Subsidence Warming Research Articles

A Theory for Nonprecipitating Convection between Two Parallel Plates. Part II: Nonlinear Theory and Cloud Field Organization

Abstract In Part I, an idealized model of nonprecipitating moist convection in a shallow conditionally unstable layer of viscous and diffusive air between two parallel plates was introduced, and the “linear” instability of an exactly saturated static state maintained by diffusion was investigated. If there are initially many clouds, the “linear” theory predicted that weaker clouds are suppressed by the subsidence warming and drying from the ever-growing stronger clouds, and the average cloud spacing becomes arbitrarily large as time goes on. Each growing cloud is surrounded by compensating subsidence, which decreases away from the cloud with a characteristic decay scale Rs, the subsidence radius, which can be understood from gravity wave arguments. In Part II, fields of finite amplitude clouds are considered. An asymptotic analysis is performed in which the moist Rayleigh number Nc2 exceeds by only a small amount μ the value Nc02 necessary for the onset of convection. This leads to a nonlinear set of “clo...

Plausibility of Substantial Dry Adiabatic Subsidence in a Tornado Core

Abstract The question of whether or not the cores of some tornadoes are significantly warmed above the moist adiabatic temperature of their parent Cb by dry adiabatic subsidence is discussed in terms of limited supporting evidence and scientific arguments both for and against. Further supporting evidence is provided by axisymmetric numerical simulations described here in which a background field of vertical vorticity is spun up at low levels by a moist convective updraft in a potentially unstable thermal environment typical for tornado events. Substantial axial subsidence heating develops in the simulations provided the vortex swirl is comparable to, or larger than, the maximum values observed in tornadoes. The significant axial forces, including buoyancy, vertical acceleration, and momentum diffusion, are examined in detail for their effect on the surface pressure deficit and the maximum low-level windspeed.

Thermal Budget of a Monsoon Depression in the Bay of Bengal during FGGE-MONEX 1979

The paper presents die results of a study of the thermal budget of a monsoon depression that developed over the Bay of Bengal during the period 3–8 July 1979. The complete thermodynamic energy equation is considered, to examine the possible role of the various terms and to evaluate the total diabatic heating. In the west-southwest quadrant of the monsoon depression where there is considerable rainfall, latent heat released by precipitation appears to account for about 80% of the total diabatic heating. This heating appears to be offset by cooling due to strong upward motion; however, the total diabatic beating over an area immediately to the north-northeast of the depression center appears to be negative, suggesting downward air motion and adiabatic warming. It is suggested that this warm sector to the north-northeast of the depression center, which is maintained by subsidence warming may serve as an effective tropospheric energy source for the monsoon depression.

An Investigation of the Development of Cumulonimbus Systems over South Florida. Part I: Boundary Layer Interactions

This paper investigates the interactions between two developing cumulonimbus systems and the boundary layer (surface winds, pressure, divergence and temperature fields) on a case study day, 25 August 1975. These two systems formed within 45 min of each other in similar locations and environments. Analysts indicate that in the case of rapid convective development, a surface pressure low develops below the convection and in the environment surrounding the convective system. The pressure low beneath the convection is hypothesized to be formed by hydrostatic and nonhydrostatic (dynamic) effects, and the extension of the low in the environment surrounding the system is hypothesized to be induced by hydrostatic adjustment resulting from subsidence warming. The environmental flow responds to the induced pressure field, causing the convergence to increase in both the convective region and the mesoscale region surrounding the convection. Once rainfall occurs at the surface, the pressure begins to rise in the convective area, reducing or reversing the pressure gradient, causing the convergence in the mesoscale and convective-sale areas to decrease. Once the precipitation stage is reached, the interaction of the outflows from the convection with the surrounding flow appears to become the dominant mechanism for enhancing convergence in the boundary layer.

Formation of easterly wind maxima at 200 mb during southwest monsoon

In three case studies presented, the formation of easterly wind maxima at 200 mb south of the monsoon depressions can be explained with the help of conservation of angular momentum when the northeasterly air stream emerging from the cloud system acquires the additional easterly momentum during Its motion towards southwest. The temperature gradients from north to south occur at 300, 250 and 200 mb across these wind maxima. In one of the case studies the subsidence warming on the equator ward leg of the monsoon cell splits the jet stream, has been elucidated in the vertical cross-section presented.

Rainbands and Symmetric Instability

Abstract A linearized two-dimensional model was used to study the formation and evolution of the rainbands which produce an intense precipitation along the Baiu front over Eastern Asia during spring and summer. This model is also applied to squall lines over the eastern part of the United States during 3–4 April 1974. Condensation is included in this nonhydrostatic model, in which the total condensation within a vertical cloud column is proportional to the convergence of moisture at cloud base, but there is no cooling in the descending branches of the circulation. The horizontal temperature gradient is introduced in order to maintain a vertical wind shear according to the thermal wind relationship. The results of this study show that rainbands propagate toward the warm side. The small, weak bands are dissipated by the subsidence warming associated with the downward motion branches of their stronger surroundings; therefore, they merge into a few large rainbands with a horizontal wavelength of a few hundred...

Convective Evolution and Merger in the FACE Experimental Area: Mesoscale Convection and Boundary Layer Interactions

Abstract This paper investigates the interactions between the various scales of motion and, specifically, the inter-actions between convection and the surface boundary layer in the development of a mesoscale convective system within the Florida Area Cumulus Experiment (FACE) experimental area. Data used in the analysis are from a surface mesonetwork covering a 1500 km2 area which consisted of wind measuring stations, raingages, hygrothermographs, microbarographs and temperature, humidity and pressure transducers. Surface convergence was shown to exist up to 2 h before the development of precipitation over the convergence area within the mesonetwork. Convergence was not being balanced by divergence within the network, which implies mesoscale and/or synoptic-scale forcing. The subsidence warming and drying in the near environment of the mesoscale convective system appeared to play an important role in its evolution from the mature to the dissipating stage. Soundings taken in the near environment of the conv...

Breakup of Temperature Inversions in Deep Mountain Valleys: Part I. Observations

Abstract The breakup of temperature inversions in the deep mountain valleys of western Colorado has been studied by means of tethered balloon observations of wind and temperature structure on clear weather days in different seasons. Vertical potential temperature structure profiles evolve following one of three patterns. Two of the patterns are special cases of the third pattern, in which inversions are destroyed by two continuous processes-upward growth of a convective boundary layer (CBL) into the base of the valley inversion, and descent of the inversion top. The three idealized patterns are described and 21 case studies of inversion breakup following the patterns are summarized. Inversion breakup begins at sunrise and is generally completed in 3½–5 h, unless the valley is snow covered or the ground is wet. Warming of the inversion layer is consistent with subsidence heating. An hypothesis is offered to explain the observations, stressing the role of the sensible heat flux in causing the CBL to grow an...

Diurnal Variation in the Troposphere's Energy Balance

The tropospheric energy balance is examined in an effort to ascertain the diurnal variation of its components. The mean local diurnal temperature change in the 850–300 mb layer was observationally determined utilizing upper air data collected at 142 Northern Hemisphere radiosonde stations. Temperature data from the GATE, ATEX, BOMEX and LIE experiments, and Operations Redwing and Hardtack in the tropical west Pacific were also analyzed. Diurnal radiative cooling values are computed for specific geographical regions, and the magnitude of the required diurnal warming for each region is determined. Required warming is composed of a variety of atmospheric processes such as condensation energy release, surface to air sensible heat transfer, horizontal advection and vertical motion. A large morning maximum and late afternoon-early evening minimum of tropospheric required warming is found for all data sets, at all latitudes, in all seasons, and for both oceanic and land areas. These morning versus late afternoon required warming differences are of magnitudes of 2–3 to 1. It is hypothesized that the subsidence warming contribution to the diurnal variation of required warming dominates the other components. Increased morning subsidence and weaker afternoon–early evening subsidence is apparently a generally operating phenomenon in the troposphere.

Formation of Mesolows or Pressure Troughs in Advance of Cumulonimbus Clouds

Abstract Mesoscale lows or pressure troughs have been observed downwind of many mid-latitude cumulonimbus cloud systems, especially those that subsequently produce severe weather such as tornadoes, large hail, or damaging wind gusts. Case studies are presented which link the formation and existence of mesolows or troughs to subsidence warming in the upper troposphere and lower stratosphere. This subsidence warming, which is believed to result from the interaction of the convective cloud with a sheared environment, leads to an instability between the meso-γ and meso-β scales by hydrostatically reducing surface pressures, thereby organizing and increasing the low-level convergence ahead of existing convection.

On the Excitation and Propagation of Zonal Winds in an Atmosphere with Newtonian Cooling

A system of equations, including Newtonian cooling and valid away from the equator, is derived for atmospheric zonal winds and the associated zonal temperature and meridional circulation fields. A time scaling is introduced to distinguish three regimes of motion. For short time scales a variation of the usual nondissipative forced symmetric vortex problem is recovered. For intermediate time scales, a “diffusive model” is defined, while for long time scales a “steady state” model is defined. Two examples of the solution of the diffusive equation are given. From the first example we infer that our model is capable of explaining many of the observed features of the downward progression of the stratospheric biennial oscillation. Momentum is carried downward by Coriolis torques resulting from propagating meridional cells. The solutions of our second example describe transient zonal winds which resemble those observed to propagate downward from the stratopause in Meteorological Rocket Network data time-height sections. In general, dissipation relaxes the constraint of conservation of potential vorticity so that wave-like motions are possible when Newtonian cooling is present. The “steady state” model equation is solved by the method of Green's functions to obtain the forced temperature. For this model, there is a direct response to heating and only momentum fluxes force meridional circulations to give subsidence heating. It is found that the domain of influence of the temperature response to momentum fluxes lies below the source point. Our model suggests that observed long-period departures from radiative equilibrium in the atmosphere below the mesopause can be explained as due either to heating by horizontal eddy heat transports or to subsidence heating forced by eddy momentum fluxes.