Relationship between summer rainfall anomalies and sub-seasonal oscillation intensity in the ChangJiang Valley in China

Sub-seasonal variability of summer (May–October) rainfall over the ChangJiang Valley exhibits two dominant timescales, one with a quasi-biweekly (QBW) period (10–20 days) and the other with an intraseasonal oscillation (ISO) period (20–60 days). A significant positive correlation (at a 99% confidence level) was found between the summer precipitation anomaly and the intensity of the QBW and ISO modes in the region. By examining the composite structure and evolution characteristics, we note that the QBW mode is characterized by a northwest–southeast oriented wave train pattern, moving southeastward. The perturbations associated with the ISO mode propagate northwestward in strong ISO years but southeastward in weak ISO years. A marked feature is the phase leading of low-level moisture to convection in both the QBW and ISO mode. When the summer rainfall is strong in the ChangJiang Valley, large-scale atmospheric conditions in the strong QBW/ISO activity region are characterized by deeper moist layer, convectively more unstable stratification and greater ascending motion. Such mean conditions favor the growth of the QBW and ISO perturbations. Thus, a significant positive correlation between the summer precipitation and the strength of sub-seasonal variability arises from the large-scale control of the summer mean flow to perturbations.     

低涡和副热带高压共同影响下的暴雨落区分析

应用常规观测资料和NCEP 1°×1°再分析资料,分析低涡和副热带高压共同影响下的暴雨过程,发现暴雨落区并不与低涡位置及路径一致,因而不能简单的按其位置和路径预报暴雨落区,应着眼于暴雨发生的物理机理,注意分析影响系统的空间结构、发展阶段和地面形势的演变特征。在有锋面系统影响时,初始对流往往由锋面触发,因此,暴雨的第一落区在锋面附近。冷锋触发的暖区暴雨随后出现,不需强的动力辐合条件,可能远离低涡中心,而是位于副热带高压边缘的高温湿舌内。另外,应密切关注周边初生的对流云团及其移入时造成的暴雨。     

一次西南低涡东移引发长江中下游暴雨的诊断研究

利用常规观测资料和NECP再分析资料,对2013年6月6—7日西南低涡东移加强发展造成长江中下游大暴雨过程进行了诊断分析,重点探讨了西南低涡东移和发展维持的物理机制以及最强降水的变化特征。结果表明,沿着700 hPa高空切变线东移的西南低涡是造成此次长江中下游地区暴雨的直接影响系统,西南低涡沿着700 hPa切变线东移发展,深厚阶段正涡度柱伸展到400 hPa高度,自下而上呈近垂直结构。西南低涡附近低层辐合与高层辐散的大尺度环境条件、西南低涡与西南低空急流耦合发展动力结构、低空暖平流和高空槽前正涡度平流输送等条件是导致西南低涡东移到长江中下游后加强发展的主要因子。与西南低涡相伴随的强降雨区主要位于低涡南部3个纬距以内,该处的西南季风和副高西南侧东南气流两支水汽输送的汇合为暴雨发生提供了充沛的水汽和对流不稳定能量,而对流层中低层携带的冷空气侵入低层低涡的后部,不仅加强了低涡的斜压性,也促进了上冷下暖不稳定层结的产生和发展,为强降水的发生提供了不稳定对流触发条件。     

THE LOW-PRESSURE SYSTEM IN THE VERTICAL VELOCITY OMEGA ON THE SCALE OF SYSTEM SENSITIVITY

In this study, the relationship between scale and vertical velocity in a low-pressure system is explored using the wave characteristics of atmospheric disturbances and the structural characteristics of low-pressure systems. The ω differential equation, as determined by the transient geopotential height field Φ, is solved to obtain an analytical solution composed only of wavelength, horizontal speed, and atmospheric stability, i.e., the ω diagnostic equation of a low-pressure system. This equation also shows that vertical velocity in the low-pressure system is very sensitive to the horizontal scale, i.e., a smaller horizontal scale means a larger vertical velocity.     

IMPACT OF TROPICAL INTRASEASONAL OSCILLATIONS ON THE PRECIPITATION OF GUANGDONG IN JUNES

The impact of tropical intraseasonal oscillations on the precipitation of Guangdong in Junes and its physical mechanism are analyzed using 30-yr (1979 to 2008), 86-station observational daily precipitation of Guangdong and daily atmospheric data from NCEP-DOE Reanalysis. It is found that during the annually first rainy season (April to June), the modulating effect of the activity of intraseasonal oscillations propagating eastward along the equator (MJO) on the June precipitation in Guangdong is different from that in other months. The most indicative effect of MJO on positive (negative) anomalous precipitation over the whole or most of the province is phase 3 (phase 6) of strong MJO events in Junes. A Northwest Pacific subtropical high intensifies and extends westward during phase 3. Water vapor transporting along the edge of the subtropical high from Western Pacific enhances significantly the water vapor flux over Guangdong, resulting in the enhancement of the precipitation. The condition is reverse during phase 6. The mechanism for which the subtropical high intensifies and extends westward during phase 3 is related to the atmospheric response to the asymmetric heating over the eastern Indian Ocean. Analyses of two cases of sustained strong rainfall of Guangdong in June 2010 showed that both of them are closely linked with a MJO state which is both strong and in phase 3, besides the effect from a westerly trough. It is argued further that the MJO activity is indicative of strong rainfall of Guangdong in June. The results in the present work are helpful in developing strategies for forecasting severe rainfall in Guangdong and extending, combined with the outputs of dynamic forecast models, the period of forecasting validity.     

Preservation of layered paleodeposits in high-latitude pedestal craters on Mars

An outstanding question in Mars’ climate history is whether or not pedestal craters represent the armored remnants of ice-rich paleodeposits. We address this question using new high-resolution images; in a survey of several hundred high-latitude pedestal craters, we have identified 12 examples in which visible and/or topographically expressed layers are exposed on the marginal scarp of the pedestal. One example, located on the south polar layered deposits, preserves ice-rich layers that have otherwise been completely removed from the polar cap. These observations provide empirical evidence that the pedestal crater formation mechanism is capable of armoring and preserving ice-rich layered paleodeposits. Although layered exposures have not yet been observed in mid-latitude pedestal craters, high-latitude instances of discontinuous, partially covered layers suggest that layers can be readily concealed, likely through mantling and/or mass wasting processes along the marginal scarp. This interpretation is supported by the observation that high-latitude pedestals with exposed layers along their margins are, on average, taller than mid-latitude examples, and have larger, steeper marginal scarps, which may help to maintain layer exposures. These observations favor the interpretation that mid- to high-latitude pedestal craters represent the armored remnants of ice- and dust-rich paleodeposits, which occurred transiently due to changes in the climate regime. Preservation of fine-scale layering of ice and dust at these latitudes implies that the climate change did not involve regional melting conditions.     

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.     

Viscous liquid film flow on dune slopes of Mars

It is shown that viscous liquid film flow (VLF-flow) on the surfaces of slopes of martian dunes can be a low-temperature rheological phenomenon active today on high latitudes. A quantitative model indicates that the VLF-flows are consistent with the flow of liquid brines similar to that observed by imaging at the Phoenix landing site. VLF-flows depend on the viscosity, dynamics, and energetics of temporary darkened liquid brines. The darkening of the flowing brine is possibly, at least partially, attributed to non-volatile ingredients of the liquid brines. Evidence of previous VLF-flows can also be seen on the dunes, suggesting that it is an ongoing process that also occurred in the recent past.     

Seasonal oscillations in mesospheric temperatures at low-latitudes

Using 3 years of high-quality temperature measurements (2002–2004) recorded from Maui, HI (20.8°N), we have investigated the characteristics of mesospheric seasonal oscillations at low-latitudes. Measurements of the near-infrared OH (6,2) and O₂ (0,1) nightglow emission layers (centered at 87 and 94 km) independently reveal a distinct semi-annual oscillation (SAO) and annual oscillation (AO) with amplitudes of 3.8 and 2.0 K, respectively. An observed asymmetry in the seasonal variation of the nocturnal mean, previously reported by Taylor et al. [2005. Characterization of the semi-annual-oscillation in mesospheric temperatures at low-latitudes. Advances in Space Research 35, doi:10.1016/j.asr.2005.05.111] from this site is shown to be due to a superposed AO of amplitude 50% of the SAO signature. Detailed investigations of the local-time variation of the SAO amplitude and phase combined with TIME-GCM simulations of the seasonal variation of the diurnal tide strongly suggest a large local-time dependence of the amplitude (but not phase) of the observed SAO. These data indicate that the true mean temperature SAO amplitude could be as high as 7 K at this latitude.     

Mineral Geochemical Compositions of Tourmalines and Their Significance in the Gejiu Tin Polymetallic Deposits, Yunnan, China

Abstract: The Gejiu tin polymetallic deposits are located in the southeastern part of Yunnan Province in China. A detailed electronic microprobe study has been carried out to document geochemical compositions of tourmalines from the deposits. The results indicate a systematic change of mineral geochemical compositions, which might be used as a mineral geochemical tracer for post-magmatic hydrothermal fluid, basin fluid and their mixture. The tourmalines from granite are schorl with Fe/(Fe+Mg) ratios of 0.912-1.00 and Na/(Na+Ca) ratios of 0.892-0.981. Tourmalines as an inclusion in quartz from the ore bodies are dravite with Fe/(Fe+Mg) ratios of 0.212-0.519 and Na/ (Na+Ca) ratios of 0.786-0.997. Tourmalines from the country rocks are dravite with Fe/(Fe+Mg) ratios of 0.313-0.337 and Na/(Na+Ca) ratio of 0.599-0.723. Tourmalines from cassiterite-tourmaline veins that occur in crannies within the country rocks show distinct optical zoning with alternate occurrence of dravite and schorl, Fe/(Fe+Mg)=0.374-0.843, Na/(Na+Ca)=0.538-0.987. It suggests that schorl in granite and dravite in carbonatite are related to magmatic fluid and basin fluid respectively. When magmatic fluid rose up and entered into crannies of the country rocks, consisting mainly of carbonatite, basin fluid would be constantly added to the magmatic fluid. The two types of fluid were mixed in structural crannies of the sedimentary basin accompanied with periodic geochemical oscillations to form material records in chemical composition zonings of tourmalines.