Air flow over and through a forest edge: A steady-state numerical simulation

A numerical model was developed to simulate neutrally stratified air flow over and through a forest edge. The spatially averaged equations for turbulent flow in vegetation canopies are derived as the governing equations. A first-order closure scheme with the capability of accounting for the bulk momentum transport process in vegetation canopies is employed. The averaged equations are solved numerically by a fractional time-step method and successive relaxation. The asymptotic solution in time is regarded as the steady-state solution. Comparisons of model output to the field measurements of Raynor (1971) indicate that the model provides a realistic mean flow.Momentum balance computations show that the pressure gradient induced by the wind blowing against the forest edge is significant and has the same order of magnitude as the drag force in the edge region. The edge effect involves the generation of drag forces, the appearance of a large pressure gradient, the upward deflection of mean flow and the transport of momentum into the edge of the canopy.     

甘肃中川地区雷暴地闪特征的初步分析

本文分析了1987年6月28日甘肃中川地区一次雷暴过程中地面4个测站的电场资料,并定量估算了4个地闪的8个回击所中和的电荷量及其高度。负地闪回击所中和的电荷离地高度为4km左右(对应的环境温度为-11.2℃),而正地闪回击所中和的电荷离地高度为2km左右(环境温度+4.9℃)。负地闪大多发生在云闪后的电场恢复阶段,且在开始时往往具有持续几百毫秒的云内放电过程。其激发过程可能是首先在云底部正电荷区产生向上传输的持续正流光,导致云中部负电荷向下移动,并引发向下传输的负先导,最终导致地闪回击的发生。     

Analysis of the surface temperature and wind forecast errors of the NCAR-AirDat operational CONUS 4-km WRF forecasting system

Investigating the characteristics of model-forecast errors using various statistical and object-oriented methods is necessary for providing useful guidance to end-users and model developers as well. To this end, the random and systematic errors (i.e., biases) of the 2-m temperature and 10-m wind predictions of the NCAR-AirDat weather research and forecasting (WRF)-based real-time four-dimensional data assimilation (RTFDDA) and forecasting system are analyzed. This system has been running operationally over a contiguous United States (CONUS) domain at a 4-km grid spacing with four forecast cycles daily from June 2009 to September 2010. In the result an exceptionally useful forecast dataset was generated and used for studying the error properties of the model forecasts, in terms of both a longer time period and a broader coverage of geographic regions than previously studied. Spatiotemporal characteristics of the errors are investigated based on the 24-h forecasts between June 2009 and April 2010, and the 72-h forecasts between May and September 2010. It was found that the biases of both wind and temperature forecasts vary greatly seasonally and diurnally, with dependency on the forecast length, station elevation, geographical location, and meteorological conditions. The temperature showed systematic cold biases during the daytime at all station elevations and warm biases during the nighttime above 1,000 m above sea level (ASL), while below 600 m ASL cold biases occurred during the nighttime. The forecasts of surface wind speed exhibited strong positive biases during the nighttime, while the negative biases were observed in the spring and summer afternoons. The surface wind speed was mostly over-predicted except for the stations located between 1,000 and 2,100 m ASL, for which negative biases were identified for most forecast cycles. The highest wind-speed errors were found over the high terrain and near sea-level stations. The wind-direction errors were relatively large at the high-terrain elevation in the Rocky and Appalachian mountain ranges and the western coastal areas and the error structure exhibited notable diurnal variability.     

Investigation on semi-direct and indirect climate effects of fossil fuel black carbon aerosol over China

A Regional Climate Chemistry Modeling System that employed empirical parameterizations of aerosol-cloud microphysics was applied to investigate the spatial distribution, radiative forcing (RF), and climate effects of black carbon (BC) over China. Results showed high levels of BC in Southwest, Central, and East China, with maximum surface concentrations, column burden, and optical depth (AOD) up to 14 μg?m^?3, 8 mg?m^?2, and 0.11, respectively. Black carbon was found to result in a positive RF at the top of the atmosphere (TOA) due to its direct effect while a negative RF due to its indirect effect. The regional-averaged direct and indirect RF of BC in China was about +0.81 and ?0.95 W?m^?2, respectively, leading to a net RF of ?0.15 W?m^?2 at the TOA. The BC indirect RF was larger than its direct RF in South China. Due to BC absorption of solar radiation, cloudiness was decreased by 1.33 %, further resulting in an increase of solar radiation and subsequently a surface warming over most parts of China, which was opposite to BC’s indirect effect. Further, the net effect of BC might cause a decrease of precipitation of ?7.39 % over China. Investigations also suggested large uncertainties and non-linearity in BC’s indirect effect on regional climate. Results suggested that: (a) changes in cloud cover might be more affected by BC’s direct effect, while changes in surface air temperature and precipitation might be influenced by BC’s indirect effect; and (b) BC second indirect effect might have more influence on cloud cover and water content compared to first indirect effect. This study highlighted a substantial role of BC on regional climate changes.     

Climate change affecting temperature and aridity zones: a case study in Eastern Inner Mongolia, China from 1960–2008

Recent climate change is substantially affecting the spatial pattern of geographical zones, and the temporal and spatial inconsistency of climatic warming and drying patterns contributes to the complexity of the shifting of temperature and aridity zones. Eastern Inner Mongolia, China, located in the interface region of different biomes and ecogeographic zones, has experienced dramatic drying and warming over the past several decades. In this study, the annual accumulated temperature above 10 °C (AAT10) and the aridity index, two key indicators in geographical regionalization, are used to assess warming and drying processes and track the movements of temperature and aridity zones from 1960 to 2008. The results show a significant warming at the regional level from 1960 to 2008 with an AAT10 increase rate of 7.89 °C·d/year (p?<?0.001) in Eastern Inner Mongolia, while the drying trend was not significant during this period. Spatial heterogeneity of warming and drying distributions was also evident. Analysis of warming and drying via piecewise regression revealed two separate, specific trends between the first 31 years (1960–1990) and the subsequent 18 years (1991–2008). Generally, mild warming and very slight wetting occurred prior to 1990, while after 1991 both warming and drying were significant and enhanced. Continuous warming drove a northward shift of temperature zones from the 1960s to 2000s, while aridity zones displayed enhanced temporal and spatial variability. Climate change effects on temperature and aridity zones imply that the patterns of cropping systems, macro-ecosystems, and human land use modes are potentially undergoing migration and modification due to climate change.     

The effects of past climate change on the northern limits of maize planting in Northeast China

Northeast China (NEC) is one of the major agricultural production areas in China and also an obvious region of climate warming. We were motivated to investigate the impacts of climate warming on the northern limits of maize planting. Additionally, we wanted to assess how spatial shifts in the cropping system impact the maize yields in NEC. To understand these impacts, we used the daily average air temperature data in 72 weather stations and regional experiment yield data from Jilin Province. Averaged across NEC, the annual air temperature increased by 0.38 °C per decade. The annual accumulated temperature above 10 °C (AAT10) followed a similar trend, increased 66 °C d per decade from 1961 to 2007, which caused a northward expansion of the northern limits of maize. The warming enabled early-maturing maize hybrids to be sown in the northern areas of Heilongjiang Province where it was not suitable for growing maize before the warming. In the southern areas of Heilongjiang Province and the eastern areas of Jilin Province, the early-maturing maize hybrids could be replaced by the middle-maturing hybrids with a longer growing season. The maize in the northern areas of Liaoning Province was expected to change from middle-maturing to late-maturing hybrids. Changing the hybrids led to increase the maize yield. When the early-maturing hybrids were replaced by middle-maturing hybrids in Jilin Province, the maize yields would increase by 9.8 %. Similarly, maize yields would increase by 7.1 % when the middle-maturing hybrids were replaced by late-maturing hybrids.     

Comparison of three methods for estimating the sea level rise effect on storm surge flooding

Two linear methods, including the simple linear addition and linear addition by expansion, and numerical simulations were employed to estimate storm surges and associated flooding caused by Hurricane Andrew for scenarios of sea level rise (SLR) from 0.15 m to 1.05 m with an interval of 0.15 m. The interaction between storm surge and SLR is almost linear at the open Atlantic Ocean outside Biscayne Bay, with slight reduction in peak storm surge heights as sea level rises. The nonlinear interaction between storm surges and SLR is weak in Biscayne Bay, leading to small differences in peak storm surge heights estimated by three methods. Therefore, it is appropriate to estimate elevated storm surges caused by SLR in these areas by adding the SLR magnitude to storm surge heights. However, the magnitude and extent of inundation at the mainland area by Biscayne Bay estimated by numerical simulations are, respectively, 22–24 % and 16–30 % larger on average than those generated by the linear addition by expansion and the simple linear addition methods, indicating a strong nonlinear interaction between storm surge and SLR. The population and property affected by the storm surge inundation estimated by numerical simulations differ up to 50–140 % from that estimated by two linear addition methods. Therefore, it is inappropriate to estimate the exacerbated magnitude and extent of storm surge flooding and affected population and property caused by SLR by using the linear addition methods. The strong nonlinear interaction between surge flooding and SLR at a specific location occurs at the initial stage of SLR when the water depth under an elevated sea level is less than 0.7 m, while the interaction becomes linear as the depth exceeds 0.7 m.     

沙尘暴数值预报技术取得新进展

（1）中国气象科学研究院自主开发的沙尘暴预报系统CUACE／Dusl与天气预报模式GRAPES实现了在线双向耦合运行。目前中国气象局业务运行的沙尘暴数值预报系统CUACE／Dust中的气象模式是MM5,大气成分中心利用我院自主开发的中尺度气象模式GRAPEs—Meso替换了MM5,建立了GRAPES_CUACE／Dust沙尘暴数值预报系统。同时把模式分辨率提高到0．5～0．5°,     

A diagnostic study of the southern hemisphere summer circulation of the CCC general circulation model

Abstract

The medium‐scale wave regime, consisting largely of zonal wavenumbers 5–7, frequently dominates the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hövmöller diagrams, space‐time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium‐scale wave regime.

The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with the observations. After comparison with the observed December‐January‐February 1979 distributions, some quantitative differences remain: the heat transport is too weak aloft and too large near the surface, whereas the momentum transport tends to be too weak. The baroclinic and barotropic conversions show a maximum in the medium‐scale waves. The time evolution of the Richardson number of the mean flow suggests that the medium‐scale wave is due to a baroclinic instability.