the improve 1 storm of 1 2 february 2001 part i development of a forward tilted cold front and a warm occlusion abstract analysis of observations and the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 are used to study the development of a forward tilted cold front off the coast of washington state the vertical velocity associated with the cold front produced a wide cold frontal rainband in the early stage of development the midtropospheric baroclinic zone or upper cold front moved forward with time over the warm sector to produce a structure similar to a split front the movement of the upper cold front was due to horizontal transport and frontogenetical propagation the frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis which resulted in a negative positive couplet of frontogenesis straddling the baroclinic zone the lower tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion in the initial occluding process the converging frontal zones tilted i

thermal structure and airflow in a model simulation of an occluded marine cyclone abstract a very fine mesh model simulation of the ocean ranger storm of february 1982 is used to study the thermal structure and airflow in an intense marine cyclone in particular the study investigates the structures of the occluded front and the secluded pool of warm air in the simulated cyclone and examines the formation of these structures with the help of a large number of air trajectories it was found that in the model simulation the largest thermal gradient occurred along the occluded front not along the warm or cold fronts and that the intense gradient was a product of strong warm and occludofrontogenesis in the inflowing air the occluded front was embedded within air that was earlier located within the baroclinic zone ahead of the low center not air that was in proximity to the historical warm and cold fronts the warm air in the seclusion likewise originated in the baroclinic zone ahead of the low the seclusion at low levels resulted from a tongue of slower moving relatively warm air be

atlantic forcing of pacific decadal variability this paper investigates the atlantic ocean influence on equatorial pacific decadal variability using an ensemble of simulations where the ictpagcm speedy is coupled to the nemo opa ocean model in the indo pacific region and forced by observed sea surface temperatures in the atlantic region it is shown that the atlantic multidecadal oscillation amo has had a substantial influence on the equatorial pacific decadal variability according to amo phases we have identified three periods with strong atlantic forcing of equatorial pacific changes namely 1 1931 1950 minus 1910 1929 2 1970 1989 minus 1931 1950 and 3 1994 2013 minus 1970 1989 both observations and the model show easterly surface wind anomalies in the central pacific cooling in the central eastern pacific and warming in the western pacific indian ocean region in events 1 and 3 and the opposite signals in event 2 the physical mechanism for these responses is related to a modification of the walker circulation because a positive negative amo leads to an overall warmer cooler tropical atlantic the warmer cooler tropical atlantic modifies the walker circulation leading to rising sinking and upper level divergence convergence motion in the atlantic region and sinking rising motion and upper level convergence divergence in the central pacific region

flow regimes of himalayan rivers of nepal nature and spatial patterns abstract a large scale perspective is provided upon the spatial distribution of river flow regimes across the nepalese himalaya by classifying long term average monthly runoff data for 28 river basins the classification methodology is shown to be a useful tool for identifying underlying spatial structure in flow regime shape timing of peak and magnitude low intermediate and high in an extreme physical environment where hydrological patterns are complex and poorly known low marked august peak regimes occur across far western nepal but also in some eastern basins which have a short summer monsoon and snow and ice melt low july august peak regimes are found in the central to eastern high mountains and high himalaya and the eastern middle mountains where the summer monsoon arrives earliest meltwaters contribute but topography limits precipitation amount low intermediate august september peak regimes dominate the central middle mountains due to an extended summer monsoon and greater groundwater contributions intermediate high magnitude regimes occur along the middle mountains high mountains boundary with july august peaks in western central areas and marked august peaks at higher elevations in eastern central and eastern nepal reflecting differences in summer monsoon penetration the practical implications of these results for assessment of water resources and prediction of runoff from ungauged basins are highlighted

hydrology of debris covered glaciers in high mountain asia abstract the hydrological characteristics of debris covered glaciers are known to be fundamentally different from those of clean ice glaciers even within the same climatological geological and geomorphological setting understanding how these characteristics influence the timing and magnitude of meltwater discharge is particularly important for regions where downstream communities rely on this resource for sanitation irrigation and hydropower such as high mountain asia the hydrology of debris covered glaciers is complex rugged surface topographies typically route meltwater through compound supraglacial englacial systems involving both channels and ponds as well as pathways that remain unknown low gradient tongues that extend several kilometres retard water conveyance and promote englacial storage englacial conduits are frequently abandoned and reactivated as water supply changes new lines of permeability are exploited and drainage is captured due to high rates of surface and subsurface change seasonal influences such as the monsoon are superimposed on these distinctive characteristics reorganising surface and subsurface drainage rapidly from one season to the next recent advances in understanding have mostly come from studies aimed at quantifying and describing supraglacial processes little is known about the subsurface hydrology particularly the nature or even existence of subglacial drainage in this review we consider in turn the supraglacial englacial subglacial and proglacial hydrological domains of debris covered glaciers in high mountain asia we summarise different lines of evidence to establish the current state of knowledge and in doing so identify major knowledge gaps finally we use this information to suggest six themes for future hydrological research at high mountain asian debris covered glaciers in order to make timely long term predictions of changes in the water they supply

connecting source and transport suspended sediments in the nepal himalayas understanding the dynamics of sediment fluxes is a key issue to constrain modern erosion rates in mountain belts and determine the still debated level of control exerted by precipitation topography and tectonics the well defined monsoon seasonality in the himalayas together with active tectonics and strong relief provide an ideal environment to assess these possible interactions for this purpose we present a new compilation of daily suspended sediment data for 12 stations of the major rivers of the nepal himalayas we analyze the relationships of sediment transport with daily river discharge and precipitation data as well as with morphometric parameters we show that suspended sediment concentrations vary systematically through the seasons and asynchronously to river discharge displaying a hysteresis effect this clockwise hysteresis effect disappears when suspended sediment fluxes are directly compared with direct storm discharge therefore we attribute the hysteresis effect to groundwater dilution rather than a sediment supply limitation we infer a rating model to calculate erosion rates directly from long river discharge chronicles we show that when normalized by drainage area and mean sediment flux all rivers exhibit the same trend this similarity implies that all river basins have the same erosion behavior independent of location size and catchment characteristics erosion rates calculated from suspended sediment fluxes range between 0 1 and 2 8 mm yr the erosion rates of the three main basins of nepal are in the range 0 9 1 5 mm yr erosion rates in the higher himalayas are relatively low 0 5 mm yr except for kali gandaki while in the lesser himalayas they range from 0 2 to 2 mm yr we propose that material transport in the rivers depends on hillslope sediment supply which is in turn controlled by those rainfalls producing direct runoff in other words the rivers in the nepal himalayas are supply limited and the hillsopes as a contributing source are transport limited we also show that erosion processes are not as much controlled by infrequently occurring extreme precipitation events than by moderate ones with a high recurrence interval

river system reformed by the eastern kunlun fault implications from geomorphological features in the eastern kunlun mountains northern tibetan plateau abstract the eastern kunlun fault is a famous sinistral strike slip fault and the eastern kunlun mountains is one of the most outstanding landmarks on the tibetan plateau comprehensive geomorphic analyses in this study constrain the activation of the eastern kunlun fault and the uplift process of the eastern kunlun mountains the three largest rivers flowing into the qaidam basin the nalenggele river the golmud river and the xiangride river all run across the mechanically strong granitoid belt and precambrian basement in the eastern kunlun mountains and reach the soft mesozoic and cenozoic hoh xil basin meanwhile the drainage patterns and longitudinal profiles of these three rivers are significantly reformed by the eastern kunlun fault the horizontal throws caused by the eastern kunlun fault inferred from drainage pattern 110 km for the golmud river and 90 km for the xiangride river are consistent with the previous estimated strike slip displacement of the eastern kunlun fault horizontal segments occur on river longitudinal profiles induced by the rapid strike slip movement of the eastern kunlun fault all the above evidences imply that the river system of the eastern kunlun mountains was built before the initiation of the eastern kunlun fault meanwhile the distribution of normalized steepness indices ksn shows that rapid rock uplift occurred between the eastern kunlun fault and the northern front of the eastern kunlun mountains

a simple model for the synoptic analysis of cold fronts a split cold front model with an over running upper front or humidity front ahead of a surface front or humidity front is shown to provide a useful representation of the principal characteristics of many katafronts the use of such a model leads to more consistent analyses especially of cloud and rain than are achieved using unmodified classical concepts

glacial and fluvial erosion in the dolpo basin western nepal abstract the landscape of western nepal has been dramatically shaped by tectonics and by both glacial and fluvial erosion here we investigate the modern geomorphic and glacial features of the dolpo basin in northern mid western nepal we analyze attributes such as glacier size slope aspect and toe head and peak elevation for 446 glaciers within and around the basin determining their relationship with basin features such as lithology slope and precipitation glaciers reflect lithology and precipitation but glacier size is strongly correlated with base level and slope our data suggest that low slope glaciers with high base level i e those that flow onto the tibetan plateau are forced to grow large because they exist at or above the equilibrium line altitude glaciers within the basin particularly in the western part can become dismembered on steep slopes an effect only partly offset by precipitation the spatial variability in regional slope and presence of a wind gap along the northern basin border suggest recent capture of the dolpo basin a plateau like morphology would have thus once extended at least 45 km southward the southward opening of the basin allowed precipitation to enter forming glaciers along the western end of the northern basin border larger glaciers resulting from high base level pushed the ridge southward over time through headward erosion our study suggests dramatic landscape reorganization in western nepal consistent with other recent studies suggesting a more extensive proto tibet

decadal variation of summer precipitation over china and associated atmospheric circulation after the late 1990s abstractin this study the authors found that the summer precipitation over china experienced different decadal variation features from north to south after the late 1990s in northeastern and north china and the lower middle reaches of the yangtze river precipitation decreased after 1999 while precipitation experienced a significant reduction over south and southwestern china and a significant increase over the southern parts of hetao region and huaihe river valley after 2003 the authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation the wind anomalies for the former exhibit a barotropic meridional dipole pattern with anticyclonic anomalies over mongolia to northern china and cyclonic anomalies over the southeastern chinese coast to the northwestern pacific for the latter there is a southeast northwest oriented dipole pattern in the middle and lower troposphere with cyclonic

the structure and evolution of a simulated midlatitude cyclone over land abstract using output from a mesoscale model simulation this paper describes the evolution of the three dimensional temperature and humidity structures of an intense cyclone that developed over the eastern half of the united states during 14 16 december 1987 some specific findings include the following the occlusion process at the surface appeared similar to the classical paradigm with the cold front catching up with the warm front aloft a warm type occlusion structure formed as an upper baroclinic zone merged with the surface based occluded front the frontogenesis associated with the cold and warm fronts was initially continuous but as the system evolved a break in the frontogenesis along the northern section of the cold front developed a single frontogenesis feature appeared to support both the warm and occluded fronts a quasi linear region of convective activity was observed in advance of the surface cold front this convective line was associated with an upper level humidity front wh

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones xi comparisons between observational and theoretical aspects of rainbands abstract comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale the warm sector flow ahead of the cold front and above the cold frontal zone aloft was unstable to conditional symmetric instability and theoretical predictions for this mechanism am consistent with several aspects of the warm sector and wide cold frontal rainbands in the case of the warm sector rainbands other mechanisms e g wave cisk and mixed dynamic convective instabilities may have also played a role the core structure of the narrow cold frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front also many aspects of the narrow cold frontal rainband were similar to a density current shear induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold frontal zone aloft the orientation of the

relationship of boreal summer 10 20 day and 30 60 day intraseasonal oscillation intensity over the tropical western north pacific to tropical indo pacific sst the present study contrasts interannual variations in the intensity of boreal summer 10 20 day and 30 60 day intraseasonal oscillations isos over the tropical western north pacific and their factors a pronounced difference is found in the relationship of the two isos to el nino southern oscillation the 10 20 day iso intensity is enhanced during el nino developing summer whereas the 30 60 day iso intensity is enhanced during la nina decaying summer the above different relationship is interpreted as follows the equatorial central and eastern pacific sst anomalies modify vertical wind shear lower level moisture and vertical motion in a southeast northwest oriented band from the equatorial western pacific to the tropical western north pacific where the 10 20 day isos originate and propagate these background field changes modulate the amplitude of 10 20 day isos preceding equatorial central and eastern pacific sst anomalies induce sst anomalies in the north indian ocean in summer which in turn modify vertical wind shear and vertical motion over the tropical western north pacific the modified background fields influence the amplitude of the 30 60 day isos when they reach the tropical western north pacific from the equatorial region a feedback of iso intensity on local sst change is identified in the tropical western north pacific likely due to a net effect of isos on surface heat flux anomalies this feedback is more prominent from the 10 20 day than the 30 60 day iso intensity change

influences of northward propagating 25 90 day and quasi biweekly oscillations on eastern china summer rainfall the present study reveals that rainfall anomalies associated with the 25 90 day quasi biweekly tropical convective activity propagates northward northwestward into northern china the yangtze river enhanced rainfall over southern china results directly from enhanced convection propagating northward from the south china sea scs western north pacific wnp strengthened northward water vapor transport and intensified upper level divergence the anomalous anticyclone over scs wnp which is induced by local lower sea surface temperature provides a favorable condition for transporting more moisture to southern china stronger subtropical westerly jet and south asia high strengthen upper level divergence the northward propagating 25 90 day oscillation over eastern china reaches farther north than the quasi biweekly oscillation qbwo the intensity of qbwo in summer rainfall has significantly strengthened over southern china after 1993 but not for the 25 90 day oscillation this is mainly contributed by an interdecadal increase in rainfall over southern china and the yangtze river basin in the wet phases of qbwo the interdecadal enhancement of rainfall associated with the qbwo is attributed to three factors one is lower tropospheric convergence caused by anomalous anticyclone over the wnp and anomalous cyclone over korean peninsula and japan the second is upper tropospheric divergence resulting from strengthening of subtropical westerly jet and the south asia high the last is enhanced ascent over south china through meridional vertical circulation

quasi biweekly oscillation over the tibetan plateau and its link with the asian summer monsoon abstract intraseasonal variation isv is especially prominent and unique in the asian summer monsoon region in this work the dominant isv mode over the tibetan plateau tp in the summer monsoon season june august together with its structure and evolution is identified using station observations global precipitation climatology project precipitation data and era interim during 1979 2011 results indicate that quasi biweekly oscillation qbwo is the dominant mode of isv over the tp and is significant in terms of the circulation precipitation and diabatic heating fields in particular the qbwo is closely related to the onset and active break phases of the tp summer monsoon in most cases the qbwo originates from the equatorial western pacific and first propagates northwestward to the bay of bengal and northern india then northward to the southeastern tp and finally eastward to the east asian area showing a clockwise propagation pathway two main mechanisms are responsible for the northward

passage of a tracer through frontal zones a model for the formation of forward sloping cold fronts it is shown that a forward sloping cold front may form in a simple two dimensional model of an atmospheric frontal zone through differential advection of dry and wet bulb potential temperature as such two frontal zones may coexist with opposing slopes this behaviour is manifest in the neutral eady edge wave which exhibits kata cold front behaviour while the unstable eady wave with an ana cold front does not generate a forward sloping cold front these results come about as a consequence of the wave like behaviour of the fronts in the model that is the ability of a front to propagate through the air locally as such they highlight the fact that a frontal zone need not be a barrier to airflow and that any air properties from water vapour to atmospheric pollution may pass through a front

airstream boundaries in numerical weather simulations abstract distinct airstreams separated by sharp boundaries are present in numerical weather simulations and can be used to identify characteristic structures in baroclinic storms to allow objective comparisons between different analyses a rigorous treatment of airstream boundaries is performed based upon a numerical procedure in which the uncertainty of individual trajectory paths is related to the strength of an airstream boundary as defined and quantified herein the properties of the procedure are then investigated via application to a numerical simulation of the erica iop 4 storm the work not only provides the necessary framework within which future analyses of airstreams can be interpreted but also provides insights into quantitative properties of atmospheric flows

contribution of the phase transition of pacific decadal oscillation to the late 1990s shift in east china summer rainfall based on our previous study the interdecadal changes in summer rainfall over east china in the late 1990s are further explored here the increased local rising motion is implicated as the dominant factor of increased rainfall in the lower huang huai river valley lhr both the observation and numerical experiments using community atmosphere model version 4 suggest that the negative pacific decadal oscillation pdo mode can result in rising anomalies and thus more rainfall in the lhr the east asian westerly jet stream eawjs is suggested as a bridge to link the pacific sea surface temperature anomalies and east asian summer rainfall model results reveal that the negative pdo mode can lead to significant easterly anomalies over east asia as a result the eawjs is weakened and shifts poleward which coincides with observed changes in eawjs after the late 1990s in addition weakened and poleward shifted eawjs can result in an anomalous ascending motion to its south in the lhr by modulating the jet related secondary meridional vertical circulation consequently rainfall increased in the lhr after the late 1990s besides the positive atlantic meridional oscillation can only induce insignificant changes over east asia and partly counteract the negative pdo effect there

low frequency equatorial waves in vertically sheared zonal flow part i stable waves abstract the mechanism by which a vertically sheared zonal flow affects large scale low frequency equatorial waves is investigated with two level equatorial plane and spherical coordinates models vertical shears couple barolinic and barotropic components of equatorial wave motion affecting significantly the rossby wave and westward propagating yanai wave but not the kelvin wave this difference results from the fact that the barotropic component is a modified rossby mode and can be resonantly excited only by westward propagating internal waves the barotropic components emanate poleward into the extratropics with a pronounced amplitude while the baroclinic components remain equatorially trapped a westerly vertical shear favors the trapping of rossby and yanai waves in the upper troposphere whereas an easterly shear tends to confine them in the lower troposphere as such their westward propagation is slowed down by both westerly and easterly shears when the strength of the vertical shear varies w

active break cycles diagnosis of the intraseasonal variability of the asian summer monsoon in this study various diagnostics have been applied to daily observed outgoing longwave radiation olr and ecmwf reanalysis era products to provide a comprehensive description of the active break cycles associated with the asian summer monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability 10 20 days and 30 60 days composite analysis of olr based on filtered daily all india rainfall air for the 40 day 30 60 days intraseasonal mode indicates that during active phases convection is significantly enhanced over the indian continent extending over the bay of bengal maritime continent and equatorial west pacific while convection is suppressed over the equatorial indian ocean and northwest tropical pacific resulting in a quadrapole structure over the asian monsoon domain in response to this heating pattern the large scale hadley lateral and the two east west transverse tropical circulations are enhanced there is also a significant impact on the extra tropical circulation through excitation and propagation of rossby waves in contrast the 15 day mode is more regional to the monsoon domain and has a prominent east west orientation in convection only the local hadley circulation over the monsoon region is modulated by this mode the evolution of these two modes as revealed by pop principal oscillation pattern analysis shows that the 40 day mode originates over the equatorial indian ocean once formed it has poleward propagation on either side of the equator and eastward propagation into the equatorial west pacific from the equatorial west pacific northward propagation over the west pacific and westward propagation into the indian longitudes are prominent the propagative features are complex and interactive and are responsible for the quadrapole structure in convection seen from the composites the interannual variability assessed from the pop coefficient time series indicates that the 40 day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic tcz tropical convergence zone the pop analysis of the 15 day mode indicates that this event originates over the equatorial west pacific associated with westward propagating rossby waves amplifies over the northwest tropical pacific and modulates both the continental and oceanic tczs over indian longitudes simultaneously this mode is pronounced during the established phase of the monsoon due to the complexity in the propagational features of both the intraseasonal modes it is concluded that understanding the subseasonal variability of one regional component of the asian summer monsoon asm requires understanding the entire asm system

structure of a frontal system over the northeast pacific ocean abstract a frontal system passed over the storm transfer and response experiment project area on 17 november 1980 as revealed by ship born rawinsonde surface and radar information this northeast pacific storm exhibited three components it linked surface and 500 mb troughs with marked surface windshift and associated confluence and horizontal wind shear but exhibiting no surface temperature change a midtropospheric cold katafront and an upper moisture front that was moving ahead more rapidly than the other components the upper moisture front was marked by a moisture gradient across the trailing edge of a large cloud region that moved with the winds at 500 mb or higher due to the horizontal thermal homogeneity of the marine boundary layer the confluence associated with the surface trough did not cause low level cold frontogenesis high w values of the upper portion of the frontal system indicate that air was advected from much farther south important characteristics of frontal system evolution

organization and structure of clouds and precipitation on the mid atlantic coast of the united states part v the role of an upper level front in the generation of a rainband abstract the origins of a rainband of moderate intensity that occurred over the eastern carolinas is investigated it is concluded that the band formed in the updraft portion of a thermodynamically direct vertical circulation about an upper level frontal zone in a region of conditional symmetrical instability csi the release of csi is presumed to have been responsible for the dimensions of the band and its orientation relative to the shear vector an adiabatic mechanism for destabilization of the environment of the upper level front to csi was explored but found to be insignificant in this case

zonal oscillation of western pacific subtropical high and subseasonal sst variations during yangtze persistent heavy rainfall events abstractthis study examines the relationship between the zonal oscillation of the western pacific subtropical high wpsh and underneath sea surface temperature sst variation on a subseasonal time scale associated with the persistent heavy rainfall phr events over the middle and lower reaches of the yangtze river valley mlyrv in china a total of 76 phr events and 45 break events in the summers of 1979 2011 are first identified over the mlyrv and divided into early and late summer groups during the phr events over the mlyrv for both groups the wpsh stretches more westward accompanied by the positive anomalies of the 500 hpa geopotential height field over east asia and its coastal region south of 30 n and the subseasonal warmer ssts beneath the wpsh western edge the time lagged composites suggest that the wpsh western edge exhibits westward then eastward migration on a subseasonal time scale for the phr events the zonal changes of the wpsh and anomalous circulation and sst anomaly ssta signal

an analysis of frontogenesis in numerical simulations of baroclinic waves abstract using a primitive equation pe model we revisit two canonical flows that were previously studied using a semigeostrophic equation sg model in a previous paper the authors showed that the pe and the sg models can have significantly different versions of the large scale dynamics here they report on the implications of this difference for frontogenesis the program for the study of frontogenesis developed by b j hoskins and collaborators is followed to show how in the pe version of the canonical cases the surface warm front develops before the cold front and why the upper level front is a long nearly continuous feature going from ridge to trough the frontogenesis experienced by an air parcel is computed following the parcel to illustrate better the mechanisms involved as the present calculations are carried out longer than most previous ones the relation of the upper frontogenesis to the formation of the upper level cutoff cyclone is also examined trajectory and three dimensional g

intraseasonal variability of the south china sea summer monsoon the objective of this study is to explore based on the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis data the intraseasonal variability of the south china sea scs summer monsoon sm in terms of its structure and propagation as well as interannual variations a possible mechanism that is responsible for the origin of the 10 20 day oscillation of the scs sm is also proposed the 30 60 day hereafter the 3 6 mode and 10 20 day hereafter the 1 2 mode oscillations are found to be the two intraseasonal modes that control the behavior of the scssm activities for most of the years both the 3 6 and 1 2 modes are distinct but may not always exist simultaneously in a particular year and their contributions to the overall variations differ among different years thus the interannual variability in the intraseasonal oscillation activity of the scs sm may be categorized as follows the 3 6 category in which the 3 6 mode is more significant in terms of the percentage of variance explained than the 1 2 mode the 1 2 category in which the 1 2 mode is dominant and the dual category in which both the 3 6 and 1 2 modes are pronounced composite analyses of the 3 6 category cases indicate that the 30 60 day oscillation of the scs sm exhibits a trough ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the scs with anomalous cyclones anticyclones along with enhanced suppressed convection migrating northward from the equator to the midlatitudes the northward migrating 3 6 mode monsoon trough ridge in the lower troposphere is coupled with the eastward propagating 3 6 mode divergence convergence in the upper troposphere it is also found that for the years in the dual category the scs sm activities are basically controlled by the 3 6 mode but modified by the 1 2 mode composite results of the 1 2 mode category cases show that the 10 20 day oscillation is manifest as an anticyclone cyclone system over the western tropical pacific propagating northwestward into the scs a close coupling also exists between the upper level convergence divergence and the low level anticyclone cyclone it is found that the 1 2 mode of the scs sm mainly originates from the equatorial central pacific although a disturbance from the northeast of the scs also contributes to this mode the flow patterns from an inactive to an active period resemble those associated with a mixed rossby gravity wave observed in previous studies

glacial runoff and transport of suspended sediment from the chhota shigri glacier western himalaya india glacial runoff and suspended sediment transport characteristics of the chhota shigri glacier were studied during the whole ablation period may october 2010 discharge patterns of this glacier show that august experiences maximum runoff accounting for 34 2 of the total runoff followed by july 32 9 june 14 3 september 12 5 may 5 2 and october 0 9 during the study period the maximum runoff qmax was observed in the evening 4 6 p m whereas minimum runoff qmin was observed in the morning 7 9 a m the lag time between generation of meltwater from the glacier and its emergence as runoff varied from 1 to 4 h depending on the condition of melting during the melt season a good correlation r2 0 71 was found between daily mean glacier specific runoff and air temperature of the study area daily mean concentrations of suspended sediment for may june july august september and october were found as 155 506 577 591 80 9 and 30 8 mg l 1 respectively showing maximum concentration of suspended sediment in august followed by july suspended sediment load also follows a similar trend diurnal variations in the concentration of suspended sediment of the investigation area are directly associated with diurnal variations in meltwater runoff there is a strong relationship between concentration of suspended sediment and discharge r2 0 79 and suspended sediment load with discharge r2 0 94 the average sediment yield of the chhota shigri glacier basin was computed as 1689 t km 2 year 1 during the melt season much higher than the worldwide and indian averages

structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation abstract the spatial and temporal structures of the northward propagating boreal summer intraseasonal oscillation bsiso are revealed based on the analysis of both the echam4 model simulation and the ncep ncar reanalysis the bsiso structure and evolution characteristics simulated by the model bear many similarities to those derived from the ncep ncar reanalysis the most notable features are the remarkable meridional asymmetries relative to the bsiso convection in the vorticity and specific humidity fields a positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center the maximum specific humidity also shows a clear northward shift in the lower troposphere two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the bsiso the first is the vertical shear mechanism the key process associated with this mechanism is the generation of barotropic vorticity due to the couplin

conceptual models of upper level frontogenesis in south westerly and north westerly flow the conceptual model proposed by shapiro as it is applied to the evolution of an upper level frontal zone within a baroclinic wave is reviewed and its limitations are investigated through previous literature and two casestudies presented in this paper the early stages in the evolutions of these two cases are used to examine specific limitations of this conceptual model 1 upper level frontogenesis in south westerly flow that evolves from a state of equivalent barotropy to a state of cold advection along the front and 2 upper level frontogenesis in northwesterly flow with along front variation in the sign of the thermal advection such that warm advection occurs upstream of cold advection in the thermal trough vector frontogenesis diagnostics for the lagrangian rate of change of the magnitude and direction of the horizontal potential temperature gradient including tilting due to vertical motion are derived these diagnostics are applied to the two cases to examine the maintenance of the potential temperature gradient and the development of cold advection along each upper level front the upper level front in south westerly north westerly flow was maintained primarily by deformation tilting frontogenesis in agreement with previous research the increasing cold advection along the upper level front in both cases was related to an upstream vorticity maximum for the case in south westerly flow the pre existing vorticity maximum approached a downstream equivalent barotropic upper level front in a manner similar to an instant occlusion resulting in cold advection along the length of the upper level front for the case in north westerly flow an intensifying vorticity maximum concentrated the cold advection in the base of the thermal trough as warm advection developed upstream these two cases are compared to upper level fronts in previous literature and a climatology of upper level fronts associated with landfalling cyclones over the eastern north pacific ocean the results indicate that these two cases are typical of early evolutions of upper level fronts that can occur in south westerly and north westerly flow therefore a revised version of the shapiro conceptual model is presented that represents more accurately the early evolutions exhibited in the present and previous studies

the structure and evolution of a continental winter cyclone part i frontal structure and the occlusion process the frontal structure and occlusion process in a cyclone of moderate intensity that affected the central united states in january 1995 is examined the deep warm frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that although characterized by cold air advection near the surface had many of the characteristics of a warm front aloft in fact this feature had a structure similar to the so called bent back fronts previously documented only in association with explosively deepening maritime cyclones the development of a warm occluded structure was investigated with the aid of a numerical simulation of the event performed using the university of wisconsin nonhydrostatic modeling system the development of the warm occluded structure was asynchronous in the vertical occurring first at midtropospheric levels and later near the surface in contrast to the classical occlusion process near the surface the warm occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical coldfrontal zone at midtropospheric levels the warm occluded structure formed as a result of the cold frontal zone approaching and subsequently ascending the warm frontal zone in accord with a component of the classical occlusion mechanism the observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper level potential vorticity pv anomaly that controls the cyclogenesis it is suggested that the occlusion process begins aloft where the associated vortex strength is greatest and gradually penetrates downward toward the surface during the cyclone life cycle additionally a characteristic treble clef shape to the upper level pv anomaly is shown to be a sufficient condition for asserting the presence of a warm occluded structure in the underlying troposphere

energy spectrum characteristics of boreal summer intraseasonal oscillations climatology and variations during the enso developing and decaying phases the geographic dependence characteristics of the energy spectrum of the boreal summer intraseasonal oscillation bsiso may october over the indo western pacific region were analyzed using 25 yr 1979 2003 observational data the bsiso energy spectrum distribution exhibits a distinctive regional characteristic the stationary and eastward propagating modes are most pronounced at the equator 5 s 5 n while the westward propagating modes are dominant in the off equatorial region 10 20 n while the eastward intraseasonal oscillation iso spectrum agglomerates on the 30 60 day period and zonal wavenumber 1 the westward mode covers wider spatial wavenumber and temporal period range along the arabian sea bay of bengal and south china sea scs latitudes the dominant wavenumber 1 mode is the eastward westward propagation at the 30 60 day 10 20 day period for zonal wavenumber 2 the dominant mode is the westward propagation at both the 30 60 day and 10 20 day periods compared to the absolute amplitude of both zonal and meridional mode energy spectrum northward propagation is the most predominant mode in boreal summer over the indo western pacific regions the strongest northwardpropagating bsiso signal appears in the eastern tropical indian ocean the variation of bsiso differs significantly in the el nino and la nina developing and decaying phases during the el nino la nina developing summer the eastward propagation is enhanced weakened at the equator while the northward propagation is also strengthened weakened over the western pacific east of 140 e during the el nino la nina decaying summer the eastward propagation weakens strengthens at the equator opposite to that in the developing summer the westward propagation off the equator and the northward propagation over scs and the western pacific are suppressed enhanced the amplitude of the bsiso variation is stronger in the decaying summer than that in the developing summer this asymmetry in bsiso variations is primarily attributed to the asymmetry of the background mean flow change associated with the developing and decaying phases of enso

the weakened intensity of the atmospheric quasi biweekly oscillation over the western north pacific during late summer around the late 1990s abstractthe interdecadal pacific oscillation ipo shifted to a negative phase around the late 1990s its impact on the atmospheric quasi biweekly oscillation qbwo intensity over the western north pacific wnp during late summer was investigated corresponding to the phase transition of the ipo la nina like sst anomalies and an enhanced walker circulation appeared in the tropical pacific which led to decreased precipitation over the equatorial central and eastern pacific the decreased precipitation induced a gill response with an anomalous anticyclone cyclone in the lower upper troposphere over the wnp this resulted in anomalous background westerly vertical shear over the tropical wnp furthermore the anomalous anticyclone induced anomalous horizontal divergence and descent motion in the planetary boundary layer which led to decreased background surface moisture over the tropical wnp these changes in background atmospheric conditions suppressed the development of qbwo perturbations over the

mechanisms for the advanced asian summer monsoon onset since the mid to late 1990s abstractunderstanding the variability and change of monsoon onset is of utmost importance for agriculture planning and water management in the last three decades the asian summer monsoon onset asmo has remarkably advanced but the physical mechanisms underlying the change remain elusive since the overall asmo occurs in may this paper focuses on the change of mean fields in may and considers enhanced mean precipitation and monsoon westerly winds as signs of advanced asmo the results reveal that the advanced asmo mainly represents a robust decadal shift in the mid to late 1990s which is attributed to the mean state change in the pacific basin characterized by a grand la nina like pattern the la nina like mean state change controls the asmo through the westward propagation of rossby waves and its interaction with the asymmetric background mean states in the indian ocean and western pacific which intensifies the northern hemispheric perturbations and westerly winds intriguingly the abrupt decadal

the improve 1 storm of 1 2 february 2001 part ii cloud structures and the growth of precipitation abstract on 1 2 february 2001 a strong cyclonic storm system developed over the northeastern pacific ocean and moved onto the washington coast this storm was one of several that were documented during the first field phase of the improvement of microphysical parameterization through observational verification experiment improve in the 1 2 february case soundings and wind profiler measurements showed that a wide cold frontal rainband was coincident with the leading edge of an upper level cold front in a classical warm occlusion ground based radar observations revealed the presence of subbands within the wide cold frontal rainband and two layers of precipitation generating cells within this rainband one at 5 7 km msl and the other at 9 10 km msl the lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km msl observations suggest a connection between the subbands and the lower layer of generating cells a research aircraft equipped

identification of homogeneous regions for flood frequency analysis abstract a procedure for classifying basins into distinct homogeneous groups for regional flood frequency analysis is described the scheme consists of an iterative search through the basin characteristic database which optimises statistics that describe the efficiency of grouping application of the procedure to basins in britain yields five groups formed on the basis of basin area average rainfall and urban fraction four of these groups are homogeneous with respect to the coefficient of variation of the annual maximum flood series the properties of these groups compare favourably with ten geographic regions hitherto used for flood analysis in britain a further procedure is described which eliminates edge effects present in quantile estimation close to region boundaries

the occlusion process in a midlatitude cyclone over land abstract a historical review highlights the lack of consensus regarding the structure and evolution of occluded fronts and cyclones in order to better understand the occlusion process this paper and its companion examine a model simulation of the 14 16 december 1987 cyclone over the eastern united states the structure of an occluded front was observed at low levels as the cold front caught up with the warm front while aloft an upperlevel frontal zone appeared to play a role in the occlusion process although the thermal structure of this storm suggested that a cold type occlusion should have formed the model simulation produced a warm type occlusion structure an extensive review of the literature on occluded fronts suggested that cold type occlusions are rare if they exist at al1 finally the occluded front simulated in this case is compared critically to the norwegian t bone and other conceptual models of cyclone evolution

mechanism of the northward propagating intraseasonal oscillation insights from a zonally symmetric model abstract the propagation and initiation mechanisms of the boreal summer intraseasonal oscillation bsiso in the south asian summer monsoon are examined with a zonally symmetric atmospheric model in the axially symmetric model the effects of zonally propagating atmospheric waves are intentionally excluded the model specifies mean flows and depicts the lowest baroclinic mode and a barotropic mode in the free troposphere the two vertical modes are coupled by the time mean vertical wind shear the model atmosphere produces a 15 20 day oscillation which is characterized by northward propagation of convection from south of the equator to the indian monsoon trough region and a reinitiation of convection in the region between 10 s and the equator the northward propagation in the model is produced by the free troposphere barotropic divergence which leads convection by about a quarter of a cycle the vertical advection of summer mean easterly vertical wind shear by perturbation vertical motion inside the con

a model for the boreal summer intraseasonal oscillation abstract the tropical intraseasonal oscillation iso exhibits pronounced seasonality the boreal summer iso is more complex than its winter counterpart due to the coexistence of equatorial eastward off equatorial westward and northward propagating low frequency modes and their interactions based on observational evidence and results obtained from numerical experiments a mechanism is proposed for the boreal summer iso in which the northern hemisphere summer monsoon nhsm circulation and moist static energy distribution play essential roles with a climatological july mean basic state the life cycle of model low frequency waves consists of four processes an equatorial eastward propagation of a coupled kelvin rossby wave packet an emanation of moist rossby waves in the western pacific a westward propagation and amplification of the rossby waves in south asian monsoon regions and a reinitiation of the equatorial disturbances over the central indian ocean the life cycle spans about one month and p

characteristics and origin of quasi biweekly oscillation over the western north pacific during boreal summer 1 this study investigates the structure energetic and origin of quasi biweekly oscillation qbwo over the western north pacific wnp using ncep reanalyses for the years 2000 2007 in the context of vorticity there appears to be a significant qbwo mode over the wnp during the summer qbwo emerges from the equatorial region and propagates northwestward its horizontal structure exhibits a slight southwest northeast tilt but mainly longitudinal elongation in the vertical the qbwo has a northwest tilt with height that gives rise to a structure of the first baroclinic mode the centers of vorticity and vertical motion near the equator show a phase lag of about one quarter wavelength consistent with the characteristics of equatorial waves whereas the cyclonic circulation is tightly coupled with anomalous convection as the wave moves away from the equator energetic analysis of the qbwo reveals that diabatic heating in the tropics and baroclinic processes in the subtropics play important roles in the generation of eddy available potential energy eape in turn the conversion from eape to eddy kinetic energy eke and the barotropic conversion are major sources for eke to compensate the loss by eke redistribution and dissipation tracing the qbwo to equatorial disturbances our results show some features of equatorially trapped n 1 rossby mode such as phase speed and group velocity this mode is generally characterized by a zonal planetary wave number of about 6 and nearly symmetric circulation about the equator a typical case from 2002 is chosen to illustrate that the origin of the qbwo is closely associated with the theoretical equatorial rossby wave

biweekly and 21 30 day variations of the subtropical summer monsoon rainfall over the lower reach of the yangtze river basin abstract the lower reach of the yangtze river basin lyrb is located at the central region of the mei yu and baiu front which represents the subtropical east asian ea summer monsoon based on the newly released daily rainfall data two dominant intraseasonal variation isv modes are identified over the lyrb during boreal summer may august with spectral peaks occurring on day 15 the biweekly mode and day 24 the 21 30 day mode these two modes have comparable intensities and together they account for above about 57 of the total intraseasonal variance both isv modes exhibit baroclinic structures over the lyrb at their extreme phases however the genesis and evolutions associated with the two modes are different considering the genesis of their extreme wet phases over the lyrb the biweekly mode is initiated by a midlatitude jet stream vorticity anomaly moving southeastward while the 21 30 day mode is primarily associated with a low level westward propagation of an anticyclonic anomaly from

classification of drainage basins according to their physical characteristics an application for flood frequency analysis in scotland abstract if the standardised floods at all sites within a given region are assumed to be samples from the same distribution all data may be combined to yield an estimate of a regional dimensionless flood frequency relationship the identification of these hydrologically homogeneous regions can be achieved on the basis of the physical characteristics of the basin which control the hydrological processes classification of the basins independent of the discharge data avoids the problems of regionalisation based on the highly variable individual site estimates of flood frequency classification of the basins can be performed using the normix multivariate clustering algorithm the homogeneity of the basins can be tested using a likelihood ratio test of whether standardised data for all basins within a region may be adequately represented by a single regional flood frequency relationship the method was applied to 168 basins in scotland and five regions were identified corresponding to physically reasonable basin types and all but one of which yield homogeneous distributions of flood frequency

structure genesis and scale selection of the tropical quasi biweekly mode the quasi biweekly mode qbm and the 30 60 day mode are two major intraseasonal oscillations isos in the tropics the qbm is known to have a major influence in determining the active and break conditions of the indian monsoon during the northern summer a westward propagating equatorial rossby wave with quasi biweekly period influences the australian monsoon during the northern winter universality between the summer and winter qbm is established through analysis of daily circulation and convection data for 10 years it is shown that the mean spatial structure of the qbm in circulation and convection resembles that of a gravest meridional mode equatorial rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4 5 m s 1 however the maximum zonal wind occurs at around 5 n 5 s during the northern summer winter the wave structure appears to be translated northward southward by about 5 during the northern summer winter the relationship between outgoing long wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons an acceptable mechanism for genesis and scale selection of the qbm has been lacking in the present study a mechanism for genesis and scale selection of the observed qbm is proposed a simple 2 layer model that includes a steady ekman boundary layer bl formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves without influence of the background mean flow moist feedback in the presence of frictional bl convergence drives the gravest meridional mode equatorial rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator potential temperature perturbation associated with the rossby wave is in phase with relative vorticity perturbation at low level the bl drives moisture convergence in phase with the relative vorticity at the top of the bl release of latent heat associated with the bl convergence enhances the potential temperature leading to a positive feedback the mean flow over the indian ocean and western pacific at low levels is such that the zero ambient absolute vorticity or the dynamic equator shifts to around 5 n 5 s during summer winter and results in a shift of the unstable rossby waves towards the north south by about 5 the resulting structure of the unstable rossby mode resembles the observed structure of the biweekly mode it is shown that neither evaporation wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode however these processes marginally modify the growth rate and make the structure of the unstable wave more realistic