Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Mary Ann Jenkins x
  • Refine by Access: All Content x
Clear All Modify Search
Mary Ann Jenkins

Abstract

The purpose of this study is to identify what effects are responsible for the observed temperature field, in particular the lower-level cold core, in the trough region of a convectively active tropical easterly wave disturbance. GATE Phase III A/B- and B-scale data were used in the analysis, and the divergence equation, a first-order balance condition proposed by Cho and Jenkins applicable to slowly evolving or near-steady-state large-scale convectively active tropical circulation systems, and the standard nonlinear and linear balance equations were chosen as the framework in which to assimilate the observational data in order to understand the spatial anticorrelation between temperature changes and latent heating. It is shown that all three balance conditions reproduced the thermal structure of the easterly waves that passed over GATE during Phase III. Despite the differences in formulation and form, the simpler standard balance equations were as accurate as the first-order divergence equation in diagnosing the temperature field. The central result of the analysis is that the lower cold-core temperature anomaly observed in the near-trough region of easterly wave disturbances is not a direct consequence of the distribution of latent heat released by cumulus clouds, but reflects instead a balance of forces that dominate the momentum field and that the agreement between the observed and diagnosed temperature fields is dominated by the rotational component of the flow. This has possible implications for a first-order model of the interaction between cumulus-scale and large-scale equatorial wave motions, and for the distinction between developing and nondeveloping easterly waves in terms of the ambient vorticity field, which may aid future numerical investigations of tropical cyclogenesis.

Full access
Mary Ann Jenkins
and
Han-Ru Cho

Abstract

GATE Phase III A/B- and B-scale data were analyzed and used to diagnostically test the first-order vorticity equation proposed by Cho et al. The results are given in the form of a composite easterly wave and the analysis shows that the first-order vorticity equation is capable of describing the vorticity dynamics of the composite easterly wave. Cumulus heating produces significant large-scale vertical motion, and it is demonstrated that the effect of cumulus clouds on the large-scale vorticity field is through vortex-tube stretching by cloud-induced vertical motion.

The composite analysis of the first-order vorticity equation also shows that the vorticity dynamics of tropical easterly waves are adequately described by a linearized first-order vorticity equation and dominated by the advection of the perturbed vorticity by the relative zonal mean wind, the advection of the zonal mean vorticity by the perturbed wind, and the vortex-tube stretching induced by cumulus activity. It is explained how, during Phase III of GATE, the westward convergence and eastward divergence of the wave trough observed just above the surface and the appearance of midlevel divergence before and convergence after the passage of the wave trough axis are associated with the vorticity dynamics of tropical easterly waves.

Full access
Han-Ruo Cho
and
Mary Ann Jenkins

Abstract

A procedure is introduced to derive a general balance condition for synoptic-scale disturbances in the tropics. The condition describes a balance between the mean pressure and momentum fields, and the pressure forcing by cumulus clouds. A simple and explicit diagnostic relationship between the mean pressure and momentum fields is obtained when this balance condition is applied to slowly evolving tropical easterly waves. This relationship suggests that the thermal structure of tropical easterly waves reflects not so much the distribution of latent heat release by cumulus clouds, but the thermal structure required to maintain the dynamic structure of the waves. The temperature field of tropical easterly waves calculated from this diagnostic relationship, using the dynamic field of the composite easterly wave presented by Thompson et al., shows reasonably good agreement with observations.

Full access
Terry L. Clark
,
Mary Ann Jenkins
,
Janice Coen
, and
David Packham

Abstract

The object of this paper is to describe and demonstrate the necessity and utility of a coupled atmosphere-fire model: a three-dimensional, time-dependent wildfire simulation model, based on the primitive equations of motion and thermodynamics, that can represent the finescale dynamics of convective processes and capture ambient meteorological conditions.

In constructing this coupled model, model resolution for both the atmosphere and the fuel was found to be important in avoiding solutions that are physically unrealistic, and this aspect is discussed. The anelastic approximation is made in the equations of motion, and whether this dynamical framework is appropriate in its usual form for simulating wildfire behavior is also considered.

Two simple experiments-the first two in a series of numerical simulations using the coupled atmosphere- fire model-are presented here, showing the effect of wind speed on fire-line evolution in idealized and controlled conditions. The first experiment considers a 420-m-long fire line, and the second considers a 1500-m-long fire fine, where wind speeds normal to the initial fire lines vary from 1 to 5 m s−1. In agreement with some general observations, the short fire line remains stable and eventually develops a single conical shape, providing the wind speed is greater than about 1–2 m s−1, while under similar conditions, the longer fire line breaks up into multiple conical shapes. In both cases, the conical shapes are attributed to a feedback between the hot convective plumes and the near-surface convergence at the fire front. The experimental results reveal a dynamical explanation for fire-line breakup and geometry, demonstrating that the model is a valuable tool with which to investigate fire dynamics, and eventually it may be able to provide a credible scientific basis for policy decisions made by the meteorological and fire-management communities.

Full access
Han-Ru Cho
,
Mary Ann Jenkins
, and
Joseph Boyd

Abstract

A first order vorticity equation tropical disturbances is derived. The equation indicates that the evolution of the vorticity field in the tropical atmosphere is influenced mainly by the horizontal advection of vorticity, and a vorticity source produced by cumulus convection.

The A/B-scale vorticity budgets during Phase III of the GARP Atlantic Tropical Experiment (GATE) are analyzed to provide a consistency check between the first order vorticity equation and observations. The results show that the equation agrees well with the observed evolution of the large-scale vorticity field, at least in the diagnostic sense.

The simple vorticity equation is also used to study the vorticity dynamics of tropical easterly waves observed in GATE; certain features of the vorticity and divergence fields are related through the first order vorticity equation. In particular, the pronounced mid-level divergence often observed in GATE daring convectively active periods is found to be associated with the vorticity advection by the west African easterly jet at the 650 mb level.

Full access