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1.
Comparison of the Influence of Interannual Vegetation Variability between Offline and Online Simulations 
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下载免费PDF全文 This study investigates the influence of interannual vegetation variability. Two sets of offline and online simulations were performed using the Community Earth System Model. The interannual Global LAnd Surface Satellite （GLASS） leaf area index （LAI） dataset from 1985 to 2000 and its associated climatological LAI were used to replace the default climatological LAI data in version 4 of the Community Land Model （CLM4）. The re- sults showed that on a global scale, canopy transpiration and evaporation, as well as total evapotranspiration in offline simulations were significantly positively corre- lated with LAI, whereas ground evaporation and ground temperature showed significant negative correlation with LAI. However, the correlations in online simulations were reduced markedly because of interactive feedbacks between albedo, changed climatic factors and atmospheric variability. In the offline simulations, the fluctuations of differences in interannual variability of evapotranspiration and ground temperature focused on vegetation growing regions and the magnitudes were smaller. Those in online simulations spread over more regions and the magnitudes were larger. These results highlight the influence of interannual vegetation variability, particularly in online simulations, an effect that deserves consideration and attention when investigating the uncertainty of climate change. 相似文献
2.
Accurately representing complex land-surface processes balancing complexity and realism remains one challenge that the weather
modelling community is facing nowadays. In this study, a photosynthesis-based Gas-exchange Evapotranspiration Model (GEM)
is integrated into the Noah land-surface model replacing the traditional Jarvis scheme for estimating the canopy resistance
and transpiration. Using 18-month simulations from the High Resolution Land Data Assimilation System (HRLDAS), the impact
of the photosynthesis-based approach on the simulated canopy resistance, surface heat fluxes, soil moisture, and soil temperature
over different vegetation types is evaluated using data from the Atmospheric Radiation Measurement (ARM) site, Oklahoma Mesonet,
2002 International H2O Project (IHOP_2002), and three Ameriflux sites. Incorporation of GEM into Noah improves the surface energy fluxes as well
as the associated diurnal cycle of soil moisture and soil temperature during both wet and dry periods. An analysis of midday,
average canopy resistance shows similar day-to-day trends in the model fields as seen in observed patterns. Bias and standard
deviation analyses for soil temperature and surface fluxes show that GEM responds somewhat better than the Jarvis scheme,
mainly because the Jarvis approach relies on a parametrised minimum canopy resistance and meteorological variables such as
air temperature and incident radiation. The analyses suggest that adding a photosynthesis-based transpiration scheme such
as GEM improves the ability of the land-data assimilation system to simulate evaporation and transpiration under a range of
soil and vegetation conditions. 相似文献
3.
The Impacts of the Interannual Variability of Vegetation on the Interannual Variability of Global Evapotranspiration: A Modeling Study 下载免费PDF全文
下载免费PDF全文 The impact of the interannual variability (IAV) of vegetation on the IAV of evapotranspiration is investigated with the Community Land Model (CLM3.0) and modified Dynamic Global Vegetation Model (DGVM). Two sets of 50-year off-line simulations are used in this study. The simulations begin with the same initial surface-water and heat states and are driven by the same atmospheric forcing data. The vegetation exhibits interannual variability in one simulation but not in the other simulation. However, the climatological means for the vegetation are the same. The IAV of the 50-year annual total evapotranspiration and its three partitions (ground evaporation, canopy evaporation, and transpiration) are analyzed. The global distribution of the evapotranspiration IAV and the statistics of evapotranspiration and its components in different ecosystems show that the IAV of ground evaporation is generally large in areas dominated by grass and deciduous trees, whereas the IAV of canopy evaporation and transpiration is large in areas dominated by bare soil and shrubs. For ground evaporation, canopy evaporation, and transpiration, the changes in IAV are larger than the mean state over most grasslands and shrublands. The study of two sites with the same IAV in the leaf area index (LAI) shows that the component with the smaller contribution to the total evapotranspiration is more sensitive to the IAV of vegetation. The IAV of the three components of evapotranspiration increases with the IAV of the fractional coverage (FC) and the LAI. The ground evaporation IAV shows the greatest increase, whereas the canopy evaporation shows the smallest increase. 相似文献
4.
Chuan Zhang Haofang Yan Haibin Shi Hideki Sugimoto 《Meteorology and Atmospheric Physics》2013,121(3-4):207-214
A field experiment was conducted in a maize field in 2006 in an arid area of the Yellow River Basin in China. The daytime evapotranspiration (ETc) and soil evaporation beneath the maize canopy (E g) were measured by Bowen ratio energy balance method and micro-lysimeters, respectively. The results showed that the total ETc during maize growth season was 696 mm, and the maximum values occurred at about 90–140 days after sowing. The crop coefficient (K c), which was calculated from the ratio of ETc to reference evapotranspiration (ET0), was quite different from the values reported by other researchers in similar climate areas, with average values of 0.34, 0.47, 1.0 and 0.9 for initial, development, mid-season and late-season stages, respectively. High correlations between leaf area index (LAI) and average K c for every 4 days were obtained. The total E g was 201.4 mm with average values ranged from 0.92 to 2.05 for four growth stages of maize; and accounted for around 28.9 % of ETc. The ratio E g/ETc showed high negative relationship with LAI. These results were very important in precise management of irrigation for maize in Yellow River Basin areas. 相似文献
5.
The sensitivity of evaporation to a prescribed vegetation annual cycle is examined globally in the Met Office Hadley Centre Unified Model (HadAM3) which incorporates the Met Office Surface Exchange Scheme (MOSES2) as the land surface scheme. A vegetation annual cycle for each plant functional type in each grid box is derived based on satellite estimates of Leaf Area Index (LAI) obtained from the nine-year International Satellite Land Surface Climatology Project II dataset. The prescribed model vegetation seasonality consists of annual cycles of a number of structural vegetation characteristics including LAI as well as canopy height, surface roughness, canopy water capacity, and canopy heat capacity, which themselves are based on empirical relationships with LAI. An annual cycle of surface albedo, which in the model is a function of soil albedo, surface soil moisture, and LAI, is also modelled and agrees reasonably with observed estimates of the surface albedo annual cycle. Two 25-year numerical experiments are completed and compared: the first with vegetation characteristics held at annual mean values, the second with prescribed realistic seasonally varying vegetation. Initial analysis uncovered an unrealistically weak relationship between evaporation and vegetation state that is primarily due to the insensitivity of evapotranspiration to LAI. This weak relationship is strengthened by the adjustment of two MOSES2 parameters that together improve the models LAI-surface conductance relationship by comparison with observed and theoretical estimates. The extinction coefficient for photosynthetically active radiation, k
par
, is adjusted downwards from 0.5 to 0.3, thereby enhancing the LAI-canopy conductance relationship. A canopy shading extinction coefficient, k
sh
, that controls what fraction of the soil surface beneath a canopy is directly exposed to the overlying atmosphere is increased from 0.5 to 1.0, which effectively reduces soil evaporation under a dense canopy. When the experiments are repeated with the adjusted parameters, the relationship between evaporation and vegetation state is strengthened and is more spatially consistent. At nearly all locations, the annual cycle of evaporation is enhanced in the seasonally varying vegetation experiment. Evaporation is stronger during the peak of the growing season and, in the tropics, reduced transpiration during the dry season when LAI is small leads to significantly lower total evaporation. 相似文献
6.
The eddy covariance technique was used to measure the CO2 flux over four differently grazed Leymus chinensis steppe ecosystems (ungrazed since 1979 (UG79), winter grazed (WG), continuously grazed (CG), and heavily grazed (HG) sites) during four growing seasons (May to September) from 2005 to 2008, to investigate the response of the net ecosystem exchange (NEE) over grassland ecosystems to meteorological factors and grazing intensity. At UG79, the optimal air temperature for the half-hourly NEE occurred between 17 and 20 °C, which was relatively low for semi-arid grasslands. The saturated NEE (NEEsat) and temperature sensitivity coefficient (Q 10) of ecosystem respiration (RE) exhibited clear seasonal and interannual variations, which increased with canopy development and the soil water content (SWC, at 5 cm). The total NEE values for the growing seasons from 2005 to 2008 were ?32.0, ?41.5, ?66.1, and ?89.8 g C m?2, respectively. Both the amounts and distribution of precipitation during the growing season affected the NEE. The effects of grazing on the CO2 flux increased with the grazing intensity. During the peak growth stage, heavy grazing and winter grazing decreased NEEsat and gross primary production (45 % for HG and 34 % for WG) due to leaf area removal. Both RE and Q 10 were clearly reduced by heavy grazing. Heavy grazing changed the ecosystem from a CO2 sink into a CO2 source, and winter grazing reduced the total CO2 uptake by 79 %. In the early growing season, there was no difference in the NEE between CG and UG79. In addition to the grazing intensity, the effects of grazing on the CO2 flux also varied with the vegetation growth stages and SWC. 相似文献
7.
Results are first presented from an analysis of a global coupled climate model regarding changes in future mean and variability of south Asian monsoon precipitation due to increased atmospheric CO2 for doubled (2 × CO2) and quadrupled (4 × CO2) present-day amounts. Results from the coupled model show that, in agreement with previous studies, mean area-averaged south Asian monsoon precipitation increases with greater CO2 concentrations, as does the interannual variability. Mechanisms producing these changes are then examined in a series of AMIP2-style sensitivity experiments using the atmospheric model (taken from the coupled model) run with specified SSTs. Three sets of ensemble experiments are run with SST anomalies superimposed on the AMIP2 SSTs from 1979–97: (1) anomalously warm Indian Ocean SSTs, (2) anomalously warm Pacific Ocean SSTs, and (3) anomalously warm Indian and Pacific Ocean SSTs. Results from these experiments show that the greater mean monsoon precipitation is due to increased moisture source from the warmer Indian Ocean. Increased south Asian monsoon interannual variability is primarily due to warmer Pacific Ocean SSTs with enhanced evaporation variability, with the warmer Indian Ocean SSTs a contributing but secondary factor. That is, for a given interannual tropical Pacific SST fluctuation with warmer mean SSTs in the future climate, there is enhanced evaporation and precipitation variability that is communicated via the Walker Circulation in the atmosphere to the south Asian monsoon to increase interannual precipitation variability there. This enhanced monsoon variability occurs even with no change in interannual SST variability in the tropical Pacific. 相似文献
8.
全球植被年际尺度动态过程的数值模拟及其评估 总被引：1，自引：1，他引：0
动态植被模型（Interactive Canopy Model,ICM）中考虑了生态系统碳氮循环过程,能够描述较短时间尺度上的植被与大气之间相互作用过程。利用21 a的GIMMS卫星观测LAI资料,与ICM模拟结果进行年际变率对比分析,评估模型对植被年际变化的模拟性能。结果表明,ICM能在一定程度上反映植被年际变率的空间分布特征,但模拟的热带部分地区植被的年际变率偏小,除此之外大部分地区模拟的年际变率偏大;模拟能够反映出全球植被年际变化的主要空间分布型,同时低纬度地区植被的时间演变特征要好于高纬度地区;ICM对寒带灌丛、北非稀树大草原、落叶针叶林的年际变化模拟较好,但对中国东部农作物的模拟表现出了明显的周期振荡现象,与实际情况差距较大。 相似文献
9.
M. Mohammadi B. Ghahraman K. Davary A. M. Liaghat M. Bannayan 《Meteorology and Atmospheric Physics》2012,117(1-2):73-83
The FAO Penman–Monteith (F-PM) method is a frequently applied approach for calculating the daily reference evapotranspiration (ET0). This method requires long records of meteorological data, which makes it quite hard to employ in locations with no or limited available data. Evaporation pans are widely used to estimate the reference ET0, but this method requires reliable estimates of the pan coefficient (K p). The objectives of this study were to determine the proper values of monthly and annual K p, as well as the best method among those available for the estimation of K p values in the study area. Measured weather data from 1992 to 2006 were obtained from 18 stations in the North and Northwest of Iran. Daily ET0 calculated using methods by Bernardo et al. and Pereira et al. were compared with those calculated by the F-PM method. The employed methods at all stations, except those located in the north of the study area with high relative humidity, overestimated the ET0 compared to the F-PM method. The constant parameters of these methods were optimized by a trial and error scheme to minimize the root mean square error. The results indicated that modified K p coefficients from Bernardo et al.’s method ranged between 0.41 and 0.87 and the optimal coefficient of Pereira et al.’s method ranged between 0.49 and 0.95. Modified monthly K p from Bernardo et al.’s method ranged between 0.3 and 1.07 and those from Pereira et al.’s method ranged between 0.4 and 1.18. Modified K p of the methods by Bernardo et al. and Pereira et al. showed the higher estimation accuracy of daily ET0 values. In general, the performance of the modified K p of Bernardo et al.’s method was higher than Pereira et al.’s method for all stations. Thus, in the study region and under the same climatic conditions [in areas with only pan evaporation (E p) records], the use of climatic monthly modified K p to calculate ET0 based on class A E p is recommended. 相似文献
10.
利用2007—2008年辽宁锦州玉米农田生态系统野外观测站资料,基于CoLM模型对玉米根分布在陆-气水热通量模拟中的影响进行研究,结果表明:模型模拟性能随年际气象条件的差异而不同,与2007年相比,2008年生长季内降水偏多,感热和潜热模拟精度明显提高;决定根分布形态的50%和95%根总量土层深度(d50和d95)两个参数中,d50比d95敏感;根分布对土壤湿度的影响在极端干旱条件下很小,在一定土壤湿度范围内随土壤湿度及土层深度的增大而减小;在水汽通量各分量中,植物蒸腾受根分布影响最大,其次是土壤蒸发,而叶片蒸发不受影响;根分布对潜热和感热模拟的影响随土壤湿度增大而减小。 相似文献
11.
Chinese temperate grasslands play an important role in the terrestrial carbon cycle. Based on the parameterization and validation of Terrestrial Ecosystem Model (TEM, Version 5.0), we analyzed the carbon budgets of Chinese temperate grasslands and their responses to historical atmospheric CO2 concentration and climate variability during 1951–2007. The results indicated that Chinese temperate grassland acted as a slight carbon sink with annual mean value of 7.3 T?g C, ranging from -80.5 to 79.6 T?g C yr-1. Our sensitivity experiments further revealed that precipitation variability was the primary factor for decreasing carbon storage. CO2 fertilization may increase the carbon storage (1.4 %) but cannot offset the proportion caused by climate variability (-15.3 %). Impacts of CO2 concentration, temperature and precipitation variability on Chinese temperate grassland cannot be simply explained by the sum of the individual effects. Interactions among them increased total carbon storage of 56.6 T?g C which 14.2 T?g C was stored in vegetation and 42.4 T?g C was stored in soil. Besides, different grassland types had different responses to climate change and CO2 concentration. NPP and RH of the desert and forest steppes were more sensitive to precipitation variability than temperature variability while the typical steppe responded to temperature variability more sensitively than the desert and forest steppes. 相似文献
12.
The deposition velocity (V
d) of nitric acid vapor over a fully leafed deciduous forest was estimated using flux/gradient theory. HNO3 deposition velocities ranged from 2.2 to 6.0cm/s with a mean V
don the order of 4.0cms-1. Estimates of V
dfrom a detailed canopy turbulence model gave deposition velocities of similar magnitude. The model was used to investigate the sensitivity of V
dto the leaf boundary-layer resistance and leaf area index (LAI). Although modeled deposition velocities were found to be sensitive to the parameterization of the leaf boundary-layer resistance, they were less sensitive to the LAI. Modeled V
d's were found to peak at LAI = 7. 相似文献
13.
In the spring of 2021, southwestern China (SWC) experienced extreme drought, accompanied by the highest seasonal-mean temperature record since 1961. This drought event occurred in the decaying phase of a La Ni?a event with negative geopotential height anomalies over the Philippine Sea, which is distinct from the historical perspective. Historically, spring drought over SWC is often linked to El Ni?o and strong western North Pacific subtropical high. Here, we show that the extreme drought in the spring of 2021 may be mainly driven by the atmospheric internal variability and amplified by the warming trend. Specifically, the evaporation increase due to the high temperature accounts for about 30% of drought severity, with the contributions of its linear trend portion being nearly 20% and the interannual variability portion being about 10%. Since the sea surface temperature forcing from the tropical central and eastern Pacific played a minor role in the occurrence of drought, it is a challenge for a climate model to capture the 2021 SWC drought beyond one-month lead times. 相似文献
14.
我国西南地区秋季降水年际变化的空间差异及其成因 总被引：3，自引：0，他引：3
使用1980～2010年全国站点降水资料、ERA-Interim再分析环流资料、哈德莱海表温度资料,运用聚类分析和旋转经验正交函数分解,对西南地区的秋季降水按照其年际变化规律进行分区,进而分析影响各区域降水变化的物理过程和机理。结果表明:西南地区被分为东、西两个区域。西南东、西区域秋季降水的年际变化、显著周期、旱涝异常年份、相关的环流系统都有明显差异。西南东部秋季降水主要与热带海温异常有关,受低纬度环流影响。当赤道东太平洋为暖海温异常,热带印度洋为西正东负的偶极子型海温异常时,分别激发出西北太平洋反气旋和孟加拉反气旋,共同向西南东部输送水汽,造成西南东部降水偏多。西南西部降水在秋季三个月份与不同的环流形势对应:9月降水由中南半岛反气旋输送的暖湿气流决定;10月降水受高原以东反气旋环流和孟加拉湾低槽共同影响;11月降水主要受中高纬环流异常的影响,与斯堪的纳维亚遥相关存在显著负相关。 相似文献
15.
Freshwater flux (FWF) directly affects sea surface salinity (SSS) and hence modulates sea surface temperature (SST) in the tropical Pacific. This paper quantifies a positive correlation between FWF and SST using observations and simulations of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) to analyze the interannual variability in the tropical Pacific. Comparisons among the displacements of FWF, SSS and SST interannual variabilities illustrate that a large FWF variability is located in the west-central equatorial Pacific, covarying with a large SSS variability, whereas a large SST variability is located in the eastern equatorial Pacific. Most CMIP5 models can reproduce the fact that FWF leads to positive feedback to SST through an SSS anomaly as observed. However, the difference in each model's performance results from different simulation capabilities of the CMIP5 models in the magnitudes and positions of the interannual variabilities, including the mixed layer depth and the buoyancy flux in the equatorial Pacific. SSS anomalies simulated from the CMIP5 multi-model are sensitive to FWF interannual anomalies, which can lead to differences in feedback to interannual SST variabilities. The relationships among the FWF, SSS and SST interannual variabilities can be derived using linear quantitative measures from observations and the CMIP5 multi-model simulations. A 1 mm d-1 FWF anomaly corresponds to an SSS anomaly of nearly 0.12 psu in the western tropical Pacific and a 0.11°C SST anomaly in the eastern tropical Pacific. 相似文献
16.
Evapotranspiration is a major component of both the energy and water balances of wetland tundra environments during the thaw season. Reliable estimates of evapotranspiration are required in the analysis of climatological and hydrological processes occurring within a wetland and in interfacing the surface climate with atmospheric processes. Where direct measurements are unavailable, models designed to accurately predict evapotranspiration for a particular wetland are used.This paper evaluates the performance, sensitivity and limitations of three physically-based, one-dimensional models in the simulation of evaporation from a wetland sedge tundra in the Hudson Bay Lowland near Churchill, Manitoba. The surface of the study site consists of near-saturated peat soil with a sparse sedge canopy and a constantly varying coverage of standing water. Measured evaporation used the Bowen ratio energy balance approach, to which the model results were compared. The comparisons were conducted with hourly and daily simulations.The three models are the Penman-Monteith model, the Shuttleworth-Wallace sparse canopy model and a modified Penman-Monteith model which is weighted for surface area of the evaporation sources.Results from the study suggest that the weighted Penman-Monteith model has the highest potential for use as a predictive tool. In all three cases, the importance of accurately measuring the surface area of each evaporation source is recognized. The difficulty in determining a representative surface resistance for each source and the associated problems in modelling without it are discussed.
List of Symbols
Models BREB Bowen ratio energy balance - P-M Penman-Monteith combination - S-W Shuttleworth-Wallace combination - W-P-M Weighted Penman-Monteith combination Other AE Available energy-all surfaces - AE c Available energy-canopy (S-W, W-P-M) - AE s Available energy-bare soil (S-W, W-P-M) - AE w Available energy-open water (W-P-M) - C p Specific heat of air - D Vapor pressure deficit - DAI Dead area index - FAI Foliage area index - LAI Leaf area index - Q * Net radiation - Q e Latent heat flux-total - Q ec Latent heat flux-canopy (S-W, W-P-M) - Q es Latent heat flux-bare soil (S-W, W-P-M) - Q ew Latent heat flux-open water (W-P-M) - Q g ground heat flux - Q h Sensible heat flux - S Proportion of area in bare soil - W Proportion of surface in open water - r a Aerodynamic resistance (P-M, W-P-M) - r c Canopy resistance - r s Generalized optimized surface resistance - r st Stomatal resistance - r c a Bulk boundary layer resistance (S-W) - r s a Aerodynamic resistance below mean canopy level (S-W) - r s s Soil surface resistance (S-W, W-P-M) Greek Bowen ratio - Psychrometer constant - Air density - Slope of saturation vapour pressure vs temperature curve 相似文献17.
Predictions of future climate change rely on models of how both environmental conditions and disturbance impact carbon cycling at various temporal and spatial scales. Few multi-year studies, however, have examined how carbon efflux is affected by the interaction of disturbance and interannual climate variation. We measured daytime soil respiration (R s) over five summers (June–September) in a Sierra Nevada mixed-conifer forest on undisturbed plots and plots manipulated with thinning, burning and their combination. We compared mean summer R s by year with seasonal precipitation. On undisturbed plots we found that winter precipitation (PPTw) explained between 77–96% of interannual variability in summer R s. In contrast, spring and summer precipitation had no significant effect on summer R s. PPTw is an important influence on summer R s in the Sierra Nevada because over 80% of annual precipitation falls as snow between October and April, thus greatly influencing the soil water conditions during the following growing season. Thinning and burning disrupted the relationship between PPTw and Rs, possibly because of significant increases in soil moisture and temperature as tree density and canopy cover decreased. Our findings suggest that R s in some moisture-limited ecosystems may be significantly influenced by annual snowpack and that management practices which reduce tree densities and soil moisture stress may offset, at least temporarily, the effect of predicted decreases in Sierran snowpack on R s. 相似文献
18.
C. J. C. Reason M. Rouault J.-L. Melice D. Jagadheesha 《Meteorology and Atmospheric Physics》2002,80(1-4):19-29
Summary The Southwestern Cape (SWC) region of South Africa is characterized by winter rainfall mainly via cold fronts and by substantial
interannual variability. Evidence is presented that interannual variability in SWC winter rainfall is related to sea-surface
temperature (SST) and sea-ice anomalies in the central South Atlantic and adjoining Southern Ocean and to large scale ocean–atmosphere
interaction in this region. During wet winters, the jet is strengthened just upstream of the SWC and significant cyclonic
anomalies extend from the SW Atlantic over the region. SST tends to be anomalously warm (cool) in the SW Atlantic and SE Atlantic
(central South Atlantic) and sea-ice extent increased in the central South Atlantic sector of the Southern Ocean. These patterns
favor increased cyclogenesis upstream, a more northward track of midlatitude depressions, local intensification near the SWC
and enhanced rainfall. Roughly the reverse patterns occur during dry winters. Some preliminary results from atmospheric GCM
experiments are presented which help support these findings.
Received November 9, 2001 Revised December 28, 2001 相似文献
19.
Roni Avissar 《Boundary-Layer Meteorology》1993,64(1-2):127-148
Plant stomata play a key role in the redistribution of energy received on vegetated land into sensible and latent heat. As a result, they have a considerable impact on the atmospheric planetary boundary layer, the hydrologic cycle, the climate, and the weather. Current parameterizations of the stomatal mechanism in state-of-the-art atmospheric models are based on empirical relations that are established at the leaf scale between stomatal conductance and environmental conditions. In order to evaluate these parameterizations, an experiment was carried out on a potato field in New Jersey during the summer of 1989. Stomatal conductances were measured within a small homogeneous area in the middle of the potato field and under a relatively broad range of atmospheric conditions. A large variability of stomatal conductances was observed. This variability, which was associated with the variability of micro-environmental and physiological conditions that is found even in a homogeneous canopy, cannot be simulated explicitly on the scale of a single agricultural field and,a fortiori, on the scale of atmospheric models. Furthermore, this variability could not be related to the environmental conditions measured at a height of 2 m above the plant canopy simultaneously with the conductances, reinforcing the concept of scale decoupling suggested by Jarvis and McNaughton (1986) and McNaughton and Jarvis (1991). Thus, for atmospheric modeling purposes, a parameterization of stomatal conductance at the canopy scale using external environmental forcing conditions seems more appropriate than a parameterization based on leaf-scale stomatal conductance, as currently adopted in state-of-the-art atmospheric models. The measured variability was characterized by a lognormal probability density function (pdf) that remained relatively stable during the entire measuring period. These observations support conclusions by McNaughton and Jarvis (1991) that, unlike current parameterizations, a parameterization of plant stomata applied on a field (or larger) scale, might not require inclusion of the complex relations found at the leaf scale between stomata and their microenvironment. 相似文献
20.
A. P. Dimri 《Climate Dynamics》2014,42(7-8):1793-1805
During the winter season (Dec., Jan., and Feb.; DJF) the western Himalaya (WH) receives one-third of its annual precipitation due to Indian winter monsoon (IWM). The IWM is characterized by eastward-moving synoptic weather systems called western disturbances. Seasonal interannual precipitation variability is positively correlated with monthly interannual variabilities. However, it was found that the monthly interannual variabilities differ. The interannual variability for Jan. is negatively correlated with that for Dec. and Feb. Because the entire seasonal interannual variability is in phase with the El Niño Southern Oscillation, it is interesting to investigate such contrasting behavior. Composite analysis based on extreme wet and dry seasons indicates that Dec. and Feb. precipitation variabilities have a high positive (low negative) correlation with eastern (western) equatorial Pacific warming (cooling), whereas Jan. precipitation variability exhibits negligible correlations. Seasonal mid/upper tropospheric cooling over the Himalayas enhances anomalous cyclonic circulation, which along with suppressed convection over the western equatorial Pacific, shifts the 200-hPa subtropical westerly jet southward over the Himalayas. Due to the upper tropospheric anomalous cyclonic circulation, mass transfer favors anticyclone formation at the mid/lower troposphere, which is enhanced in Jan. due to a warmer mid troposphere and hence decreases precipitation compared with Dec. and Feb. Additionally, a weakening of meridional moisture flux transport from the equatorial Indian Ocean to WH is observed in Jan. Further analysis reveals that mid-tropospheric and surface temperatures over WH also play dominant roles, acting as local forcing where the preceding month’s surface temperature controls the succeeding month’s precipitation. 相似文献