Evaporation and energy balance of a wet grassland at Tadham Moor on the Somerset Levels

Wet grasslands are important both for their conservation value and for their important hydrological function. Evaporation is an important component of the water balance of a wetland. Where water is limited rainfall and/or surface (or sub‐surface) inflows are required to balance the summer evaporation and thus become a significant factor in the maintenance and environmental health of a wetland. This study presents an almost complete year of measurements of the water and energy balance of a wet grassland in the Somerset Levels in southwest England. The majority of the radiant energy at this site goes into evaporation. There is a strong seasonality of the controls on evaporation; the roughness length varies by a factor of 10 between winter and summer. The surface resistance to evaporation is low, close to zero, during the winter when the water table is at, or just below the surface. In the summer the water table drops to 80 cm below the surface; there is no sign of soil water stress on the evaporation but a clear effect of the senescence of grass during seed head production and of the subsequent harvest. There is clear evidence that water for evaporation is provided by the drainage ditches through sub‐surface flow. Standard evaporation formulae—such as the Penman‐Monteith equation with constant and standard parameters—provide a reasonable simulation of the total evaporation at this site, although they miss some of the seasonal detail. Copyright © 2008 John Wiley & Sons, Ltd.     

梯度扩散理论在能量和物质输送计算中的若干问题

本文用近年来的大量观测结果,揭示了被长期使用的梯度扩散理论的问题和局限性。讨论了它的成立条件:只有当湍流的涡旋尺度远小于梯度变化的空间尺度时,梯度扩散理论才可以正确使用。对于几种典型的近地层条件,计算了它们的湍流的涡旋尺度,从而说明了梯度扩散理论在这些条件下失败的原因。     

Accuracy and precision of different sampling strategies and flux integration methods for runoff water: comparisons based on measurements of the electrical conductivity

Because of their fast response to hydrological events, small catchments show strong quantitative and qualitative variations in their water runoff. Fluxes of solutes or suspended material can be estimated from water samples only if an appropriate sampling scheme is used. We used continuous in‐stream measurements of the electrical conductivity of the runoff in a small subalpine catchment (64 ha) in central Switzerland and in a very small (0·16 ha) subcatchment. Different sampling and flux integration methods were simulated for weekly water analyses. Fluxes calculated directly from grab samples are strongly biased towards high conductivities observed at low discharges. Several regressions and weighted averages have been proposed to correct for this bias. Their accuracy and precision are better, but none of these integration methods gives a consistently low bias and a low residual error. Different methods of peak sampling were also tested. Like regressions, they produce important residual errors and their bias is variable. This variability (both between methods and between catchments) does not allow one to tell a priori which sampling scheme and integration method would be more accurate. Only discharge‐proportional sampling methods were found to give essentially unbiased flux estimates. Programmed samplers with a fraction collector allow for a proportional pooling and are appropriate for short‐term studies. For long‐term monitoring or experiments, sampling at a frequency proportional to the discharge appears to be the best way to obtain accurate and precise flux estimates. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance of various similarity functions for nondimensional wind and temperature profiles in the surface layer in stable conditions

The linear functions for non-dimensional wind and temperature profiles are commonly used to describe the surface layer fluxes in atmospheric models. However, their applicability is limited to smaller values of the stability parameter z/L (where z is the height above ground and L is the Obukhov length) i.e. z/L < 1.0. These linear functions have been modified (Webb 1970, Quart. J. Roy. Meteor. Soc. 96, 67–90; Clarke 1970, Quart. J. Roy. Meteor. Soc. 96, 91–114; Hicks 1976, Quart. J. Roy. Meteor. Soc. 102, 535–551; Beljaars and Holtslag 1991, J. Appl. Meteorol. 30, 327–341; Cheng and Brutsaert 2005, Boundary-Layer Meteorol. 114, 519–538) over the years for calculating fluxes when z/L > 1.0 under strongly stable conditions. In view of this, the objective of the present study is to analyze the performance of these similarity functions to compute surface fluxes in stable conditions.The meteorological observations from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) experiment are utilized for computing the surface fluxes in stable conditions. The computed fluxes are found to be reasonably close to those observed. The ratio of observed to computed fluxes reveals that the computed fluxes are close to the observations for all the similarity functions for z/L < 1.0 whereas the computed values show relatively a large scatter from observations for z/L > 1.0. The computed values of u and heat flux do not show significant differences from those observed at 99% confidence limit. The performance of all the similarity functions considered here is found to be comparable to each other in strongly stable conditions.     

A Note on Reviving the Goddard Satellite-Based Surface Turbulent Fluxes （GSSTF） Dataset

Accurate sea surface flux measurements are crucial for understanding the global water and energy cycles. The oceanic evaporation, which is a major component of the global oceanic fresh water flux, is useful for predicting oceanic circulation and transport. The global Goddard Satellite-based Surface Turbulent Fluxes Version-2 (GSSTF2; July 1987--December 2000) dateset that was officially released in 2001 has been widely used by scientific community for global energy and water cycle research, and regional and short period data analyses. We have recently been funded by NASA to resume processing the GSSTF dataset with an objective of continually producing a uniform dataset of sea surface turbulent fluxes, derived from remote sensing data. The dataset is to be reprocessed and brought up-to-date (GSSTF2b) using improved input datasets such as a recently upgraded NCEP/DOE sea surface temperature reanalysis, and an upgraded surface wind and microwave brightness temperature V6 dataset (Version 6) from the Special Sensor Microwave Imager (SSM/I) produced by Remote Sensing Systems (RSS). A second new product (GSSTF3) is further proposed with a finer temporal (12-h) and spatial (0.25^ox0.25^o) resolution. GSSTF2b (July 1987--December 2008) and GSSTF3 (July 1999--December 2009) will be released for the research community to use by late 2009 and early 2011, respectively.     

Responses of Vertical Structures in Convective and Stratiform Regions to Large-Scale Forcing during the Landfall of Severe Tropical Storm Bilis （2006）

The responses of vertical structures,in convective and stratiform regions,to the large-scale forcing during the landfall of tropical storm Bilis(2006) are investigated using the data from a two-dimensional cloud-resolving model simulation.An imposed large-scale forcing with upward motion in the mid and upper troposphere and downward motion in the lower troposphere on 15 July suppresses convective clouds,which leads to ～100% coverage of raining stratiform clouds over the entire model domain.The imposed forci...     

地表通量输送对飑线过程影响的数值模拟研究

使用水平分辨率20km的RSM-RANAL再分析资料和WRF模式,对2004年7月12日影响上海地区的一次飑线过程进行分析和数值模拟,结果表明:(1)雷暴发生区域的西北侧约300km处有一冷锋,地面风场辐合;中低层为一致西南急流,雷暴发生区域上空300hPa位于西风急流右后侧辐散区中;地面存在明显的温度梯度。(2)雷暴发生前浙皖交界山区处的CAPE增至1000J/kg,CIN减至30J/kg以下;杭州湾附近的CAPE值大于2000J/kg。这些地方之后均有强对流单体生成,并成为飑线的一部分。(3)WRF模式对这次飑线过程的主要特征模拟较好,包括地面风场和飑线的结构。(4)敏感性试验指出陆面过程对本次飑线过程的形成发展起了重要作用。一方面通过潜热通量输送增大边界层湿度;另一方面通过感热通量输送改变了边界层的层结结构,使得低层辐合(或抬升)比较容易释放不稳定。在强对流天气的预报中地表条件不可忽略。