强对流天气分析与预报中的若干基本问题

使用探空观测资料、NCEP再分析资料及多普勒天气雷达产品等多种气象资料,对2015年4月3日湖南出现的一次大范围不同类型强对流天气过程进行了分析,结果表明：强对流天气过程发生前大气环流出现明显调整;低空西南急流的发展加强、中层干冷空气的卷入为大范围强对流天气提供了有利的环境条件;高层辐散、低层辐合的抽吸作用及气旋性切变、湿斜压强迫作用,是湘东北发生混合对流和极端强降水的主要原因。汉寿在逆温层之上暖湿气流抬升和强垂直风切变作用下产生的冰雹,具有高架雷暴特征。新晃具有较高热力不稳定,在冷暖空气交汇时产生冰雹。雷达产品分析表明,冰雹、雷雨大风发生时分别具有典型的三体散射与中高层较强辐散特征,而短时强降水发生时具有低质心和高效降水效率及列车效应特征。

     

2017年8月大气环流和天气分析

2017年8月北半球500 hPa极涡呈单极型分布,强度强于常年同期;亚欧洲大陆中高纬为多波动;西北太平洋副热带高压位置偏西,强度接近常年略偏强。8月全国平均降水量126.6 mm,较常年同期（105.3 mm）偏多20%;全国平均气温为21.4℃,较常年同期（20.8℃）偏高0.6℃。月内共出现了8次主要的区域性强降水过程,多站日降水量超历史同期极值。8月共有5个台风在西北太平洋和南海海域活动,其中1713号台风天鸽、1714号台风帕卡4天内先后登陆珠三角。月内,我国南方地区出现大范围持续高温天气,江淮、江汉等地出现阶段性伏旱。     

大尺度形势对中尺度对流风暴影响的研究

2014年5月10日08时左右,在广东阳江市附近有分散对流单体出现并发展合并形成为尺度约200 km的准静止东西向线状MCS（Meso-scale Convective System）,持续近16 h,造成广东阳江以东至珠江入海口以西沿海强降水。通过实况分析、WRF-ARW（the Advanced Research WRF）模式模拟及地形敏感试验发现,在有利的大尺度水汽热力条件背景下,特殊的中尺度动力热力结构与云雾山地形的持续相互作用是该MCS触发和维持机制的关键因素。在大气低层出现的SLLJ（Super-low Level Jet）构成了向北输送的暖湿气流“通道”,增强了云雾山南侧上干冷与下暖湿的大气对流不稳定层结,使得暖湿空气在云雾山地形的阻挡作用下被强迫抬升达到自由对流高度,气块的CAPE（Convective Available Potential Energy）被触发释放,在云雾山附近生成分散的对流单体,配合中层偏西引导气流的作用,对流单体向东移动发展。由于以上大气中尺度动力热力结构特征的持续维持,配合云雾山地形抬升作用,在此后的十余小时内,云雾山附近不断触发新对流单体,在中层引导气流作用下向东移动的同时,持续的SLLJ为对流的发展供给大气不稳定能量使得对流单体逐渐发展合并,以此往复,形成了西端位于云雾山附近的准东西向线状MCS并长时间维持。

     

热带海温异常对东南亚夏季风爆发的影响

通过季风指数Im定义了能表征东南亚地区降水实况的东南亚夏季风指数,根据东南亚夏季风指数测算出东南亚夏季风爆发的平均时间为5月7日.利用东南亚夏季风指数分析热带海温场及垂直速度场的变化后发现,在东南亚夏季风爆发的前期秋、冬季节,中东太平洋地区以及中西印度洋地区的冷海温有利于东南亚地区夏季风的提前爆发.当中东太平洋地区是冷(暖)海温时,对应着纬向的Walker环流及季风环流圈强(弱),东南亚地区的对流也强(弱),则东南亚地区夏季风爆发早(迟).     

Using a Convection Model to Predict Altitudes of White Stork Migration Over Central Israel

Soaring migrants such as storks, pelicans and large birds of prey rely on thermal convection during migration. The convection model ALPTHERM was designed to predict the onset, strength, duration and depth of thermal convection for varying topographies for glider pilots, based on atmospheric conditions at midnight. We tested ALPTHERM predictions as configured for two topographies of central Israel, the Coastal Plains and the Judean and Samarian Mountains in order to predict altitudes of migrating white storks (Ciconia ciconia). Migrating flocks of white storks were tracked with a motorized glider, to measure maximum altitudes of migration during spring 2000. A significant positive correlation was found between the maximum daily altitudes of migration measured and the predicted upper boundary of thermal convection for the Coastal Plains and Samarian Mountains. Thirty-minute predictions for the Coastal Plains and Samarian Mountains correlated positively with measured maximum migration altitudes per thermal. ALPTHERM forecasts can be used to alter flight altitudes in both civil and especially military aviation and reduce the hazard of serious aircraft collisions with soaring migrants.     

Excess of bottom-released methane in an Arctic shelf sea polynya in winter

Latent heat polynyas are regions generating strong ice formation, convection and extensive water mass formation. Here we report on the effects of these processes on resuspension of sediments and subsequent methane release from the seafloor and on the resulting excess methane concentration in surface water on a polar shelf during winter. The study is based on measurements of concentration and δ¹³C values of methane, water temperature, salinity, light transmission and sea ice data collected in March 2003 in Storfjorden, southern Svalbard. In winter, strong and persistent northeasterly winds create polynyas in eastern Storfjorden and cause ice formation. The resulting brine-enriched water cascades from the Storfjordbanken into the central depression thereby enhancing the turbulence near the seafloor. A distinct benthic nepheloid layer was observed reflecting the resuspension of sediments by the cascading dense bottom water. High concentrations of ¹³C-depleted methane suggest submarine discharge of methane with the resuspended sediments. As the source of the submarine methane, we propose recent bacterial methanogenesis near the sediment surface because of extremely high accumulation rates of organic carbon in Storfjorden. Convective mixing transports newly released methane from the bottom to the sea surface. This eventually results in an excess concentration in surface water with respect to the atmospheric equilibrium, and a sea-air flux of methane during periods of open water. When a new ice cover is formed, methane becomes trapped in the water column and subsequently oxidized. Thus, the residual methane is strongly enriched in ¹³C in relation to the δ¹³_CCH4${\delta }^{13}{\mathrm{C}}_{{\mathrm{CH}}_4}$ signature of atmospheric methane. Our results show that latent heat polynyas may induce a direct pathway for biogases like methane from sediments to the atmosphere through coupling of biogeochemical and oceanographic processes. Extrapolating these processes to all Arctic ocean polynyas, we estimate a transfer of CH₄ between 0.005 and 0.02 Tg yr⁻¹. This is not a large contribution but the fluxes from the polynyas are 20–200 times larger than the ocean average and the methane evasion process in polynyas is certainly one that can be altered under climate change.     

Long‐term annual surface heat and water balances over Canada and the United States South of 60°N: Reconciliation of precipitation,run‐off and temperature fields

Abstract

The analysis compares the observed field of run‐off (assumed correct) with adjusted precipitation over North America (as amended by den Hartog and LeDrew over Canada) and derives the principal hydroclimatological ratios for each five‐degree latitude‐longitude square. The amended precipitation field yields values of the Budyko dry ness index close to values suggested by the vegetation distribution. The Priestley‐Taylor parameter, α, lies between unity (equilibrium) and potential (1.26) values over much of humid North America, but exceeds these values in the northwest Pacific squares, where advective heating may be the cause. Other regions of strong seasonal advective heating (e.g. the Great Plains) do not appear to influence the distribution strongly. A weighted convective forcing temperature is derived, varying from 298 K in the extreme south to below 285 K in the north. This function (and the Bowen ratio) achieve improbable values in northern Labrador‐ Ungava. The precipitation, run‐off and net radiation régimes appear still to be out of balance in these squares. An adjustment of either precipitation or net radiation by about a tenth corrects the imbalance, but the method is not capable of deciding which field (or both) is in error. Over the rest of the continent the adjusted precipitation field now appears to be in balance with observed run‐off and temperature distributions.     

An improvement of the mass flux convection parameterization scheme and its sensitivity tests for seasonal prediction over China

A modified cumulus parameterization scheme, suitable for use in a seasonal forecast model, is presented. This parameterization scheme is an improvement of the mass flux convection scheme developed by Gregory and Rowntree (1989; 1990). This convection scheme uses a “bulk” cloud model to present an ensemble of convective clouds, and aims to represent shallow, deep, and mid-level convection. At present,this convection scheme is employed in the NCC T63L20 model (National Climate Center, China Meteorological Administration). Simulation results with this scheme have revealed some deficiencies in the scheme,although to some extent, it improves the accuracy of the simulation. In order to alleviate the deficiencies and reflect the effect of cumulus convection in the actual atmosphere, the scheme is modified and improved.The improvements include (i) the full estimation of the effects of the large-scale convergence in the lower layer upon cumulus convection, (ii) the revision of the initial convective mass flux, and (iii) the regulation of convective-scale downdrafts. A comparison of the results obtained by using the original model and the modified one shows that the improvement and modification of the original convection scheme is successful in simulating the precipitation and general circulation field, because the modified scheme provides a good simulation of the main features of seasonal precipitation in China, and an analysis of the anomaly correlation eoetfieient between the simulation and the observations confirms the improved results.     

Spatially intermittent fields in photospheric magnetoconvection

Motivated by recent high-resolution observations of the solar surface, we investigate the problem of non-linear magnetoconvection in a three-dimensional compressible layer. We present results from a set of numerical simulations which model the situation in which there is a weak imposed magnetic field. This weak-field regime is characterized by vigorous granular convection and spatially intermittent magnetic field structures. When the imposed field is very weak, magnetic flux tends to accumulate at the edges of the convective cells, where it forms compact, almost 'point-like' structures which are reminiscent of those observed in the quiet Sun. If the imposed field is slightly stronger, there is a tendency for magnetic flux to become concentrated into 'ribbon-like' structures which are comparable to those observed in solar plages. The dependence of these simulations upon the strength of the imposed magnetic field is analysed in detail, and the concept of the fractal dimension is used to make a further, more quantitative comparison between these simulations and photospheric observations.