Low Static Stability Research Articles

Air motions and precipitation growth in Alpine storms

AbstractThis study documents two strong Mesoscale Alpine Programme (MAP) storms. Each occurred ahead of a strong baroclinic trough. Even though their 500 and 850 hPa flow patterns were strikingly similar upstream of the Alps, their flow at lower levels was different and resulted in different forms of orographic enhancement of the precipitation associated with baroclinic systems by the Alpine terrain. During Intensive Observing Period (IOP) 2b, the low‐level flow and thermodynamic conditions over the Alps north‐west of the Lago Maggiore region constituted an unstable atmosphere with Froude number Fr>1 (i.e. unblocked or flow‐over regime), while in IOP 8 the flow was stable with Fr<1 (i.e. blocked or flow‐around regime in the region immediately upstream of the slopes on the western side of Lago Maggiore). For IOP 2b (IOP 8) the wind field and precipitation patterns north‐west of the Lago Maggiore were very similar to the high‐ (low‐)Fr autumn climatology. Thus the two cases represent fundamentally distinct regimes of orographic modification of the baroclinic precipitation.During IOP 8 (blocked case), the strong stability and weak wind speed at low levels prevented the airflow from rising over the slopes north‐west of the Lago Maggiore region and forced it to turn away from the mountains. The precipitation over the mountains was produced as the strong flow above 900 hPa was forced over the blocked low‐level air as well as the terrain. In contrast, during IOP 2b (unblocked case) the flow was strong at all levels with low static stability; therefore the low‐level air rose easily over the abruptly rising terrain. Since the lower‐level air rose together with the upper‐level air, it could transport upward moisture unavailable in the blocked case; this moisture condensed to add significantly to the precipitation production in the unblocked case. In addition to the high Fr, the air was potentially unstable; the slight instability of the airstream impinging upon the upslopes favoured the development of convective cells over the lower slopes which were embedded in the stratiform background lifting, thus further enhancing the formation of cloud and precipitation on the lower windward slopes.Examination of all the cases observed by polarimetric radar in MAP confirmed the microphysical processes seen in IOPs 2b and 8, and suggests fundamental microphysical differences between the unstable unblocked, and stable blocked cases. In both cases precipitation forms by a simple stratiform process in the form of dry snow aloft, becoming wet snow at the melting layer and falling out as rain below. However, in the unstable unblocked cases rainfall is enhanced by the participation of the low‐level flow in the orographic lifting; when the low‐level air rises easily over the first high peaks of terrain, raindrops grow rapidly by coalescence at low levels, and graupel forms just above the 0°C level. The coalescence‐produced drops and melted graupel particles contribute to heavy rain on the lower Alpine slopes. The orographic enhancement in the stable blocked cases is fundamentally limited since these low‐altitude precipitation processes cannot add significantly to the background stratiform precipitation. Copyright © 2003 Royal Meteorological Society.

Shear Instabilities as a Probe of Jupiter's Atmosphere

Linear wave patterns in Jupiter's clouds with wavelengths strongly clustered around 300 km are commonly observed in the planet's equatorial atmosphere (F. M. Flasar and P. J. Gierasch, 1986, J. Atmos. Sci. 43, 2683–2707). We propose that the preferred wavelength is related to the thickness of an unstable shear layer within the clouds (A. P. Ingersoll and D. W. Koerner 1989, Bull. Am. Astron. Soc. 21, 943). We numerically analyze the linear stability of wavelike disturbances that have nonzero horizontal phase speeds in Jupiter's atmosphere and find that, if the static stability in the shear layer is very low (but still nonnegative), a deep vertical shear layer like the one measured by the Galileo probe (D. H. Atkinson et al. 1998, J. Geophys. Res. 103, 22911–22928) can generate the instabilities. The fastest growing waves grow exponentially within an hour, and their wavelengths match the observations. Close to zero values of static stability that permit the growth of instabilities are within the range of values measured by the Galileo probe in a hot spot (A. Seiff et al. 1998, J. Geophys. Res. 103, 22857–22889). Our model probes Jupiter's equatorial atmosphere below the cloud deck and suggests that thick regions of wind shear and low static stability exist outside hot spots.

A Composite Study of Explosive Cyclogenesis in Different Sectors of the North Atlantic. Part I: Cyclone Structure and Evolution

General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.58 resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland‐Iceland region is higher than previously thought, and some of the events can be extremely violent. Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle. For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a ‘‘parent cyclone’’ with appreciable strength exists to the north/northeast of the incipient system.

The Large-Scale Environments of the Global Populations of Mesoscale Convective Complexes

Abstract The mean genesis environment was constructed for each of five mesoscale convective complex (MCC) population centers around the world: Africa, Australia, China, South America, and the United States. It is found that the environments are very similar and exhibit many of the same dynamic and thermodynamic structures that are present with systems in the United States. In particular, MCCs initiate within prominent baroclinic zones characterized by locally large values of lower-tropospheric vertical wind shear and convective available potential energy (CAPE). Typically, a low-level jet of air with low static stability, high equivalent potential temperature, oriented nearly perpendicular to the baroclinic zone, intrudes into the genesis region and is forced to ascend over a relatively shallow, surface-based layer of relatively cool air. Pronounced warm advection accompanied by strong lower-tropospheric veering overlays the surface-based cool layer. A local maximum in absolute humidity and a local minimu...

Discrete Frontal Propagation in a Nonconvective Environment

Abstract Surface discrete frontal propagation in a wintertime, nonconvective environment is documented using conventional surface and upper-air data and simulated using the PSU–NCAR mesoscale model. Synoptic and mesoscale surface analyses show a cold front associated with a synoptic-scale low-pressure system propagating from northwest to southeast across the central United States. Apparently discrete frontal propagation occurs when the surface front dissipates and a new front forms approximately 500 km ahead of the original front, with no compelling evidence of frontal passage in the intervening space. Upper-air analyses indicate the infusion of three different airstreams into the frontal region, resulting in the formation of a ribbon of low static stability air parallel to and several hundred kilometers in advance of the original front. This static stability structure appears to be involved in the observed evolution of the front. The development of precipitation over the intervening zone between the old ...

A Diagnostic Study of Three Heavy Precipitation Episodes in the Western Mediterranean Region

Abstract A diagnostic evaluation of three project ANOMALIA case studies involving heavy precipitation in the western Mediterranean region has been carried out. The evaluation shows the unique characteristics of each event, as well as some limited similarities. Heavy precipitation events in the western Mediterranean region typically occur downstream of a significant cyclone aloft (often, but not always, exhibiting “cutoff” cyclone characteristics), but important structural and evolutionary differences are found among these cases. At low levels, a long fetch of flow over the Mediterranean Sea frequently interacts with terrain features to produce persistent heavy precipitation. Although most heavy precipitation events occur during the fall season, they can develop at other times. In the first case, the synoptic-scale environment produced low static stability and substantial storm-relative environmental helicity, thereby supporting both heavy rain in the vicinity of Valencia on mainland Spain and on Ibiza in ...

Recovering spatial features in the ocean: performance of isopycnal vs. isobaric analysis

This study compares the properties of isopycnal and isobaric coordinate systems as a basis for spatial interpolation of oceanographic observations. The effect of 2D interpolation between observations sampled on either isobaric or isopycnal surfaces is investigated using both real and simulated vertical soundings whose locations correspond to CTD casts carried out during an intensive survey in the Alborán Sea (western Mediterranean). In the case of hydrographic data, a spatial analysis of 3D features via interpolation on regularly spaced isopycnal surfaces is clearly superior to one based on isobaric interpolation within a stably stratified ocean. The isopycnal distribution of geostrophic velocity can be derived via interpolation of a Montgomery potential function, sample values of which are easily obtained from hydrographic data. The advantage of isopycnal interpolation of current data appears to be significant only within frontal regions characterized by a pycnocline of high static stability separating water masses of relatively low static stability. In regions of more uniform stratification, isobaric spatial interpolation is equally effective in resolving baroclinic features sampled by observed current profile data.

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.

Turbulence and gravity waves in the vicinity of a midtropospheric warm front: A case study using VHF echo‐intensity measurements and radiosonde data

It is well known that VHF echo‐intensity data obtained at nonvertical beam directions can be used to retrieve the refractive‐index structure parameter C2n and that vertical profiles of C2n can be used to identify and monitor frontal structures in the free troposphere. In a case study, VHF radar observations of C2n are compared with vertical profiles of M2g (the square of the vertical gradient of the generalized potential refractive index) obtained from the data of two radiosondes launched from 4 km east of the radar site. The experiment was carried out in the Harz Mountains, north Germany, on October 18, 1993, during a high‐pressure period under clear‐sky conditions. Well‐defined maxima of C2n and M2g are found at the surface of a warm front at altitudes slightly below 5 km. At altitudes in the immediate vicinity of the front, echo‐intensity oscillations are observed, which have an amplitude of up to 9 dB and a period of about 40 min and which are attributed to gravity wave‐induced vertical displacements of the frontal surface. Vertical profiles of the outer scale of turbulence, L0, are obtained from the profiles of C2n and M2g. In the stably stratified region below the front (at altitudes between 3 and 5 km), 5–20 m is typical, while in the less stably stratified altitude region between 5 and 7 km, 20–50 m is more representative. It is found that large values of L0 are correlated with low static stability; a relationship between L0 and the Richardson number (dynamic stability), however, was not observed. A continuous temporal increase of C2n at altitudes between 5 and 7 km was observed. It is attributed to a temporal increase of the vertical variability of the humidity above the front.

Study on the ethanol sensitivity of La0.7Sr0.3FeO3 nanocrystalline

By comparing the characteristics of ethanol sensitivity of nanocrystalline LaFeO3 and Lao0.7Sr0.3FeO3 in this work, we find that nanocrystalline Lao0.7Sro0.3FeO3 sensors have many advantages, such as low static electric resistance, good stability, selectivity and obvious response to ethanol gas as low as 10ppm

An idealized study of African easterly waves. III: More realistic basic states

AbstractThe nature of unstable easterly waves growing on three different easterly jets has been examined. the ‘shearjet’ with strongest horizontal mean shear on the equatorward side of the jet is characterized by linearly unstable waves with maximum amplitude equatorward of the jet maximum and are dominated by barotropic energy conversions associated with positive horizontal momentum fluxes. the normal modes of the shear‐jet have very little amplitude at the surface.The ‘desert‐jet’ with lower mean static stability better represents the atmospheric conditions on the poleward side of the jet, and is characterized by linearly unstable waves with maximum amplitude at the surface. the modes have much larger baroclinic energy conversions than those of the shear‐jet, although barotropic energy conversions are still larger. About 25% of the value of the barotropic energy conversions is attributable to the vertical Reynolds stress term associated with vertical momentum fluxes.The ‘combined‐jet’ with both enhanced equatorward horizontal shear and low static stability combined, has also been examined. the linear normal modes associated with this jet are characterized by significant amplitudes at both the jet‐level and at the surface. the ratio of barotropic to baroclinic energy conversions is between that associated with the modes growing on the shear‐ and desert‐jets.The nonlinear behaviour of 3000 km wavelength easterly waves growing on the jets has been examined. All life‐cycles are characterized by a transition from initial barotropic energy conversions dominance to baroclinic energy conversions dominance. the nonlinear part of the shear‐jet life‐cycle is dominated by negative horizontal momentum fluxes on the poleward side of the jet and maximum amplitude at the surface in the region of the temperature gradients. In disagreement with observations only very weak nonlinear positive horizontal momentum fluxes exist on the equatorward side of the jet. the nonlinear desert‐jet life‐cycle as for the normal mode continues to be characterized by maximum amplitudes at low‐levels and becomes dominated by baroclinic energy conversions sooner. the nonlinear combined‐jet life‐cycle evolves in a similar manner to the desert‐jet life‐cycle but with more significant jet‐level amplitudes.The synoptic evolution of the relative vorticity in the life‐cycles is examined. A single surface relative vorticity maximum, associated with the meridional surface temperature gradient has been identified in all idealized integrations examined, including those with latent heating, in disagreement with observations which indicate two maxima north and south of the jet. At about 850 mb however, two positive vorticity centres do exist; a weak equatorward one associated with the developments at the jet level and a stronger poleward one associated with the low‐level temperature gradients.The shear life‐cycle is re‐examined with a simple moist parametrization included. Baroclinic energy conversions increase but barotropic energy conversions are almost unaffected by the latent heating and the jet‐level structure almost unchanged. the surface vorticity maxima is increased. the relationship between moisture availability and the jet is shown to be important in determining the nature of surface developments and the ascent pattern as well as the magnitude of the baroclinic energy conversions.It is suggested that differences between observations and the nonlinear equatorward structures identified here, may be reduced with a more accurate representation of diabatic processes.

Internal Gravity Wave Generation and Hydrodynamic Instability

Two mechanisms are proposed whereby internal gravity waves (IGW) may radiate from a linearly unstable region of Boussinesq parallel flow that is characterized in the far field by constant horizontal velocity and Brunt-Väisälä frequency. Through what is herein referred to as “primary generation,” IGW may be directly excited by linear instability of the initial-state parallel shear flow. Characteristically, these waves propagate with horizontal phase speed and wavenumber equal to that of the most unstable mode of linear stability theory. Through the second mechanism, referred to as “secondary generation,” IGW may be excited via nonlinear modification of the initial instability into a form that couples strongly to a large amplitude outgoing internal wave field. The authors propose that the primary generation of IGW may occur provided a penetration condition, which is derived on the basis of linear theory, is satisfied. The penetration condition provides a limit on the growth rate of a disturbance of any particular frequency that is capable of propagating into the far field. This hypothesis is supported by a sequence of representative nonlinear numerical simulations in two spatial dimensions for both free mixing layer and jet flows with horizontal velocity profiles U(z) = tanh (z) and U(z) = sech2(z), respectively. For the purpose of these analyses, the fluid density is taken to be such that the square of the Brunt–Väisälä frequency is given by N2(z) = J tanh2(z/R). Such stratification allows both for the development of large-scale eddies in the region of low static stability and, in the far field where N2 ≈ J is positive and approximately constant, for the radiation of a broad frequency spectrum of IGW.

Observations of a Mesoscale Ducted Gravity Wave

Abstract This paper reports coordinated observations of a mesoscale gravity wave made during the FRONTS 84 field experiment conducted in southwestern France in the summer of 1984. The observations were unique in the sense that all relevant wave characteristics, including its detailed vertical structure, could be directly ascertained and shown to be mutually consistent. The wave was observed to be ducted. The static stability structure of the atmosphere was favorable for the existence of such a duct. A critical level (where the phase velocity of the wave matches the wave-parallel background wind speed) was present in or just above a layer of low static stability that capped a stabler layer at a still lower level. The observed vertical structure was that of a standing wave with the top of the stable lower layer corresponding roughly to an antinode and one-half the vertical wavelength below the critical level. The wave was observed when there was extensive activity associated with two line of deep convection...

Cumulus Convection and CISK in Midlatitudes. Part II: Comma-Cloud Formation in Cyclonic Shear Regions

Abstract Observational studies show that polar air stream cyclones form preferentially to the north of the polar front in regions of high vorticity and low static stability, although the baroclinicity may be stronger elsewhere. This phenomenon is investigated by considering a semigeostrophic linear stability analysis of a constant potential vorticity zonal jet in the presence of parameterized cumulus heating in the cold air mass. Three types of unstable modes are found for different amounts of heating. With relatively small heating rates, the usual baroclinic instability occurs, with the disturbance centered on the jet axis. With moderate heating, the fastest growing mode is again a mainly baroclinic system, but with significant amplitude only in the region of heating on the cold-air side of the jet. Finally, for sufficiently large heating rates, a small-scale disturbance, which is driven primarily by diabatic processes, forms in the cold air mass. There is a continuous transition between the three types ...

Contrasting the impact of dynamic data assimilation on the numerical simulations of cyclogenesis during GALEIOP 10 and IOP 1

The nudging assimilation scheme described in the companion paper by Brill et al. is applied to study oceanic cyclogenesis during GALEIOP 10 on 27–28 February 1986. A 36-h control simulation statically initialized from 0000 UTC 27 February 1986 data moves the cyclone too far north and east in the 12-h period of most rapid deepening limiting the usefulness of the simulation for diagnostic study. The use of nudging to dynamically assimilate special 3-h and routine 12-h rawindsonde and dropsonde data into the model during the entire 36-h forecast period failed to deepen the cyclone as it moved northeast off the Atlantic seaboard beyond the area covered 3-h by sounding data. Subjectively analyzed mean sea-level pressures (MSLP) were included in the data base to allow the model to nudge toward 3-h surface pressure analyses extended to cover the region of cyclogenesis over the ocean. The assimilation of 3-h surface data over the ocean is insufficient to produce a realistic simulation of cyclogenesis. This result motivated the use of the nudging technique to assimilate surface pressure and upper air data over land during the 12-h pre-cyclogenetic period (i.e. dynamic initialization) and compare the subsequent 24-h simulation with one initialized statically at the same synoptic time. Dynamic initialization produced the best simulation of the occanic cyclone based upon the standard statistical scores and positions of the MSLP minima. This simulation is used to diagnose differences between cyclogenesis during GALEIOP 1 and IOP 10. Isentropic analyses and vertical cross sections are derived from the model simulations and are used to contrast the strength of the upper tropospheric forcing and the low-level static stability associated with each case. The results of the diagnostic analyses reveal that stronger surface response (based upon MSLP minima) to weaker upper-level forcing during GALEIOP 10 (compared with GALEIOP 1) was associated with differences in the lower tropospheric static stability and thermal advection patterns and their interaction with upper tropospheric features.

Three-dimensional baroclinic instability and summertime frontogenesis in the Australian region

The structure of a mature three-dimensional baroclinic wave and its attendant cold front is examined using an idealized numerical model with a simple parametrization of dry buoyant convection resulting from diurnal heating. Relative-flow isentropic analyses of the numerical solution and of an observed Australian summertime frontal system are compared. It is shown that the model well represents the typical synoptic environment accompanying frontogenesis in the Australian region. A diagnosis of the three-dimensional vertical circulation is obtained using a technique involving a vector streamfunction. The circulation in a cross-sectional plane parallel with the low-level temperature gradient is found to be highly two-dimensional, with geostrophic confluence constituting the principal frontogenetical forcing at low levels, while at upper levels the frontogentical forcing is dominated by geostrophic horizontal shear. In a second cross-section, taken zonally, the frontogenetic forcing is found to consist almost solely of geostrophic horizontal shear throughout the entire troposphere. However, in this case the circulation in the plane of cross-section is found to be only qualitatively two-dimensional, as a significant proportion of the low-level postfrontal subsidence is associated with circulation normal to the cross-section. It is shown that the frontal structure analysed is sensitive to the location of the chosen cross-sectional plane, and that this in turn provides an explanation for the observed relative shallowness of Australian summertime fronts. The effects of diurnal heating are important but confined to the surface-based mixed layer. With the addition of diurnal heating, the prefrontal updraught amplifies almost threefold and the frontal temperature contrast over the heated region approximately doubles. The diabatic forcing term in the well-known Sawyer-Eliassen equation is shown to be negligible. This suggests that the increased low-level convergence is a reflection of the model's enhanced response to frontogenetic forcing in the presence of low static stability in the surface-based well-mixed layer. Finally, the numerical model solution is compared with a simpler two-dimensional model specifically developed to study Australian summertime fronts.

Three-dimensional baroclinic instability and summertime frontogenesis in the Australian region

The structure of a mature three-dimensional baroclinic wave and its attendant cold front is examined using an idealized numerical model with a simple parametrization of dry buoyant convection resulting from diurnal heating. Relative-flow isentropic analyses of the numerical solution and of an observed Australian summertime frontal system are compared. It is shown that the model well represents the typical synoptic environment accompanying frontogenesis in the Australian region. A diagnosis of the three-dimensional vertical circulation is obtained using a technique involving a vector streamfunction. The circulation in a cross-sectional plane parallel with the low-level temperature gradient is found to be highly two-dimensional, with geostrophic confluence constituting the principal frontogenetical forcing at low levels, while at upper levels the frontogentical forcing is dominated by geostrophic horizontal shear. In a second cross-section, taken zonally, the frontogenetic forcing is found to consist almost solely of geostrophic horizontal shear throughout the entire troposphere. However, in this case the circulation in the plane of cross-section is found to be only qualitatively two-dimensional, as a significant proportion of the low-level postfrontal subsidence is associated with circulation normal to the cross-section. It is shown that the frontal structure analysed is sensitive to the location of the chosen cross-sectional plane, and that this in turn provides an explanation for the observed relative shallowness of Australian summertime fronts. The effects of diurnal heating are important but confined to the surface-based mixed layer. With the addition of diurnal heating, the prefrontal updraught amplifies almost threefold and the frontal temperature contrast over the heated region approximately doubles. The diabatic forcing term in the well-known Sawyer-Eliassen equation is shown to be negligible. This suggests that the increased low-level convergence is a reflection of the model's enhanced response to frontogenetic forcing in the presence of low static stability in the surface-based well-mixed layer. Finally, the numerical model solution is compared with a simpler two-dimensional model specifically developed to study Australian summertime fronts.

Surface heat fluxes and marine boundary layer modification in the Agulhas Retroflection region

The Subtropical Convergence Agulhas Retroflection Cruise, from February 12 to March 4, 1987, provided an opportunity for studying variability of surface heat fluxes and marine boundary layer modification within an ocean area where few atmospheric measurements have been made. From 3‐hourly surface observations it has been ascertained that surface heat flux processes are enhanced under cold air outbreak, postfrontal synoptic conditions and over warmer ocean areas. A maximum turbulent heat loss of 828 W/m2, 4 times the climatological value, was found. Over the cooler Subtropical Convergence waters and over the Agulhas Bank, heat fluxes were comparatively low. Two transect lines were performed across the Subtropical Convergence‐Agulhas SST front. In both cases, air temperatures, dew point temperatures, and particularly wind speeds increased over the warmer water owing to enhanced vertical mixing resulting from oceanic heat losses. The second transect revealed marked vertical shear of the zonal wind. The essential elements for cyclogenesis, namely, low static stability and strong low level baroclinicity, were found to be characteristic of the region.

A Rapid Cyclogenesis Event during GALE IOP 9

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems. The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

A Rapid Cyclogenesis Event during GALE IOP 9

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems. The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.