role of narrow mountains in large scale organization of asian monsoon convection abstract the asian summer monsoon is organized into distinct convection centers but the mechanism for this organization is not well understood analysis of new satellite observations reveals that narrow mountain ranges are an important organizing agent anchoring monsoon convection centers on the windward side the bay of bengal convection in particular features the heaviest precipitation on its eastern coast because of orographic lifting as the southwest monsoon impinges on the coastal mountains of myanmar also known as burma this is in contrast to the widely held view that this convection is centered over the open ocean as implied by coarse resolution datasets a view that would require an entirely different explanation for its formation narrow in width and modest in height 1 km these mountains are hardly mentioned in conceptual depictions of the large scale monsoon and poorly represented in global climate models the numerical simulations of this study show that orographic rainbands are not a

limitations on the role of pore pressure in gravity gliding a simple model has been developed to evaluate the gravity gliding of nappes in sedimentary terranes three factors play a critical role in the model 1 the ratio of horizontal to vertical stress 2 the rate of fluid flow to the detachment horizon and 3 the permeability of lithologies immediately above that horizon if the ratio of horizontal to vertical stress is less than one this ratio limits the maximum attainable pore pressure through hydrofracture existing in situ stress measurements suggest that vertical hydrofracturing rather than gravity gliding might be the result of elevated pore pressures unless a low strength cap rock exists if the stress ratio exceeds one a relatively simple equation relates the cap rock permeability and fluid flow necessary for gravity gliding effective cap rock permeabilities less than 10 sup 4 to 10 sup 5 md are required for gravity gliding in a sedimentary basin based on available in situ permeability measurements only shales and evaporites could have sufficiently low permeabilities on a regional scale 24 references

interaction of low level flow with the western ghat mountains and offshore convection in the summer monsoon abstract seven year averaged values of percent frequency of occurrence of highly reflective cloud for the months june july and august indicate that offshore convection is a major component of the cloudiness of the southwest monsoon principal areas of convection occur off of the western coats of india burma thailand and the philippines this study concentrates on the area upstream of the western ghats mountains of india analysis of a special boundary layer mission flown during the wmo icsu summer monsoon experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection data obtained on this mission allow a large scale momentum budget computation for the subcloud layer which shows pressure deceleration to be significant the budget dominated by advection predicts an increase of average wind speed which is observed the pressure decelera

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

large scale atmospheric forcing by southeast pacific boundary layer clouds a regional model study abstract a regional model is used to study the radiative effect of boundary layer clouds over the southeast pacific on large scale atmosphere circulation during august october 1999 with the standard settings the model simulates reasonably well the large scale circulation over the eastern pacific precipitation in the intertropical convergence zone itcz north of the equator and marine boundary layer stratocumulus clouds to the south in a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern pacific artificially removed boundary layer clouds south of the equator almost disappear and precipitation in the itcz is reduced by 15 20 indicating that the stratocumulus clouds over the southeast pacific have both local and cross equatorial effects examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer in response to this cloud induced cooling an in situ anom

hydraulic modeling of bed load transport in gravel bed laba river on the basis of a field study of a piedmont braided reach of the gravel bed laba river in the northern caucasus russia an undistorted 1 50 scale mobile bed hydraulic model was constructed in an outdoor experimental plot of the state hydrological institute st petersburg russia channel and hydraulic regimes of the laba river were studied in this model attention was paid to the regime of bed load transport and its relation to the hydraulic characteristics of the flow the same problems were investigated in an 100 m long 1 m wide flume with glass walls also located at the state hydrological institute and in an 18 m long 2 46 m wide steel tilting flume at hr wallingford ltd u k quantitative characteristics of bed load transport were obtained from the laboratory experiments and compared with the field data proximity of the field and scaled up laboratory data demonstrates that the processes of bed load transport of gravel bed rivers can be faithfully reproduced in small scale models and flumes

effects of the andes on eastern pacific climate a regional atmospheric model study abstract a regional atmospheric model is used to study the effects of the narrow and steep andes on the eastern pacific climate in the southern hemisphere cold season i e august october 1999 the model reproduces key climatic features including the intertropical convergence zone itcz north of the equator and an extensive low level cloud deck capped by a temperature inversion to the south blocking the warm easterly winds from south america the andes help maintain the divergence and temperature inversion and hence the stratocumulus cloud deck over the southeast pacific off south america in an experiment where the andean mountains are removed the warm advection from the south american continent lowers the inversion height and reduces the low level divergence offshore leading to a significant reduction in cloud amount and an increase in solar radiation that reaches the sea surface in march and early april 1999 the model simulates a double itcz in response to the seasonal warming on and south

variability of bedload transport and channel morphology in a braided river hydraulic model this paper investigates variability in bedload transport and channel morphology for 11 replicate experimental runs in an approximately 1 50 braided river model the experiments each of 90 h duration were carried out in a 20 3m tilting flume all the experiments started with the same initial conditions bedload transport was measured at 5 min intervals in a collection drum at the exit from the flume the model showed reasonable hydraulic similarity when compared to prototype rivers results show that mean bedload transport rates for the 11 runs vary in the range 0 98 to 1 49gs 1 mean 1 21 coefficient of variation 11 per cent within run transport rates commonly vary from close to zero to two and occasionally three or four times the mean rate within the bedload series several irregular phases of transport intensity can be observed but time series analysis of the data show little underlying serial structure an ar 2 autoregressive model is appropriate channel patterns are narrow braided are established quickly and remain relatively stable throughout the runs although channel widths increase between 20 and 103 per cent over the 11 runs channel behaviour varies from aggradational to transitional between aggradation and degradation time averaged bedload transport rate is weakly correlated with braiding intensity in general these results demonstrate that for a given set of controlling variables bedload transport and channel morphology can be approximately replicated

far reaching effects of the hawaiian islands on the pacific ocean atmosphere system using satellite data we detected a wind wake trailing westward behind the hawaiian islands for 3000 kilometers a length many times greater than observed anywhere else on earth this wind wake drives an eastward ocean current that draws warm water from the asian coast 8000 kilometers away leaving marked changes in surface and subsurface ocean temperature standing in the path of the steady trade winds hawaii triggers an air sea interaction that provides the feedback to sustain the influence of these small islands over a long stretch of the pacific ocean

regional model simulations of marine boundary layer clouds over the southeast pacific off south america part ii sensitivity experiments abstract the sensitivity of a regional climate model to physical parameterizations and model resolution is investigated in terms of its simulation of boundary layer stratocumulus scu clouds over the southeast pacific specifically the physical schemes being tested include shallow cumulus convection subgrid vertical mixing cloud droplet number concentration cdnc and drizzle as described in part i the model with standard settings captures the major features of the boundary layer in the region including a well mixed marine boundary layer a capping temperature inversion scu clouds and the boundary layer regime transition from the well mixed layer near the coast of south america to a decoupled cloud layer over warmer water to the west turning off the shallow cumulus parameterization results in a dramatic increase in the simulated scu clouds while the boundary layer structure becomes unrealistic losing the decoupled regime over warm water with reduced penetrative mixing at the top of shallow cu

effect of the arakan mountains in the northwestern indochina peninsula on the late may asian monsoon transition by simulations using a global climate model with and without the arakan mountains in the northwest of myanmar we demonstrated that this mesoscale meridionally elongated mountain range has a substantial effect on anchoring and enhancing precipitation in the region during the bay of bengal bob monsoon onset in late may in this period the presence of the arakan mountains in the model significantly improves the simulation of the thermal and dynamical atmospheric structure of the monsoon by substantially enhancing precipitation deepening the midtropospheric trough and the southwesterly flow over the bob and strengthening the upper tropospheric anticyclone atop the trough these mountain induced changes essentially improve the simulation of the late may asian summer monsoon transition prior to the bob monsoon onset the blocking and deflecting effects of the arakan mountains on the low level flow are marked which enhance the moisture convergence and also probably influence the oceanic forcing over the bob as well as the mountain effect on the moisture convergence inclusion of the arakan mountains apparently initiates an upstream troughing effect which in turn enhances the synoptic and large scale circulation during the monsoon transition furthermore the presence of the arakan mountains induces a large scale wavelike perturbation which likely leads to an improvement of meiyu baiu simulation this study reveals that a narrow mountain like the arakan mountains likely contributes markedly to the characteristics of the asian summer monsoon during its early seasonal march such effects need to be reasonably resolved in the models

mountain uplift and surface temperature changes mountain uplift significantly affects both sea surface temperature sst and land surface temperature this was studied using a coupled atmosphere mixed layer ocean model with and without mountains the global mean sst dropped 1 4 c with mountain uplift mainly due to increased lower tropospheric clouds in the subtropical eastern pacific increased clouds hinder solar radiation into the ocean and lower sst the increased frequency of stratus incidence is related to subtropical anticyclones intensified by a strong temperature contrast between the continent and ocean in mountainous regions land surface temperature drops due to the lapse rate effect when this effect is eliminated the continental interior does not become as cool with mountain uplift because clouds become fewer and the surface drier due to decreased moisture transport southern asia becomes cooler because monsoon induced precipitation wets the ground and increases clouds our result showed greater northern high latitude temperature changes than the previous study indicating the importance of cloud related feedback in paleoclimate modeling

an observational and modeling study of the interaction of low level southwesterly flow with the olympic mountains during coast iop 4 abstract this paper presents an observational and modeling study of the three dimensional flow and precipitation around the olympic mountains of northwest washington state for a case of relatively steady south southwesterly flow this study utilized data from coast iop 4 the fourth intensive observing period during the coastal observation and simulation with topography field experiment that took place during 9 10 december 1993 one of the most important data sources was the noaa p 3 aircraft which provided flight level data reflectivity and doppler winds as it flew around the olympics features that were mapped successfully with the noaa p 3 included flow splitting around the windward pressure ridge strong southeasterlies to approximately 25 m s 1 northeast of the olympics caused by downgradient acceleration into the lee trough the rapid transition from strong southeasterlies to lighter easterlies 5 10 m s 1 in the strait of juan de fuca strong downslope flow and a hydraulic jump like transiti

the non layering of gravel streambeds under ephemeral flood regimes abstract the two layer format common to perennial streambeds in which a relatively coarse armour overlies a finer subarmour develops as a function of both the ingress and subsequent near surface winnowing of interstitial material and the selective non entrainment or slower transport velocity of coarse clasts ephemeral streams appear to lack such vertical layering or are characterized by weak layer development some of this may be due to the degree of mixing associated with the scour and fill process however continuous monitoring of bedload discharge in the nahal yatir in the northern negev desert reveals that sediment transport rates are extremely high so that the chance of armour layer development through selective non entrainment is much reduced indeed a comparison of the bedload and bed material size distributions confirms a high degree of similarity and hints at equal mobility regardless of clast size the monitoring programme also indicates that the bed becomes highly mobile at comparatively modest fluid shear so that practically all floods are associated with high transport rates consequently the winnowing that might be brought about by low transport rate events does not occur even within a single event winnowing is precluded by the rapid nature of flow recession that is so characteristic of flash floods the high degree of bed material mobility is attributable in part to the lack of strength that would otherwise be a corollary of armour development however it also highlights the divergent nature of the feedback loops that govern the relationship between flow and channel deposit in ephemeral and perennial systems

mechanisms of anabranch avulsion within gravel bed braided rivers observations from a scaled physical model the physical modelling of gravel bed braided rivers using a 1 20 froude scale model permits analysis of the types and relative occurrence of different avulsion mechanisms anabranch avulsion within braided rivers involves three main mechanisms choking avulsion caused by blockage of one channel by a sediment lobe constriction avulsion produced by deflection confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends each avulsion mechanism is described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers three factors are found to control anabranch avulsions namely flow discharge sediment flux and floodplain topography these are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium

model experiments on mobile paved gravel bed streams laboratory experiments which model streams with poorly sorted gravel beds at reduced scale reveal that pavement is a mobile bed phenomenon it acts to decrease the inherent difference in mobility between large and small grains by overrepresenting the percentage of large grains exposed to the flow pavement can coexist with the motion of all available grain sizes because motion is sporadic so that at any given time only a small percentage of surface grains are actually in motion particles in the immediate subpavement are only occasionally moved and deeper particles are hardly ever moved the experiments were conducted at constant discharges they provide accurate quantitative models of two field streams in that they obey the same bedload law as field data both sediment feed and recirculating configurations were utilized the only systematic difference between the two is in the behavior of the finer grains

swiftness of biomorphodynamics in lilliput to giant sized rivers and deltas physical experiments of self formed river channels and floodplains with live vegetation are pathways for understanding that complement numerical modelling recent experiments succeeded in creating braided rivers and dynamic meandering systems with clastic and vegetated floodplains however application of the insights gained from such experiments to natural systems depends on understanding potential scale effects temporal and spatial here we combine review analysis and experiments to identify fundamental problems of biomorphological river pattern formation that are open for further research in experiments we first show by review and analysis that physics based linear bar theory predicts negligible spatial scale effects in bar and bend wavelength relative to channel width time scaling on the other hand remains problematic because it integrates multiple processes of sediment transport floodplain formation and bank failure affected by bank stratigraphy and riparian vegetation as a tentative solution we secondly present experimental methods to assess bank strength effects that can be used in the design of river pattern experiments the third issue is that riparian vegetation has often been represented in experiments by uniformly seeded sprouts of a single plant species whilst spectacularly different patterns are obtained with contrasting seeding protocols showing the need for other experimental procedures and alternative riparian species the main challenge for future experiments is better understanding of temporal scaling of biomorphodynamics

channel morphology and bedload pulses in braided rivers a laboratory study bedload pulses in gravel bed rivers have been widely reported in recent years and attempts have been made to relate them to channel morphology bedload transport and channel morphology were measured in a small scale generic model of braided gravel bed streams two experiments are described in which braided channels developed in a 14 m 3 m sand tray total bedload output from the tray was weighed every 15 minutes stream bed geometry was surveyed every four hours pulses were observed in the bedload output time series and were qualitatively related to the channel morphology immediately upstream of the measuring section the bagnold 1980 bedload equation generally overpredicts measured bedload transport rates when applied to channels that were in equilibrium or aggrading underprediction occurred when applied to degrading channels aggradation was associated with channel multiplication and bar deposition channel pattern simplification occurred when degradation took place and bars emerged from the water flow development of phases of aggradation and degradation is dependent upon the three dimensional geometry of the stream beds spatial and temporal feedback loops can be identified enabling links between channel morphology and bedload transport rate to be directly identified

the influence of mesoscale orography on a coastal jet and rainband abstract the role of mesoscale orography along the central california coast in the development and evolution of a coastal jet and rainband is investigated using a high resolution triply nested nonhydrostatic numerical model comparison of the model simulations which use horizontal grid increments of 5 and 2 km on the inner computational meshes with a coastal mesoscale observation network indicates that the finescale structure of the jet and rainband dynamics are adequately simulated although phase and orientation errors occur the observed and simulated near surface winds have maximum speeds that exceed 22 m s 1 and a direction nearly parallel to the coastline and topography force balance analysis indicates that blocking in the lowest 500 m and flow over the coastal range above this layer contribute to mesoscale pressure perturbations including pressure ridging upstream of the coastal mountains which forces the ageostrophic dynamics of the coastal jet pressure perturbations associated with the to

eocene seismicity in the pyrenees from megaturbidites of the south pyrenean basin spain abstract the triggering mechanism for emplacement of nine calcareous megaturbidites mt in the clastic turbidite eocene basin of the pyrenees is examined the thickness of each mt is 10 to 200 m and their volume may reach 200 km 3 their mobilization is thought to result from seismic activity related to thrust faults along the northern border of the basin we attempt to define the seismic characteristics for a volume of 100 km 3 the dimensions of the slope failures are estimated to be 100 5 10 0 1 0 2 km and related to a fault rupture length of about 50 100 km and liquefaction of basal clayey layers given these two constraints superficial earthquakes of m 7 would be required to initiate landslides

critical shear stress for incipient motion of sand gravel streambeds results of an experimental study of the incipient motion of streambeds composed of sand gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments the experiments were conducted in an 8 m long by 0 30 m wide glass walled tilting flume and an 18 m long by 0 80 1 10 m wide trapezoidal concrete channel a reference transport method is used to define the beginning of bed material movement the experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size intergranular effects mixture standard deviation effect of the shape of grain size distribution absolute value of median size absolute size effect and bed slope effect of relative depth on overall flow resistance the shear stress at incipient motion of median sized grains in mixtures is found to be the same as for uniform sediment of this size the present findings are consistent with available flume and field data a technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal weakly bimodal sediment mixtures is proposed

role of the indochina peninsula narrow mountains in modulating the east asian western north pacific summer monsoon abstractunrealistic topographic effects are generally incorporated in global climate simulations and may contribute significantly to model biases in the asian monsoon region by artificially implementing the arakan yoma and annamese cordillera two south north oriented high mountain ranges on the coasts of the indochina peninsula in a 1 global climate model it is demonstrated that the proper representation of mesoscale topography over the indochina peninsula is crucial for realistically simulating the seasonality of the east asian western north pacific eawnp summer monsoon presence of the arakan yoma and annamese cordillera helps simulate the vertical coupling of atmospheric circulation over the mountain regions in late may the existence of the arakan yoma enhances the vertically deep southwesterly flow originating from the trough over the bay of bengal the ascending southwesterly flow converges with the midlatitude jet stream downstream in the southeast of the tibetan plateau and transports moistur

simple model for downstream variation of median sediment size in chilean rivers a simple theory is developed to account for the observed downstream variation of the median sediment size in chilean rivers based on a reach wise equilibrium sediment transport concept the theory makes use of a bedload transport equation linked with a resistance equation to estimate the median sediment diameter as a function of channel slope with the flow reynolds number and bedload concentration as parameters both meyer peter and muller s and ackers and white s formulas are used alternatively as bedload equations a manning strickler type of formulation is used as a resistance relationship the resulting model is validated against field data corresponding to 150 rivers in central chile covering slopes in the range of 0 04 8 61 with median sediment size in the range of 0 3 250 mm despite the simplicity of the present theory and the somewhat bold assumptions made in its derivation the estimated variation of the median sediment size with channel slope follows the same trend as the field data most of the scatter of these data falls within the theoretical limits given by the estimated range of values of the parameters of the model

effects of central american mountains on the eastern pacific winter itcz and moisture transport the intertropical convergence zone itcz is displaced to the south edge of the eastern pacific warm pool in boreal winter instead of being collocated a high resolution regional climate model is used to investigate the mechanism for this displaced itcz under the observed sea surface temperature sst and lateral boundary forcing the model reproduces the salient features of eastern pacific climate in winter including the southward displaced itcz and gap wind jets off the central american coast as the northeast trades impinge on the mountains of central america subsidence prevails off the pacific coast pushing the itcz southward cold sst patches induced by three gap wind jets have additional effects of keeping the itcz away from the coast in an experiment in which both the central american mountains and their effect on sst are removed the itcz shifts considerably northward to cover much of the eastern pacific warm pool the central american mountains are considered important to freshwater transport from the atlantic to the pacific ocean which in turn plays a key role in global ocean thermohaline circulation the results of this study show that this transport across central america is not very sensitive to the fine structure of the orography because the increased flow in the mountain gaps in a detailed topography run tends to be compensated for by broader flow in a smoothed topography run implications for global climate modeling are discussed

a method for estimating the mean bed load flux in braided rivers prediction of bed load flux remains a significant problem in understanding braided river morphodynamics for geomorphic and engineering applications two sets of data from laboratory experiments on braided networks performed at the university of trento italy and at the university of alberta canada provide the basis for development of a dimensionless bed load function and for testing a simple predictive model measured total sediment transport rates time averaged at equilibrium channel configuration collapse to a single dimensionless relationship based on dimensionless stream power bed load fluxes predicted by the parker and bagnold functions and cross section average hydraulic parameters under predict the bed load flux particularly at low shear stress this is consistent with previous observation and theory demonstrating the significant influence of transverse variability of the hydraulic parameters in controlling and predicting bed load flux a simple method for adjusting for this lateral variation is proposed for computing the sediment transport rate using topographic cross sections of braided rivers results show good agreement with the measured values suggesting that simple assumptions combined with the mean morphology of the channel may be sufficient to estimate mean bed load flux model predictions also highlight the significance of active bed width rather than bed shear stress as a fundamental ingredient in the prediction of the bed load rate and therefore as an important morphological property to be modelled the model also predicts similar behaviour in the at a station variability of bed load and in the importance of variation of active width relative to bed shear stress in the transport process

forced planetary waves in the northern hemisphere winter wave coupled orographic and thermal forcings abstract a more complete and new formulation of the orographic forcing and new thermal forcings are included in a steady state model of the northern hemisphere planetary waves when both forcings are included the simulation produces excellent results which are compared in detail with observations in particular the siberian high the tropospheric east asian trough and subtropical tropospheric east asian jet stream maxima are well reproduced even though the forcing is primarily extratropical in origin the modes uses a lower boundary condition in which the orographic forcing is determined by the effects of the total flow not just the zonal mean basic state consequently the net orographic forcing changes when thermal forcing is added and the tow solution is not equal to the linear sum of the solutions with orographic and thermal forcings separately the thermally induced orographic forcing is found to be very significant and in the troposphere there is strong interaction between the two forcings with

the nahal yatir bedload database sediment dynamics in a gravel bed ephemeral stream the nahal yatir bedload transport database arises from the northern negev river sediment monitoring programme and represents the first body of bedload information to be collected during flash floods in desert gravel bed streams bedload flux was established automatically with three slot samplers of the birkbeck type this was complemented by hydraulic measurements that allow sediment transport to be rated against channel average shear stress and specific stream power bed material and bedload grain size distributions are also provided the database is given in both printed and electronic tabular format

orographically forced planetary waves in the northern hemisphere winter steady state model with wave coupled lower boundary formulation abstract a planetary wave model has been developed in which the orographic forcing at the lower boundary arising from the kinematically induced vertical motion is due to the total flow impinging on the mountains rather than just the zonal mean basic state component of the flow over the mountains used in previous models consequently the effects of the vertical motions produced by the eddies at the lower boundary are included and are found to be as large if not larger than the zonal mean component the model remains linear mathematically but all the planetary waves become coupled through the lower boundary condition lbc and the model wave equations have to be solved for simultaneously a contrast is drawn between the wave coupled solutions and the solutions using the traditional lower boundary formulation in which the planetary waves are decoupled the model is symmetric about the equator and uses the linear balance set of equations on the sphere with full spherical geometry and spherical harmonic f

the effect of mesoscale mountain over the east indochina peninsula on downstream summer rainfall over east asia abstractthe mesoscale mountain over the east indochina peninsula named annam cordillera plays a key role in shaping the south china sea scs summer climate in both the atmosphere and the ocean however its effect is not limited to the scs ensemble simulations using a high resolution regional atmospheric model with or without the mountain reveals that the annam cordillera has a significant impact on regional climate as far as 3000 km over south and east china and western northwest pacific wnp by blocking lifting the warm and moist air from the bay of bengal the annam cordillera forces upward motion and precipitation on the windward side and subsidence on the leeward side and a low level southwesterly jet to the southeast tip of the indochina peninsula over the scs the latter gives rise to coastal upwelling and cold sea surface temperature sst filaments in the western scs reducing surface sensible and latent heat fluxes and thus suppressing convection over the scs heating associated with the

evaluation of an e e and three other boundary layer parameterization schemes in the wrf model over the southeast pacific and the southern great plains abstractto accurately simulate the atmospheric state within the planetary boundary layer pbl by pbl parameterization scheme in different regions with their dominant weather climate regimes is imp

tropical rainfall measuring mission observation and regional model study of precipitation diurnal cycle in the new guinean region 1 the diurnal cycle of precipitation in the new guinean region is studied on the basis of satellite observations from tropical rainfall measuring mission trmm precipitation radar pr measurements and regional atmospheric model simulations the study focuses on the effects of both the land sea breeze and the orographic forcing on the diurnal evolution of precipitation during the rainy season january march in the region the 7 year trmm pr data composite reveals several distinct features of the precipitation diurnal cycle in the region precipitation bands develop in the inland coastal region in the late morning to early afternoon and migrate inland from both northeast and southwest sides of the new guinean island following the inland penetration of the sea breeze fronts a separate convective rainband develops over the central mountain ridge in the early afternoon as a result of the development of the upslope winds due to the elevated surface warming over the mountain in the morning hours this mountain ridge rainband intensifies and becomes the dominant rainband as the coastal rainbands associated with the sea breeze fronts weaken during the late afternoon and the early evening in the midnight to the early morning the rainband over the mountaintop weakens as downslope winds develop and splits into two rainbands propagating away from the mountain ridge one to the north and one to the south and weakens over the lowland some distance away from the coasts meanwhile a coastal rainband develops offshore on each side of the island in the late evening to midnight and remains strong through early morning before it migrates offshore as a result the rainfall rate peaks in the late afternoon to early evening in most land areas except for in the lowland regions between the coastlines and the mountain where the rainfall rate peaks during the midnight while the rainfall rate peaks in the late evening to early morning in most coastal regions offshore the distribution of the diurnal amplitude shows two maxima one over the mountains and the other in the coastal regions offshore convective rainfall rate peaks in the late afternoon while stratiform rainfall rate peaks in the midnight to early morning the latter dominates the large diurnal amplitude over the mountain areas in the early morning the above broad features are simulated reasonably well in a control experiment with a high resolution regional atmospheric model a sensitivity experiment with the terrain removed is conducted to elucidate the role of orographic forcing in the diurnal evolution of both the local circulation and rainfall patterns the results show that the orographic forcing affects the diurnal precipitation through three major processes first the orography increases the moisture convergence at low levels by blocking and deflecting the mean flow second the upslope winds help initiate convection in the afternoon at the mountaintop finally the deep convection over the mountain acts as a source of propagating gravity waves which help initiate rainbands in the coastal regions offshore in the late evening to early morning implication of the results is discussed

topographic effects on a wintertime cold front in taiwan abstract this paper describes an observational and numerical study of an intense wintertime cold front that occurred in taiwan on 8 january 1996 the front was associated with rope clouds at the leading edge and a broad area of stratiform clouds behind the front was blocked by the central mountain range of taiwan and divided into two sections on each side of the mountain range as the cold air moved southward along the east coast the increasing westward coriolis force induced a landward acceleration after the cold air piled up against the mountains a coastal pressure ridge developed the cold air damming yielded a geostrophic balance between the westward coriolis force and the eastward component of the pressure gradient force in the x direction and a southward acceleration in the y direction mainly caused by the southward pressure gradient force component over the taiwan strait southward pressure gradient forces increased when the low level stable cold air was confined over the taiwan strait lead

summer upwelling in the south china sea and its role in regional climate variations seasonal and interannual variations of summer upwelling off the south vietnam coast and the offshore spread of cold water are investigated using a suite of new satellite measurements in summer as the southwesterly winds impinge on annam cordillera a north south running mountain range on the east coast of indochina a strong wind jet occurs at its southern tip offshore east of saigon resulting in strong wind curls that are important for ocean upwelling off the coast in july and august an anticyclonic ocean eddy develops to the southeast advecting the cold coastal water offshore into the open south china sea scs the center of this cold filament is located consistently north of the wind speed maximum indicating that open ocean upwelling helps to cool the ocean surface corroborating evidence for the cold filament is found in ocean color observations that reveal a collocated tongue of high chlorophyll concentration the development of this cold filament disrupts the summer warming of the scs and causes a pronounced semiannual cycle in sst moreover the cold filament is an important player in interannual variability in the summer scs in 1998 the cold filament and mid summer cooling never took place giving rise to a strong basin wide surface warming interannual sst variance has a local maximum over the climatological cold filament and is much greater than the variance over the adjacent indian and western pacific oceans a cold filament index is constructed which displays significant lagged correlation with sst in the eastern equatorial pacific and indian oceans indicative of a teleconnection from el nino

a comprehensive mass flux scheme for cumulus parameterization in large scale models abstract observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large scale models but this approach through the introduction of a spectral cloud ensemble leads normally to rather complex schemes in this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions this is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection gate marshall islands tradewind cumuli atex bomex and extratropical organized convection sesame 79 and then in a nwp model the scheme considers a population of clouds where the cloud ensemble is described by a one dimensional bulk model as earlier applied by yanai et al in a diagnostic study of tropical convection cumulus scale downdrafts are included various types of convection are represented i e penetrative convection in c

effects of mountain uplift on east asian summer climate investigated by a coupled atmosphere ocean gcm abstract to study the effects of progressive mountain uplift on east asian summer climate a series of coupled general circulation model cgcm experiments were performed eight different mountain heights were used 0 no mountain 20 40 60 80 100 control run 120 and 140 the land sea distribution is the same for all experiments and mountain heights are varied uniformly over the entire globe systematic changes in precipitation pattern and circulation fields as well as sea surface temperature sst appeared with progressive mountain uplift in summertime precipitation area moves inland on the asian continent with mountain uplift while the pacific subtropical anticyclone and associated trade winds become stronger the mountain uplift resulted in an sst increase over the western tropical pacific and the maritime continent and an sst decrease over the western indian ocean and the central subtropical pacific there is a drastic change in the east asian circulations with the threshold value

the incidence and nature of bedload transport during flood flows in coarse grained alluvial channels a continuous record reveals that the incidence of bedload in a coarse grained river channel changes from flood to flood long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood wave but when floods follow each other closely the bed material is comparatively loose and offers less resistance to entrainment in this case substantial amounts of bedload are generated on the rising limb this is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season this produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse grained channels besides this the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion this adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory bedload is shown to be characterized by a series of pulses with a mean periodicity of 1 7 hours in the absence of migrating bedforms it is speculated that this well documented pattern reflects the passage of kinematic waves of particles in a slow moving traction carpet the general pattern of bedload including pulsations is shown to occur more or less synchronously at different points across the stream channel