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

an atmospheric solitary gust observed with a doppler radar a tall tower and a surface network abstract doppler radar and a 444 m tall instrumented tower provide a detailed view of the kinematic and thermodynamic structure of a solitary gust a study of the data fields and comparison with theoretical and laboratory work leads to the conclusion that the gust is an internal solitary wave of permanent form launched by a thunderstorm outflow onto an inversion layer formed by the passage of an earlier storm comparisons with results from fluid experiments and numerical models are made and the similarities are striking both observations show turbulent breakdown at the rear of the wave the ease with which solitary waves can be generated in experiments gives reason to believe that these nonlinear waves might be of considerable interest in the context of geophysical fluid dynamics

bioprotection explored the story of a little known earth surface process abstract bioprotection is identified as an earth surface process however it has been little studied little acknowledged and yet may have major implications for the operation and management of geomorphic systems key early observations are traced back to darwin s voyage of the beagle geikie in the natural environment and watson for the built environment recent field observations and experimental work examining bioprotection are reviewed with a specific focus on lichens and the landscape as are its complex interactions with other processes a conceptual model of bioprotection is presented for the case of an epilithic lichen on a limestone surface

travelling waves and bores in the lower atmosphere the morning glory and related phenomena abstract observations of solitary wave and travelling bore like disturbances in the lower atmosphere are reviewed together with relevant theoretical laboratory and numerical studies concerning the structure propagation and genesis of such disturbances primary emphasis is given to a discussion of the morning glory phenomenon in northern australia the example par excellence of these types of disturbance and the best documented observationally to date observations of related phenomena elsewhere are reviewed briefly while such occurrences appear to be less frequent in any single location than in the southern gulf of carpentaria region of northern australia reported occurrences are widespread and there is mounting evidence that such disturbances are more common than is generally supposed the evidence suggests that whenever a suitable wave guide exists for the propagation of disturbances the latter can be generated as the natural response of quite general forcing mechanisms examples being the collision of two sea breezes from opposite directions on a peninsula the interaction between a thunderstorm outflow and a sea breeze front or simply the transformation of a thunderstorm or squall line outflow as it moves into a low level stable layer because of the large vertical displacements they produce the larger amplitude disturbances typically with surface pressure jumps of 1 mb or more must be considered as potential triggers for deep convection if the atmosphere is sufficiently conditionally unstable and the large low altitude wind shears associated with them present a serious hazard to low flying aircraft

turbulent mixing processes in atmospheric bores and solitary waves deduced from profiling systems and numerical simulation abstract families of solitary waves solitons associated with two atmospheric bores on the same day were observed by an unprecedented number of ground based and airborne profiling systems during the international h2o project ihop in addition a very high resolution numerical weather prediction model initialized with real data was used with success to simulate one of the bores and the evolving soliton the predicted wave amplitude phase speed wavelength and structure compared well to these extraordinarily detailed observations the observations suggest that during the active phase when turbulent mixing was active which was prior to bore collapse the bores and waves vigorously mixed dry air from above a nocturnal boundary layer down to the surface refractivity computed from near surface radar observations showed pronounced decreases due to sudden drying during the passage of the bores in this phase but refractivity increases appeared during the period of bore collapse during both phases the

finescale vertical structure of a cold front as revealed by an airborne doppler radar abstract in the afternoon of 24 may 2002 a well defined and frontogenetic cold front moved through the texas panhandle detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary this paper primarily uses reflectivity and doppler velocity data from an airborne 95 ghz radar as well as flight level thermodynamic data to describe the vertical structure of the cold front as it intersected with the dryline the prefrontal convective boundary layer was weakly capped weakly sheared and about 2 5 times deeper than the cold frontal density current the radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail 40 m the echo structure and dual doppler inferred airflow in the vertical plane reveal typical features such as a nose a head a rear inflow current and a broad current of rising prefrontal air that feeds the accelerating front to rear current over the head the 2d cross fro

measuring erosion with the micro erosion meter contributions to understanding landform evolution abstract we review the contribution made to understanding landform evolution through the use of the micro erosion meter mem and the variant the traversing micro erosion meter tmem the mem has allowed the direct measurement of bedrock erosion in terrestrial and coastal settings as well as of building stone as recorded in mem measurements world wide the average rate of surface lowering on shore platforms is 1 486 mm a 1 and on terrestrial rock surfaces measured lowering rates are on average 0 903 mm a 1 reported means obscure significant variation in rates from a range of environmental settings and lithologies the mem has in a number of examples allowed the identification of the contribution of individual processes in polygenetic settings particularly in shore platform studies the dynamic nature of rock weathering has been highlighted where the tmem has revealed rock surfaces swelling and contracting at time scales ranging from hours to years the mem will continue to provide useful data on erosion rates and processes but the most fruitful investigations will be those that combine the t mem with other techniques that transcend time and space scale issues in erosion and process measurement

early miocene anatexis identified in the western syntaxis pakistan himalaya abstract evidence for typical himalayan early to middle miocene anatexis has remained elusive in the nanga parbat massif in the western himalaya of pakistan previous work has identified only young plutonism 10 1 ma new u th pb data from the southern chichi granite a leucogranite in southern nanga parbat reveal that crustal melting occurred during the early miocene this largely undeformed fine grained pluton intrudes the indian metasedimentary cover sequence adjacent to the rupal shear a major shear zone at nanga parbat th pb ion microprobe analyses of monazites from the chichi granite yield ages between 22 ma and 16 ma with the majority of analyses lying at 19 18 ma u pb zircon analyses yield ages which fall along a chord with a lower intercept age of 19 ma the zircons also contain an 1850 ma inherited component these data indicate that the early miocene anatexis that is ubiquitous in central portions of the himalayan orogen unreported anywhere in the nw himalaya also occurred in the western himalayan syntaxis and demonstrates that nanga parbat has a protracted melting history a small granitic dike that cross cuts the outer portion of the rupal shear yields monazite ages between 22 ma and 9 ma where the young ages correlate with high u concentrations 40ar 39ar biotite ages from adjacent gneisses indicate cooling by 10 ma requiring significant displacement on this portion of the rupal shear to be older than 10 ma and possibly as old as 20 ma

microscale biopitting by the endolithic lichen verrucaria baldensis and its proposed role in mesoscale solution basin development on limestone data are reported demonstrating the potential role of microscale morphologies induced by endolithic lichen communities specifically verrucaria baldensis in the initiation and development of mesoscale solution basin formation on limestone in the burren county clare a biophysical model is proposed outlining the different microscale stages leading to solution basin initiation with a progression from initial lichen colonization and growth associated perithecial biopitting followed by biopit coalescence to form biotroughs their subsequent enlargement and eventual incipient solution basin formation this model provides one explanation for solution basin development as this end state may also be achieved through simple solutional means without biological input the complexity of interactions at the lichen rock interface are identified with emphasis on the spatial and temporal variability of these underlining the point that as with macrotopographies at the landscape scale rock surface microtopographies also reflect historical weathering legacies copyright 2011 john wiley sons ltd

surface hardness as a proxy for weathering behaviour of limestone heritage a case study on dated headstones on the isle of portland uk this study estimates stone weathering rates using in situ surface hardness testing surface hardness changes are precursors to erosion and may be utilized to describe stone weathering behaviour the method proposed here complements previous approaches to determining stone weathering rates by surface loss change a time series covering 1 248 years of exposure is investigated using a sample of 12 headstones in two nearby cemeteries using an equotip d surface hardness tester rates of change in surface hardness for top and bottom sections of the headstones were determined and the data evaluated using robust nonparametric statistical methods when considering all headstones as one time series nonlinear behaviour is observed with rates of decline in surface hardness slowing over time however breakpoint analysis shows a breakpoint at c 100 years with higher rates of surface hardness decline as measured by qc 50 the regression coefficient for 0 50 quantile regression up to that point and lower rates thereafter up to c 100 years surface hardness declines more rapidly in the top versus bottom sections possible explanations for the differing rates in surface hardness changes are a inherent natural stone variability and or different weathering stress history b the use of two different portland limestone varieties c synergistic effects of microclimates and lichen cover in order to gain a deeper insight into stone weathering behaviour future studies could combine surface hardness measurements with surface change methods such as micro erosion meter and lead plug index over short and long term time series on architectural heritage under real world conditions

decadal scale micro erosion rates on shore platforms abstract knowledge of the long term rates of erosion of shore platforms are needed to assist modeling platform development understand rates and modes of development and decipher issues of inheritance from former sea level few studies report rates from more than 2 years of monitoring we report decadal scale erosion rates using both a micro erosion meter and a traversing micro erosion meter with measurements from 30 years and 10 years respectively gross mean surface lowering rates measured over 10 years with a traversing micro erosion meter were 0 901 s e 0 116 mm yr compared with 1 130 s e 0 217 mm yr over two years over thirty years 1973 2003 12 micro erosion meter sites provide a mean surface lowering rate of 1 09 s e 0 126 mm yr compared to 1 43 s e 0 128 mm yr calculated over twenty years 1973 1993 from 15 bolt sites and 1 53 s d 1 45 mm yr from 31 bolt sites rates calculated over longer time periods are reduced compared to short term rates as a result of faster eroding bolt sites being removed from the data set comparison of means between short and long time periods for existing bolts show no significant difference that is short term measurements over two years are representative of decadal scale erosion rates in fact decadal scale rates may under represent rates of platform development because of the loss of faster eroding bolt sites the loss of such bolts means that obtaining long term erosion rates is problematic future modeling of shore platform development can use mem rates with more certainty but we recommend using higher rates from shorter term studies unless longer records are complete

initiation of convective storms at radar observed boundary layer convergence lines abstract the origin of 653 convective storms occurring over a 5000 km2 area immediately east of the colorado rocky mountains from 18 may to 15 august 1984 was examined seventy nine percent of the 418 storms that initiated within the study area occurred in close proximity to radar observed boundary layer convergence lines this percentage increased to 95 when only the more intense storms 60 dbze were considered colliding convergence lines initiated new storms or intensified existing storms in 71 of the cases a new storm took a median time of 24 min to grow to 30 dbz following line collision the convergence lines ranged in length between ten and several hundred kilometers both radar and mesonet stations indicated that the primary convergence was concentrated in a zone 0 5 to 5 km in width these lines were characterized on doppler radar as thin lines of enhanced reflectivity between 0 and 20 dbze and as a line of strong radial or azimuthal gradient in doppler velocity these lines were observed ev

simulation of the dissolution of weathered versus unweathered limestone in carbonic acid solutions of varying strength a simulation was undertaken within a climatic chamber to investigate limestone dissolution under varied carbonic acid h2co3 strengths as a possible analogue for future increases in atmospheric co2 arising from global warming twenty eight samples cut from a block of bath box hill limestone from somerville college oxford which had been removed during restoration after 150 years in an urban environment were weighed and placed in closed bottles of thin plastic containing varying concentrations of h2co3 half of the stone samples were derived from exposed surfaces of the stone block weathered while the others were obtained from the centre of the block on unexposed surfaces unweathered the purpose of this was to compare dissolution of previously weathered versus unweathered surfaces in strong ph 4 73 versus weak ph 6 43 solutions of h2co3 a temperature of c 19 c was maintained within the chamber representing a plausible future temperature in oxford for the year 2200 given current warming scenarios the simulation lasted 25 days with a few stone samples being removed midway stone samples show reduced weight in all cases but one there was greater dissolution of stone samples in a strong h2co3 solution as conveyed by higher concentrations of total hardness and ca2 in the water samples as well as enhanced microscopic dissolution features identified using sem the simulation confirms that enhanced atmospheric co2 under global warming given adequate moisture will accelerate dissolution rates particularly of newly replaced limestone building stones however previously weathered surfaces such as those on historical stone exposed for a century or more appear to be less susceptible to the effects of such increased rainfall acidity conservation techniques which remove weathered surfaces such as stone cleaning may accelerate future decay of historical limestone structures by increasing their susceptibility to dissolution copyright 2006 john wiley sons ltd

initiation of a solitary wave family in the demise of a nocturnal thunderstorm density current abstract this paper describes the characteristics and evolving nature of a vigorous thunderstorm density current very early in the morning of 9 may 1981 in oklahoma because the ambient lower atmosphere was stratified interesting interactions between the outflow current and the ambient environment resulted the leading portion of the current was modulated by at least three gravity wavelike perturbations of horizontal spacing 12 km which initially coexisted with it however as the current evolved it initiated an undular borelike disturbance which propagated ahead into the stable boundary layer carrying cold outflow air in large amplitude rolls eventually the wave family left the decelerating outflow air in its wake this borelike disturbance resembles the australian morning glory phenomenon and appears to represent an early stage in the development of a solitary wave family the observations resemble other reported morning glories and solitary waves as well their laboratory and numerically simulated

the effects of lichen cover upon the rate of solutional weathering of limestone abstract the contribution of lichens to the biomodification of limestone surfaces is an area of conflict within bioweathering studies with some researchers suggesting a protective effect induced by lichen coverage and others a deteriorative effect induced by the same organisms data are reported demonstrating the potential role of endolithic lichen in particular of bagliettoa baldensis in the active protection of carboniferous limestone surfaces from rainfall induced solutional weathering during a 12 month microcatchment exposure period in the west of northern ireland average dissolutional losses of calcium are greater from a lichen free limestone surface compared with a predominantly endolithic lichen covered surface by just under 1 25 times during colder winter months the lichen free surface experiences calcium loss almost 1 5 times greater than the lichen covered surface using extrapolation to upscale from the micro catchment sample scale for the year of sample exposure the rate of calcium loss is 1 001 g m 2 a 1 from lichen covered limestone surfaces and 1 228 g m 2 a 1 from lichen free bare limestone surfaces this research has implications for our understanding of karst environments the contribution of lichens to karren development and the conservation of lichen colonised dimension stone within a cultural setting

the influence of the rocky mountains on the 13 14 april 1986 severe weather outbreak part ii evolution of a prefrontal bore and its role in triggering a squall line abstract in this paper part ii of a series the evolution of a prefrontal bore on the leeside of the rockies and its subsequent propagation and initiation of convection farther downstream over eastern colorado and western nebraska are investigated the observational evidence for this sequence of events was obtained from combined analyses of high resolution goes satellite imagery and program for regional observing and forecasting services mesonetwork data over the colorado region for the severe weather event that occurred during 13 14 april 1986 a 2d nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection analysis of satellite imagery and mesonet data indicated that an internal bore ahead of a cold front a moderate downslope windstorm and a quasi stationary hydraulic jump were generated within a few hours along the iceslope as a pacific cold front and its attendant upper level jet streak advanced o

structure of an internal bore and dissipating gravity current as revealed by raman lidar abstract detailed moisture observations from a ground based raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented a synthesis of the lidar data with conventional meteorological data in conjunction with theoretical calculations and comparison to laboratory studies leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore the disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current analysis suggests that the bore was generated by the collapse of the gravity current into a stable nocturnal inversion layer and subsequently propagated along this wave guide at nearly twice the speed of the gravity current the raman lidar provided detailed measurements of the vertical structure o

a nonclassical cold front observed during cops 91 frontal structure and the process of severe storm initiation abstract this case study addresses the issue of gravity current and bore development at surface cold fronts and the role of these phenomena in the generation of severe frontal convection the event investigated occurred on 27 april 1991 during the cooperative oklahoma profiler studies 1991 field project the development of a bore from a gravity current like structure along a cold front the subsequent propagation of the bore ahead of the front on a low level inversion and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory the bore developed after a nocturnal inversion which acted as a waveguide had become established the bore and gravity current were both evident as fine lines in the radar reflectivity displays a microscale envelope of enhanced water vapor with an embedded roll cl

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development

a numerical and analytical study of atmospheric undular bores summary in this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis the numerical model is first used to simulate a density current propagating into an unstratified environment a detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory when the density current is allowed to propagate into a low level stable layer it is found that for certain parameter regimes an undular bore forms ahead of the current the results for two different types of stable layers are examined for a layer with a step profile of potential temperature comparison is made with both classical bore theory and the theory of density currents in two layer fluids and encouraging agreement is found when a linear profile is specified in the stable layer it is found that energy conserving solutions can be obtained the numerical fields are then compared with measurements of the morning glory an undular bore observed in north eastern australia and again agreement is good finally preliminary results of model simulations with stratification above the stable layer are discussed