Alan J. Thorpe

Baroclinic Instability in an Environment of Small Stability to Slantwise Moist Convection. Part I: Two-Dimensional Models

Abstract In the semigeostrophic system, the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity, which acts as the effective static stability. Recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero. In this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward (saturated) displacements but large in regions of descent. Analytic solutions for two-dimensional disturbances in a two-layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion, growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet. In the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2.5 times the...

The Fronts and Atlantic Storm-Track Experiment (FASTEX): Scientific Objectives and Experimental Design

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as ...

Frontogenesis in the presence of small stability to slantwise convection

Abstract It is often observed that, despite the existence of near neutrality to slantwise convection, rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases. This is probably due to the presence of active frontogenesis which, as shown here, maintains such structures. In order to investigate this process, we use two-dimensional semigeostrophic theory to solve the deformation-forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive. In this case, numerical simulation is necessary to determine the evolution of the front. Several previous studies have described the role of diabatic forcing in modifying the cross-frontal circulation by use of the Sawyer-Eliassen equation. In particular, analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis. Numeri...

Conditional Convective Heating in a Baroclinic Atmosphere: A Model of Convective Frontogenesis

Abstract It is shown here that there exists a regime of balanced frontogenesis that is forced almost entirely by the diabatic hating due to convection at a front. This theory is explored in the context of the two-dimensional semigeostrophic equations with an Eady basic state: convection is parameterized to be dependent on the low-level moisture convergence of the cross-frontal ageostrophic flow, in accordance with recent diagnostic studies. The significant result is that the growth rate of the convective frontal system becomes independent of the total wavelength of the domain once the diabatic heating exceeds a relatively large threshold magnitude. In this regime the frontal zone has a width and structure dependent on the heating magnitude but not on the wavelength. The system is described as “solitary” or “isolated” since the dynamics are self-contained and independent of the far field. The energetics of the system have a diabatic conversion that is an order of magnitude greater than that due to the larg...

Synoptic Scale Disturbances with Circular Symmetry

Abstract Ate balanced flow structure of various classic synopitc-scale disturbances is reviewed using the invertibility principle for isentropic potential vorticity (IPV) distributions. Complete solutions are shown for cold and warm core structures of various types. The basic model imagines the tropopause to be the interface between the lower potential vorticity of the troposphere and the approximately six-fold larger value typical of the lower stratosphere. The sensitivity of the structure of the potential temperature variation along the tropopause and at the surface is described. Results are presented in diagrammatic form to allow easy diagnosis of the vortex structure from synoptic data available at perhaps only a few levels. The point is made that upper air IPV and surface potential temperature distributions are often the most crucial in accounting for the balanced flow structure.

Comments on “The Evolution and Dynamical Role of Reduced Upper-Tropospheric Potential Vorticity in Intensive Observing Period One of FASTEX”

Abstract The existence and production of reduced upper-tropospheric potential vorticity (RUPV) by heating is considered. An objective technique is used that identifies anomalies of PV arising from a particular physical process (here latent heat release). The evolution of two RUPV anomalies and a related diabatically increased lower-tropospheric PV (ILPV) anomaly occurring during Intensive Observing Period One of the cyclones from the Fronts and Atlantic Storm Track Experiment (FASTEX) is examined using model analyses, sounding data, and trajectory calculations. Three distinct airflows are identified emanating from the ILPV anomaly each with a different evolution. Results show that RUPV anomalies exist in the atmosphere and, in a weaker form, in numerical models. The dynamical role of RUPV anomalies is examined using a nonlinear balance PV inversion and reruns of the U.K. Meteorological Office Limited Area Model. This shows that instantaneously the flow and temperature perturbations associated with RUPV an...

Mechanisms for pollutant transport between the boundary layer and the free troposphere

Pollutants are longer-lived in the free troposphere than the boundary layer, hence the transport of pollutants from the boundary layer to the free troposphere has significant implications for long-range transport and global warming. It is important to quantify the transport of air between the boundary layer and the free troposphere and to understand the role different meteorological mechanisms play. Idealised passive tracer experiments, with tracer initially only in the boundary layer, are performed in a numerical model for three case study days with different synoptic conditions. After 24 hours, more than 50% of the tracer resides in the free troposphere for the two frontal cases, and 40% resides there for the high-pressure case. The tracer was transported to maximum heights of 8 km. To elucidate the role of different mechanisms for each case, the tracer amount transported by advection only, advection and turbulent mixing, and advection and convection was calculated. Advection is found to be the most important mechanism in transporting the tracer to the free troposphere; however, the addition of upright convection and turbulent mixing increases the amount by up to 24% with convection transporting the tracer to heights of 5 km. The inclusion of convection and turbulent mixing to the advection are not linearly additive processes. This study shows the possibility of a large proportion of the pollutant emitted in the boundary layer being transported to the free troposphere in a short time and the importance of representing all the meteorological processes.

Feedback between the Land Surface and Rainfall at Convective Length Scales

Abstract The surface fluxes of heat and moisture in semiarid regions are sensitive to spatial variability of soil moisture caused by convective rainfall. Under conditions typical of the Sahel, this variability may persist for several days after a storm, during which time it modifies the overlying boundary layer. A model of the land surface is used to quantify the dependence of surface fluxes of heat and moisture on antecedent rainfall amount, time since rainfall, and surface properties. Next, a coupled model of the land and atmosphere is used to characterize the boundary layer variability that results from this surface variability, and its dependence on factors including the length scale of the surface variability. Finally, two- and three-dimensional modeling of squall lines is used to examine the sensitivity of rainfall to boundary layer variability. Boundary layer variability tends to be greater for surface variability on long length scales, but squall-line rainfall shows the strongest response for anom...

Use of Ducting Theory in an Observed Case of Gravity Waves

Abstract This paper attempts to explain the properties of a gravity wave event observed on Mallorca (Balearic islands) using an array of microbarographs. The waves propagated coherently for a relatively long distance with a speed of about 29 m s−1 and were nondispersive. In the absence of a continuous forcing, long-lived atmospheric gravity waves can only exist if some mechanism prevents the vertical leakage of energy through wave propagation, trapping the gravity wave in a duct layer near the surface. Lindzen and Tung showed the necessary conditions for trapping and discussed the properties of neutral modes for constant stability and wind in the duct. The role of shear in the wind profile near the ground is examined here by first finding three distinct neutral mode types using a matrix eigenvalue method. Applying the wave theory given by Booker and Bretherton, those neutral modes in the sheared duct that have critical levels within the stable duct will be mostly absorbed at their critical level. Therefor...

Frontal Wave Stability during Moist Deformation Frontogenesis. Part I: Linear Wave Dynamics

Abstract It has been shown that lower tropospheric potential vorticity zones formed during moist deformation frontogenesis will support growing waves if at some time the frontogenesis ceases. In this paper, the ways in which these waves are affected by the frontogenetic process are identified. Observations show that fronts in the eastern Atlantic commonly feature saturated ascent regions characterized by zero moist potential vorticity. Furthermore, in many cases the horizontal temperature gradient in the lowest one to two kilometers of the atmosphere is rather weak. These features are incorporated in an analytical archetype. The dynamical implications of saturated ascent in conditions of zero moist potential vorticity are represented in the model by assuming that adiabatic temperature changes are precisely balanced by diabatic tendencies. The observed small temperature gradient at low levels is represented in the model by taking it to be zero in the lowest two kilometers. Consequently, the forcing of the ...