The spectral composition of fluxes and variances over land and sea out to the mesoscale

We discuss the accuracy requirements for measuring mesoscale (roughly horizontal scales > 10 km or 5 to 10 times the planetary boundary-layer (PBL) depth) fluxes in the convective PBL, and the ability of current research aircraft to achieve this accuracy. We conclude that aircraft equipped with inertial nagivation systems capable of < 3 km hr⁻¹ navigational accuracy are able to resolve mesoscale fluctuations in velocity, and thus variances and fluxes on the mesoscale. We then discuss measurements of velocity and scalar spectra, and cospectra of vertical velocity with horizontal velocity components and scalars, obtained from long flight legs with the National Center for Atmospheric Research Electra aircraft over the boreal forest of Canada in summer during the BOreal Ecosystem-Atmosphere Study (BOREAS), over the tropical Pacific Ocean from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and over the East China Sea during wintertime cold-air outbreaks from the Air Mass Transformation Experiment (AMTEX). Each of these studies has somewhat different forcings and boundary conditions, so we can compare their consequences on the spectra and cospectra. On average, we found no significant scalar or momentum fluxes for horizontal scales > 10 km. We also develop a simple model based on observed thermal structure to explain the phase angle between vertical velocity and the along-wind horizontal velocity as a function of height, which shows good agreement with the observed phase angle in AMTEX. The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

四川地形扰动对降水分布影响

引入一维加权平均的谱分析方法定量研究四川地形强迫对该区域降水分布的影响。结果表明:纬向地形和冬季降水谱峰锁相于同一波长（475.8 km）,呈共振关系,地形与其他季节降水呈漂移关系,这与经向和纬向上环流变动有关,即冬季纬向环流占主导,纬向地形触发的大气波动对冬季降水策动作用大;夏季降水是各种不同尺度系统相互作用的结果,地形是重要因素之一。经向和纬向地形特征尺度分别为296.8 km和475.8 km,反映了地形强迫的中尺度特征,且纬向地形谱峰比经向大1个数量级,纬向强迫更明显。夏季降水谱峰比冬季大2个数量级,降水系统纬向特征尺度比冬季小约150 km,说明夏季在纬向地形强迫下,降水系统尺度减小的同时其强度大大增加,这在一定程度上可以解释中尺度对流性降水在夏季偏多。四川夏季最大降水位于雅安地区,其地形扰动比四川整体扰动更明显,故产生的降水也更大。夏季降水和经向地形锁相于同一波长（37.1 km）,经向地形对雅安夏季强降水起关键作用。     

Mesoscale Meteorological Features Associated with Heavy Precipitation in the Southern Alpine Region

Summary  Several episodes of heavy precipitation, which occurred in the region south of the Alps, have been simulated by means of the mesoscale model BOLAM3. Each case was run at 3 different resolutions, up to 4 km grid interval. The quantitative precipitation “forecast” fields are compared with available rain data. In general, satisfactory results are obtained in terms of spatial distribution and timing of precipitation, except in cases in which pre-frontal convection is dominant. The diagnostics of phenomena involved in orographic precipitation identify the different mesoscale atmospheric features associated with the interaction with topography, like the formation of low level jets, convergence zones, rainbands, and organized convective systems. These appear as “ingredients” common to all the cases considered and are shown to be sensitive to orographic forcing, as well as to the latent heat exchange processes. Received March 2, 1999/Revised June 1, 1999     

Grundzüge der allgemeinen atmosphärischen Zirkulation auf der Südhalbkugel

Zusammenfassung Nach vergleichenden Untersuchungen der bei der “Schwabenland”-Expedition 1938/39 verwendeten Radiosondentypen wird die — vom Strahlungseinflu? befreite — mittlere Temperaturverteilung auf einem Schnitt durch die freie Atmosph?re der Südhalbkugel im Sommer bis 20 km H?he für 10°-Breitenzonen berechnet und mit allen vorliegenden Mittelwerten anderer Stationen verglichen. Eine ?hnliche Zusammenstellung für den Winter liefert wenigstens unterhalb 5 km einigerma?en vergleichbare Resultate. Für die relative und die spezifische Feuchtigkeit in der Troposph?re (bis 8 km) werden gleichfalls schematische Meridionalschnitte abgeleitet. Aus der Verteilung der (virtuellen) Temperatur werden die Topographien der Hauptisobarenfl?chen bis zur 17 mb-Fl?che berechnet und hieraus ein Meridionalschnitt der Zonalkomponente des H?henwindes für den Südsommer bis 30 km H?he abgeleitet. Hierbei ergab sich eine Berichtigung der vertikalen Temperaturverteilung über der ?quatorialregion und ihre Erg?nzung bis 26 km, sowie eineeinwandfreier Zusammehang zwischen Temperatur und Windrichtung. Auch für den Südwinter werden angen?herte Mittelwerte der Zonalkomponente berechnet und mit den entsprechenden Werten der Nordhalbkugel verglichen. Abschlie?ende Betrachtungen befassen sich mit den geringfügigen zonalen Unterschieden auf der Südhalbkugel, mit dem Vergleich zwischen. Arktis und Antarktis, welch letztere im Sommer wie im Winter um 5 bis 12° k?lter ist als die Arktis, sowie mit den jahreszeitlichen Unterschieden in der Zirkulation der Hochstratosph?re und ihren Ursachen.

---

Summary After discussion of the different types of radiosondes during the German Antarctic Expedition in 1938/39, the mean distribution of temperature — corrected for radiation — will be given in a cross-section of the free atmosphere up to a level of 20 kms., over the Southern Hemisphere, and will be compared with all available mean values of other aerological stations in the Southern Hemisphere. A similar collection of winter temperature data, at least below the 5 km. level, also shows comparable results. Merdional cross-sections are derived for the relative and specific humidity below 8 kms. From the distribution of virtual temperature are computed the heights of the standard isobaric surfaces up to 17 mbs, of which is derived a meridional cross-section up to 30 kms. of the zonal component of upper winds for the summer in the Southern Hemisphere. Hereof follows a correction of the vertical distribution of temperature over the equatorial belt and its extrapolation up to 26 kms. as well as an indisputable relationship between temperature and wind direction. Similarly, there are computed for the southern winter approximate mean values of the zonal component, and are compared with the appropriate data for the Northern Hemisphere. Concluding investigations deal with minor zonal differences in the Southern Hemisphere, with a comparison of the Arctic and the Antarctic — which in summer and winter is 5–12° colder than the Arctic — and with seasonal variations in the circulation system of the upper stratosphere, and their causes.

---

Résumé Après avoir comparé les types de radio-sondes employées lors de l'expédition du “Schwabenland” en 1938/39, l'auteur a établi la distribution moyenne de la température de l'atmosphère libre (débarrassée de l'influence du rayonnement) le long d'un profil méridien de l'hémisphère austral, en été jusqu'à 20 km. de hauteur, et pour des zones de 10° de latitude; il compare ces valeurs à toutes celles dont on dispose dans d'autres stations aérologiques. Un profil analogue se rapporte à l'hiver et fournit des résultats à peu près comparables, au moins jusqu'à 5 km. Le même procédé est appliqué à l'humidité relative et spécifique de la troposphère (jusqu'à 8 km). La distribution de la température virtuelle permet le calcul de la topographie des principales surfaces isobares jusqu'à celle de 17 mb., ce qui conduit à une coupe méridienne des composantes zonales du vent pendant l'été austral jusqu'à une hauteur de 30 km. Ce matériel a permis de donner une meilleure distribution verticale de la température au-dessus des régions équatoriales et de la compléter jusqu'à 26 km.; on a en outre établi une correspondance parfaite entre la température et la direction du vent. On a de plus calculé les mêmes valuers moyennes approximatives des composantes zonales pour l'hiver austral et on les a comparées à celles de l'hémisphère boréal. Quelques considérations finales se rapportent à certaines divergences zonales de l'hémisphère sud, à la comparaison entre l'Arctique et l'Antarctique laquelle est, hiver comme été, plus froide de 5° à 12° que la région boréale, et enfin aux variations saisonnières de la circulation de la haute stratosphère et à leurs causes.

---

---

Mit 12 Textabbildungen.     

资料同化中二维特征长度随模式分辨率变化的分析研究

特征长度是资料同化中的重要参量,决定了观测信息在空间的传递特征,而特征长度随模式水平分辨率增减而变化的特点与背景误差湍流功率谱分布特点密切相关。通过对不同来源实际资料计算获得的特征长度数据分析和对理想数据数值试验分析,结果表明随着模式分辨率的提高,特征长度会按照二次根的规律递减。特征长度的这种变化由背景误差湍流功率谱,特别是与次天气尺度（20～60波）到中尺度波（大于60波）的湍流功率谱斜率特征决定。当湍流功率谱斜率从-5/3变化到-4时,特征长度随模式分辨率变化的敏感性降低。作者估计出的温度场的实际背景误差湍流功率谱斜率在次天气尺度到中尺度大约在-2.8左右。对特征长度的估计除传统方法外,可以根据背景误差的湍流功率谱斜率特征来更方便地给出,该方法可作为传统方法的补充来匹配应用。     

Vertical Variation of Madden-Jullian Oscillations in the Normal Monsoon Season as Revealed Through MST Radar Wind Data

Summary  Mesosphere-Stratosphere-Troposphere (MST) Radar wind data for the period June through September 1996 have been examined to study vertical variation of Madden-Jullian Oscillations in wind and eddy kinetic energy (eke) in the normal monsoon season. The domain of analysis in the vertical is from 6 to 20 km with a height resolution of 150 m. Fast-Fourier-Transformation (FFT) has been applied to zonal (u), meridional(v) components of wind to extract the Madden-Jullian oscillations and eke. There are three dominant modes viz., 50–70, 30–40 and 10–20 day periodicity, which contain considerable fraction of energy and show high degree of vertical variability. The peak amplitude of 50–70 day mode in u, 30–40 mode in v and eke were observed at 16–17 km just below the tropopause level. The peak amplitudes of 30–40 day mode in u and 50–70 day mode in v were found in the height region of 13–16 km. To understand the origin and propagation of these waves, wave energy is calculated. The wave energy is higher at tropospheric heights than at lower stratospheric heights indicating that the origin of these waves is in the troposphere, and a part of the energy leaks into the stratosphere. Received September 17, 1998/Revised September 26, 1999     

Evaluation of Heavy Rainfall Model Forecasts over the Korean Peninsula Using Different Physical Parameterization Schemes and Horizontal Resolution

In this study, the accuracy of a Pennsylvania State University-National Center for Atmospheric Research mesoscale model (PSU/NCAR MM5) for predicting heavy summer precipitation over the Korean Peninsula was investigated. A total of 1800 simulations were performed using this model for 30 heavy rainfall events employing four cumulus parameterization schemes (CPS), two grid-scale resolvable precipitation schemes (GRS), and two planetary boundary layer (PBL) schemes in three model resolutions (90 km, 30 km, and 10 km). The heavy rainfall events were mesoscale convective systems developed under the influence of mid-latitude baroclinic systems with low-level moisture transport from the ocean. The predictive accuracy for maximum rainfall was approximately 80% for 10-km resolution and was 60% for 30-km resolution. The predictive accuracy for rainfall position extended to ～150 km from the observed position for both resolutions. Simulated rainfall was most sensitive to CPS, then to PBL schemes, and then to GRS. In general, the Grell (GR) scheme and the Anthes and Kuo (AK) scheme showed a better prediction capability for heavy rainfall than did the Betts-Miller (BM) scheme and the Kain-Fritsch (KF) scheme. The GR scheme also performed well in the 24-h and 12-h precipitation predictions: the parameterized convective rainfall in GR is directly related to synoptic-scale forcing. The models without CPS performed better for rainfall amounts but worse for rainfall position than those with CPS. The MM5 model demonstrated substantial predictive capacity using synoptic-scale initial conditions and lateral boundary data because heavy summer rainfall in Korea occurs in a strong synoptic-scale environment.     

不同阶曲线拟合扰动场对下平流层重力波气候特征影响研究

重力波参数气候特征是确定大气模式中重力波参数化方案的重要条件之一,高垂直分辨率探空资料扰动场是获取重力波参数气候特征的基础数据;目前,获取扰动场的方法较多,但基于不同方法计算的扰动场对重力波参数气候特征影响的研究较少.基于2014—2017年山西太原气象台高垂直分辨率探空资料,利用2—4阶曲线拟合方法获取下平流层(17...     

Some physical aspects of summer monsoon clouds—comparison of cloud model results with observations

The physical characteristics of the summer monsoon clouds were investigated. The results of a simple cloud mod-el were compared with the aircraft cloud physical observations collected during the summer monsoon seasons of 1973,1974,1976 and 1981 in the Deccan Plateau region.The model predicted profiles of cloud liquid water content (LWC) are in agreement with the observed profiles. There is reasonable agreement between the model predicted cloud vertical thickness and observed rainfall.The observed cloud-drop spectra were found to be narrow and the concentration of drops with diameter >20μm is either low or absent on many occasions. In such clouds the rain-formation cannot take place under natural atmos-pheric conditions due to the absence of collision-coalescence process. A comparison of the model predicted and ob-served rainfall suggested that the precipitation efficiency in cumulus clouds of small vertical thickness could be as low as 20 per cent.The clouds forming in the Deccan Plateau region during the summer monsoon are, by and large, cumulus and strato-cumulus type. The vertical thickness of the cumulus clouds is in the range of 1.0-2.0 km. The LWC is found to be more in the region between 1.6-1.9 km A. S. L., which corresponds to the level at almost 3 / 4 th of the total verti-cal thickness of the cloud and thereafter the LWC sharply decreased. Nearly 98 per cent of the tops of the low clouds in the region are below freezing level and the most frequent range of occurrence of these cloud-tops is in the range of 2.0-3.0 km A. S. L.. The dominant physical mechanism of rain-formation in these summer monsoon clouds it the col-lision-coalescence process.