Low-level Baroclinicity Research Articles

Polar Low Genesis over the East Coast of the Asian Continent Simulated in an AGCM

This report shows genesis of a polar low by an AGCM without specialized initial condition. A case of polar low genesis over the eastern coast of the Asian Continent, simulated in the seasonally varying climatological SST run by an AGCM (T42L52, primitive equation spectral model with 42 wave-number and 52 layers), is presented. A polar low simulated in January 22-23 of the fourth year (Y04) integration after the 10-year period of the spin-up is studied in comparison with polar lows described in several observational studies. In January Y04, large-scale circulation systems, such as Asian winter monsoon, extratropical cyclones and upper cold lows are reasonably simulated. A typical polar low is formed in January 22 over the coastal sea area ∼1500 km west of the major extratropical cyclone that developed over the Northwestern Pacific Ocean, under the influence of a short wave trough, which propagates along the rim of an upper cold low. The polar low genesis takes place first in connection with a deepening of the surface trough, which extends westward from the major cyclone. The deepening of the surface trough in the zone of strong low-level thermal gradient over the coastal sea area suggests the important role of the low-level baroclinicity for the polar low genesis. The strong heating due to the energy supply from the sea surface contributes for the genesis of the polar low through the decreasing of the vertical stability, and the sustaining thermal gradient. Meanwhile, the heating around 700 hPa associated with the precipitation concentrated within the polar low indicates the influence of the condensation heating for the development of the polar low. Aforementioned various processes contribute together to generate and develop the polar low. The structure and the evolution process of the simulated polar low are consistent with those of the observed polar lows. It is concluded that the realistic polar low genesis takes place in the model, when the large-scale phenomena such as the upper cold low, the short-wave trough, parent major cyclone and polar air outbreak are reasonably simulated. The present study is significant in presenting an AGCM simulation of a polar low for the first time.

Climatology of Cyclogenesis Mechanisms in the Mediterranean

Abstract A general climatology of the main mechanisms involved in Mediterranean cyclogenesis is presented. A diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons, and in different cyclogenetic regions within the same season. It is shown that cyclones that developed over the three most active areas in winter—the Gulf of Genoa, the Aegean Sea, and the Black Sea—are essentially subsynoptic lows, triggered by the major North Atlantic synoptic systems being affected by local orography and/or low-level baroclinicity over the northern Mediterranean coast. It is also suggested that cyclones in two, or all three, of these regions often occur consecutively, linked to the same synoptic system. In spring and summer, thermally induced lows become progressively more important, despite the existence of other factors, such as the Atlas Mountains contributing to lee cyclogenesis in northern Africa, or the extension of the Asian monsoon into the eastern ...

A Baroclinic Mechanism for the Eddy Feedback on the Zonal Index

A baroclinic mechanism for the positive eddy feedback on the zonal index is proposed. The author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies. The transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation, but over time, the action of surface drag leads to enhanced baroclinicity at these latitudes. The steady-state response has positive baroclinicity at the latitude of eddy generation, if the eddies propagate away from this latitude before they dissipate. This reinforcement of the low-level baroclinicity provides a positive feedback, if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low-level baroclinicity. The proposed mechanism explains observed and modeled features of zonal index variations: the frequency and drag dependence of eddy feedback, the bandedness in latitude of zonal wind variations, and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time.

The Use of Pre-Storm Boundary-Layer Baroclinicity in Determining and Operationally Implementing the Atlantic Surface Cyclone Intensification Index

The lateral motion of the Gulf Stream off the eastern seaboard of the United States during the winter season can act to dramatically enhance the low-level baroclinicity within the coastal zone during periods of offshore cold advection. The ralative close proximity of the Gulf Stream current off the mid-Atlantic coast can result in the rapid and intense destabilization of the marine atmospheric boundary layer directly above and shoreward of the Gulf Stream within this region. This airmass modification period often precedes either wintertime coastal cyclogenesis or the cyclonic re-development of existing mid-latitude cyclones. A climatological study investigating the relationship between the severity of the pre-storm, cold advection period and subsequent cyclogenic intensification was undertaken by Cione et al. in 1993. Findings from this study illustrate that the thermal structure of the continental airmass as well as the position of the Gulf Stream front relative to land during the pre-storm period (i.e., 24–48 h prior to the initial cyclonic intensification) are linked to the observed rate of surface cyclonic deepening for storms that either advected into or initially developed within the Carolina-southeast Virginia offshore coastal zone. It is a major objective of this research to test the potential operational utility of this pre-storm low level baroclinic linkage to subsequent cyclogenesis in an actual National Weather Service (NWS) coastal winter storm forecast setting. The ability to produce coastal surface cyclone intensity forecasts recently became available to North Carolina State University researchers and NWS forecasters. This statistical forecast guidance utilizes regression relationships derived from a nine-season (January 1982–April 1990), 116-storm study conducted previously. During the period between February 1994 and February 1996, the Atlantic Surface Cyclone Intensification Index (ASCII) was successfully implemented in an operational setting by the NWS at the Raleigh-Durham (RAH) forecast office for 10 winter storms. Analysis of these ASCII forecasts will be presented.

Observations of Low-Level Baroclinity Generated by Anvil Shadows

Abstract Low-level cooling beneath the cirrus anvil canopies of supercell thunderstorms is documented in two Verification of the Origins of Rotation in Tornadoes Experiment cases and in the 17 May 1981 Arcadia, Oklahoma, supercell. Surface temperature decreases of 3°C or more occurred beneath the anvils within 45 min of the onset of overcast conditions. Cooling was confined to the lowest few hundred meters of the boundary layer, and believed to be due mainly to a deficit in the energy budget following a reduction of incoming shortwave radiation. In the three cases studied, the vertical wind shear was strong; thus, mixing prevented the formation of an inversion layer. Strong insolation at the ground outside of the anvil shadows coupled with the cooling beneath the cirrus canopies led to corridors of baroclinity along the shadow edges. It is shown that residence times in these baroclinic zones may be long enough for parcels to acquire considerable horizontal vorticity (e.g., ∼10−2 s−1) en route to a storm u...

A Numerical Modeling Study of Mesoscale Cyclogenesis to the East of the Korean Peninsula

Numerical simulations and the analysis of observational data are employed to understand the mesoscale cyclogenesis in a polar airstream that occurred over the sea to the east of the Korean peninsula on 28‐29 January 1995. The observational analysis shows that a mesoscale low develops over the southeastern East Sea (Japan Sea) on 29 January 1995. Satellite imagery also indicates that a meso-b-scale vortex forms on the lee side of the northern Korean mountain complex (KMC), which is located in the northern Korean peninsula, and that a meso-a-scale cyclone develops over the southeastern East Sea at a later time. The mesoscale cyclone forms in the lower troposphere with strong baroclinicity and cyclonic circulation under the influence of an upper-level synoptic-scale cold vortex. Numerical simulation has captured major features of the observed cyclogenesis very well. The cyclogenesis occurs in a progressive manner. Basically, four distinctive stages of the cyclogenesis are identified. 1) First, a surface pressure trough forms on the lee side of the KMC under a northwesterly synoptic-scale flow that is deflected anticyclonically over the KMC. 2) Second, the lee trough deepens further into a strong convergence zone and a meso-b-scale vortex. 3) Next, the meso-b-scale vortex develops into a meso-a-scale vortex as the vortex and the trough begin to move southeastward from the lee of the KMC. 4) Finally, the surface trough deepens into a closed low and the meso-a-scale vortex becomes collocated with this deepening surface low to form a meso-a-scale cyclone over the southeastern East Sea. Several sensitivity experiments are performed to isolate the effects of a topography, warmer sea surface, diurnal thermal forcing, and latent heat release. During stages 1 and 2, it is found that the KMC and low-level baroclinicity are responsible for generating the strong lee trough and vortex. During stage 3, the development of the meso-a-scale vortex is brought on by the tilting of horizontal vorticity and vertical stretching in a synopticscale cyclonic circulation. In the final stage, the condensational heating plays the key role for the development of the meso-a-scale cyclone under the influence of an upper-level synoptic-scale cold vortex. The presence of the warm sea surface is found to be a necessary condition for the development of a polar air convergence zone and the mesoscale cyclone. It is also found that the low-level baroclinicity is essential for the present case of mesoscale cyclogenesis.

Poleward Deflection of Storm Tracks

An analysis of 11 years of European Centre for Medium-Range Weather Forecasts data focuses primarily on the vertically averaged high-frequency transients. The conclusions are discussed in the context of (a) the winter storm track, (b) monthly variability, and (c) interannual variability. (a) Winter storm track: Results show that the pattern of the forcing by the high-frequency eddies along the storm track is highly correlated with the stationary circulation, and the forcing itself is primarily responsible for the location of the trough–ridge system associated with the stationary flow. The results also clarify the role of wind component covariance terms u′υ′ and (υ′2 − u′2) in the column-averaged vorticity forcing. The simpler term u′υ′ has the well-known effect of intensifying the anticyclonic (cyclonic) tendencies on the southern (northern) side of the jet, thereby producing an increase in the barotropic component of the zonal jet. The (υ′2 − u′2) term displays a quadrupole pattern, which is also approximately in phase with the trough–ridge system associated with the stationary flow. (b) Monthly variability: Eddy activity has been shown to possess a seasonal life cycle, increasing during the early fall and reaching a maximum around the month of November, then decaying for most of the winter months. Month-to-month variations in eddy activity over the Pacific Ocean show that energy levels increase up through November, decreasing thereafter, at the same time the trough–ridge circulation pattern is intensifying. By December, baroclinicity in the western Pacific has increased substantially, and low-level eddies are found to break by the middle of the ocean. Upper-level eddies start to break well before reaching the west coast of North America, resulting in a displacement of the maximum in (υ′2 − u′2) westward from its November position and increasing the trough–ridge forcing by the high-frequency eddies. (c) Interannual variability: Wintertime eddy kinetic energy is seen to extend further eastward through the Pacific Ocean during the warm phase but displays an abrupt termination during the cold phase. Anomalies in the eddy transient forcing tend to be quite similar to that of the Pacific–North American pattern itself. The extension of the storm track during the warm phase resembles that of fall conditions and is present in the winter season because the source of low-level baroclinicity is extended well into the eastern Pacific for this El Niño–Southern Oscillation phase.

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.

A Polar-Low Development over the Bering Sea: Analysis, Numerical Simulation, and Sensitivity Experiments

Abstract A polar low that developed over the western Bering Sea on 7 March 1977 and tracked across St. Paul Island is investigated using observations and the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). A series of fine-mesh (20 km) simulations are performed in order to examine the structure of the cyclone and the airflow within it and to determine the physical processes important for its development. Observations show that the low formed near the ice edge in a region of moderate low-level baroclinicity and cold-air advection when an upper-level trough, or lobe of anomalously large potential vorticity (PV), broke off from a migratory, upper-level cold low over Siberia and advanced into the region. A full physics model experiment, initialized 24 h prior to the appearance of the polar low, produced a small, intense cyclone having characteristics similar to the observed low. The simulated low more closely resembled an extratropical cyclone than a typ...

A numerical case study of a polar low in the Labrador Sea

A mesoscale (15 km) version of the Canadian regional finite-element model is used to study a polar low that developed in the Labrador Sea on 11 January 1989, in the wake of an intense cold air outbreak associated with a major synoptic-scale system located to the east of Greenland. The rapid evolution of the polar low is well revealed from satellite imagery showing a complex structure with strong surface winds near the vortex and deep convection nearby during the mature stage. The simulated structure of the polar low agrees quite well with observed features. Based on the detailed mesoscale model outputs, the evolution of the Labrador Sea polar low is discussed at the initiation and mature stages. The polar low developed under a combination of baroclinic and convective processes. At an early stage, baroclinic development takes place in conditions of reversed shear flow, marked by low-level baroclinicity near the ice edge and a mobile upper-level short wave. Rapid modification of the Arctic boundary layer by strong surface heat fluxes is similar to that observed in other areas. Sensitivity experiments indicate that the mutual interaction between the upper-level potential vorticity anomaly and the low-level baroclinicity at the Arctic front, favoured by the deep convective boundary layer, appears to trigger the polar low. At the onset of the mature stage, the approach of a cold air dome favours the outbreak of deep convection, in agreement with satellite imagery. As shown by sensitivity experiments, latent heat release from organized convection contributes to the major part of the rapid deepening of the polar low in its mature stage, and sea surface evaporation is the primary feeding mechanism for condensation processes. The structure of the polar low is characterized by a warm core due to the combined effects of warm air seclusion and diabatic heating. Comparisons with previously studied polar lows developing in similar conditions of reversed shear at other locations are discussed. DOI: 10.1034/j.1600-0870.1996.t01-2-00003.x

A numerical case study of a polar low in the Labrador Sea

A mesoscale (15 km) version of the Canadian regional finite-element model is used to study a polar low that developed in the Labrador Sea on 11 January 1989, in the wake of an intense cold air outbreak associated with a major synoptic-scale system located to the east of Greenland. The rapid evolution of the polar low is well revealed from satellite imagery showing a complex structure with strong surface winds near the vortex and deep convection nearby during the mature stage. The simulated structure of the polar low agrees quite well with observed features. Based on the detailed mesoscale model outputs, the evolution of the Labrador Sea polar low is discussed at the initiation and mature stages. The polar low developed under a combination of baroclinic and convective processes. At an early stage, baroclinic development takes place in conditions of reversed shear flow, marked by low-level baroclinicity near the ice edge and a mobile upper-level short wave. Rapid modification of the Arctic boundary layer by strong surface heat fluxes is similar to that observed in other areas. Sensitivity experiments indicate that the mutual interaction between the upper-level potential vorticity anomaly and the low-level baroclinicity at the Arctic front, favoured by the deep convective boundary layer, appears to trigger the polar low. At the onset of the mature stage, the approach of a cold air dome favours the outbreak of deep convection, in agreement with satellite imagery. As shown by sensitivity experiments, latent heat release from organized convection contributes to the major part of the rapid deepening of the polar low in its mature stage, and sea surface evaporation is the primary feeding mechanism for condensation processes. The structure of the polar low is characterized by a warm core due to the combined effects of warm air seclusion and diabatic heating. Comparisons with previously studied polar lows developing in similar conditions of reversed shear at other locations are discussed. DOI: 10.1034/j.1600-0870.1996.t01-2-00003.x

A Numerical Investigation of the Effects of Timing of Diabatic Processes in the Coastal Cyclogenesis of GALE IOP 2

Abstract Sensitivity of coastal cyclogenesis to the effects of timing of diabatic processes is investigated using the Naval Research Laboratory mesoscale model. Numerical experiments were conducted to examine the sensitivity of the intensification and propagation of a coastal cyclone to changes in the timing of latent heat release due to cumulus convection, surface fluxes, and low-level baroclinicity. The NMC Regional Analysis and Forecast System analysis of the GALE IOP 2 coastal cyclone was unable to resolve the initial subsynoptic-wale cyclogenesis. Hence, tracking and identification of a well-defined coastal cyclone was difficult operationally. However, the control model experiment having full physics, initialized with the NMC analyses, was able to properly simulate the development of the coastal cyclone. Results from the control experiment agree with the more accurate Fleet Numerical Oceanographic Center low-level analysis. The numerical experiments suggest the development of the surface cyclone was ...

Genesis of Mobile Troughs in the Upper Westerlies

Stability calculations on basic-state velocity profiles representative of the preferred regions for the development of the upper-level disturbances active in Type B cyclogenesis show that conditions in these regions (weak low-level baroclinicity, large low-level static stability, and large surface roughness) are favorable for the growth of baroclinic waves with maximum amplitude near the tropopause. The structure of these waves compares favorably with observations of developing short-wavelength upper-level troughs in the atmosphere. Basic states characteristic of the storm track regions (strong low-level baroclinicity and small surface roughness) favor the development of baroclinic waves with maximum amplitude at the surface. The dynamics of both the surface-trapped and the upper-tropospheric waves can be interpreted concisely using concepts of potential vorticity. Based on these results, a possible mechanism for Type B cyclogenesis in the storm track regions is proposed that involves the propagation and structural modification of baroclinic wave packets in a zonally varying basic flow.

Midwinter Suppression of Baroclinic Wave Activity in the Pacific

Abstract Seasonal variations in baroclinic wave activity and jet stream structure in the Northern Hemisphere are investigated based upon over 20 years of daily data. Baroclinic wave activity at each grid point is represented for each day by an envelope function, the lowpass-filtered time series of the squared highpass-filtered geopotential height. Baroclinic wave activity over the Atlantic exhibits a single maximum in January, whereas in the Pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter, which is more evident at the tropopause level than near the surface. This suppression occurs despite the fact that the low-level baroclinity and the intensity of the jet stream are strongest in midwinter.Based on the analysis of 31-day running mean fields for individual winters, it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper-tropospheric jet for wind speeds up to ∼45 m s−1. When the strength of t...

Study of Explosive and Nonexplosive Cyclogenesis during FGGE

Abstract Explosive cyclogenesis during the winter of the First Global GARP Experiment (January–February 1979) is analyzed using the revised European Centre for Medium Range Weather Forecasts (ECMWF) analyses. Explosive cyclogenesis is defined as a decrease in the sea level pressure at the rate of 1 mb h−1 for at least 12 h. Diagnostics for 23 explosively developing cases and 16 nonexplosive cases are evaluated. Parameters compared include the dry static stability, low-level relative vorticity, vorticity advection, upper-level divergence, kinematic vertical velocities, and the strength of the low-level baroclinity. These parameters are compared statistically at the initial, 12-, and 24-h time periods. Parameters for which the explosive and nonexplosive cyclone ensembles were statistically separable are the kinematic vertical velocity and the upper-level divergence and vorticity advection. The strong upper-level processes for the explosive cases at the initial time indicate the importance of upper-troposphe...

A Numerical Investigation of East Coast Cyclogenesis during the Cold-Air Damming Event of 27–28 February 1982. Part II: Importance of Physical Mechanisms

This study presents a series of numerical experiments designed to examine the role of several physical processes on the evolution of and interaction between atmospheric phenomena having different scales, each of which contributed to the development of the storm of February 27-28, 1982 along a Carolina coastal front. The physical processes include: the role of diabatic heating associated with convective and grid-scale precipitation, the role of a thermally direct transverse circulation about the entrance region of a strong polar jet streak, and modification of the marine planetary boundary layer by fluxes of heat and moisture over the Gulf Stream. The diabatic heating associated with precipitation is found to have the most significant impact on storm development. Without latent heating, cyclogenesis does not occur along the Carolina coastal front despite the presence of strong low-level baroclinicity and cyclonic vorticity. A less dramatic but still important relationship is found between storm formation and the other two physical mechanisms.

A Diagnostic Study of Two Intense Oceanic Cyclones

A diagnostic study of two intense oceanic cyclones (16–18 November 1972 and 9–11 January 1982) along the east coast of North America is presented. Both cyclones formed along a strong low-level baroclinic zone and deepened in response to an approaching tropospheric short-wave trough aloft. The 16–18 November 1972 storm developed along a Carolina coastal front in a region of enhanced low-level convergence and cyclonic vorticity generation favorable for incipient cyclogenesis. Significant oceanic heat and moisture fluxes northeast of the cyclone helped to enhance the low-level baroclinicity and to destabilize the warm, moist air mass along the cyclones’ path. Explosive deepening [35 mb(12 h)−1] commenced when a rather ordinary midtropospheric trough was about 300 km upstream of the surface low. A noteworthy aspect of the storm was the existence of deep convection near the center throughout the deepening phase and the exceptionally tight inner pressure gradient, suggestive of the role of diabatic process in the extreme development. The 9–11 January 1982 cyclone formed and deepened in response to a vigorous mid- and upper-tropospheric trough/jet-streak system of greater intensity and large-scale baroclinicity than seen for the November 1972 cyclone. Oceanic heat fluxes were small except in the cyclones' wake. No widespread areas of convection and convective instability were observed as the cyclone explosively deepened. The 1982 storm achieved peak intensity over cold water while the 1972 storm reached maximum strength over the warm waters of the Gulf Stream. Upper-level frontogenesis may have contributed to the intensity of the upper-tropospheric trough and ensuing downstream development in the 1982 case.

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 Numerical Study of the East Coast Snowstorm of 10–12 February 1983

A numerical study of the East Coast snowstorm of 10–12 February 1983 has been conducted with the NRL mesoscale model. The three-dimensional, hydrostatic, primitive equation model has 91 × 51 horizontal grid points with a half degree resolution in a verification domain of 100°W to 60°W and 25°N to 45°N. There are ten layers in the vertical of equal σ(=p/ps) thickness. The model uses a split-explicit method for temporal integration and a second-order accurate spatial finite differencing. Model physics include precipitation on the resolvable scale and parameterized boundary layer and cumulus convection. The NMC 2.5 degree hemispheric analyses are used as the basic dataset (called NMC analysis hereafter). Because significant details in the initial conditions contained in the original rawinsonde observations may have been smoothed out by the NMC analysis algorithm, the analyses are also altered to provide a closer fit to the rawinsonde data. Original rawinsonde data are used in this enhancement of the NMC analyses (called enhanced). Forecasts are made from both the NMC analysis and the enhanced analyses to determine whether the enhancement can improve the forecasts. The Barnes analysis scheme with parameters suitable for reducing the short wavelength noise on the model grid scale is used to enhance the NMC analyses with the original soundings. Three types of lateral boundary treatments—constant, tendency damping, and temporal relaxation boundary conditions—are tested and compared. Results from forecast experiments show that the boundary treatment has a great impact on the model performance. The constant boundary condition produces an unusable forecast after 12 h as judged by the S1 scores, while the relaxation boundary condition produces an excellent forecast. The enhancement of the initial conditions has a negligible effect on predictions when reasonable boundary updates are used for the snowstorm case. The enhanced dataset produces a slightly better but still useless forecast when constant boundary conditions are used. Numerical experiments have also been conducted to test the sensitivity of the cyclogenesis to physical processes by suppressing one or more physical processes in the model. It is found that the evaporation from the ocean modulates the location and amount of precipitation. Without the evaporation, the-intensity of the cyclone remains the same but the center stays on the coast instead of staying off shore. The track of the snowstorm is such that the sensible beating from the ocean dampens the development of the cyclone by reducing the low-level baroclinicity. Without the sensible heating, the minimum surface pressure of the cyclone is 11 mb lower. The latent heating is found to be important for this case in which the maximum beating rate is 15°–20°C/day. When latent heating is suppressed, the cyclone translates at a much reduced rate and its central pressure is 10 mb higher after two days of simulation. These results from the sensitivity tests are of course case-dependent.

A Numerical Investigation of Low-Level Processes in Rapid Cyclogenesis

Abstract Several physical processes and properties of the initial state that affect marine cyclogenesis are examined using a mesoscale numerical model. The sensitivity of an idealized cyclone to the effects of latent heat release, surface heat and moisture fluxes as well as the initial meridional temperature gradient and static stability is examined by comparing various numerical stimulations of cyclogenesis in a baroclinic channel-flow model. Idealized initial conditions are derived analytically and are characterized by strong low-level baroclinity and a very weak upper-level trough. These initial conditions are used to examine which factors in baroclinic cyclogenesis are most important for rapid development (1 m h−1 for 24 h or more) and how diabatic processes modify the development rate. A strong low-level meridional gradient (40°C/2000 km) and low static stability (a mean lapse rate of 6.0°C km−1) resulted in rapid development of the model cyclone. The model cyclogenesis is more sensitive to small cha...