2000年6月4日沙尘暴过境时敦煌地面气象要素及地表能量平衡特征的变化

利用在敦煌进行的"我国典型干旱区陆-气相互作用试验"观测资料,分析了2000年6月4日在我国西北所发生的沙尘暴过境时的地面气象要素的变化以及地表能量平衡变异特征.结果表明,在沙尘暴过境时地面水平风风向经历了从稳定到多次调整,再到稳定,最后崩溃的演变.风向调整时,风速减弱,而调整之后,风速加大,同时伴有很强的上升气流.沙尘暴天气来临前,地面空气呈干热状态,处于低气压控制之下;而在沙尘暴过境时和过境后,地面空气显得相对湿冷,地面气压急剧上升,并处于高压控制之下.沙尘暴过境前后,地表的能量平衡关系遭到破坏,净辐射大幅减少,地气间的感热和潜热输送变弱,土壤全天向上释放热量.     

1998年中国大洪水时期的水汽收支研究

文中首先通过水汽通量的势函数和流函数的计算 ,分析了 1998年中国大洪水时期的全球水汽背景 ,然后从雨情分析入手 ,将 1998年 5～ 8月长江、松花江流域洪水期分为 7个降水阶段、11个区域 ,对各时段、各区域的水汽收支作了诊断分析 ,得到中国大洪水时期部分水汽收支图像 ,揭示了水汽循环的一些规律 ,主要结果如下 :( 1) 1998年 5～ 8月 ,中国东部地区是全球最强的水汽汇区 ,这与 1991年夏季的情况相似。水汽通量的势函数极小值区 (最大辐合区 )对应强降水区 ,并且暴雨区的水汽辐合是由半球尺度的水汽输送造成 ,这表明 ,即使对于区域性大洪水 ,它必须从极大范围地区获得水汽供应。分析还表明 ,南海季风的爆发及其区域内西南方向水汽流的增强与印度洋势函数 (水汽辐散 )的增强关系密切。( 2 )大气的水汽收支表明 ,降水主要来自水汽的辐合项 ,辐合主要发生在大气低层 ;用余差法计算出的局地蒸发项一般为降水量的 13 ～ 12 ,因而水汽的再循环过程也十分重要 ;垂直输送项把低层的水汽向中上层输送 ,增加高层的水汽积累 ,为积云的发展和潜热释放提供条件。( 3 )南海地区的水汽输送情况与中国强降水密切相关 ,南海季风爆发后 ,其强劲南风气流输送水汽的区域往往是强降水发生区。对于整个中国东部大陆区而言 ,来     

SEDIMENT BUDGET OF THE ILLINOIS RIVER

1 BACKGROUND The Illinois River is the most significant river in the state of Illinois. The river drains nearly half of the state and has a drainage area of 28,906 mi2 (74,867 km2). Except for about a 4,000 mi2 (10,360 km2) area in Indiana and Wisconsin, the watershed is located in Illinois (see Fig. 1). The watershed contains the drainage basins of several of the state's significant rivers such as the Sangamon, LaMoine, Spoon, Mackinaw, Vermilion, Fox, Kankakee, and Des Plaines Ri…     

我国大陆地区和近海海域能量收支分布及其季节变化的数值模拟研究

应用美国宇航局Goddard地球观测系统四维资料同化系统、计算了我国大陆地区和近海海域1998年各月月平均能量收支各项和10m气温、比湿及风矢量的地理分布特征. 模式计算结果表明,地表短波净辐射最强出现在夏季(7月)新疆和西藏中部地区,高值中心区可达275W/m2,在黄海东海海域春季(4月)最大,其值为250W/m2左右. 地表长波净辐射最强出现在夏季(7月)我国西北地区,中心区值为125W/m2,我国近海海域在冬季(1月)最强,其值为75-100W/m2. 我国近海海面,冬季(1月)潜热通量值高于一般月份,中心区值可达250W/m2,夏季我国大陆西南、华北和东北一带为潜热通量高值区,其值为125W/m2. 月平均能量收支计算结果显示,在黄海、东海海域冬季(1月)净通量为海洋向大气输送,夏季(7月)则反之,新疆和西藏高原中部夏季为净通量正值区. 综合温度、湿度和风矢量场分布发现,夏季从南海向华东地区,孟加拉湾向印度次大陆有明显的水汽平流输送,西藏西南部也有来自西南方向的水汽输送.     

密云水库沉积物-水界面磷的地球化学作用

以密云水库为例，研究了水库沉积物中有机质的含量与有机磷的关系，表明表层沉积物中的总有机碳和有机磷的含量都明显高于底层，是由于一些以磷为营养素的富营养化指示藻类在水体中逐年增加，它们死亡后的残骸经分解-矿化后在底积物中累积的结果。沉积物和孔隙水中磷的剖面特征研究表明，表层沉积物和孔隙水中的磷都有明显上升的趋势，是由于沉积物的表层微生物和活性有机碎屑层比较丰富，有机质降解和含磷的有机化合物分解，使溶解性磷酸盐进入孔隙水。对沉积物磷形态、总有机碳及孔隙水中总磷对磷释放的影响进行了线性相关分析，结果表明上覆水中的磷主要来自沉积物的铁结合态磷、铝结合态磷、溶解态磷三种形态，孔隙水中总磷以及总有机碳含量对上覆水中总溶解磷浓度存在较大的影响，这些为研究水库沉积物作为水库内污染源对水质的影响提供了基础资料。     

The effect of idealized water waves on the turbulence structure and kinetic energy budgets in the overlying airflow

The influence of an idealized moving wavy surface on the overlying airflow is investigated using direct numerical simulations (DNS). In the present simulations, the bulk Reynolds number is Re = 8000 (; where U₀ is the forcing velocity of the flow, h the height of the domain and v the kinematic viscosity) and the phase speed of the imposed waves relative to the friction velocity, i.e., the wave age varies from very slow to fast waves. The wave signal is clearly present in the airflow up to at least 0.15λ (where λ is the wave length) and is present up to higher levels for faster waves. In the kinetic energy budgets, pressure transport is mainly of importance for slow waves. For fast waves, viscous transport and turbulent transport dominate near the surface. Kinetic energy budgets for the wave and turbulent perturbations show a non-negligible transport of turbulent kinetic energy directed from turbulence to the wave perturbation in the airflow. The wave-turbulent energy transport depends on the size, tilt, and phase of the wave-induced part of the turbulent Reynolds stresses.According to the DNS data, slow waves are more efficient in generating isotropic turbulence than fast waves.Despite the differences in wave-shape as well as in Reynolds number between the idealized direct numerical simulations and the atmosphere, there are intriguing similarities in the turbulence structure. Important information about the turbulence above waves in the atmosphere can be obtained from DNS—the data must, however, be interpreted with care.     

Groundwater inflow and associated transport of phosphorus to a hypereutrophic lake

Hydrogeochemical data from lake, sediment pore, and well waters were used to quantify groundwater seepage and the associated transport of phosphorus to Lake Persimmon, Florida, USA. The data show that lake chloride concentrations vary as a function of lake elevations that are controlled by groundwater inflow. A whole-lake average seepage rate, estimated using a simple one dimensional advection-diffusion model fitted to the lake chloride profile, currently averages 2.3 ± 0.3 cm yr^-1 and is in reasonable agreement with the rate of advective flow obtained from the pore water chloride profile. The ratios of nutrient regeneration versus sulfate consumption indicate that the phosphorus enrichment in deeper portions of sediment pore water is most likely a result of groundwater phosphorus transport through sediment. Thus, the net inputs of groundwater phosphorus to the lake, calculated using the deep pore water phosphorus concentration, are about 7.4 ± 4.3 mg P m^-2 yr^-1 and comparable with recent in situ estimates from seepage meters. This study provides a simple hydrogeochemical method for estimating hydrologic and phosphorus inputs via groundwater to the lake, thereby supporting current efforts for lake management.     

Geomorphic Effects of the Extreme Rainfall Event of 20–21 July, 2004 in the Latnjavagge Catchment, Northern Swedish Lapland

Mass transfers triggered by a rare rainfall event on 20–21 July, 2004, with 58.4 mm of rain within 24 h and 71.7 mm of rain within 48 h in the Latnjavagge catchment (9 km² , 950–1440 m a.s.l.; 68°20'N, 18°30'E) in the higher Abisko mountain region (Swedish Lapland), are quantified and analysed in direct comparison with mean annual mass transfers in this drainage basin. In years without rare rainfall events the Latnjavagge catchment is characterized by restricted sediment availability resulting in low mechanical denudation and mass transfers. During the rare rainfall event of 20–21 July, 2004, major stability thresholds on the slope systems (triggering debris flows and slides) and in the channel systems (break‐up of channel debris pavements and step–pool systems) in the Latnjavagge catchment were passed and mass transfers by debris flows, slides and fluvial debris transport in creeks and channels were several times higher than the mean annual mass transfers in Latnjavagge. In the calculation of longer‐term mass transfers and sediment budgets, rare events like the 20–21 July, 2004 rainfall event have to be considered as essential components. A reliable estimation of the recurrence intervals of such rare events is especially problematic. The general problem of defining an adequate length of process monitoring programmes is pointed out.     

Effects of high-magnitude/low-frequency fluvial events generated by intense snowmelt or heavy rainfall in arctic periglacial environments in northern Swedish Lapland and northern Siberia

Abstract In the Latnjavagge drainage basin (68°21′N, 18°29′E), an arctic‐oceanic periglacial environment in northernmost Swedish Lapland, the fluvial sediment transport and the characteristics and importance of high‐magnitude/low‐frequency fluvial events generated by intense snowmelt or heavy rainfall have been investigated and compared with snowmelt‐ and rainfall‐induced discharge peaks in the Levinson‐Lessing Lake basin (Krasnaya river system) on the Taimyr Peninsula, an arctic periglacial environment in northern Siberia (74°32′N, 98°35′E). In Latnjavagge (9 km²) the intensity of fluvial sediment transport is very low. Most of the total annual sediment load is transported in a few days during snowmelt generated runoff peaks. Due to the continuous and very stable vegetation covering most areas below 1300 m a.s.l. in the Latnjavagge catchment, larger rainfall events are of limited importance for sediment transport in this environment. Compared to that, in the c. 40 times larger Krasnaya riversystem rainfall‐generated runoff peaks cause significant sediment transport. The main sediment sources in the Latnjavagge drainage basin are permanent ice patches, channel debris pavements mobilized during peak discharges and exposing fines, and material mobilized by slush‐flows. In the Krasnaya river system river bank erosion is the main sediment source. In both periglacial environments more than 90% of the annual sediment yield is transported during runoff peaks. The results from both arctic periglacial environments underline the high importance of high‐magnitude/low‐frequency fluvial events for the total fluvial sediment budgets of periglacial fluvial systems. Restricted sediment availability is in both arctic environments the major controlling factor for this behaviour.     

Process-based snowmelt modeling: does it require more input data than temperature-index modeling?

Modeling snow hydrology for cold regions remains a problematic aspect of many hydro-environmental models. Temperature-index methods are commonly used and are routinely justified under the auspices that process-based models require too many input data. To test this claim, we used a physical, process-based model to simulate snowmelt at four locations across the conterminous US using energy components estimated from measured daily maximum and minimum temperature, i.e. using only the same data required for temperature-index models. The results showed good agreement between observed and predicted snow water equivalents, average R²>0.9. We duplicated the simulations using a simple temperature-index model best fitted to the data and results were poorer, R²<0.8. At one site we applied the process-based model without substantial parameter estimation, and there were no significant (=0.05) differences between these results and those obtained using temperature-estimated parameters, despite relatively poorly predicted specific energy budget components (R²<0.8). These results encourage the use of mechanistic snowmelt modeling approaches in hydrological models, especially in distributed hydrological models for which landscape snow distribution may be controlled by spatially distributed components of the environmental energy budget.