山脉偏低于实况可达3℃,而华北地区偏高于实况3℃以上.为了减小GRAPES_RAFS系统偏差对2m温度预报的影响,本文采用平均法、双权重平均法、滑动平均法和滑动双权重平均法分别对GRAPES_RAFS系统2 m温度预报产品进行偏差订正,并对订正前后的结果进行检验分析和对比.结果表明:2 m温度订正后的平均误差大部地区减小到(一1～1℃),而均方根误差大部地区降低到2.5℃内.对于偏差较大地区,订正效果更为明显,如西藏东部川西高原,经过订正,平均误差绝对值由订正前3℃以上降低到1℃内,而RMSE由订正前4℃以上控制到3℃内.对比四种订正方法,双权重订正方法与平均法订正整体效果接近,但对个别站点,双权重订正法要优于平均法,经过滑动的订正方法比无滑动的订正方法订正效果更好,订正效果最好的是滑动双权重平均法,全国平均误差大部分在(-0.5～0.5℃)内,不超过(-1～1℃)的范围.     

沈阳大气气溶胶光学特性及其影响因子

利用2010年3—10月沈阳大气成分监测站CE－318太阳光度计观测资料,计算沈阳大气气溶胶光学厚度和波长指数等大气光学特性参数,结合地面气象观测资料,分析大气气溶胶光学特性及其影响因子。结果表明：沈阳气溶胶光学厚度在3—6月较高,8月较低,9—10月气溶胶光学厚度略有增加;除4月和8月外,气溶胶光学厚度与风速基本呈反相关;气溶胶光学厚度与可吸入颗粒物（particulate matter,PM）质量浓度变化趋势基本一致;气溶胶光学厚度日平均值距平的绝对值、改变率均与降水强度成正比;地面能见度与气溶胶光学厚度呈负相关。由气溶胶浑浊度系数计算的能见度在4—6月与实际观测的能见度基本吻合,由气溶胶标高计算的水平能见度整体小于实际观测的水平能见度。     

The problem of underpowered rivers

This study has hypothesized that for many rivers the trade-off between flow accumulation and the decrease in slope along channel length means that stream power increases downstream and, moreover, that given the low slope angles in headwater and low-order streams, they would have insufficient stream power to erode let alone transport sediment. The study considered the stream power profile, the particle travel distances and the application of the Hjulström curve based on the velocity profile of nine, large UK catchments. The study showed that:

1. Some rivers never showed a maximum in their longitudinal stream power profile, implying that some rivers never develop a deposition zone before they discharge at the tidal limit.
2. Particle travel distances during a bankfull discharge event showed that for some rivers 91% of the upper main channel would not be cleared of sediment. Furthermore, while some rivers could transport a 2 mm particle their entire length in one bankfull event, for another river it would take 89 such events.
3. The Hjulström curve shows that for three of the study rivers the upper 20 km of the river was not capable of eroding a 2 μm particle.
4. The study has shown that for all rivers studied, erosion is focused downstream and deposition upstream. Many UK rivers have a dead zone where, on time scales of the order of centuries, no erosion or transport occurs and erosion only occurs in the lower courses of the channel where discharge rather than slope dominates – we propose these as underpowered rivers.

© 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Tephra isochrons and chronologies of colonisation

This paper demonstrates the use of tephrochronology in dating the earliest archaeological evidence for the settlement of Iceland. This island was one of the last places on Earth settled by people and there are conflicting ideas about the pace and scale of initial colonisation. Three tephra layers, the Landnám (‘land-taking’) tephra layer (A.D. 877 ± 1), the Eldgjá tephra (A.D. 939) and the recently dated V-Sv tephra (A.D. 938 ± 6) can be found at 58% of 253 securely-dated early settlement sites across the country. The presence of the tephras permits both a countrywide comparison, and a classification of these settlement sites into pre-Landnám, Landnám and post-Landnám. The data summarised here for the first time indicate that it will be possible to reconstruct the tempo and development of the colonisation process in decadal resolution by more systematically utilising the dating potential of tephrochronology.     

A method of searching for radial velocity anomalies in the lower mantle using interfering signals

Radial velocity anomalies in the lower mantle that give rise to triplications in the travel-time curve for short-periodP waves will produce arrivals havingdT/dmg src="/content/g03u40u21502m278/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"> values that differe by roughly 0.2–0.5 s/deg. The first two arrivals associated with such triplications will be separated by less than one second over a distance range of 4°–10° they may not, therefore, be separable visually on single seismograms, so that their presence can only be inferred from some measurable property that depends on their mutual interference. If there are lateral variations in the regions of anomalous velocity gradients, the interfering signals will also have different azimuths of arrival. Using two synthetic wavelets we have investigated the effect of interference on bothdT/dmg src="/content/g03u40u21502m278/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"> and azimuth measurements at the Yellowknife Array. We found that if the interfering pulses have a dominant frequencyv, there is a range of time separations (0.30/vmg src="/content/g03u40u21502m278/xxlarge8594.gif" alt="rarr" align="BASELINE" BORDER="0">0.55/v) over which the measureddT/dmg src="/content/g03u40u21502m278/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"> and azimuth values may fluctuate by much more than the differences indT/dmg src="/content/g03u40u21502m278/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0"> and azimuth between the interfering signals. We have evaluated the following empirically defined functions for three different primary signals, and for three different relative amplitudes of the interfering signals:f _{mg src="/content/g03u40u21502m278/xxlarge964.gif" alt="tau" align="BASELINE" BORDER="0">}(t), the drift function, which expresses how the measured slownesses,p, and azimuths, mg src="/content/g03u40u21502m278/xxlarge981.gif" alt="phiv" align="MIDDLE" BORDER="0">, differ from the slownesses and azimuths of the primary wavelets; mg src="/content/g03u40u21502m278/xxlarge916.gif" alt="Delta" align="BASELINE" BORDER="0">f(mg src="/content/g03u40u21502m278/xxlarge964.gif" alt="tau" align="BASELINE" BORDER="0">), the range function, which describes the behaviour of the upper and lower bounds ofp and mg src="/content/g03u40u21502m278/xxlarge981.gif" alt="phiv" align="MIDDLE" BORDER="0"> as a function of the difference in arrival times of the signals, mg src="/content/g03u40u21502m278/xxlarge964.gif" alt="tau" align="BASELINE" BORDER="0"> andf _{mg src="/content/g03u40u21502m278/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">} ^{mg src="/content/g03u40u21502m278/xxlarge964.gif" alt="tau" align="BASELINE" BORDER="0">} , studied the properties of these functions, and have outlined how these properties provide criteria based on the numerical and statistical characteristics of the arrival vectors, and on the waveform of the signal that will enable small radial velocity anomalies to be more clearly delineated.Contribution No. 863 from the Earth Physics Branch.