澳大利亚植被覆盖对亚澳季风影响的数值模拟(Ⅱ): 对东亚夏季风环流的影响

在文献[1]的基础上,对澳大利亚大陆植被覆盖变化对北半球夏季越赤道气流和东亚季风环流季节变化的影响进行了研究.结果表明,植被覆盖变化对东亚季风建立前后南风越赤道气流建立时间、强弱和南半球主要环流系统都有显著的影响.绿化导致了索马里越赤道气流的建立提前,增强了不同时期南风越赤道气流的强度,但对90°E以东来自澳大利亚高压的几支越赤道气流影响不大.同时,绿化促使南半球澳大利亚高压和马斯克林高压提前建立,西太平洋副热带高压北进提前,且强度减弱,导致西南气流更容易深入东亚内陆和西太平洋.这些影响促使盛夏期西南亚季风的影响区域和强度都有所扩展,对东南季风则影响不大.沙漠化则使索马里气流略微减弱,西太平洋副热带高压在春夏季节则一直偏强,至7月中旬,才有明显东撤,阻碍了越赤道气流的北上,西南季风在此影响下强度和影响范围均有所缩减.     

An assessment of the potential impact of a downward shift of tropospheric water vapor on climate sensitivity

This work uses an energy balance climate model (EBCM) with explicit infrared radiative transfer, parametrized tropospheric temperature and humidity profiles, and separate stratosphere, troposphere, and surface energy balances, to investigate claims that a downward redistribution of tropospheric water vapor in response to surface warming could serve as a strong negative feedback on climatic change. A series of sensitivity tests is carried out using: (1) a variety of relationships between total precipitable water in the troposphere and temperature; (2) feedbacks between surface temperature and the vertical distribution of tropospheric water vapor at low latitudes; and (3) feedback between surface temperature or meridional temperature gradient and lapse rate. Fixed relative humidity (RH) enhances the global mean surface temperature response to a CO₂ doubling by only 50% compared to fixed absolute humidity, giving a response of 1.8 K. When water vapor is assumed to be redistributed downward between 30°S–30°N such that a 1 K surface warming reduces total precipitable water above 600 hPa by 10%, the global mean surface air temperature response is reduced to 1.2 K. Assuming a stronger downward redistribution in relation to surface temperature change has a rapidly diminishing marginal effect on global mean and tropical surface temperature response, while slightly increasing the warming at high latitudes due to the parametrized dependence of middle-to-high latitude lapse rate on the meridional temperature gradient. A modest downward water vapor redistribution, such that absolute humidity in the upper troposphere at subtropical latitudes is constant as total precipitable water increases, can reduce the tropical temperature sensitivity to less than 1 K, while increasing the equator-to-pole amplification of the surface air temperature response from a factor of about three to a factor of four. However, it is concluded that whatever changes in future GCM response might occur as a result of new parametrizations of subgrid-scale processes, they are exceedingly unlikely to produce a climate sensitivity to a CO₂ doubling of less than 1 K even if there is a strong downward shift in the water vapor distribution as climate warms. Received: 23 February 1998 / Accepted: 1 November 1999     

A note on the occurrence of wind direction differences below 1 km during episodes of extreme vertical Windshear for Berlin,Germany and Centreville,Alabama, U.S.A.

Summary Windshear is critical to aeronautical activities such as aircraft takeoffs and landings and the ascending and descending phase phases of missile launch. The probability of extreme vertical windshear below 1 km at Centreville, Alabama (U.S.A.) and Berlin, Germany has been studied. Windshear (total vector difference) was derived from radiosonde ascents using both windspeed and wind direction differences between two altitudes. The wind direction differences are used to compute the angular shear magnitude.The wind direction differences between the surface and specified altitude as well as the contribution of the angular shear magnitude to the total vector difference during episodes of extreme vertical windshear were quantified. For example, wind direction changes of 60 degrees or more for cases of extreme windshear (windshear > 15m/s per 900m) in the layer surface to 900m occurred with a relative frequency of only 8% at Berlin in contrast to 34% at Centreville. The ratio of the angular shear magnitude to the total vector difference squared (times 100%) exceeded 40% five times more often at Centreville as compared with Berlin for this layer. Analysis using the Kolmogorov-Smirnov test confirmed that these differences (between the two locations) in wind direction change during episodes of extreme windshear are statistically significant. Backing vs. veering winds in the boundary-layer and the 500 mb wind directions are discussed in order to relate the occurrence of extreme vertical windshear to characteristics of two contrasting geographic locations, one in the transition region between sub-tropics and mid-latitudes (Centreville), and the other well-entrenched in the westerlies (Berlin).There were considerable day-night differences in the occurrence of extreme shears at Centreville. For example, windshear > 10m/s per 600 m in the layer surface to 600 m were more than three times as frequent at 1200 UTC (morning) than at 0000 UTC (evening). This is due to larger wind direction differences in the boundary-layer in addition to the nocturnal rise in windspeed at 300 m (low-level jet).It should also be noted that extreme windshear near the surface was not always associated with strong surface winds. Vertical windshear below 1 km was found to increase with increasing surface windspeed up until 98% probability. Above 98% probability this relationship breaks down, as the second largest maximum windshear in the layer surface to 900 m was observed for a surface wind of 3 m/s at Berlin.The seasonal variation of vertical windshear below 1 km was also illustrated, indicating winter to be the season of maximum shears, summer the season of minimum shears. An exception was that above 99% probability the shear in the spring usually exceeded the winter shear.With 14 Figures     

Covariations in oceanic dimethyl sulfide,its oxidation products and rain acidity at Amsterdam Island in the Southern Indian Ocean

Simultaneous measurements of rain acidity and dimethyl sulfide (DMS) at the ocean surface and in the atmosphere were performed at Amsterdam Island over a 4 year period. During the last 2 years, measurements of sulfur dioxide (SO₂) in the atmosphere and of methane sulfonic acid (MSA) and non-sea-salt-sulfate (nss-SO₄ ^2-) in rainwater were also performed. Covariations are observed between the oceanic and atmospheric DMS concentrations, atmospheric SO₂ concentrations, wet deposition of MSA, nss-SO₄ ^2-, and rain acidity. A comparable summer to winter ratio of DMS and SO₂ in the atmosphere and MSA in precipitation were also observed. From the chemical composition of precipitation we estimate that DMS oxidation products contribute approximately 40% of the rain acidity. If we consider the acidity in excess, then DMS oxidation products contribute about 55%.     

一堆气候模式的灵敏度分析

在本文中我们设计了一个只有垂直方向的一维气候模式。该模式可以用来研究大气中CO_2气体浓度从330增加到660ppm时地面温度的变化。同时我们将分析几个反馈机制对气候系统灵敏度的贡献,其中包括大气湿度反馈、云反馈和地面反射率反馈等机制。     

Terrain-influenced local wind forecasting for the Sasebo typhoon haven: Improved empirical techniques

Although the Sasebo, Japan harbor is usually a “typhoon haven” from tropical cyclone (TC) winds due to terrain-blocking effects, in rare cases damaging winds occur that may be attributed to terrain channeling. An empirical parametric model technique is developed and tested that includes consideration of the TC wind structure, land frictional effects, and terrain influences affecting the maximum wind speeds in the harbor when TCs pass within 200 nautical miles of Sasebo. The terrain influence is represented by two sets of wind direction-dependent acceleration factors. The first set, which is directly from the ratio of the local wind to the adjusted parametric wind for TCs passages during 2003–2010, provides mean values that represent the terrain blocking and channeling effects, but the variability with wind direction may be suspect. The second set derived from a large sample of reanalysis winds not limited to TCs has better variability properties, but is not easily related to just the TC passages. A new nomogram modified to include TC wind structure has higher estimates of Sasebo sustained winds for some TC tracks that may be related to terrain influences, but is limited due to the number of TC structure estimates in the developmental sample. These empirical models have the advantage of ease and low cost for future use in also estimating the combined uncertainty in the local winds in Sasebo harbor due to TCs.     

Storm waves in Canadian waters: A major marine hazard

Abstract

In this paper, an overview of storm waves associated with intense weather systems affecting the east and west coasts of Canada is presented. The paper presents the wave climatology of the east and west coasts in terms of the 100‐year significant and maximum wave heights and further analyses the directional distribution of wave heights at selected locations in the Canadian east and west coasts offshore. The paper also analyses wave hazards associated with storm waves in the Beaufort Sea as well as the Canadian Great Lakes region. A section on ocean wave modelling provides a brief history of the development of ocean surface wave models and its present status. The paper further considers the impact of climate change scenarios on wave hazards and finally examines mitigation measures in terms of wave products available from operational wave models and related wave climatology.     

Climate change and soil freezing dynamics: historical trends and projected changes

Changes to soil freezing dynamics with climate change can modify ecosystem carbon and nutrient losses. Soil freezing is influenced strongly by both air temperature and insulation by the snowpack, and it has been hypothesized that winter climate warming may lead to increased soil freezing as a result of reduced snowpack thickness. I used weather station data to explore the relationships between winter air temperature, precipitation and soil freezing for 31 sites in Canada, ranging from the temperate zone to the high Arctic. Inter-annual climate variation and associated soil temperature variation over the last 40 years were examined and used to interpolate the effects of projected climate change on soil freezing dynamics within sites using linear regression models. Annual soil freezing days declined with increasing mean winter air temperature despite decreases in snow depth and cover, and reduced precipitation only increased annual soil freezing days in the warmest sites. Annual soil freeze–thaw cycles increased in both warm and dry winters, although the effects of precipitation were strongest in sites that experience low mean winter precipitation. Overall, it was projected that by 2050, changes in winter temperature will have a much stronger effect on annual soil freezing days and freeze–thaw cycles than changes in total precipitation, with sites close to but below freezing experiencing the largest changes in soil freezing days. These results reveal that experimental data relevant to the effects of climate changes on soil freezing dynamics and changes in associated soil physical and biological processes are lacking.     

珠峰和曲宗站2005年4,5月近地辐射能收支初步分析

利用中国科学院又一次珠穆朗玛峰地区科学考察(2005年4,5月)期间获得的辐射资料分析该地区地表辐射能的日变化,以及不同海拔高度、不同下垫面辐射各分量的异同,同时分析了不同海拔高度地表反照率在4月和5月的月变化和日变化。     

Tests of a robust eddy correlation system for sensible heat flux

Summary Sensible heat flux estimates from a simple, one-propeller eddy correlation system (OPEC) were compared with those from a sonic anemometer eddy correlation system (SEC). In accordance with similarity theory, the performance of the OPEC system improved with increasing height of the sensor above the surface. Flux totals from the two systems at sites with adequate fetch were in excellent agreement after frequency response corrections were applied. The propeller system appears suitable for long periods of unattended measurement. The sensible heat flux measurements can be combined with net radiation and soil heat flux measurements to estimate latent heat as a residual in the surface energy balance.With 6 Figures