the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

a concentrated outbreak of tornadoes downbursts and microbursts and implications regarding vortex classification abstract a remarkable case of severe weather occurred near springfield illinois on 6 august 1977 aerial and ground surveys revealed that 17 cyclonic vortices an anticyclonic vortex 10 downbursts and 19 microbursts occurred in a limited 20 km 40 km area associated with a bow shaped radar echo about half of the vortices appeared to have occurred along a gust front some of the other appear to have occurred within the circulation of a mesocyclone accompanying the bow echo but these vortices seem to have developed specifically in response to localized boundary layer vorticity generation associated with horizontal and vertical wind shears on the periphery of microbursts some of these vortices and other destructive vortices in the literature do not qualify as tornadoes as defined in the glossary of meteorology a moon pragmatic definition of a tornado is suggested

high winds generated by bow echoes part i overview of the omaha bow echo 5 july 2003 storm during bamex abstract an analysis of a bow echo that produced damaging winds exceeding f1 in intensity on the fujita scale near omaha nebraska is shown part i of this study presents a combination of airborne doppler derived wind syntheses with a comprehensive damage survey in order to document the generation of strong winds at the surface a detailed kinematic analysis of the evolution of a quasi linear convective system into a bow shaped and subsequently a spearhead echo is shown for the first time it is hypothesized that a large cyclonic bookend vortex 70 80 km in diameter north of the bow apex enhanced the rear inflow jet and initiated the bowing process a hook shaped echo and mesovortex formed at the apex of a bowed segment of the convective line and was located to the north of the swath of strong damage rated greater than f1 in damage intensity the peak single doppler radial velocity recorded by the tail radar was 43 m s 1 in the low level outflow near the apex of the bow echo the regions of the str

doppler radar observations of an asymmetric mesoscale convective system and associated vortex couplet abstract a study of the 28 may 1985 asymmetric mesoscale convective system mcs observed during pre storm is presented dual doppler analysis revealed a well defined cyclonic mesovortex in the northern portion of the stratiform region in accord with previous studies on this type of circulation at maximum extent the closed cyclonic circulation had a diameter of approximately 80 km and a depth of 7 km a smaller anticyclonic circulation was present to the south of the cyclonic vortex counterrotating vortices in asymmetric mcss have been identified in recent modeling studies however prior to this study observational confirmation of the anticyclonic vortex has been elusive

middle to late holocene fluctuations of c3 and c4 vegetation in a northern new england salt marsh sprague marsh phippsburg maine abstract a 3 1 m sediment core was analyzed for stable carbon isotope composition of organic matter and higher plant leaf wax hplw lipid biomarkers to determine holocene shifts in c 3 higher high marsh and c 4 low and or high marsh plant deposition at the sprague river salt marsh phippsburg maine the carbon isotope composition of the bulk sediment and the hplw parallel each other throughout most of the core suggesting that terrestrial plants are an important source of organic matter to the sediments and diagenetic alteration of the bulk sediments is minimal the current salt marsh began to form 2500 cal yr bp low and or high c 4 marsh plants dominated deposition at 2000 cal yr bp 700 cal yr bp and for the last 200 cal yr bp expansion of higher high marsh c 3 plants occurred at 1300 and 600 cal yr bp these major vegetation shifts result from a combination of changes in relative sea level rise and sediment accumulation rates average annual carbon sequestration rates for the last 2500 years approximate 40 g c yr 1 m 2 and are in strong agreement with other values published for the gulf of maine given that maine salt marshes cover an area of 79 km 2 they represent an important component of the terrestrial carbon sink more detailed isotopic and age records from a network of sediment cores at sprague marsh are needed to truly evaluate the long term changes in salt marsh plant communities and the impact of more recent human activity including global warming on salt marsh vegetation

low level mesovortices within squall lines and bow echoes part i overview and dependence on environmental shear this two part study proposes fundamental explanations of the genesis structure and implications of lowlevel meso g scale vortices within quasi linear convective systems qlcss such as squall lines and bow echoes such mesovortices are observed frequently at times in association with tornadoes idealized simulations are used herein to study the structure and evolution of meso g scale surface vortices within qlcss and their dependence on the environmental vertical wind shear within such simulations significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s 21 or greater over a 0 2 5 or 0 5 km agl layer as similarly found in observations of qlcss these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger scale bow shaped system they generally develop first near the surface but can build upward to 6 8 km agl vortex longevity can be several hours far longer than individual convective cells within the qlcs during this time vortex merger and upscale growth is common it is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme straight line wind damage as has also been observed with some qlcss surface vortices are also produced for weaker shears but remain shallow weak and short lived although similar in size and strength to mesocyclones associated with supercell storms and also sometimes producing similar hooklike structures in the rain field it is also shown that the present vortices are quite distinct structurally and dynamically most critically such vortices are not associated with long lived rotating updrafts at midlevels and the associated strong dynamically forced vertical accelerations as occur within supercell mesocyclones

deep sea coral aragonite as a recorder for the neodymium isotopic composition of seawater deep sea corals have been shown to be useful archives of rapid changes in ocean chemistry during the last glacial cycle their aragonitic skeleton can be absolutely dated by u th data freeing radiocarbon to be used as a water mass proxy for certain species of deep sea corals the growth rate allows time resolution that is comparable to ice cores an additional proxy is needed to exploit this opportunity and turn radiocarbon data into rates of ocean overturning in the past neodymium isotopes in seawater can serve as a quasi conservative water mass tracer and initial results indicate that deep sea corals may be reliable archives of seawater nd isotopes here we present a systematic study exploring nd isotopes as a water mass proxy in deep sea coral aragonite we investigated five different genera of modern deep sea corals caryophyllia desmophyllum enallopsamia flabellum lophelia from global locations covering a large potential range of nd isotopic compositions comparison with ambient seawater measurements yields excellent agreement and suggests that deep sea corals are reliable archives for seawater nd isotopes a parallel study of nd concentrations in these corals yields distribution coefficients for nd between seawater and coral aragonite of 1 10 omitting one particular genus enallopsamia the corals and seawater did however not come from exactly the same location and further investigations are needed to reach robust conclusions on the incorporation of nd into deep sea coral aragonite lastly we studied the viability of extracting the nd isotope signal from fossil deep sea corals by carrying out stepwise cleaning experiments our results show that physical removal of the ferromanganese coating and chemical pre cleaning have the highest impact on nd concentrations but that oxidative reductive cleaning is also needed to acquire a seawater nd isotope signal

numerical simulation of thunderstorm outflow dynamics part i outflow sensitivity experiments and turbulence dynamics abstract in this first paper of a two part series a two dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows by focusing only on the outflow and using essentially inviscid equations and high spatial resolution we are able to explicitly represent important physical processes such as turbulent mixing to simplify interpretation of the results the model atmosphere used in all experiments is calm and dry adiabatic this approach allows us to establish basic characteristics of modeled outflows in simple physical settings and provides a foundation for future studies using more realistic environments all simulated outflows are initialized by prescribing a controlled horizontal flux of cold air into the model domain through a lateral boundary in a series of sensitivity tests we examine three parameters of the cold air source region 1 the vertical temperature deficit profile 2 the magnitude of the temperature deficit and 3 the cold air depth by hol

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

the role of convectively generated rear inflow jets in the evolution of long lived mesoconvective systems abstract in this study the structure of convectively generated rear inflow jets and their role in the evolution of long lived mesoconvective systems are investigated through an analysis of idealized three dimensional simulations using a nonhydrostatic cloud model rear inflow jets are generated within these systems in response to the upshear tilting of the convective circulation as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear within this framework a wide range of rear inflow strengths and structures are produced depending on the magnitude of the ambient convective available potential energy cape and the vertical wind shear in general for environments characterized by weak to moderate vertical wind shear and weak to moderate cape the rear inflow jet descends and spreads along the surface well behind the leading edge of the gust front and the subsequent convective activity becomes weaker however for e

a high plains squall line associated with severe surface winds abstract the characteristics of a severe squall line are examined using data acquired from the 1981 cooperative convective precipitation experiment ccope the case is unusual in that the squall line was decoupled from an immediate surface based inflow source due to a meso scale 20 200 km outflow pool produced by a separate mesoscale convective system both systems participated in the development of a mesoscale convective complex which subsequently produced sustained severe surface winds throughout its entire life cycle despite the absolutely stable presquall atmospheric boundary layer the squall line produced radar reflectivity values of 70 dbz and storm induced outflow of 30 m s 1 aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high valued c air overriding the cold dome produced by the developing mcc the strongest surface winds were located upshear from the convective line and consisted of a northerly alongline component because the

the global population of mesoscale convective complexes a global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective the data set includes date of occurrence time of first storms initiation maximum extent termination duration cold cloud shield areas and tracks from initiation to termination it is found that the typical convective complex is nocturnal generates a cold cloud shield area of approximately 350 000 km2 and persists for about 10 h the largest systems and most persistent systems tend to occur near the summer solstices for the globe about 400 systems occur each year primarily over land areas most systems develop in favoured zones although some activity occurs over every continent except antarctica and all major oceans the concentration of activity into favoured zones indicates that there must be special dynamic and or thermodynamic conditions necessary for convection to organize into convective complexes activity is strongly tied to the solar day and shifts from 35 s in early january to about 50 n during the boreal summer and back to 35 s by december within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward relatively little migration occurs in the ocean dominated southern hemisphere where the subtropical jet remains quasi stationary over the convective complex regions the nocturnal life cycles copious rainfall large cloud shields and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth system energy budget

structure and evolution of the 22 february 1993 toga coare squall line aircraft observations of precipitation circulation and surface energy fluxes abstract this study documents the precipitation and kinematic structure of a mature eastward propagating oceanic squall line system observed by instrumented aircraft during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare doppler radar and low level in situ observations are used to show the evolution of the convection from an initially linear nnw sse oriented convective line to a highly bow shaped structure with an embedded low to midlevel counterclockwise rotating vortex on its northern flank in addition to previously documented features of squall lines such as highly upshear tilted convection on its leading edge a channel of strong front to rear flow that ascended with height over a rear inflow that descended toward the convective line and a pronounced low level cold pool apparently fed from convective and mesoscale downdrafts from the convective line rearward the observations of this system showed distinct multiple maxima in updraft strength with

vortex structure and evolution within bow echoes part i single doppler and damage analysis of the 29 june 1998 derecho abstract single doppler radar along with damage observations are examined to investigate the structural evolution of vortices observed within the 29 june 1998 derecho event that propagated through southeastern iowa into central and eastern illinois a total of 13 meso scale vortices observed primarily at low levels 0 3 km agl along the leading edge of the convective system were detected by the weather surveillance radar 1988 doppler wsr 88d radars at davenport iowa and lincoln illinois all but one of the vortices formed after the system evolved into a bow echo ten of the vortices formed north of the apex while three formed south of the apex seven of the vortices produced tornadoes that created f0 f1 surface damage none of the vortices exhibited appreciable upscale growth careful analysis of the radar data suggests that it may be possible to discern between the tornadic and nontornadic vortices the tornadic vortices tended to be stronger longer lived and deeper than their nontornadic count

tornadoes and downbursts in the context of generalized planetary scales abstract in order to cover a wide range of horizontal dimensions of airflow the author proposes a series of five scales maso meso miso to be read as my so moso and muso arranged in the order of the vowels a e 1 o u the dimensions decrease by two orders of magnitude per scale beginning with the planet s equator length chosen to be the maximum dimension of masoscale for each planet mesoscale highs and lows were described on the basis of mesoanalyses while sub mesoscale disturbances were depicted by cataloging over 20 000 photographs of wind effects taken from low flying aircraft during the past 15 years various motion thus classified into these scales led to a conclusion that extreme winds induced by thunderstorms are associated with misoscale and mososcale airflow spawned by the parent mesoscale disturbances

a mesoscale vortex couplet observed in the trailing anvil of a multicellular convective complex abstract observations collected during the oklahoma kansas pre storm experiment are used to document the evolution and structure of a mesoscale vortex couplet that developed in the mesoscale convective system that occurred on 16 17 june 1985 the evolution of the circulations was captured by dual doppler radar observations for 1 4 hours this allowed an evaluation of the various terms of the vertical vorticity equation which give insight into the mechanisms that are important in the generation of the circulations the primary mechanism responsible for the formation of the observed vortices was the interaction of the larger scale flow with low level momentum transported to higher levels by multiple convective updrafts as a consequence vertical shear of the horizontal wind was important to initial vorticity production the vorticity generated in this manner was subsequently increased in strength due to middle level convergence when the convection weakened and dissipated the primary source of vorticity w

rear inflow in squall lines with trailing stratiform precipitation abstract the relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of rear inflow i e the intrusion of environmental air into these storms across the trailing precipitation boundary three cases from kansas and oklahoma provide examples of strong rear inflow which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s 1 the vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar with the rear inflow confined to a jet like layer centered at about 550 mb doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region where it merged with outflow from convective downdrafts to bolster the leading gust front strong front to rea

high winds generated by bow echoes part ii the relationship between the mesovortices and damaging straight line winds airborne radar analysis of a mesovortex that developed near the apex of a bow echo is presented the mesovortex was shown to play a critical role in determining the location of intense straight line wind damage at the surface the perturbation pressure gradient force in natural coordinates along the parcel path accelerated the horizontal winds however intense mesovortices modified the low level outflow and largely determined the locations where the strongest winds occurred regions of maximum winds are accounted for as a superposition of the vortex and the flow in which it is embedded the strongest winds occur on the side of the vortex where translation and rotation effects are in the same direction this model explains the observed tongue of high wind speeds that were confined to the periphery of the mesovortex the origin of the mesovortex is also examined similarities and differences of this bow echo event with recent modeling studies are presented

low level mesovortices within squall lines and bow echoes part ii their genesis and implications this two part study proposes a fundamental explanation of the genesis structure and implications of lowlevel meso g scale vortices within quasi linear convective systems qlcss such as squall lines and bow echoes such mesovortices are observed frequently at times in association with tornadoes idealized experiments with a numerical cloud model show that significant low level mesovortices develop in simulated qlcss especially when the environmental vertical wind shear is above a minimum threshold and when the coriolis forcing is nonzero as illustrated by a qlcs simulated in an environment of moderate vertical wind shear mesovortexgenesis is initiated at low levels by the tilting in downdrafts of initially crosswise horizontal baroclinic vorticity over a 30 min period the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and more notably planetary vorticity is stretched vertically hence the coriolis force plays a direct role in the low level mesovortexgenesis a downward directed vertical pressure gradient force is subsequently induced within the mesovortices effectively segmenting the previously nearly continuous convective line in moderate to strong environmental shear the simulated qlcss evolve into bow echoes with straight line surface winds found at the bow echo apex and additionally in association with and in fact induced by the lowlevel mesovortices indeed the mesovortex winds tend to be stronger more damaging and expand in area with time owing to a mesovortex amalgamation or upscale vortex growth in weaker environmental shear in which significant low level mesovortices tend not to form damaging surface winds are driven by a rear inflow jet that descends and spreads laterally at the ground well behind the gust front

initiation and development of rear inflow within the 28 29 june 1989 north dakota mesoconvective system abstract five dual doppler analyses spanning a period of 1 5 h are used to document the initiation and development of rear inflow within a high plains squall line the squall line developed as part of the 28 29 june 1989 mesoconvective system that passed through the north dakota thunderstorm project observational network this storm was explosively initiated along a preexisting isolated gust front in an environment of much instability and weak upper air flow and moved within dual doppler coverage soon after its initiation the kinematic analyses were performed over a 90 min period as the squall line continued to develop and move to the east northeast the doppler analyses reveal that the rear inflow was initiated near the high reflectivity cores of the squall line within 20 min of the formation of the system with time the rear inflow expanded rearward increased in intensity and descended to near the surface behind the northern section of the squall line in regions where the squall line dissipated

the genesis of severe long lived bow echoes abstract a series of idealized simulations using a nonhydrostatic cloud model is used to investigate the genesis of bow echoes a bow shaped system of convective cells that is especially noted for producing long swaths of damaging surface winds it is hypothesized that severe long lived bow echoes represent a dynamically unique form of mesoconvective organization being produced for a restricted range of environmental conditions including a convective available potential energy cape of at least 2000 m2 s2 and vertical wind shears of at least 20 m s 1 over the lowest 2 5 5 km agl the key structural features include a 40 100 km long bow shaped segment of convective cells with a strong rear inflow jet extending to the leading edge of the bow at 2 3 km agl and cyclonic and anticyclonic eddies referred to as bookend vortices on the northern and southern flanks of the bowed segment respectively this structure characteristically develops three to four hours into the lifetime of a convective system a