observations of a cut off low over southern australia the structure and evolution of an intense cut off low is documented in detail unprecedented for the australian region by combining high time resolution radiosonde profiles with co located wind profiler observations and routine australian bureau of meteorology analyses the cut off low develops over the ocean south of australia and subsequently interacts with a subtropical frontal system over the central part of the continent throughout the period of observation the strongest temperature gradients lay through the subtropics moreover the subtropical temperature gradients strengthened while the midlatitude temperature gradients associated with the cut off low weakened the australian bureau of meteorology s 150 km regional assimilation and prognosis system analyses capture the broad scale structure and evolution of the system reasonably well the system documented here is similar in structure to the instant occlusion investigated by browning and hill 1985 the major difference being that our cut off low develops poleward of a subtropical front the cut off low was cold cored and produced a very deep tropopause fold over adelaide the tropopause descended to an altitude of about 5 km and the temperature at 500 hpa fell to 30 c the western edge of the low level cold dome was marked by a strong secondary warm front which passed over adelaide about 8 hours after the tropopause height minimum the warm front was characterized at the surface by an absolute minimum in the pressure and a temperature rise of about 7 c significantly this increase in temperature took place in the early hours of the morning against the diurnal trend the cut off low produced very high rainfalls resulting in flash flooding and much damage in the adelaide hills area the most severe convection took place in the region behind the lowered tropopause around the passage of the warm front

an experimental investigation of kinetic fractionation of open water evaporation over a large lake the oxygen isotopes of water h218o and h216o are tracers widely used for the investigation of earth science problems the tracer applications are based on the premise that the 18o 16o ratio of open water evaporation 18oe can be calculated from environmental conditions a long standing issue concerns the role of kinetic fractionation or diffusion transport in the evaporation process here we deployed an optical instrument at a large lake area 2400 km2 to make in situ measurement of 18o and d of atmospheric vapor then determined 18o and d of open water evaporation using the gradient diffusion method our results show a much weaker kinetic effect than suggested by the kinetic factor ek adopted in some previous studies of lake hydrology 14 2 by incorporating into the h218o isotopic mass balance of the lake a lower ek value about 6 2 used for ocean evaporation in global climate models we obtain an annual lake evaporation rate that agrees with an independent eddy covariance observation but the rate is 72 higher than if the commonly used lake ek value of 14 2 is applied the applicability of this results to small lakes is uncertain and in need of field based assessment

simulations of low level convergence lines over north eastern australia we describe high resolution numerical model simulations of low level convergence lines over north eastern australia using the pennsylvania state university national center for atmospheric research mesoscale model mm5 the simulations are for selected events that were documented during the gulf lines experiment held in september october 2002 the calculations provide further insights into the dynamics of the convergence lines and the mechanisms involved in their formation in particular they show two clearly distinct convergence lines one that corresponds to the morning glory and one which corresponds to the north australian cloud line the former originates from the east coast sea breeze over cape york peninsula south of about 14 s while the latter originates from the east coast sea breeze north of this latitude they support also a recently proposed conceptual model for the generation of southerly morning glories and show for the first time the separation of a bore like disturbance following the collision of a nocturnal cold front to the south of the inland trough with a sea breeze front to the north of the trough moreover they show the progressive transition of the east coast sea breeze front and the inland cold front from gravity current like flows to bore like disturbances overnight to form north easterly and southerly morning glories respectively copyright 2006 royal meteorological society

mesoscale analysis of the activation of a cold front during cyclogenesis summary the archetypal mesoscale structure of an ana cold front comprises a narrow cold frontal rainband ncfr associated with line convection embedded within a wide cold frontal rainband wcfr due to the slantwise ascent associated with a strong transverse front mesoscale circulation whereas most mesoscale observational studies have concentrated on this mature stage the present paper explores by means of a detailed case study how such a mature frontal system develops from pre existing features the study is based upon the synthesis of data spanning many scales in the form of satellite imagery observations from conventional and multi parameter doppler radar and surface reports together with diagnostics from an operational mesoscale forecast model the cold frontal zone in this case is characterized by multiple cold fronts the dominant second cold front is studied in detail from the point of view of its evolution and its relationship to neighbouring fronts especially the leading front associated with the initial polar front cloud band the dominant cold front started as a shallow rope cloud in the cold sector and at a fairly early stage it produced a ncfr due to line convection then over a period of only a few hours a dry intrusion and an associated strong direct transverse front circulation led to the formation of a canopy of mid level cloud and a wcfr the wcfr was attributable not only to the rearward ascending outflow from the line convection but also to the re invigoration of ascent within the initial polar front cloud band that the line convection was undercutting although the transverse circulation was probably intensified by precipitation processes it is inferred that it was mainly a manifestation of the larger scale environmental flow that existed independently of the major precipitation features

wave cloud lines over northwest australia the frequency of occurrence and formation mechanisms of wave cloud lines off the northwest coast of australia are investigated prior to the present study little was known about these wave cloud lines apart from being spectacular atmospheric phenomenon in their own right these wave clouds can have a role in the secondary initiation of convection and can be a hazard to low flying aircraft a climatology of wave clouds produced from visible satellite imagery suggests two main types of cloud lines form over northwest australia the first are bore like waves similar in structure to the morning glory of northeast australia and occur at least 2 to 3 times per month throughout the entire year the second type are convectively generated cloud lines which are more circular in shape appear to originate from convective storms and occur at least 0 5 to 1 5 times per month during the wet season high resolution nested simulations are performed with the met office unified model for case studies of each type of wave the bore like waves occurred in the presence of synoptic scale low level southeasterly flow and a heat low along the northwest coast of australia at night the offshore southeasterlies accelerate into the heat low and collide with the onshore sea breeze the southeasterlies override the sea breeze and the wave cloud lines form at the leading edge of this front the convectively generated waves radiate outwards from the convective storms producing compensating subsidence and adiabatic warming these waves take the form of n 2 mode wave fronts which span the entire depth of the troposphere and are similar in structure to waves produced by deep convection which are described in previous studies

regional trends in evaporation loss and water yield based on stable isotope mass balance of lakes the ontario precambrian shield surveys abstract stable isotopes of water oxygen 18 and deuterium were measured in water samples collected from a network of 300 lakes sampled in six 100 km 2 blocks centred at 49 72 n 91 46 w 48 49 n 91 58 w 50 25 n 86 62 w 49 78 n 83 98 w 48 24 n 85 49 w 47 73 84 52 w within precambrian shield drainages in the vicinity of lake superior northern ontario canada additional sampling was also conducted within the turkey lakes watershed 47 03 n 84 38 w a research basin situated in the algoma region located 50 km north of sault saint marie ontario the studies were undertaken to gain a better understanding of hydrology and geochemistry of watersheds in the region in order to better predict acid sensitivity of lakes the main objective of this paper is to describe the hydrologic variations observed based on stable isotope results evaporative isotopic enrichment of lake water was found to be systematic across the region and its deviation from the isotopic composition of precipitation was used to estimate the evaporation inflow to the lakes as well as runoff or water yield based on a simple isotope mass balance model the analysis illustrates significant variability in the water yield to lakes and reveals a pattern of positively skewed distributions in all six widely spaced blocks suggesting that a high proportion of lakes have relatively limited runoff whereas relatively few have greater runoff such basic information on the drainage structure of an area can be valuable for site specific hydrologic assessments but also has significant implications for critical loads assessment as low runoff systems tend to be less buffered and therefore are more sensitive to acidification importantly the turkey lakes sampling program also suggests that isotope based water yield is comparable in magnitude to hydrometric gauging estimates and also establishes that uncertainty related to stratification can be as high as 20 or more for individual lakes although it likely has only a minor influence on regional survey results while further analysis in gauged lake watersheds would be beneficial to constraining the accuracy of the method or calibrating it for operational use it is nevertheless a powerful tool in its present form for lake to lake and regional runoff inter comparisons

climatology of strong intermountain cold fronts abstract motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern utah this paper presents a 25 yr climatology of strong cold frontal passages over the intermountain west and adjoining western united states using conventional surface observations and the north american regional reanalysis strong cold frontal passages are identified based on a temperature fall of 7 c or greater in a 2 3 h period a concurrent pressure rise of 3 hpa or greater and the presence of a large scale 700 hpa temperature gradient of at least 6 c 500 km 1 the number of strong cold frontal passages exhibits a strong continental signature with very few events 10 along the pacific coast and more than 200 events east of the continental divide the number of events increases dramatically from the cascade mountains and sierra nevada to northern utah indicating that the intermountain west is a frequent cold front breeding ground a composite of th

central australian cold fronts abstract this paper presents an observational study of the structure and behavior of cold fronts over central australia during the late dry season a time of year when the prefrontal convectively well mixed layer is particularly deep the study is based on the results of the central australian fronts experiments cafe held in 1991 three fronts were documented in unprecedented detail for the australian region using a greatly enhanced surface observing network and a boundary layer wind profiler as well as serial upper air soundings data on the surface energy balance were obtained also a common feature of the fronts observed during cafe was that they were dry shallow 1 km deep and moved into a deep 4 km convectively well mixed boundary layer one of them initiated major dust storms across central australia a prominent feature of the fronts was the marked diurnal variation of their surface signature as they moved through the network noteworthy was the tendency during the night for the initiatio

the dynamics of heat lows a numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea of particular interest is the evolution of the distributions of relative vorticity and potential vorticity while the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land the relative vorticity is strongest in the early morning hours following a prolonged period of low level convergence thus the heat low is not approximately in quasi geostrophic balance the low level convergence is associated with the sea breeze and later with the nocturnal low level jet the effects of differing sea area land area and coriolis parameter on various aspects of the heat low are investigated although a cyclonic vortex the heat low is characterized by an anticyclonic potential vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well mixed boundary layer however the surface temperature maximum over land corresponds with a cyclonic potential vorticity anomaly at the surface the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential temperature gradient make a non negligible contribution and of opposite sign to the potential vorticity in certain flow regions two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low level atmospheric phenomena over the arid interior of australia these are the deep convective mixing over land during the daytime and the development of a nocturnal low level jet which leads to convergence in the trough such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores it is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere

nonlinear waves ahead of fronts in the great australian bight abstract this study investigates nonlinear waves ahead of cold fronts in the great australian bight south of the australian continent these waves often form a series of roll clouds on their crests analogous to the morning glory which is observed around the gulf of carpentaria in northeastern australia high resolution visible satellite imagery from nasa s polar orbiting aqua and terra satellites between 23 october 2004 and 29 february 2008 is used to determine how frequently these cloud lines occur ahead of cold fronts a total of 14 cases are identified with the most cases occurring in summer and none occurring in winter the authors hypothesize that the summer maximum is due to a combination of lower cloud amounts associated with summertime cold fronts and a stronger maritime stable layer which is produced as hot continental air is advected offshore three cloud line events are modeled using the fifth generation pennsylvania state university national center for atmospheric research psu ncar me

isotopic variation in modern lake waters from western greenland the stable isotope composition of lake waters along sindre strimfjord in western greenland have been measured both spatially coast to ice sheet and temporally up to five times between 1998 2000 and the data compared to local climatic information there is a climatic gradient along sindre strimfjord with the head of the fjord characterised by a low arctic continental climate while the coast has a more maritime climate increased precipitation and reduced annual temperature range the change in climate from continental to maritime is reflected in the isotopic composition of the lakes the inland lakes have 18o and dd values which lie on evaporation trends away from meteoric water composition the coastal lakes have 18o and dd compositions close to meteoric water short term seasonal trends are superimposed on longer term variation for example the relatively dry years precipitation lower than the long term annual mean of 1998 and 1999 are seen in the smaller lakes as isotopic enrichment but the

on frontal zone analysis in the tropical region of the northeast brazil a frontal structure in the tropical region is different than in the extratropical region and traditional methods of frontal identification are not accurate the use of a horizontal distribution of the equivalent potential temperature and its advection is the new aspect in frontal identification and will support forecasters in their daily operational practice six fronts over 3 years were identified by the method used in operational practice twenty fronts were registered by cloud bands together with the equivalent potential temperature and its advection maps a cold front separated from the cyclone s center and affected weather on the northeast brazil

a review of cold fronts with prefrontal troughs and wind shifts the conceptual model of a classical surface based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface many cold fronts however do not conform to this model time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front hereafter called a prefrontal trough and prefrontal wind shift respectively although many authors have recognized these prefrontal features previously a review of the responsible mechanisms has not been performed to date this paper presents such a review ten disparate mechanisms with different frontal structures have been identified from the previous literature these mechanisms include those external to the front i e those not directly associated with the cold front itself synoptic scale forcing interaction with lee troughs drylines interaction with fronts in the mid and upper troposphere and frontogenesis associated with inhomogeneities in the prefrontal air mechanisms internal to the front i e those directly associated with the structure and dynamics of the front include the following surface friction frontogenesis acting on alongfront temperature gradients moist processes descent of air ascent of air at the front and generation of prefrontal bores gravity waves given the gaps in our knowledge of the structure evolution and dynamics of surface cold fronts this paper closes with an admonition for improving the links between theory observations and modeling to advance understanding and develop better conceptual models of cold fronts with the goal of improving both scientific understanding and operational forecasting

climatology and dynamics of the link between dry intrusions and cold fronts during winter part ii front centred perspective the conceptual picture of an extratropical cyclone typically includes a cold front and a dry intrusion di behind it by objectively identifying fronts and dis in ecmwf era interim data for 1979 2014 part i quantified the climatological relationship between cold fronts and dis driven by the finding that front intensity and frontal precipitation are enhanced in the presence of dis here we employ a front centred perspective to focus on the dynamical and thermodynamical environment of cold fronts with and without dis in the northern hemisphere winter distinguishing between trailing fronts that connect to a parent cyclone and isolated fronts examples of dis behind each type illustrate the baroclinic environment of the trailing front and the lack of strong temperature gradients across the isolated front composite analyses of north atlantic and north pacific fronts outline the major differences in the presence of dis compared to similar fronts but without dis in their vicinity the magnitude and spatial structure of the modification by dis depends on the front intensity yet generally with dis trailing fronts occur with stronger slp dipole deeper upper tropospheric trough stronger 10 m wind gusts enhanced ocean sensible and latent heat fluxes in the cyclone cold sector and heavier precipitation isolated weak fronts exhibit similar behaviour with different spatial structure this study highlights the central role of dis for shaping the variability of fronts and their associated environment and impact

diurnally forced convergence lines in the australian tropics observations and analyses of low level convergence lines over the gulf of carpentaria region of northeastern australia made during the second and third gulf lines experiments glex ii and glex iii are reported during glex ii the initiation of the north australian cloud line nacl was documented using a small autonomous aircraft and two doppler sodars nacls are long convergence lines that form on the western side of northern cape york peninsula and are typically marked by a line of contiguous cumulus or cumulus congestus cloud the observations show that the degree of asymmetry between west coast and east coast sea breezes as characterized by the wind field depends on the strength of the background easterlies and that nacls develop only when the background easterlies are sufficiently large 5 m s 1 glex iii focused on morning glories which are mostly southwestward moving bore like convergence lines that originate over the southern part of the gulf region the field experiment involved doppler sodar measurements as well as high temporal resolution data from operational automatic weather stations the sodar measurements showed that undular bore like morning glory cloud lines develop only when the background easterlies are sufficiently weak 10 m s 1 and that strong bore like morning glories develop when the background winds exceed about 10 m s 1 if the background easterlies are too strong no morning glories develop numerical simulations show that the structure of convergence line produced depends on the strength of the collision between the sea breezes from each side of the cape york peninsula which in turn depends on the strength of the background easterlies when the easterlies are weak 10 m s 1 the sea breezes have similar depths and strengths and their collision is relatively violent whereas when the background easterlies are strong 10 m s 1 the sea breezes have very different depths and strengths and their collision is comparatively benign

the dynamics of heat lows in simple background flows a three dimensional numerical model is used to study the evolution of an idealized heat low that develops over an isolated level land mass surrounded by sea calculations are carried out without a background flow and for simple background flows in all cases the flow shows a significant diurnal variation in which the sea breeze circulations are the prominent feature during the daytime and the nocturnal low level jet is the prominent feature at night if the land area is large enough so that the sea breezes do not cover it entirely during the diurnal cycle strong convergence associated with the low level jet leads to the formation of intense but shallow cold fronts along the inland boundaries of sea breeze air these fronts decay rapidly after sunrise when surface heating leads to renewed vertical mixing which destroys the low level stable layer the presence of a uniform easterly flow leads to the formation of a west coast trough similar to the situation commonly observed over western australia the trough broadens during the day due to the heating of the land while at night it sharpens and again frontogenesis occurs at low levels near the leading edge of the sea breezes to form shallow cold fronts in these calculations there is an east west asymmetry in the sea breeze circulations the cool air behind the west coast sea breeze is shallower than that behind the east coast sea breeze but the vertical circulation associated with the west coast sea breeze is deeper the east coast sea breeze front penetrates further inland but is more diffuse and is recognizable more by its signature in the relative vorticity than by that in the horizontal temperature gradient the presence of a horizontal shear flow leads to the deformation of the heat trough a process that appears to play an important role in the formation of cold fronts over central australia again the calculations show the formation of shallow fronts over the land during the night which frontolyse rapidly after sunrise despite the idealized nature of the calculations the associated patterns of low level vorticity divergence and horizontal temperature gradient that develop overnight show remarkable similarity to those observed over central australia indeed the calculations help to interpret the observations of nocturnal frontogenesis over this region copyright 2005 royal meteorological society

groundwater mixing dynamics at a canadian shield mine temporal and spatial variations in geochemistry and isotopes in mine inflows at the con mine yellowknife are studied to access the impact of underground openings on deep groundwater flow in the canadian shield periodic sampling of inflow at 20 sites from 700 to 1615 m depth showed that salinities range from 1 4 to 290 g l with tritium detected at all depths three mixing end members are identified 1 ca na cl shield brine 2 glacial meltwater recharged at the margin of the retreating laurentide ice sheet at 10 ka and 3 modern meteoric water mixing fractions calculated for inflows on five mine levels illustrate the infiltration of modern water along specific fault planes tritium data for the modern component are corrected for mixing with brine and glacial waters and interpreted with an exponential piston flow model results indicate that the mean transit time from surface to 1300 m depth is about 23 years in the early period after drift construction in 1979 but decreases to about 17 years in the past decade the persistence of glacial meltwater in the subsurface to the present time and the rapid circulation of modern meteoric water since the start of mining activities underline the importance of gradient in addition to permeability as a control on deep groundwater flow in the canadian shield q 2000 elsevier science b v all rights reserved

on the significance of δ13c correlations in ancient sediments a graphical analysis of the correlations between c and etoc was introduced by rothman et al 2003 to obtain estimates of the carbon isotopic composition of inputs to the oceans and the organic carbon burial fraction applied to cenozoic data the method agrees with independent estimates but with neoproterozoic data the method yields results that cannot be accommodated with standard models of sedimentary carbon isotope mass balance we explore the sensitivity of the graphical correlation method and find that the variance ratio between c and o is an important control on the correlation of c and e if the variance ratio c o 1 highly correlated arrays very similar to those obtained from the data are produced from independent random variables the neoproterozoic data shows such variance patterns and the regression parameters for the neoproterozoic data are statistically indistinguishable from the randomized model at the 95 confidence interval the projection of the data into c e space cannot distinguish between signal and noise such as post depositional alteration under these circumstances there appears to be no need to invoke unusual carbon cycle dynamics to explain the neoproterozoic c e array the cenozoic data have c o 1 and the c vs e correlation is probably geologically significant but the analyzed sample size is too small to yield statistically significant results

observation of a nonlinear wave disturbance in the marine atmosphere by the synthetic aperture radar aboard the ers 1 satellite a nonlinear wave disturbance in the marine atmosphere associated with an atmospheric cold front has been identified on a spaceborne radar image of the north sea by its imprint on the sea surface this nonlinear wave disturbance is either a solitary wave disturbance or an undular bore propagating in the lower marine atmosphere the radar image showing sea surface manifestations of this atmospheric wave disturbance was acquired over the north sea by the synthetic aperture radar sar aboard the first european remote sensing satellite ers 1 on march 8 1992 atmospheric waves become visible on radar images of the sea surface because they are associated with a varying wind field at the sea surface which modulates the sea surface roughness and thus the backscattered radar power the identification of the wave pattern visible on the ers 1 sar image as a sea surface manifestation of an atmospheric nonlinear wave disturbance has been possible because this disturbance was also captured by meteorological sensors mounted on an 80 m high meteorological mast located on the island of heligoland geophysical parameters characterizing the atmospheric wave disturbance are derived from the ers 1 sar image and from the in situ meteorological data the wave period in the reference frame moving with the average wind is 9 5 min the average wavelength is 2300 m and the propagation velocity is 4 5 4 9 m s 1 estimates of the amplitude of the wind speed fluctuations in the wave propagation direction derived from the meteorological measurements at heligoland and from the gray level variations in the sar image yield values between 1 3 and 3 0 m s 1 the experimental data are interpreted in terms of theories of atmospheric nonlinear gravity waves

kinematic and moisture characteristics of a nonprecipitating cold front observed during ihop part i across front structures abstract a wide array of ground based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west central kansas on 10 june 2002 during the international h2o project ihop this study the first of two parts is focused on describing structures in the across front dimension coarsely resolved observations from the operational network and dropsondes deployed over a 200 km distance centered on the front are combined with higher resolution observations from in situ sensors doppler radars a microwave radiometer and a differential absorption lidar that were collected across a 40 km swath that straddled a 100 km segment of the front the northeast southwest oriented cold front moved toward the southeast at 8 10 m s 1 during the morning hours but its motion slowed to less than 1 m s 1 in the afternoon in the early afternoon the cold front separated cool air with a northerly component flow of 2 4 m s 1 from a 10 km wide ban

dynamics of solitary waves observed over the north indian ocean during the indian ocean experiment indoex 1999 during the indian ocean experiment indoex experiment january march 1999 mesoscale solitary waves have been observed and tracked over the north indian ocean on a series of meteosat 5 satellite images these solitary waves have a horizontal wavelength of 10 15 km and propagate westward at low level at a speed of 10 18 m s unlike similar wave phenomena observed mainly over land they have a long lifetime which can exceed 48 h a key element explaining the existence and longevity of the solitary waves is the presence of an inversion layer acting as a waveguide and separating the boundary layer into two sublayers a lower layer over the ocean marine boundary layer and an upper layer originating from the indian subcontinent land plume layer profiles from radiosondes launched from the ron brown and from dropsondes from the hercules c 130 airplane helped to determine this waveguide structure a suggested mechanism leading to the generation of solitary waves is a collision between the sea breeze in the vicinity of the west indian coast and the easterly northeasterly winter monsoon winds with a possible contribution of convection reinforced by topography western ghats range during the local afternoon another phenomenon related to sea breeze and local convection in this coastal area the injection of bubbles of moisture into the drier upper boundary layer has also been identified on european centre for medium range weather forecasts analyses these bubbles form daily during the afternoon and drift westward over the north indian ocean at latitudes around 12n and progressively subside and dissipate or become integrated into larger air masses copyright 2011 by the american geophysical union

analysis of the slope of isentropic surfaces and its tendencies over the north atlantic the maintenance of baroclinicity along mid and high latitude storm tracks is a matter of ongoing debate using an isentropic framework a novel diagnostic based on the tendency equation for the slope of isentropic surfaces a measure of the potential for baroclinic development is presented the tendency comprises contributions from dynamic processes latent heat release radiation and subgrid scale turbulence which incorporates the effect of sensible heat fluxes a climatology of these tendencies over the north atlantic is compiled for the winters of 2009 and 2010 it is found that adiabatic tilting flattens the isentropic surfaces reflecting the action of growing baroclinic cyclones this tendency is balanced climatologically by the generation of slope by diabatic processes in the lower troposphere the most intense diabatic increase of slope is found along the oceanic frontal zone associated with the gulf stream and at higher latitudes in the labrador sea the nordic seas and the barents sea latent heat release and sensible heat fluxes both contribute substantially in these regions a quantitative analysis of cold air outbreaks emphasizes their important role in restoring the slope in the lower troposphere over the gulf stream region and off the sea ice edge at high latitudes in the upper troposphere latent heat release due to cloud microphysical processes is the dominant mechanism maintaining the slope

runoff to boreal lakes linked to land cover watershed morphology and permafrost thaw a 9 year isotope mass balance assessment stable isotopes of oxygen and hydrogen were measured in water samples collected annually from a representative suite of 50 lakes in northeastern alberta over a 9 year period and are interpreted using a steady state isotope mass balance model to determine water yield and runoff ratios for the lake watersheds and residence time of the lakes this isotopic perspective on hydrology of the region provides new insight into the role of land cover watershed morphometry climatic drivers and permafrost thaw on lakes bog cover permafrost and presence of thaw features in bogs are found to be the dominant hydrologic drivers although morphometric properties such as elevation lake area and drainage basin area are also influential in addition to quantifying the hydrologic fluxes the analysis establishes contrasting conditions in more southerly lakes located in the stony mountains and west of fort mcmurray as compared with more northerly sites in the birch mountains caribou mountains and northeast of fort mcmurray mainly because of contributions from thawing permafrost at the northerly sites distinct hydrologic conditions are also noted for shield systems north of lake athabasca where bogs and permafrost are absent while permafrost thaw is not directly labelled by oxygen and hydrogen isotope composition isotope mass balance calculations suggest that contributions of up to several hundred millimetres per year are occurring in 14 of the 50 lake watersheds under study several of these lakes have water yields in excess of precipitation in some years and regional groups of lakes display significant correlations between water yield and percentage of bogs that have collapsed copyright 2015 john wiley sons ltd

southerly nocturnal bores over northeastern australia analyses of 1 min data from automatic weather stations awss in the gulf of carpentaria region are used to study southerly nocturnal bores over northeastern australia the data cover the time period 1 august to 31 december in the three years 2012 2014 a total of 12 days on which a bore passed over the region of interest were found ten of the disturbances occurred between august and mid october in the presence of synoptic scale ridging across the continent behind a frontal trough the other two occurred in early november in the presence of localized ridging behind an inland trough separate from a frontal trough six of the bores were followed some hours later by a clear air mass change four of the 12 cases are examined in detail wherein interpretations of the aws data are aided by analysis fields from the european centre for medium range weather forecasts ecmwf the results indicate that the longevity of the bore is related to the time of day that the cold front interacts with the stable maritime layer to the north of it the ecmwf analyses are able to capture the frontal troughs as well as the timing of their passage through the aws network in the analyses the troughs are seen as narrow strips of elevated cyclonic vorticity at 950 mb vertical cross sections intersecting the troughs showed structural characteristics of a shallow cold front but the horizontal resolution of the analyses is inadequate to capture the generation of bores ahead of the fronts

a climatology of atmospheric pressure jumps over southeastern australia black saturday provided the first evidence of an atmospheric bore affecting the behaviour of a bushfire as the bore passed the fire unexpectedly strengthened this behaviour highlighted the lack of understanding of how common bores are in the southeastern part of australia a region of relatively high bushfire risk the present study addresses that lack of understanding pressure jumps are identified in the one minute records at four automatic weather stations in southeastern australia by correlating the pressure time series with a large amplitude step function these jumps are then separated in two classes bores and frontal pressure jumps bores are defined as pressure jumps without a change in relative humidity whereas frontal pressure jumps are defined by jumps with an accompanying decrease in temperature greater than 3 oc about 15 pressure jumps per station per year are found most jumps are found in the spring and summer and fewest in winter bores are found most frequently in the early morning and late evening at most stations whereas frontal pressure jumps are most frequently found in the late afternoon or early evening following their passage frontal pressure jumps are associated with higher 30 min mean wind speeds than bores 9 0 m s 1 and 6 1 m s 1 respectively both of which are higher than climatology 4 7 m s 1

lake landscape position relationships to hydrologic connectivity and landscape features to improve our understanding of lake landscape position we compared four metrics based on different aspects of lake surface hydrologic connections 1 lake hydrology which is a general measure of lake surface hydrologic position 2 lake order which measures connections to streams by stream order 3 lake network number which measures connections to other lakes and 4 lake network complexity which measures the complexity of connections to other lakes in a chain or branched we sampled 71 lakes in northern michigan u s a and measured lake landscape position and landscape characteristics around each lake e g land use cover and geology to answer two questions 1 which metric of landscape position explains the most variation in lake water chemistry clarity and 2 what landscape and physical features are also related to landscape position all four landscape position metrics explained significant variation in some water chemistry clarity variables however lake order the metric based on stream order consistently explained the most variation ranging from 22 dissolved organic carbon to 53 conductivity and calcium with lake hydrology the metric based on both streams and lakes explaining similar amounts of variation to lake order but less overall landscape position was also significantly related to both lake morphometry and the proportion of wetland types in buffer areas which may help explain why landscape position is related to lake water chemistry and clarity variables

remote sensing of volumetric storage changes in lakes three dimensional remote sensing promises a giant leap forward for surface water hydrology in much the same way that radar altimetry transformed physical oceanography however the complex geometries of small terrestrial water bodies introduce difficulties particularly with respect to trade offs between changing water depth and inundation area we use in situ measurements of water surface stage h dt and remotely sensed area a to compute time varying storage changes s in nine lakes of the peace athabasca delta canada despite their identical geomorphic setting regression slopes between h and a vary significantly between lakes primarily from a predictable area effect but also small bathymetric variations between basins on average lateral contraction expansion versus stage adjustment contributes as little as 7 versus 93 to as much as 76 versus 24 of overall storage change s we conclude that both surface area and h dt rather than just either alone must be measured to confidently estimate s from space copyright 2009 john wiley sons ltd

the australian nocturnal jet and diurnal variations of boundary layer winds over mt isa in north eastern australia an analysis of radiosonde data shows the nocturnal boundary layer jet over australia to be continental in scale the detailed temporal development of the jet is described using two years of boundary layer wind profiler data from mt isa in north eastern australia a distinct isallobaric wind is found which has a spectral peak near the inertial period this is associated with the diurnal pressure oscillation and developments in the heat low over central and western australia however it is not important for the development of the nocturnal jet the nocturnal jet begins as a shallow disturbance and grows through the night most of the year the wind maximum is at about 500 m above the ground and has a well defined jet profile in the summer monsoon season the vertical extent of the wind maximum increases but has weaker vertical shear the jet results from an inertial response but is affected by other processes the amplitude of the jet is less than would be expected from a straightforward response to the daytime ageostrophic flow and there is super inertial rotation and damping of the wind vector late in the night

the dynamics of heat lows over flat terrain the numerical model for a heat low developed by racz and smith is extended to include a representation of radiative heating and cooling the model is run with a higher horizontal resolution than the original version and is used to investigate additional dynamical aspects of the structure and evolution of a heat low over a subcontinental or continental scale circular island surrounded by sea of particular interest is the diurnal and day to day evolution of the upper and lower level circulations and the degree of balance that exists in these the heat low is surmounted by an anticyclone the development of which is closely tied to the outflow branch of the sea breeze the anticyclone has a much smaller diurnal variation than the heat low and unlike the heat low is largely in balance except in the region affected by the upward propagating gravity wave induced by the inland penetrating sea breeze there is a strong analogy to certain aspects of tropical cyclones which have a warm core a shallow unbalanced boundary layer and which are surmounted also by an anticyclone principles governing the absolute angular momentum budget are the same as those relating to the tropical cyclones and to the zonal mean flow over antarctica implications of these principles for obtaining a realistic steady state in long term integrations of axisymmetric models are discussed copyright 2008 royal meteorological society

a coupled isotope tracer method to characterize input water to lakes summary we develop a new coupled isotope tracer method for characterizing the isotopic composition of input water to lakes and apply it in the context of ongoing hydrological process studies in the peace athabasca delta a large remote riparian ecosystem in the boreal region of western canada the region has a highly seasonal climate with floodplain lakes typically receiving input only during the 4 6 month open water season from varying proportions of spring snowmelt summer rains and river flooding these possible input sources have distinct ranges of isotopic compositions that are strongly constrained to a well defined local meteoric water line thus affording the opportunity to derive lake specific estimates of the integrated isotopic composition of input waters after accounting for the effects of secondary evaporative isotopic enrichment as shown by comparison of the results of isotopic surveys of delta lakes prior to freeze up in 2000 and 2005 this isotopic characterization of input waters can be combined with other data and field observations to provide new insight into spatial and temporal variability in delta lake recharge processes this includes evidence that summer rainfall in 2000 played an important role in replenishing shallow basins delta wide especially in the central low lying region compensating for below average snow accumulation during the previous winter in contrast 2005 was marked by greater relative contributions from both snowmelt and river flooding because of high winter snow accumulation and a spring ice jam that caused river floodwaters to enter some basins in the southern part of the delta the method is readily transferable to investigations in other remote regions that are sparsely monitored by conventional hydrometric networks

an observational and modeling study of an atmospheric internal bore during name 2004 abstract observations from the 2004 north american monsoon experiment name have been used to identify an atmospheric internal bore that occurred over the gulf of california goc on 31 july 2004 this bore disturbance was identified at bahia kino along the northwest coast of mexico during the late evening of 31 july it was hypothesized to have originated from the interaction of a gravity current from a large mesoscale convective system mcs which formed along the western slopes of the sierra madre occidental during the afternoon and a surface stable layer that developed from a sea breeze circulation it is suggested that the bore s energy was trapped at low levels by an elevated stable layer the vertical structure and undular nature of the bore was initially identified from 915 mhz wind profiler data at bahia kino results show a series of waves along the bore s leading edge and turbulent mixing of air from above the stable layer to the surface on the downstream face of the leading undulation the

the diurnal evolution of cold fronts in the australian subtropics the effect of the diurnal cycle on the evolution of cold fronts in the australian subtropics is investigated in two high resolution numerical simulations the simulations are made using the pennsylvania state university national center for atmospheric research mesoscale model mm5 with the initial and boundary conditions taken from the operational analyses of the european centre for medium range weather forecasts these simulations are compared with the observations of two cold fronts taken during the central australian fronts experiment in 1991 the simulations show a number of features that have been suspected but never confirmed or quantified i although the wind field in the boundary layer is frontogenetic daytime turbulent mixing is strongly frontolytic which accounts for the weakening and deceleration of the fronts during the late morning and afternoon when convective mixing in the boundary layer is most vigorous ii when the mixing subsides in the early evening the low level winds increase along with the deformation and convergence leading to a strengthening and acceleration of the fronts iii bore like disturbances are generated during the early hours of the morning as the nocturnal inversion strengthens these bores propagate ahead of the front developing a series of large amplitude waves at their leading edge copyright c 2009 royal meteorological society

a case study of saharan cyclogenesis a case of saharan cyclogenesis associated with the equatorward intrusion of a trough at the end of the atlantic storm track is investigated it is shown that a potential vorticity anomaly resulting from a baroclinic wave life cycle associated with the polar jet interacts with the low level baroclinicity over subtropical africa beneath the subtropical jet it is suggested that low latitude synoptic scale cyclogenesis events of this type can be triggered only by upper level potential vorticity anomalies if they have a sufficient depth scale and that the interaction may be aided by the presence of low static stability associated with a well mixed boundary layer a weak cold front also forms during the cyclogenesis event associated with the convergence of the baroclinicity of the polar trough front with the baroclinicity of the subtropical heat low

controls on recent alaskan lake changes identified from water isotopes and remote sensing 1 high latitude lakes are important for terrestrial carbon dynamics and waterfowl habitat driving a need to better understand controls on lake area changes to identify the existence and cause of recent lake area changes in the yukon flats a region of discontinuous permafrost in north central alaska we evaluate remotely sensed imagery with lake water isotope compositions and hydroclimatic parameters isotope compositions indicate that mixtures of precipitation river water and groundwater source 95 of the studied lakes the remaining minority are more dominantly sourced by snowmelt and or permafrost thaw isotope based water balance estimates indicate 58 of lakes lose more than half of inflow by evaporation for 26 of the lakes studied evaporative losses exceeded supply surface area trend analysis indicates that most lakes were near their maximum extent in the early 1980s during a relatively cool and wet period subsequent reductions can be explained by moisture deficits and greater evaporation

terrain influences on the dynamics of heat lows the hydrostatic primitive equation numerical model we used earlier to study the basic dynamics of heat lows in an idealized flow configuration is extended to investigate the effects of orography calculations are carried out over a heated land mass with an elevated plateau for the case with no surrounding sea as well as with the sea present we investigate the effect of the shape of the terrain on the mature heat low in the presence of orography the mixed layer over land becomes deeper than without orography and the induced flow is stronger also the width of the plateau influences among other things the relative vorticity distribution and the nocturnal jet development the flow behaviour in the presence of orography provides an explanation for the different vertical profiles of vertical velocity in heat lows discussed in the literature copyright 2003 royal meteorological society

the iberian peninsula thermal low statistics of the thermal low above the iberian peninsula are presented for the period between 1979 and 1993 based on gridded data from the european centre for medium range weather forecasts reanalysis project the sample of days with a thermal low above the iberian peninsula was selected objectively using criteria applied to mean sea level pressure and 925 hpa geopotential fields the synoptic and regional scale horizontal structure is characterized by the climatology of the 500 hpa geopotential and mean sea level pressure distributions the diurnal evolution of the mean sea level pressure is portrayed by mean fields at 00 06 12 and 18 u tc the climatological vertical structure of the thermal low is shown by relation to meridional and zonal cross sections passing through the thermal low s centre the diurnal evolution of the divergence relative vorticity potential temperature and vertical velocity is investigated statistics are presented also for the monthly frequency geographical location vertical extent and intensity of the iberian thermal low copyright 2003 royal meteorological society

the effect of the continental boundary layer on the dynamics of fronts in a 2d model of baroclinic instability ii surface heating and cooling the effect of surface heating and cooling on the structure and evolution of the surface front that develops in the finite amplitude eady wave is investigated spatially uniform short wave radiative heating of the surface is weakly frontolytic although it appears this result may be sensitive to the way in which the boundary layer turbulence is parametrized the cross front potential temperature gradient is weakened principally by the differential heating produced by horizontal variations in the depth of the mixed layer and to a smaller extent by horizontal variations in the surface sensible heat flux the direct effect of enhanced daytime turbulent stresses is to retard the winds in the boundary layer reducing the convergence in the cross front plane while the indirect effect is to produce a strong cross isobaric flow towards the trough thereby increasing the convergence the net effect is to reduce the rate of frontogenesis near the surface front during the day the front is oriented vertically in the boundary layer because of the strong mixing nocturnal cooling through long wave radiative emission from the surface is strongly frontogenetic at night as the turbulence decays in the trough ahead of the surface front air parcels accelerate down the pressure gradient towards the centre of the trough producing low level cross front surges on both sides of the trough axis this enhanced nocturnal convergence rapidly strengthens the cross front potential temperature gradient and tilts the front westward with height such nocturnal frontogenesis is especially intense when it follows strong daytime turbulent mixing copyright 2005 royal meteorological society

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

a six year isotopic record of lake evaporation at a mine site in the canadian subarctic results and validation an isotopic method is applied in conjunction with a water balance method and the penman combination method to estimate evaporation from a small high closure low outflow lake near yellowknife northwest territories canada 62803 0 n 111824 0 w the study provides baseline hydrological information for assessment of tailings pond design and management at nearby mine sites and notably enables intercomparison of several field based evaporation methods and a standard climate approach in a subarctic setting a non steady isotope mass balance method is applied to estimate evaporation over time intervals ranging from five days to three weeks based on isotopic surveys of lakewater groundwater precipitation and atmospheric moisture during the open water periods of 1991 to 1996 use of a relatively high precision non steady technique in contrast to the commonly employed approach assuming steady state is feasible in the present setting owing to pronounced seasonal evaporative enrichment in lake water 20 30 times analytical uncertainty of d 18 o a comparative analysis reveals that the isotopic method is conservative relative to the penman combination method but less conservative than standard water balance although estimates for the open water period are in agreement to within 20 in both cases interannual variability in evaporation is revealed to be 30 50 greater than predicted from standard pan to lake algorithms and of the same order of magnitude as the annual snow water equivalent 155 175 mm which has important implications for the design and management of tailings ponds in the area 1998 john wiley sons ltd