John H. E. Clark

Resonant Planetary Waves in a Spherical Atmosphere

Abstract A global model of planetary wave propagation in a spherical atmosphere is used to examine the spectrum of free or resonant planetary waves of the solstitial stratosphere. These free modes are located by forcing the model with a weak periodic vertical velocity along the lower boundary and looking for a resonant response in wave amplitude. The modes correspond to the natural traveling oscillations in the earths atmosphere, of which the 5-day wave is the best known example. The 15-day wave observed by Madden (1978) and others is found to be such a resonant mode. We find that the strong stratospheric winds cause the 15-day wave to become baroclinic by trapping the wave between the earths surface and the strong winds at the stratopause. The strong winds effectively reduce the atmospheric damping which greatly reduces the amplitude of barotropic waves with periods >10 days. The computed meridional structure of the 15-day wave is in reasonable agreement with Maddens (1978) observations at extratropic...

A QUASI-GEOSTROPHIC MODEL OF THE WINTER STRATOSPHERIC CIRCULATION

Abstract A six-level quasi-geostrophic model including radiative and photochemical processes in the manner suggested by Lindzen and Goody is run from a state of joint radiative-photochemical equilibrium for midwinter conditions. The spectral method is used to integrate the equations where all dependent variables are represented by a set of spherical harmonics with east-west wave numbers 0, 1, 2, 3, and 6 included. The winter storage of ozone in the polar lower stratosphere is simulated, and the importance of horizontal planetary scale transports and the vertical eddy diffusion of ozone is demonstrated. The rapid dissipation of upper stratospheric temperature disturbances by joint radiative-photochemical relaxation is discussed, and the importance of tropospheric forcing and nonlinear exchanges of kinetic energy between the planetary scale waves is demonstrated. The energetics of downward-propagating spontaneous warmings is discussed. A full-scale warming is triggered by strengthening the north-south lower...

Environment and Evolution of a Cold-Frontal Mesoscale Convective System

Abstract Observations from the Oklahoma-Kansas Preliminary Regional Experiment for STORM-CENTRAL (OK PRE-STORM) have been used to study the evolution and propagation characteristics of a long-lived (≥16 h) mesoscale convective system (MCS) that produced locally heavy (50–100 mm) rainfall during 26–27 June 1985. The MCS formed in association with a synoptic-scale cold front and upper-level trough system. Mesoscale ascent contributed to an increase in convective available potential energy (CAPE) and a decrease in convective inhibition, facilitating the development of deep convection. During the late morning and early afternoon hours convection was present along and within an ∼200-km zone in advance of the cold front. In advance of the main precipitation area, a series of nearly parallel rainbands formed from in situ boundary-layer cloud streets. The development and organization of these rainbands was aided by the moderate-to-large CAPE, small convective inhibition, and moderate unidirectional shear at low l...

A Reexamination of the Mechanisms Responsible for Banded Precipitation

Abstract The processes responsible for a banded snowfall region during a December 1997 East Coast storm are examined. Conventional data plus a numerical simulation with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used. Calculations of slantwise potential area near the bands suggest that the release of conditional symmetric instability played a role in their formation. The location and timing for the appearance of negative moist potential vorticity (MPV) cannot, however, be reconciled with band formation. A balanced MPV model based on the geostrophic momentum approximation is developed. It provided new insights into the mechanisms of MPV generation. A swath of negative balanced MPV now coincides with the snowbands. MPV sources are proposed that are linked to a vigorous mesoscale updraft near the bands. The updraft occurred on the warm, moist side of a zone of midtroposphere frontogenesis. Negative MPV develops through differential ageostrophic transports of geostrophic...

The Transport of Conservative Trace Gases by Planetary Waves

Abstract An analytical model for the horizontal and vertical transport of a conservative trace gas by a stationary planetary wave is developed. The wave is associated with a constant background zonal flow. In the absence of dissipation, transports are zero if the wave is trapped, i.e., the background flow is easterly or strong westerly. Propagating waves can carry out appreciable transports. The vertical transport is proportional to the latitudinal gradient of the background mixing ratio and the horizontal transport is proportional to the vertical gradient. With dissipation in the form of Newtonian cooling, transports are permitted by trapped waves. For strong westerlies, they are very small but for easterlies they can be appreciable. In fact, substantial southward ozone transports in the summertime lower stratosphere are suggested. The simplicity of the flux expressions suggests that they might be exploited in two-dimensional models of ozone.

The Vertical Propagation of Forced Atmospheric Planetary Waves

Abstract Adiabatic, inviscid, quasi-geostrophic perturbations on a β plane are forced at some initial time by the switch-on of a vertical velocity or temperature disturbance at the base of a stratified atmosphere. The vertical variation of the basic zonal wind is represented by three simple layered models. A Laplace-transformed potential yorticity equation is solved in each layer, and after solutions are matched across interfaces, inversion integrals are evaluated numerically and asymptotically. After initial high-frequency planetary waves have died out by radiating energy to infinity, the response primarily consists of a standing-wave mode and a traveling barotropic Rossby mode. In spite of the fact that the barotropic model propagates energy vertically with an infinite group velocity, a signfficant build-up of wave energy is not realized until many days after the switch-on. The time dependence of energy and heat fluxes is caused by the interaction of these modes. The role of unstable modes and other ver...

The Diagnosis of Vertical Motion within Dry Intrusions

Dry intrusions play an important role in modulating precipitation patterns both in the midlatitudes and in the Tropics. The lifting of unsaturated air aloft often leads to destabilization and the enhancement of precipitation rates, and may occasionally contribute to the occurrence of severe weather. A method for qualitatively diagnosing vertical motion in a region of elevated dry advection is presented. The procedure measures the rate of propagation of relative humidity isopleths relative to the flow and deduces the sign of the vertical velocity. Changes in static stability are inferred, leading to the possibility of improved short-term forecasting of precipitation associated with dry intrusions. The procedure is illustrated with a case study involving heavy snowfall associated with a dry intrusion in the mid-Atlantic region. A diagnosis of ascent within the dry intrusion is obtained from satellite imagery and confirmed using numerical model output.

A Flash Flood from a Lake-Enhanced Rainband

Abstract An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land–lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence....

The Catalina Eddy Event of July 1987: A Coastally Trapped Mesoscale Response to Synoptic Forcing

Abstract The Catalina eddy that existed from 5 July to 12 July 1987 during FIRE (First ISSCP Regional Experiment) over offshore California is analyzed. There were two stages to the eddys lifecycle. During the first, from 5 July to 1200 UTC 9 July, the eddy formed just south of Santa Barbara and drifted southeastward parallel to the coastline. This motion is attributed to an equivalent β effect associated with gradients of marine layer depth perpendicular to the coast. The eddys thermal structure was characterized by an elevated marine inversion with surface temperatures 2°–4°C higher than beyond the periphery. Over offshore regions a sharp edge to the eddy was noted with a sudden change in mixed layer depth, wind speed and direction, and temperature. The eddys influence on coastal winds was most notable during the nighttime and early morning. The strong local sea-breeze circulation overwhelmed the coastal eddy circulation during daytime. A pronounced diurnal wind fluctuation was observed at San Nicolas...

The Role of Kelvin Waves in Evolution of the Catalina Eddy

Abstract The response to pressure falls near a coastline is studied with a linear shallow-water representation of the inversion-capped marine layer. The aim is to examine Catalina eddy development in the southern California bight region. Of particular interest is response sensitivity to background flow parallel to the coastline, as quantified by a Froude number (Fr). A conceptual model of eddy evolution is formulated that focuses on a pair of Kelvin waves triggered by the pressure falls. For subcritical Fr,|Fr| < 1, the waves move out of the regions of most favored growth located to the immediate north and south of the pressure fall center. Only a weak residual circulation remains in the bight region and an eddy does not form, For supercritical Fr, the waves tend to be held in place by the background flow. They thus undergo considerable amplification before dissipation eventually halts growth. A vigorous eddy circulation results in the bight region that resembles observed structures.