The diagnosis of middle latitude synoptic development

Abstract

SUMMARYThe use of diagnostics based on different forms for the forcing term in the omega equation is explored. These forms are the two-level and continuous versions of the approximation used by Sutcliffe (1947) in his development theory, the usual dynamical meteorology version involving vorticity and thermal advection, and that involving the so-called Q-vectors which was introduced by Hoskins ef al. (1978). The diagnostics are applied to a model baroclinic wave and to a subjectively analysed real data case. The Sutcliffe form is simplest and gives a global view of the system movement and development, but details such as active frontal regions are missed. The vorticity and thermal advection form has few advantages. It is demonstrated that the Q-vector analysis can provide more information than the Sutcliffe form in describing details of system development, particularly with respect to (a) a vectorial view of the horizontal ageostrophic motion field, and (b) some indication of the intensity of frontal circulations. A case is presented for including Q-vector fields in low-and mid-tropospheric forecast charts. INTRODUCT~ONThe purpose of this paper is to further explore the use of diagnostics which were discussed in Hoskins et af. (1978, hereafter denoted Q). There various forms of the forcing term F in the quasi-geostrophic 'omega equation'(1) were considered, N being the buoyancy frequency and f the Coriolis parameter. Before recalling these forms we first note that if w is approximately sinusoidal in the three space dimensions then F negative is expected to give w positive, i.e. upward motion.Neglecting the variation offwith latitude and diabatic effects, the form of F given the name 'Sutcliffe' in Q iswhere tg is the vertical component of geostrophic relative vorticity.Using a two-level representation with vg = vo at z = 0 and vg = vo+v' at z = H , This is the form for F, actually introduced by Sutcliffe (1947) in his development theory. Further analysis of this theory is given in Appendix A.In R it was shown that the Sutcliffe form Fs, though convenient, neglects a term which formally is of the same order as those retained. The neglected term is proportional to the square of the deformation times the rotation of the dilatation axis with height and may be important in frontal regions and in jet entrance and exit regions. The usual form of the omega equation used by dynamical meteorologists makes no such approximation in writing 707

A Composite Perspective of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones

“…To assess regions of frontogenesis and forcing for vertical motion associated with recurving WNP TCs, 700-hPa Q vectors and Q-vector divergence, respectively, are computed in pressure coordinates following Hoskins and Pedder (1980), except that the nondivergent wind is used instead of the geostrophic wind. In this formulation, the Q vector describes the time rate of change of the vector horizontal potential temperature gradient due to the nondivergent wind.…”

Extratropical cyclones, frontal waves, and Mars dust: Modeling and considerations

“…[19] The circulation within an cyclone/anticyclone disturbance and its accompanying frontal wave is richly 3D due to the requirement for thermal and dynamical balance within an individual weather system [Hoskins and Pedder, 1980;Sanders and Hoskins, 1990]. Associated with the convergent/divergent and stretching/deforming quasi-horizontal circulations in a frontal system, is a secondary, cross-frontal (ageostrophic) circulation [Hoskins and Pedder, 1980;Sanders and Hoskins, 1990] which results in ascending (descending) motion ahead (behind) the frontal zone.…”

“…Associated with the convergent/divergent and stretching/deforming quasi-horizontal circulations in a frontal system, is a secondary, cross-frontal (ageostrophic) circulation [Hoskins and Pedder, 1980;Sanders and Hoskins, 1990] which results in ascending (descending) motion ahead (behind) the frontal zone. The occurrence of this secondary circulation can be assessed in terms of the so-called Q vector where Q = (Q (x) , Q (y) , 0) is a horizontal vector whose components are comprised of the products of horizontal shears in the horizontal wind and directional derivatives in potential temperature, [Kurz, 1992] …”

“…Further, Q is a good dynamical indicator of frontogenetic/frontolytic processes within a front, and the cross-frontal (secondary) circulation can be shown to be related to it by w / −r · Q [Sanders and Hoskins, 1990]. Under quasi-geostrophic theory, the quantity Q · r is proportional to the Langrangian time derivative of the squared temperature gradient [Hoskins and Pedder, 1980], a quantitative measure of development or decay of a frontal wave.…”

Rainfall: Features and Variations over Saudi Arabia, A Review

“…In spite of the fact that the Mediterranean and neighboring areas have been extensively explored throughout the last few decades [1][2][3][4], the SA region has received little attention. Brief and sound climatological information on the Arabian Peninsula (AP) was provided by [5][6][7], and they demonstrate that there is a wide climatic spectrum on the AP, from the snows of the Asir Province in SA to the over powering humidity of the Arabian Gulf, from the searing heat of the Rub Al Khali to the monsoon precipitation in the Qara mountains in Dhofar.…”