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1.
AMIP model simulations of the east China (5–50°N; 105–122°E) monsoon system are analyzed to study coherent relationships
between rainfall and wind annual cycle biases. A comparison with observed interannual variability patterns is carried out
to identify the physical processes that explain the biases. The analyses show that poleward displacement of the simulated
east Asian jet stream causes the ascending branch of the jet-induced transverse circulation to move north and, as a consequence,
produces negative (positive) rainfall biases occur in central (northeast) China. The model simulations show decreased southwesterly
flow and ITCZ rainfall over the South China Sea when weaker (versus observations) summer Hadley and Walker circulations are
present. This results from diminished model tropical disturbance activity, and highlights the importance of air-sea interactions.
In addition, during October–January, intensified model low-level easterlies enhance moisture transport and produce positive
local rainfall biases over central and northeast China. Biases in the east China monsoon system are concurrently reflected
in the planetary circulation. Enhanced northeast China rainfall results from increased surface pressure over the North Pacific
and an amplified zonal pressure gradient along the east China coast. This bias pattern is associated with differences in model
representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall
bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as
well as a barotropic wave train that extends from Australia to the Antarctic, where the teleconnection is likely a direct
atmospheric response to tropical convective heating.
Received: 20 June 2000 / Accepted: 17 September 2000 相似文献
2.
Performance assessment of three convective parameterization schemes in WRF for downscaling summer rainfall over South Africa 总被引：2，自引：1，他引：1
Satyaban B. Ratna J. V. Ratnam S. K. Behera C. J. deW. Rautenbach T. Ndarana K. Takahashi T. Yamagata 《Climate Dynamics》2014,42(11-12):2931-2953
Austral summer rainfall over the period 1991/1992 to 2010/2011 was dynamically downscaled by the weather research and forecasting (WRF) model at 9 km resolution for South Africa. Lateral boundary conditions for WRF were provided from the European Centre for medium-range weather (ECMWF) reanalysis (ERA) interim data. The model biases for the rainfall were evaluated over the South Africa as a whole and its nine provinces separately by employing three different convective parameterization schemes, namely the (1) Kain–Fritsch (KF), (2) Betts–Miller–Janjic (BMJ) and (3) Grell–Devenyi ensemble (GDE) schemes. All three schemes have generated positive rainfall biases over South Africa, with the KF scheme producing the largest biases and mean absolute errors. Only the BMJ scheme could reproduce the intensity of rainfall anomalies, and also exhibited the highest correlation with observed interannual summer rainfall variability. In the KF scheme, a significantly high amount of moisture was transported from the tropics into South Africa. The vertical thermodynamic profiles show that the KF scheme has caused low level moisture convergence, due to the highly unstable atmosphere, and hence contributed to the widespread positive biases of rainfall. The negative bias in moisture, along with a stable atmosphere and negative biases of vertical velocity simulated by the GDE scheme resulted in negative rainfall biases, especially over the Limpopo Province. In terms of rain rate, the KF scheme generated the lowest number of low rain rates and the maximum number of moderate to high rain rates associated with more convective unstable environment. KF and GDE schemes overestimated the convective rain and underestimated the stratiform rain. However, the simulated convective and stratiform rain with BMJ scheme is in more agreement with the observations. This study also documents the performance of regional model in downscaling the large scale climate mode such as El Niño Southern Oscillation (ENSO) and subtropical dipole modes. The correlations between the simulated area averaged rainfalls over South Africa and Nino3.4 index were ?0.66, ?0.69 and ?0.49 with KF, BMJ and GDE scheme respectively as compared to the observed correlation of ?0.57. The model could reproduce the observed ENSO-South Africa rainfall relationship and could successfully simulate three wet (dry) years that are associated with La Niña (El Niño) and the BMJ scheme is closest to the observed variability. Also, the model showed good skill in simulating the excess rainfall over South Africa that is associated with positive subtropical Indian Ocean Dipole for the DJF season 2005/2006. 相似文献
3.
Atmospheric Intercomparison Project simulations of the summertime diurnal cycle of precipitation and low-level winds over subtropical China by Intergovernmental Panel on Climate Change Fifth Assessment Report models were evaluated. By analyzing the diurnal variation of convective and stratiform components, results confirmed that major biases in rainfall diurnal cycles over subtropical China are due to convection parameterization and further pointed to the diurnal variation of convective rainfall being closely related to the closure of the convective scheme. All models captured the early-morning peak of total rainfall over the East China Sea, but most models had problems in simulating diurnal rainfall variations over land areas of subtropical China. When total rainfall was divided into stratiform and convective rainfall, all models successfully simulated the diurnal variation of stratiform rainfall with a maximum in the early morning. The models, overestimating noon-time （nocturnal） total rainfall over land, generally simulated too much convective rainfall, which peaked close to noon （midnight）, sharing some similarities in the closures of their deep convection schemes. The better performance of the Meteorological Research Institute atmospherer. ocean coupled global climate model version 3 （MRI-CGCM3） is attributed to the well captured ratio of the two kinds of rainfall, but not diurnal variations of the two components. Therefore, a proper ratio of convective and stratiform rainfall to total rainfall is also important to improve simulated diurnal rainfall variation. 相似文献
4.
Regional coupled modeling is one of the frontiers of regional climate modeling, but intercomparison has not been well coordinated. In this study, a community regional climate model, WRF4, with a resolution of 15 km, was coupled with a high-resolution(0.1°) North Pacific Ocean model(LICOM_np). The performance of the regional coupled model,WRF4_LICOM, was compared to that of another regional coupled model, RegCM4_LICOM, which was a coupling of version 4 of the Regional Climate Model(RegCM4) with LICOM_np. The analysis focused on the 2005 western North Pacific summer monsoon rainfall. The results showed that the regional coupled models with either RegCM4 or WRF4 as their atmospheric model component simulated the broad features over the WNP reasonably well. Quantitative intercomparison of the regional coupled simulations exhibited different biases for different climate variables.RegCM4_LICOM exhibited smaller biases in its simulation of the averaged June–July–August SST and rainfall, while WRF4_LICOM better captured the tropical cyclone(TC) intensity, the percentage contributions of rainfall induced by TCs to the total rainfall, and the diurnal cycle of rainfall and stratiform percentages, especially over land areas. The different behaviors in rainfall simulated by the two models were partly ascribed to the behaviors in the simulated western North Pacific subtropical high(WNPSH). The stronger(weaker) WNPSH in WRF4_LICOM(RegCM4_LICOM) was driven by overestimated(underestimated) diabatic heating, which peaked at approximately 450 hPa over the region around the Philippines in association with different condensation–radiation processes. Coupling of WRF4 with LIOCM is a crucial step towards the development of the next generation of regional earth system models at the Chinese Academy of Sciences. 相似文献
5.
Diurnal variations of precipitation over the South China Sea 总被引：1，自引：0，他引：1
In this study, the diurnal variations of precipitation and related mechanisms over the South China Sea (SCS) are studied using the TRMM and other auxiliary atmospheric data. We have found that: (1) the amplitude and peak time of the diurnal precipitation over SCS exhibit remarkable regional features and seasonal variations. Diurnal variations are robust all the year around over the southern SCS especially over the Kalimantan Island and its offshore area. Over the middle to northern SCS, however, diurnal variations are noticeable only in the summer and autumn; (2) over the northern SCS precipitation peaks in early morning, while over the southern SCS it has two diurnal peaks: one in the early morning and another in the late afternoon; (3) the diurnal variations of precipitation over the SCS are related to the activity of the SCS summer monsoon and the ENSO events. The late afternoon precipitation increases remarkably after the onset of the SCS summer monsoon over the northern SCS. The early-morning rainfall peak is much more significant during La Nina years than during El Nino years; (4) the land–sea breeze is responsible for the diurnal cycle over the Kalimantan Island and its offshore area while the “static radiation–convection” mechanisms may result in the early-morning rainfall peak over the SCS. 相似文献
6.
This study investigates the roles of the boreal summer intraseasonal oscillation (BSISO) in the diurnal rainfall cycle over Hainan Island during the warm season (April-September) using 20-year satellite-based precipitation, ERA5 and the outgoing longwave radiation data with the phase composite analysis method. Results show that the spatial distributions of the hourly rainfall anomaly significantly change under the BSISO phases 1-8 while no clear variations are found on the daily and anomaly daily area-averaged rainfall over the island. During the BSISO phase 1, the rainfall anomaly distinctly increases in the morning over the southwest and late afternoon over the northeast of the island, while suppressed convection occurs in the early afternoon over the southwest area. Under this circumstance, strong low-level westerly winds bring abundant moisture into the island, which helps initiate the nocturnal-morning convection over the south coastal area, and drives the convergence region of sea breeze fronts to concentrate into the northwest. Opposite to Phase 1, an almost completely reversed diurnal cycle of rainfall anomaly is found in Phase 5, whereas a positive anomalous rainfall peak is observed in the early afternoon over the center while negative peaks are found in the morning and late afternoon over the southwest and northeast, owing to a strong low-level northeasterly anomaly flow, which causes relatively low moisture and enlarges a sea-breeze convergence area over the island. During Phase 8, strongest moisture is found over the island all through the day, which tends to produce highest rainfall in the afternoon with enhanced anomalous northerly. These results further indicate that multiscale interactions between the large-scale circulations and local land-sea breeze circulations play important roles in modulating diurnal precipitation cycles over the tropical island. 相似文献
7.
Uncertainties in simulating the seasonal mean atmospheric water cycle in Equatorial East Africa are quantified using 58 one-year-long experiments performed with the Weather Research and Forecasting model (WRF). Tested parameters include physical parameterizations of atmospheric convection, cloud microphysics, planetary boundary layer, land-surface model and radiation schemes, as well as land-use categories (USGS vs. MODIS), lateral forcings (ERA-Interim and ERA40 reanalyses), and domain geometry (size and vertical resolution). Results show that (1) uncertainties, defined as the differences between the experiments, are larger than the biases; (2) the parameters exerting the largest influence on simulated rainfall are, in order of decreasing importance, the shortwave radiation scheme, the land-surface model, the domain size, followed by convective schemes and land-use categories; (3) cloud microphysics, lateral forcing reanalysis, the number of vertical levels and planetary boundary layer schemes appear to be of lesser importance at the seasonal scale. Though persisting biases (consisting of conditions that are too wet over the Indian Ocean and the Congo Basin and too dry over eastern Kenya) prevail in most experiments, several configurations simulate the regional climate with reasonable accuracy. 相似文献
8.
M. A. Tadross W. J. Gutowski Jr B. C. Hewitson C. Jack M. New 《Theoretical and Applied Climatology》2006,86(1-4):63-80
Summary Two cumulus convection and two planetary boundary layer schemes are used to investigate the climate of southern Africa using
the MM5 regional climate model. Both a wet (1988/89) and a dry (1991/92) summer (December–February, DJF) rainfall season are
simulated and the results compared with three different observational sources: Climate Research Unit seasonal data (precipitation,
2 m surface temperature, number of rain days), satellite-derived diurnal precipitation and the Surface Radiation Budget diurnal
short-wave fluxes and optical depth.
Using the ETA model boundary layer in MM5 simulates too much incident short-wave radiation at the surface at 12 UTC, whereas
the medium range forecast model boundary layer yields a diurnal cycle of short-wave radiation closer to the observed. The
Betts-Miller convection scheme in MM5 simulates peak rainfall later in the day and less rain days than observed, whereas when
using the Kain-Fritsch convection scheme a peak rainfall earlier in the day and more rain days than observed are simulated.
The intensity of the hydrological cycle is therefore dependent on the choice of convection scheme, which in turn is further
modified by the boundary layer scheme. Precipitation during the wet 1988/89 season is reasonably captured by most simulations,
though using the Betts-Miller scheme more accurately simulates rainfall during the dry 1991/92 season. Mean DJF biases in
the surface temperature and diurnal temperature range are consistent with biases in the number of rain days and the diurnal
cycles of surface moisture and energy. 相似文献
9.
The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981?C1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally. 相似文献
10.
利用2009~2017年7~9月福建省逐小时地面加密自动站资料和2015~2017年7~9月厦门站的探空资料,通过K均值聚类法和中尺度数值模式(WRF3.9.1.1版本)理想数值模拟,分析了我国东南沿岸及复杂山地(福建)后汛期降水日变化特征,揭示了地形热力环流以及海陆风环流在热对流降水日变化形成中的作用,探讨了环境温湿廓线及风垂直廓线对热对流降水日峰值强度和日峰值出现时间的影响。结果发现:我国东南沿岸复杂山地(福建)后汛期降水日变化受地形热力环流和海陆风环流的影响和调制,白天辐射加热在复杂山地形成的局地热力环流激发出对流降雨带,午后受海风环流的影响,对流降雨带组织发展达到峰值,之后随着地形热力环流和海风环流减弱雨带逐渐减弱。武夷山及周边复杂山地的降水日变化主要受地形热力环流的影响,在午后对流降水达到峰值,夜间减弱几近消失。理想数值试验进一步证实了我国东南沿岸复杂山地地形热力环流对对流降雨的触发以及海陆风环流在山地对流雨带组织发展中的作用,环境温湿廓线以及风垂直廓线对热对流降水日峰值强度以及日峰值出现的时间具有重要影响,其中环境温湿廓线的大气抬升凝结高度、大气可降水量、大气的对流不稳定度以及大气中低层湿度分布的不同,会影响热对流降水日峰值强度,并通过影响山地热力对流触发时间,改变热对流降水日峰值时间,而环境风垂直廓线的低层气流强度和方向、中低层垂直风切变的不同,会影响地形热力对流系统的启动、组织发展和移动等特征,进而影响热对流降水日峰值强度以及热对流降水日峰值时间。 相似文献
11.
The Indian subcontinent witnessed a severe monsoon drought in 2002, which largely resulted from a major rainfall deficiency
in the month of July. While moderate El Nino conditions prevailed during this period, the atmospheric convective activity
was anomalously enhanced over northwest and north-central Pacific in the 10–20°N latitude belt; and heavy rainfall occurred
over this region in association with a series of northward moving tropical cyclones. Similar out-of-phase rainfall variations
over the Indian region and the northwest (NW) Pacific have been observed during other instances of El Nino/Southern Oscillation
(ENSO). The dynamical linkage corresponding to this out-of-phase rainfall variability is explored in this study by conducting
a set of numerical experiments using an atmospheric general circulation model. The results from the model simulations lend
credence to the role of the tropical Pacific sea surface temperature anomalies in forcing the out-of-phase precipitation variability
over the NW Pacific and the Indian monsoon region. It is seen that the ENSO induced circulation response reveals an anomalous
pattern comprising of alternating highs and lows which extend meridionally from the equatorial region into the sub-tropic
and mid-latitude regions of west-central Pacific. This meridional pattern is associated with an anomalous cyclonic circulation
over NW Pacific, which is found to favor enhanced tropical cyclonic activity and intensified convection over the region. In
turn, the intensified convection over NW Pacific induces subsidence and rainfall deficiency over the Indian landmass through
anomalous east-west circulation in the 10–20°N latitude belt. Based on the present findings, it is suggested that the convective
activity over NW Pacific is an important component in mediating the ENSO-monsoon teleconnection dynamics. 相似文献
12.
Andi Walther Marc Schröder Jürgen Fischer Ralf Bennartz 《Theoretical and Applied Climatology》2014,118(4):627-640
Observational data and simulations of the regional climate system Baltic integrated model system (BALTIMOS) were used to study precipitation in the Baltic Sea and its drainage basin with a special focus on the diurnal cycle. The study includes a general evaluation of BALTIMOS precipitation, showing that BALTIMOS has too many light rain events causing an overestimation of the total annual precipitation amount. The diurnal cycle as well as its spatial distribution was analysed. BALTIMOS captures the broad characteristics: a significant diurnal variability with an afternoon peak above land and weak variability with a nocturnal peak above sea. An algorithm to distinguish between frontal and convective precipitation was applied to examine the diurnal cycle more thoroughly. The local solar time of maximum rain in summer is about 1 to 2 h earlier in BALTIMOS than in radar observations of precipitation. 相似文献
13.
This paper summarizes the recent progress in studies of the diurnal variation of precipitation over con- tiguous China. The main results are as follows. （1） The rainfall diurnal variation over contiguous China presents distinct regional features. In summer, precipitation peaks in the late afternoon over the south- ern inland China and northeastern China, while it peaks around midnight over southwestern China. In the upper and middle reaches of Yangtze River valley, precipitation occurs mostly in the early morning. Summer precipitation over the central eastern China （most regions of the Tibetan Plateau） has two diurnal peaks, i.e., one in the early morning （midnight） and the other in the late afternoon. （2） The rainfall diurnal variation experiences obvious seasonal and sub-seasonal evolutions. In cold seasons, the regional contrast of rainfall diurnal peaks decreases, with an early morning maximum over most of the southern China. Over the central eastern China, diurnal monsoon rainfall shows sub-seasonal variations with the movement of summer monsoon systems. The rainfall peak mainly occurs in the early morning （late afternoon） during the active （break） monsoon period. （3） Cloud properties and occurrence time of rainfall diurnal peaks are different for long- and short-duration rainfall events. Long-duration rainfall events are dominated by strat- iform precipitation, with the maximum surface rain rate and the highest profile occurring in the late night to early morning, while short-duration rainfall events are more related to convective precipitation, with the maximum surface rain rate and the highest profile occurring between the late afternoon and early night. （4） The rainfall diurnal variation is influenced by multi-scale mountain-valley and land-sea breezes as well as large-scale atmospheric circulation, and involves complicated formation and evolution of cloud and rainfall systems. The diurnal cycle of winds in the lower troposphere also contributes to the regional differences 相似文献
14.
This study examines cloud radiative forcing (CRF) in the Asian monsoon region
(0o--50oN,60o--150oE) simulated by Intergovernmental Panel on
Climate Change (IPCC) Fourth Assessment Report (AR4) AMIP models. During boreal winter,
no model realistically reproduces the larger long-wave cloud radiative forcing (LWCF) over
the Tibet Plateau (TP) and only a couple of models reasonably capture the larger short-wave
CRF (SWCF) to the east of the TP. During boreal summer, there are larger biases for central
location and intensity of simulated CRF in active convective regions. The CRF biases are closely
related to the rainfall biases in the models. Quantitative analysis further indicates that the
correlation between simulated CRF and observations are not high, and that the biases and diversity
in SWCF are larger than that in LWCF. The annual cycle of simulated CRF over East Asia (0o--50oN,
100o--145oE) is also examined. Though many models capture the basic annual cycle in
tropics, strong LWCF and SWCF to the east of the TP beginning in early spring are underestimated
by most models. As a whole, GFDL-CM2.1, MPI-ECHAM5, UKMO-HadGAM1, and MIROC3.2 (medres) perform
well for CRF simulation in the Asian monsoon region, and the multi-model ensemble (MME) has improved
results over the individual simulations. It is suggested that strengthening the physical
parameterizations involved over the TP, and improving cumulus convection processes and model
experiment design are crucial to CRF simulation in the Asian monsoon region. 相似文献
15.
Many coupled ocean–atmosphere general circulation models (GCMs) suffer serious biases in the tropical Atlantic including a
southward shift of the intertropical convergence zone (ITCZ) in the annual mean, a westerly bias in equatorial surface winds,
and a failure to reproduce the eastern equatorial cold tongue in boreal summer. The present study examines an ensemble of
coupled GCMs and their uncoupled atmospheric component to identify common sources of error. It is found that the westerly
wind bias also exists in the atmospheric GCMs forced with observed sea surface temperature, but only in boreal spring. During
this time sea-level pressure is anomalously high (low) in the western (eastern) equatorial Atlantic, which appears to be related
to deficient (excessive) precipitation over tropical South America (Africa). In coupled simulations, this westerly bias leads
to a deepening of the thermocline in the east, which prevents the equatorial cold tongue from developing in boreal summer.
Thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical Atlantic
climate. 相似文献
16.
Simulating the diurnal cycle of rainfall in global climate models: resolution versus parameterization 总被引：1，自引：1，他引：0
Paul A. Dirmeyer Benjamin A. Cash James L. Kinter III Thomas Jung Lawrence Marx Masaki Satoh Cristiana Stan Hirofumi Tomita Peter Towers Nils Wedi Deepthi Achuthavarier Jennifer M. Adams Eric L. Altshuler Bohua Huang Emilia K. Jin Julia Manganello 《Climate Dynamics》2012,39(1-2):399-418
The effects of horizontal resolution and the treatment of convection on simulation of the diurnal cycle of precipitation during boreal summer are analyzed in several innovative weather and climate model integrations. The simulations include: season-long integrations of the Non-hydrostatic Icosahedral Atmospheric Model (NICAM) with explicit clouds and convection; year-long integrations of the operational Integrated Forecast System (IFS) from the European Centre for Medium-range Weather Forecasts at three resolutions (125, 39 and 16 km); seasonal simulations of the same model at 10 km resolution; and seasonal simulations of the National Center for Atmospheric Research (NCAR) low-resolution climate model with and without an embedded two-dimensional cloud-resolving model in each grid box. NICAM with explicit convection simulates best the phase of the diurnal cycle, as well as many regional features such as rainfall triggered by advancing sea breezes or high topography. However, NICAM greatly overestimates mean rainfall and the magnitude of the diurnal cycle. Introduction of an embedded cloud model within the NCAR model significantly improves global statistics of the seasonal mean and diurnal cycle of rainfall, as well as many regional features. However, errors often remain larger than for the other higher-resolution models. Increasing resolution alone has little impact on the timing of daily rainfall in IFS with parameterized convection, yet the amplitude of the diurnal cycle does improve along with the representation of mean rainfall. Variations during the day in atmospheric prognostic fields appear quite similar among models, suggesting that the distinctive treatments of model physics account for the differences in representing the diurnal cycle of precipitation. 相似文献
17.
Kyu-Myong Kim William K.-M. Lau Yogesh C. Sud Gregory K. Walker 《Climate Dynamics》2010,35(1):115-126
Effects of aerosol radiative forcing on the diurnal and seasonal cycles of precipitation over West Africa and eastern Atlantic
Ocean are investigated for the boreal summer season: June–July–August. An eight year (2000–2007) average of GCM simulated
rainfall data is compared with the corresponding TRMM rainfall data. The comparison shows that the amplitude of the diurnal
cycles of rainfall over land and ocean are reasonably well simulated. Over land, the phase of the simulated diurnal cycle
of precipitation peaks several hours earlier than that of the TRMM data. Corresponding differences over the ocean(s) are relatively
smaller. Some of the key features of the aerosol induced model simulated field anomalies are: (a) aerosol direct radiative
forcing which increases the atmospheric stability and reduces the daytime moist convection and convective precipitation; (b)
the aerosol induced changes in the diurnal cycle of precipitation are out of phase with those of the TRMM data over land,
but are in-phase over the ocean; (c) aerosols reduce the amplitude of the diurnal cycle of precipitation over land and enhance
it over ocean. However, the phase of the diurnal cycle is not affected much by the aerosol radiative forcing both over land
and ocean. During the boreal summer, aerosol radiative forcing and induced circulation and precipitation cool the Sahel and
the southern part of Sahara desert more than the adjacent areas to the north and south, thereby shifting the peak meridional
temperature gradient northward. Consequently, an anomalous easterly jet is found north of its climatological location. This
anomalous jet is associated with increased cyclonic circulation to the south of its axis, resulting in an anomalous monsoon
rain belt in the Sahel. 相似文献
18.
Frédéric Hourdin Jean-Yves Grandpeix Catherine Rio Sandrine Bony Arnaud Jam Frédérique Cheruy Nicolas Rochetin Laurent Fairhead Abderrahmane Idelkadi Ionela Musat Jean-Louis Dufresne Alain Lahellec Marie-Pierre Lefebvre Romain Roehrig 《Climate Dynamics》2013,40(9-10):2193-2222
Based on a decade of research on cloud processes, a new version of the LMDZ atmospheric general circulation model has been developed that corresponds to a complete recasting of the parameterization of turbulence, convection and clouds. This LMDZ5B version includes a mass-flux representation of the thermal plumes or rolls of the convective boundary layer, coupled to a bi-Gaussian statistical cloud scheme, as well as a parameterization of the cold pools generated below cumulonimbus by re-evaporation of convective precipitation. The triggering and closure of deep convection are now controlled by lifting processes in the sub-cloud layer. An available lifting energy and lifting power are provided both by the thermal plumes and by the spread of cold pools. The individual parameterizations were carefully validated against the results of explicit high resolution simulations. Here we present the work done to go from those new concepts and developments to a full 3D atmospheric model, used in particular for climate change projections with the IPSL-CM5B coupled model. Based on a series of sensitivity experiments, we document the differences with the previous LMDZ5A version distinguishing the role of parameterization changes from that of model tuning. Improvements found previously in single-column simulations of case studies are confirmed in the 3D model: (1) the convective boundary layer and cumulus clouds are better represented and (2) the diurnal cycle of convective rainfall over continents is delayed by several hours, solving a longstanding problem in climate modeling. The variability of tropical rainfall is also larger in LMDZ5B at intraseasonal time-scales. Significant biases of the LMDZ5A model however remain, or are even sometimes amplified. The paper emphasizes the importance of parameterization improvements and model tuning in the frame of climate change studies as well as the new paradigm that represents the improvement of 3D climate models under the control of single-column case studies simulations. 相似文献
19.
Microphysical and radiative effects of ice clouds on diurnal variations of tropical convective and stratiform rainfall are examined with the equilibrium simulation data from three experiments conducted with a two-dimensional cloud resolving model with imposed temporally and zonally invariant winds and sea surface temperature and zero mean vertical velocity. The experiment without ice radiative effects is compared with the control experiment with ice microphysics (both the ice radiative and microphysical effects) to study effects of ice radiative effects on diurnal rainfall variations whereas it is compared with the experiment without ice microphysics to examine ice microphysical effects on the diurnal rainfall variations. The ice radiative processes mainly affect diurnal cycle of convective rainfall whereas the ice microphysical processes have important impacts on the diurnal cycles of both convective and stratiform rainfall. Turning off the ice radiative effects generally enhances convective rainfall during the morning and evening and suppresses convective rainfall in the afternoon whereas turning off the ice microphysical effects generally suppresses convective and stratiform rainfall during the morning and enhances convective and stratiform rainfall in the afternoon and evening. The ice radiative and microphysical effects on the diurnal cycle of surface rainfall are mainly associated with that of vapor condensation and deposition, which is controlled by air temperature through saturation specific humidity. The ice effects on the diurnal cycle of local temperature tendency are largely explained by that of latent heating since the diurnal cycle of radiation is insensitive to the ice effects. 相似文献
20.
This study evaluates the performance of a regional climate model in simulating two types of synoptic tropical weather disturbances: convectively-coupled Kelvin and easterly waves. Interest in these two wave modes stems from their potential predictability out to several weeks in advance, as well as a strong observed linkage between easterly waves and tropical cyclogenesis. The model is a recent version of the weather research and forecast (WRF) system with 36-km horizontal grid spacing and convection parameterized using a scheme that accounts for key convective triggering and inhibition processes. The domain spans the entire tropical belt between 45°S and 45°N with periodic boundary conditions in the east–west direction, and conditions at the meridional/lower boundaries specified based on observations. The simulation covers 6 years from 2000 to 2005, which is long enough to establish a statistical depiction of the waves through space-time spectral filtering of rainfall data, together with simple lagged-linear regression. Results show that both the horizontal phase speeds and three-dimensional structures of the waves are qualitatively well captured by the model in comparison to observations. However, significant biases in wave activity are seen, with generally overactive easterly waves and underactive Kelvin waves. Evidence is presented to suggest that these biases in wave activity (which are also correlated with biases in time–mean rainfall, as well as biases in the model’s tropical cyclone climatology) stem in part from convection in the model coupling too strongly to rotational circulation anomalies. Nevertheless, the model is seen to do a reasonable job at capturing the genesis of tropical cyclones from easterly waves, with evidence for both wave accumulation and critical layer processes being importantly involved. 相似文献