FIFTH INTERNATIONAL WORKSHOP on TROPICAL CYCLONES
Topic 1 TROPICAL CYCLONE STRUCTURE AND STRUCTURE CHANGE
[Russ Elsberry (USA)]
Topic 1.3: Role of convective processes
Rapporteur: Roger K. Smith
University of Munich
Theresienstr. 37, 80333, Munich, GERMANY
Fax: +49 89 2180 4383
Working Group: M. Bister, C. Davis, D. Möller, C. M. Nguyen, H. Zhu
This report summarizes research carried out since IWTC-IV that contributes to an improved understanding of the role of convective processes in tropical cyclones (TCs).
This brief review of research pertaining to convective processes in TCs since IWTC-IV is divided into three sections. First we consider the results of simple numerical model calculations that relate mainly to symmetric processes of TC intensification and go on to examine the rapidly-developing field of investigation into asymmetric processes. Finally we touch upon a range of observational studies including those using analyses of lightning data.
1.3.2 Review of deep convection in tropical cyclones
A recent review of the state of knowledge about the role of deep convection on the dynamics of tropical cyclones (TCs) and its representation in tropical cyclone models is the subject of a paper by Smith (2000).
"Thought experiments" carried out using simple models can play an important role in providing basic understanding of TC behaviour. A series of papers by Zehnder (2001), Zhu et al. (2001), Nguyen et al. (2002) and Zhu and Smith (2002a, b) contain numerical studies of TC intensification using such minimal models. Zehnder studies vortex evolution in a three-layer, constant-density, shallow-water model, while the other papers use a three-layer model formulated in sigma-coordinates, which allows a more complete representation of moist processes. Zehnder (2001) and Zhu et al. (2001) compared vortex evolution for various closures on the parameterized cloud-base mass flux of deep convection. Calculations starting with an initially symmetric vortex focus on the symmetric process of spin-up. Both models show a similar evolution (see Fig. 1.3.1) with a gestation stage followed by a rapid intensifying stage and a mature stage, but the time of rapid intensification depends on the closure used and spans a two-day period. If the same sensitivity exists in operational models, it would have implications for forecasting TC intensity change. Zhu and Smith (2002a) investigated the role of shallow convection, precipitation-cooled downdrafts, and the vertical transport of momentum by deep convection on vortex evolution, while Nguyen et al. (2002) and Zhu and Smith (2002b) investigated, inter alia, the formation of flow asymmetries during the rapid-development stage.
Fig. 1.3.1: Maximum wind speed in the middle layer of the calculation by Zhu et al. (2001) for three different closures on the cloud-base mass flux of parameterized convection (curves labeled 2 - 4) and for explicit convection only (curve 1). See Zhu et al. for details.
Bister (2001) investigated the effects of peripheral convection on TC formation using an axisymmetric model with explicit convection. Results suggest that peripheral convection causes changes in the vortex structure that are unfavourable for future inner convection, thereby delaying the onset of rapid intensification into a TC. Weak inner convection is associated with slow moistening of the inner region, which seems to be a prerequisite for the onset of rapid intensification of the vortex in the model. The negative impact of the peripheral convection increases with latitude.
1.3.3 Asymmetric convection
Since IWTC-IV, there has been a significant increase in research focused on the role of asymmetric convection in all phases of TC life cycles. Particular attention has been given to convectively-induced asymmetries of rainfall with an emerging perspective that processes within TCs are manifestations of coherent structures that undergo their own life cycle and may ultimately decay in favor of the symmetric circulation. Studies suggest that asymmetries arise in various ways, but nearly all involve the imposition of non-trivial vertical wind shear across a developing or mature vortex (Frank and Ritchie 2001, Reasor et al. 2001). The asymmetries often evolve into coherent sub-system-scale vortices that can persist for one or more revolutions about the parent vortex and induce significant intensity changes. Part of the intensity change observed in storms such as Danny (1997) (Blackwell, 2000) may be due to the superposition of strong asymmetries, manifested as intense mesovortices, on the symmetric circulation. Supporting evidence comes from the Doppler radar synthesis from Danny as well as earlier studies by, for instance, Black and Marks (1991). Cloud-resolving simulations of tropical storm Diana (1984) (Davis and Bosart, 2002) reveal convectively induced mesovortices in numerical simulations. The coherence of these structures is consistent with "hot tower" ideas. While over four decades old, the ideas have been revived somewhat recently (Heymsfield et al., 2001, Simpson et al., 1998, Braun, 2002). Collectively, the emerging concept is one of intensity and structural change (including warming in the eye) occurring through bursts, fundamentally stochastic in nature, associated with life cycles of asymmetries, rather than though a continuous "slow" evolution caused by the symmetric, secondary circulation.
The relationship of convectively induced asymmetries to vertical wind shear has been further quantified through the concept of balanced lifting resulting from the presence of a lower-tropospheric vortex in shear. Frank and Ritchie (1999) showed that mesoscale ascent achieved saturation in the lower troposphere and caused a shift of the rainfall from the downshear-right quadrant of the storm to the downshear-left quadrant. Davis and Bosart (2001, 2002) showed that balanced motion within a weak, subtropical baroclinic system functions in a similar way to focus rainfall and to generate potential vorticity (PV) anomalies. Such anomalies can merge to form a new center or a single, dominant anomaly can form a distinct center and subsume the surrounding vortices. The mesovortices occurred over a range of scales from 10 to perhaps 100 km. The smaller ones resemble more the classic hot towers; the larger ones are mesoscale convective vortices, analogous to the cousins from continental convection (Rogers and Fritsch, 2001). Larger ones appear capable of forming a new center; smaller vortices distort and can amplify an existing center. Importantly, all such vortices share a common property of large cyclonic vorticity in the lower troposphere because of the vigorous organized convection that produced them. Thus, they contribute vorticity to low levels of an existing vortex or can form a low-level circulation by themselves. Either way, the center of circulation becomes increasingly able to tap the latent reservoir of energy contained in the upper ocean.
Fig. 1.3.2: of Basic-state tangential velocity (m s-1) at the bottom of the domain for the narrow PV anomaly initialization for the tropical storm vortex initially (solid line), initialization inside the RMW after 48 h (dashed line), and initialization at the RMW after 48 h (dotted line). From Möller and Montgomery, 2000.
The mechanism of vortex intensification by vortex Rossby (PV-) waves has been investigated with a range of idealized numerical experiments that have provided fundamental insights into the influence of convectively induced PV asymmetries on the intensification of tropical storms. Möller and Montgomery (1999) carried out two-dimensional calculations with relatively high wave amplitudes and identified a wave-induced eigenmode that interacts with the hurricane-like vortex. The vortex can sustain the eigenmode, which itself can interact with convection and then feed back to the vortex. A subsequent study by Möller and Montgomery (2000) confirmed the important role of convectively induced asymmetric flow features in determining the structure and intensity of TCs. Simple "axisymmetrization" experiments in three dimensions with mono-chromatic azimuthal-wavenumber disturbances and single-cluster PV anomalies show that vortex Rossby waves propagate both radially and vertically. When persistent convection is simulated by adding double-cluster PV anomalies to the PV fields, one after another (so-called "pulsing"), the tropical storm intensifies to hurricane strength with the final intensity dependent on the location and extent of the anomaly (see Fig. 1.3.2). The results support the existence of an alternate means of TC intensification to the symmetric mode.
To incorporate the feedback between PV anomalies, a hurricane vortex and convection, requires a model that includes moist physics. Shapiro (2000) used a three-layer numerical model including a convergence-based convective parameterization scheme to investigate the role of cumulus convection and the boundary layer in the interactions between asymmetric PV anomalies and a hurricane vortex. The study shows that convection plays an important role in determining how a hurricane responds to the flow asymmetries in its environment. In particular, the location of the local symmetric PV maximum inside the radius of maximum wind controls the response to the extent that moving the PV anomaly radially inward or outward has no qualitative effect on the response.
Frank and Ritchie (2001) used the MM5 mesoscale model to investigate the effects of vertical wind shear on the intensity and structure of hurricanes. The storms in shear weaken with time and eventually reach an approximately steady intensity that is well below their theoretical maximum potential intensity. It is hypothesized that the weakening of the storm occurs via the following sequence of events: (1) the shear causes the structure of the eyewall region to become highly asymmetric throughout the storm; (2) the asymmetries in the upper troposphere, where the storm circulation is weaker, become sufficiently strong that air with high values of PV and equivalent potential temperature are mixed outward rather than into the eye; (3) the asymmetric features at upper levels are advected by the shear, causing the upper portions of the vortex to tilt approximately downshear. The storm weakens from the top down, reaching an approximate steady-state intensity when the ventilated layer can descend no farther due to the increasing strength and stability of the vortex at lower levels.
Convectively induced asymmetries can have an effect also on storm motion. Davis and Bosart (2002) showed that deep convection can dramatically alter the distribution of PV near the tropopause. These changes then alter the ventilation flow and change the track of the TC. Wu and Wang (2001) generalized the effect of asymmetries on track from a PV perspective capable of distinguishing diabatic effects from advection. They show how the track of the storm is influenced both by advection of symmetric (PV) by heating-induced asymmetric flow and through the direct generation of a positive PV tendency by asymmetric heating.
Recent work by Emanuel (1999) using a highly simplified, axisymmetric, coupled atmosphere-ocean model to forecast TCs in real time suggests that basic knowledge of the symmetric circulation and its interaction with the ocean could is sufficient to obtain remarkably accurate intensity forecasts in many cases. However, the model is too crude to adequately represent the effects of asymmetries, and performs poorly when strong vertical shear is present, presumably owing to the effects of asymmetric convection in the real storms. The model has also not been used to predict the formation of tropical cyclones, wherein asymmetric effects are known to be crucial.
1.3.3 Cloud buoyancy
There has been a recent interest in the buoyancy of eyewall convection (Zhang et al., 2000; Braun, 2002). Braun (2002) carried out a numerical simulation of Hurricane Bob (1991) using the MM5 model with a horizontal grid spacing of 1.3 km on the finest nested mesh. The time-mean asymmetric vertical motion is comprised of small-scale convective updrafts that at any given time cover only a small portion of the eyewall area, but account for a majority of the updraft mass flux, consistent with the concept of hot towers. It is found that eyewall updrafts are positively buoyant with respect to an environment that includes the vortex-scale warm core structure. Calculations along the trajectories show sufficient buoyancy to account for the simulated vertical velocities. A key source for the buoyancy is the energy gained from surface fluxes of moisture and heat by select parcels that originate from outside of the eyewall in the lowest part of the boundary layer, penetrate farthest into the eye, and then accelerate outward sharply while rising out of the boundary layer. Occasionally, air within the eye is drawn into the eyewall updrafts, suggesting episodic rather than continuous venting of the eye air into the eyewall. The concept of buoyancy in rapidly rotating vortices is reviewed in a recent paper by Smith et al. (to be submitted).
1.3.3 Observational studies
a) Lightning and 85-GHz satellite data
Studies of the convective structure of hurricanes using lightning data have been a focus of studies by Molinari et al. (1999), Cecil and Zipser (1999), Cecil et al. (2002), and Corbosiero and Molinari (2002). Molinari et al.s results suggest that precipitation in the hurricane can be divided into three regimes: the eyewall, which although unique, shares some attributes of weakly electrified monsoonal convection; the region outside and under the central dense overcast that has characteristics of the trailing stratiform region of mesoscale convective systems with a relatively high fraction of positive polarity flashes; and the outer rainbands, which contain the vast majority of ground flashes in storms. Cecil and Zipser (1999) and Cecil et al. (2002) examined relationships between TC intensity and satellite-based indicators of inner core convection including the 85-GHz ice-scattering signature and lightning. Up to a point, the more intense storms had larger ice-scattering signatures, but the inner-core lightning observations showed no clear relationship to TC intensification. Corbosiero and Molinari (2002) investigated the effects of vertical wind shear on the distribution of convection in TCs, based on lightning data. Their results suggest that in convectively active TCs, deep divergent circulations oppose the vertical wind shear and act to minimize the tilt. This allows convection to remain downshear rather than rotate with time.
b) Other studies
Fig. 1.3.3: Radar echoes of Typhoon 9414 at 0000 UTC 13 August. Storm-relative coordinates with range rings at 100 km intervals are used. The upper side of the figure is the northern side of storms. Solid and double hatching indicate reflectivities = 35 and 25 dBz, and a contour of 15 dBz is drawn. Thick broken lines represent data processing areas of radars and thin broken lines coast lines (From Shimazu, 1998).
For substantiation of the formation and early TC development process in the context of interaction between convection and larger-scale motions, it is important to investigate how the convection is organized within a TC in the early developing stage based on continuous radar observation. In the work of Mori et al. (1999) the structure and evolution of convection in the major part of Yancy in the early developing stage were investigated, mainly using radar, upper air and sea surface data. To cover a shortage of wind data around Yancy, cell echo tracking winds were evaluated. Recently available satellite data were utilized also.
The implied heating and PV generation in TC rainbands was derived by May and Holland (1999) from observed vertical motion profiles. High levels of PV generation were found in the stratiform rain regions, sufficient to generate substantial wind maxima along the bands within a couple of hours. Such generation may represent a significant source of PV for the system as a whole and may have implications for cyclone intensity.
Shimazu (1998) studied the size, shape, location, lifetime, and motion of precipitation system in 16 mature and developing typhoons around Japan using conventional radar network data. The precipitation systems, remote from mid-latitude frontal zones, were classified into inner and outer rain shields, inner and outer rainbands, and eyewalls (see Fig. 1.3.3). Based on these features, a modified version of Rockney's, and Senn and Hizer's, classification was proposed. Delta-shaped precipitation systems, called delta rain shields, were found to characterize typhoons approaching mid-latitude frontal zones.
This article provides a brief review of recent research relevant to understanding convective processes in TCs. Space limitations did not allow for a more comprehensive discussion of the literature, although a more complete set of references is provided. The main focuses of recent research has been into symmetric processes of TC intensification using relatively simple numerical models; asymmetric processes using a range of models; the quantification of cloud buoyancy, and various observational studies using radar, wind profiler, lightning and satellite data.
Acknowledgments Support from the US Office of Naval Research enabled the participation of D. Möller and R. K. Smith in the Workshop.
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