Tropical Cyclone Development Research Articles

Simulation of Tropical Cyclone Circulation Over the Bay of Bengal Using the Arakawa-Schubert Cumulus Parameterization. Part II: Some Sensitivity Experiments

— The role of sea-surface temperature (SST) and Coriolis parameter in the evolution and intensification of tropical cyclones has been examined using the ten-level axi-symmetric primitive equation model described in the companion paper (Bhaskar Rao and Ashok, 1999). Two experiments have been conducted using the ten-level model to assess the role of Coriolis parameter “f” in tropical cyclone intensity and the size of the storm generated. Six experiments have been performed to assess the importance of Sea-Surface Temperature (SST) in tropical cyclogenesis and intensification. The initial thermodynamic field and the initial vortex are the same as that used to simulate the Bay of Bengal tropical cyclone discussed in the companion paper. Further sensitivity experiments indicated a strong dependency of the model on SSTs. The model initial vortex could not intensify with an SST of 299 K but could with an SST of 300 K. The increase of SST from 300 K to 300.5 K shows rapid intensification with a minimum central surface pressure of 910 hPa and a maximum tangential wind of 80 m/s. Further increase of SST only shows a marginal increase in intensity and a larger radius of maximum wind. Sensitivity experiments to assess the role of the Coriolis parameter suggest that tropical cyclones develop more intensity and are faster at relatively lower latitudes.

Potential Vorticity Asymmetries and Tropical Cyclone Evolution in a Moist Three-Layer Model

Abstract The role of potential vorticity (PV) asymmetries in the evolution of a tropical cyclone is investigated using a three-layer model that includes boundary layer friction, surface moisture fluxes, and a convergence-based convective parameterization. In a benchmark experiment, a symmetric vortex is first spun up on an f plane for 24 h. The symmetric vortex has a realistic structure, including a local PV maximum inside its radius of maximum wind (RMW). A weak azimuthal-wavenumber 2 PV asymmetry confined to the lower two layers of the model is then added to the vortex near the RMW. After an additional 2 h (for a total 26-h simulation), the asymmetric PV anomaly produces changes in the symmetric vortex that have significant differences from those in dry experiments with the present model or previous barotropic studies. A diagnosis of the contributions to changes in the symmetric wind tendency due to the asymmetry confirm the dominance of horizontal eddy fluxes at early times. The barotropic eddy kick pr...

Tropical Cyclone Evolution via Potential Vorticity Anomalies in a Three-Dimensional Balance Model

Abstract A new mechanism of vortex intensification by convectively forced vortex Rossby waves was proposed by Montgomery and Kallenbach. As demonstrated by them, the axisymmetrization process is described by vortex Rossby waves that eventually propagate outward before their symmetrization. Montgomery and Kallenbach were able to relate these waves to intensity changes in barotropic hurricane-like vortices. In the present work these ideas are applied to better understand structure change and intensification of hurricanes in a baroclinic setting. The work of Moller and Montgomery, who examined the wave kinematics and wave–mean flow interaction of vortex Rossby waves in a barotropic model, is extended here to three dimensions. The model is based on the asymmetric balance theory of Shapiro and Montgomery. A nonlinear prognostic model is used to examine the effect of convectively generated potential vorticity (PV) disturbances on the evolution of a hurricane-like vortex on an f plane. This investigation general...

Interaction of binary tropical cyclones in a coupled tropical cyclone‐ocean model

The motion and evolution of binary tropical cyclones was investigated using a coupled tropical cyclone‐ocean movable nested grid model. The model consists of eight‐layer atmospheric and seven‐layer ocean primitive equation models. Several regimes of binary storm interaction have been identified, depending on the initial separation distance (d) and differences in storm strengths. At d less than a few hundred kilometers, interacting storms experienced complete merger (CM) or partial merger (PM). At larger d (between about 600 km and 1000 km), three regimes of storm interaction have been found: PM, straining out (SO), characterized by complete disintegration of the weaker storm, and mutual straining out (MSO), characterized by weakening and dissipation of both storms. SO occurred when the interacting storms had substantially different intensities and strengths. MSO was observed when the interacting storms were comparable in size and intensity. In the latter case the storms were unable to approach each other at distances smaller than a certain minimum distance (of about 450–500 km) without being mutually stretched out. Moreover, initial attraction of the storms in this regime was replaced by repulsion, as frequently observed in the western Pacific. At d exceeding about 1000 km, elastic interaction (EI) was found, when the storms interact without any significant changes in their intensity and structure. In additional experiments with a conditional instability of the second kind (CISK) type parameterization of convective heating the storm interaction was very different: The storms were nearly axisymmetric and very compact, and they continued approaching each other until they merged. Thus more realistic simulations of binary storm interaction can be achieved by using a physically more reasonable convective parameterization.

Origins and Mechanisms of Eastern Pacific Tropical Cyclogenesis: A Case Study

The genesis of Hurricane Hernan (1996) in the eastern Pacific was investigated using gridded analyses from the European Centre for Medium-Range Weather Forecasts and gridded outgoing longwave radiation. Hernan developed in association with a wave in the easterlies that could be tracked back to Africa in longitude‐time plots of the filtered y component of the wind (2‐6-day period) at 700 mb. The wave crossed Central America near Lake Nicaragua with little change in its southwest‐northeast tilt, but the most intense convection shifted from near the wave axis in the Caribbean to west of the wave axis in the Pacific. The wave intensified as it moved through a barotropically unstable background state (defined by a low-pass filter with a 20-day cutoff ) in the western Caribbean and eastern Pacific. A surge in the southwesterly monsoons and enhanced convection along 108N occurred to the west of the 700-mb wave in the Pacific and traveled with the wave. This had the effect of enhancing low-level vorticity over a wide region ahead of the 700-mb wave. Available evidence suggests that additional low-level vorticity was produced by enhanced flow from the north through the Isthmus of Tehuantepec as the 700-mb wave approached. Depression formation did not occur until 6‐12 h after the 700mb wave reached this region of large low-level vorticity in the Gulf of Tehuantepec. Eastern Pacific SST and vertical wind shear magnitude are typically favorable for tropical cyclone development in Northern Hemisphere summer and early fall. Because the favorable mountain interaction and the surge in the low-level monsoons appear to relate directly to the wave in the easterlies, it is argued that the strength of such waves reaching Central America from the east is the single most important factor in whether subsequent eastern Pacific cyclogenesis occurs. Possible parallels with western Pacific cyclogenesis are discussed.

Simulation of Tropical Cyclone Circulation over Bay of Bengal Using the Arakawa-Schubert Cumulus Parametrization. Part I -- Description of the Model, Initial Data and Results of the Control Experiment

—A ten-level axi-symmetric primitive equation model with cylindrical coordinates is used to simulate the tropical cyclone evolution from a weak vortex for the Bay of Bengal region. The physics of the model comprises the parameterization schemes of Arakawa-Schubert cumulus convection (Lord et al., 1982) and Deardorff’s (1972) planetary boundary layer. The initial conditions have been taken from the climate mean data for November of Port Blair (92.4 E, 11.4 N) in the Bay of Bengal, published by the India Meteorological Department. An initial vortex has been designed to have tangential wind maximum of 10 m/s at 120-km radius with a central surface pressure of 1008 hPa. As a control experiment, referred to as ASBB1, the model is integrated for 240 h maintaining the sea-surface temperature (SST) constant at 301 K. The results of the control experiment reveal a slow decrease of the Central Surface Pressure (CSP) from the initial value of 1008 hPa to 970 hPa at 156 h. After 156 h the CSP decreased sharply until 186 h, attaining 890 hPa. The tangential wind at 1 km level attained the Cyclone Threshold Intensity (CTI) of 17 m/s around 78 h and a maximum of 87 m/s was found at 210 h. These features indicate a predeveloping stage up to 156 h, a deepening stage of 30 h from 156–186 h followed by the mature stage. The mature stage is characterized by the simulation of the central eye region, warm core, strong cyclonic circulation in the central 300 km with low-level inflow; strong vertical motion at the eye wall and outflow aloft. The convection features of the different cloud types conform with the circulation features. The control experiment clearly indicates the evolution of a cyclone with hurricane intensity from a weak vortex. In part two of the paper, results from sensitivity experiments with respect to variations in latitude, SST and initial thermodynamic state have been presented.

Synoptic environment of composite tropical cyclones in the South-West Indian Ocean

The composite structure of the tropical cyclone (TC) environment in the South-West Indian Ocean is analysed using European Community medium-range weather data. Conditions suitable for the development of tropical cyclones are present between 5 and 15°S and between 40 and 100°E. Groups of westward-moving and poleward-recurving TCs are used to construct system-following composites from three days before maximum intensity to one day after. The synoptic fields surrounding the westward-moving composite TC remain steady, whereas the recurving composite environment supports rapid vortex growth. Interaction with a subtropical trough is prominent in the recurving TC. The results offer useful insights to weather interactions in the South-West Indian Ocean.

Atlantic Sea Surface Temperatures and Tropical Cyclone Formation

It has long been accepted that interannual fluctuations in sea surface temperature (SST) in the Atlantic are associated with fluctuations in seasonal Atlantic basin tropical cyclone frequency. To isolate the physical mechanism responsible for this relationship, a singular value decomposition (SVD) is used to establish the dominant covarying modes of tropospheric wind shear and SST as well as horizontal SST gradients. The dominant SVD mode of covarying vertical shear and SST gradients, which comprises equatorially confined near-zonal vertical wind shear fluctuations across the Atlantic basin, is highly correlated with both equatorial eastern Pacific SST anomalies (associated with El Niño) and West African Sahel rainfall. While this mode is strongly related to tropical storm, hurricanes, and major hurricane frequency in the Atlantic, it is not associated with any appreciable Atlantic SST signal. By contrast, the second SVD mode of covarying vertical shear and horizontal SST gradient variability, which is effectively uncorrelated with the dominant mode, is associated with SST fluctuations concentrated in the main tropical cyclone development region between 10° and 20°N. This mode is significantly correlated with tropical storm and hurricane frequency but not with major hurricane frequency. Statistical tests confirm the robustness of the mode, and lag correlations and physical reasoning demonstrate that the SST anomalies are not due to the developing tropical cyclones themselves. Anomalies of SST and vertical shear during years where the mode has substantial amplitude confirm the resemblance of the individual fields to the modal structure, as well as the association of hurricane development with the warmer SSTs. Although SSTs are of secondary importance to vertical shear in modulating hurricane formation, explaining only ∼10% of the interannual variability in hurricane frequency over the ∼50% explained by vertical shear, the results support the conclusion that warmer SSTs directly enhance development. The lack of correlation with major hurricanes implies that the underlying SSTs are not a significant factor in the development of these stronger systems.

Eastern Hemisphere Tropical Cyclones of 1995

Abstract This paper is designed to be an annual summary of the Eastern Hemisphere tropical cyclones of 1995. The tropical cyclone statistics presented are those of the Joint Typhoon Warning Center, Guam. The text focuses primarily upon the tropical cyclones that occurred in the western North Pacific during 1995; however, since the area of responsibility of the Joint Typhoon Warning Center covers the entire Eastern Hemisphere, brief summaries of the tropical cyclone activity within the north Indian Ocean, south Indian Ocean, and the southwest Pacific are also presented. Overall, 1995 was a relatively quiet year in the Eastern Hemisphere: the 22 tropical cyclones of the Southern Hemisphere were only one shy of the record low of 21, and for the first time since 1988 the number of tropical storms and typhoons in the western North Pacific was below normal. In the western North Pacific, there was a marked shift to the west of the preferred region for the genesis and development of tropical cyclones. This is con...

Statistical evidence links exceptional 1995 Atlantic Hurricane season to record sea warming

Tropical cyclones rank above earthquakes as the major geophysical cause of loss of life and property (Bryant, 1991; Houghton, 1994). In the United States alone, the damage bill from mainland landfalling hurricanes over the last 50 years averages $2.0 billion per year (Hebert et al., 1996). Years with high numbers of hurricanes provide new insight on the environmental factors influencing interannual variability; hence the interest in the exceptional 1995 Atlantic season which saw 11 hurricanes and a total of 19 tropical storms, double the 50‐year average. While most environmental factors in 1995 were favourable for tropical cyclone development, we show that a factor not fully explored before, the sea surface temperature (SST) was the most significant. For the 10°–20°N, 20°–60°W region where 93% of the anomalous 1995 hurricanes developed, ∼45 year statistical regressions show that SST is the dominating influence, independent of all known other factors, behind the interannual variance in Atlantic hurricance numbers. With this SST experiencing record warm levels in 1995, 0.66°C above the 1946–1995 mean, these regressions indicate that sea warming explains 61±34% of the anomalous hurricane activity in 1995 to 95% confidence.

Tropical cyclone simulation with Emanuel’s convection scheme

ABSTRACT. A new convection parameterization scheme proposed by Emanuel (1991) is used to simulate the evolution of tropical cyclone. The numerical model used for this study is a 19 level axi-symmetric primitive equation, hydrostatic model in a z co-ordinate system. The vertical domain ranges from 0 to 18 km and the horizontal domain ranges upto 3114 km with a resolution of 20 km. in the central 400 km radius and with increasing radial distance thereafter.
 The evolution of an initially balanced vortex with an initial strength of 9 m/sec is studied. It is shown that Emanuel's convection scheme is successful in simulating the development of the initial vortex into a mature, intense cyclonic storm. At the mature stage, a minimum surface pressure of 930 hPa is attained with the associated low level maximum tangential wind speed of 70 m/sec. The simulated circulation features at the mature stage show the formation of an intense cyclone.
 
 Two different sensitivity experiments were performed. A set of experiments with the variation of sea surface temperature (SST) from 300.5° to 302° K in steps of 0.5° K have shown that the intensity of model cyclone increases with the increase of SST. Another set of experiments with variation of latitude has shown that the cyclonic storm is more intense at lower latitudes.
 

Motion and Evolution of Binary Tropical Cyclones in a Coupled Atmosphere–Ocean Numerical Model

Abstract The interaction of binary tropical cyclones (TC) is investigated using a coupled TC-ocean movable nested-grid model. The model consists of an eight-layer atmospheric model in the sigma coordinate system and a three-layer primitive equation ocean model. There are five meshes in the TC model. The outermost domain (3840 km × 3840 km) is motionless. For the description of each TC in a TC pair, two telescopically nested meshes of finer resolution are used. The pair of the middle (1600 km × 1600 km) and innermost (800 km × 800 km) meshes move with the center of a corresponding TC. The space increments of the outermost domain and the middle and finest meshes are 160, 80, and 40 km. The oceanic domain contains 107 × 107 grid points, with the spatial increment of 40 km. In all numerical experiments a pair of equal strength axisymmetric vortices was located at different separation distances. Experiments show that the rate of development of interacting TCs is different, mainly due to the difference in the v...

The Flow during TOGA COARE as Diagnosed by the BMRC Tropical Analysis and Prediction System

The evolution of the large-scale flow through the four-month intensive observing period of TOGA COARE is documented from large-scale numerical analyses and GMS cloud imagery produced by the Australian Bureau of Meteorology and transmitted to the field stations during the experiment. The evolution of the flow is dominated by the following phenomena: 1) the normal seasonal evolution of the tropical flow over this region, including a southward and eastward progression of the tropical convective heat source as the Southern Hemisphere monsoon developed and matured; 2) a more eastward than normal progression of this monsoon circulation, associated with a warm event of the ENSO phenomenon; 3) the existence of a major westerly–easterly–westerly cycle of the Madden–Julian low-frequency wave occurring during the latter half of the experimental period, and 4) the development and subsequent movement of tropical cyclones in both (northern and southern) hemispheres. The Madden–Julian event consisted of two eastward progressions across the domain of satellite-observed cloud, south of the equator. The horizontal scale of the cloud regions is approximately 10° latitude × 40° longitude and the eastward phase speed is approximately 3.7 m s−1. Linear correlation studies substantiate the eastward movement of both cloud and zonal wind across the domain. The correlation analysis reveals a strong relationship between cloud and low-level zonal wind, with the cloud variations leading those in wind by approximately five days. Time-longitude sections of relative vorticity show that the synoptic activity also progressed eastward with the cloud, and its structure is suggestive that the controlling dynamics (for the synoptic activity) may be the energy dispersion mechanism of Davidson and Hendon. The development of each westerly event was accompanied by a major change in the Southern Hemisphere deep-layer mean flow from easterly to westerly. Examination of flow fields and satellite imagery for individual days shows that the peak of the first westerly event is associated with the flow patterns surrounding two Southern Hemisphere tropical cyclones. The subsequent rapid evolution to an easterly state occurs as the cyclones move eastward and southward, and the monsoon flow collapses in their wake. There is an accompanying ridging at low levels in the subtropics and the establishment of the Southern Hemisphere subtropical jet. The subsequent reestablishment of the monsoon (the second westerly event) occurs from west to east with the eastward moving cloud bands. There is also a suggestion that an equatorward extension of a Southern Hemisphere upper-level trough may have played a role. Major active and break periods are identified over four tropical subdomains over the TOGA COARE region. These are most easily defined in the Southern Hemisphere subdomains. They are characterized by a slowly ,varying signal in the satellite-observed average cloud-top temperature. Superimposed on this is a rapid transition between the active and break states.

TOGA COARE IOP期間中に捉えられた赤道太平洋域の4-20日周期擾乱の特徴

Several types of westward-propagating equatorial disturbances within a 4-to-20-day-period range in the equatorial Pacific are detected from an objective analysis data during November and December 1992. Their horizontal structures are determined by composite analysis. A signal of 4-to-5-day period is detected in the 850 hPa meridional wind field and a signal of a 7-day period is detected in the 200 hPa meridional wind field. The associated structure of both signals has a characteristic of the mixed Rossby-gravity wave. Further, a 15-to-20-day signal is detected in equatorial zonal wind variation, both at 200 hPa and 850 hPa. The structure and westward phase speed can be interpreted as the equatorial Rossby wave of n = 1. The signals at the 200 hPa and 850 hPa levels are coherent and baroclinic near the equator but nearly barotropic in the subtropics. These disturbances have a relation to the cloud activity and have a equivalent depth of the order of 10 m. However, the equivalent depth of the 15-day signal in the 200 hPa level is estimated to be an order of magnitude larger. It is suggested that the development of tropical cyclones in the tropical western Pacific is affected by both the 4-5-day mixed Rossby-gravity type waves and 15-day Rossby type waves. A possible excitation mechanism of the 15-day Rossby type waves through an intensification of 200 hPa trough by midlatitude forcing is also suggested.

夏季モンスーン循環と45日周期擾乱とのインタラクション

The influence of the low-level mean summer monsoon flow on 45-day transient eddies through barotropic interaction is diagnosed by a scalar product between the horizontal shear vector and Eliassen-Palm (E-P) flux, while the dry (moist) baroclinic interaction is measured by a vector product between the vertical wind shear and the eddy sensible (latent) heat transport, which represents the vertical component of E-P fluxes. Activity of 45-day transient eddies was monitored by the evolution of kinetic energy during the 9-year period of 1985-1993. The 45-day wave activity is relatively weak over the SEAM (Southeast Asian monsoon) domain. Absence of geographically fixed forcing makes the WNPM (western North Pacific monsoon) more violent, with the peak 45-day wave activity occurring in late August, the time of frequent development of vigorous tropical cyclones. Near the WNPM updraft center (15°N, 140°-150°E), the E-P wave energy fluxes are oriented in a direction which causes weakening of the sheared mean zonal and meridional winds, implying barotropic amplification of 45-day waves. The weak mean temperature gradient relegates the dry baroclinic instability to that of secondary importance over the WNPM domain. This is contrasted with the significant moist baroclinic process due to an approximate perpendicular relationship between the eddy moisture transport and the strong vertical shear of the monsoon flow. Curiously, 45-day waves are quite active near Japan even during August's dry, hot climate. Neither barotropic nor dry (moist) baroclinic instability can account for late-summer disturbance activity. Evidence is presented that 45-day convective oscillations occuring over the WNPM domain induce a strong 45-day response near Japan and further eastward, possibly indicating an intraseasonal Rossby wave dispersion along the westerly jet stream. Near the Aleutian Islands, a pronounced poleward eddy sensible heat flux is directed down the mean temperature gradient, resulting in dry baroclinic amplification of mid-latitude 45-day waves via action of the Coriolis torque. An import of moisture from the WNPM source region brings about a moist baroclinic instability of extratropical 45-day waves poleward of the Pacific High.

On the interaction of tropical‐cyclone‐scale vortices. IV: Baroclinic vortices

AbstractThe binary interaction of tropical cyclones is investigated using a three‐dimensional primitive‐equation model. The extended anticyclonic circulations in the upper troposphere merge at very large vortex‐separation distances. For the cyclonic component in the lower troposphere, we find three fundamental modes of interaction separated by two critical separation distances: a mutual approach separation (MAS) and a mutual merger separation (MMS). We suggest that failure to identify these modes may have caused some confusion in interpreting previous baroclinic interaction studies.The MAS delineates vortices that approach each other from those that move on divergent orbits. The approach phase consists of steady radial movement and gradual acceleration, with deformation of the outer vorticity structure of each vortex but little change to their cores. In contrast to barotropic studies, the MAS is much larger than the radius at which the potential‐vorticity gradient of each vortex changes sign. Vortex tilting associated with the vertical shear of the azimuthal winds from the opposing vortex and secondary circulations associated with diabatic heating increases the mutual vortex attraction. The presence of an earth‐vorticity gradient reduces this attraction slightly, but also introduces considerable sensitivity to vortex orientation. When all physical processes are included, we find an MAS of around 1000 km with a scatter of several hundred km, which agrees well with observational studies.Approach occurs with little change in the vortex cores until they reach the MMS. Merger then occurs very rapidly, usually within several hours, and follows that described in parts II and III for barotropic vortices. The MMS is approximately three times the equivalent vortex‐patch radius for the cyclones; it is slightly reduced by diabatic heating, but it is largely independent of the earth‐vorticity gradient. The vortices experience a slight weakening during the approach and initial merger stages. However, with diabatic heating, rapid intensification follows merger; such intensification may have implications for rapid development of tropical cyclones.

The seclusion intensification of the New Year’s day storm 1992

The Bergen School meteorologists realized that not all cyclones follow their conceptual cyclone model. In particular they found cases with re-generation of occluded cyclones. In their literature, for instance as found in Godske and co-workers, 2 kinds of re-generation were considered challenging to weather forecasting: a thermodynamic intensification with similarities to the development of tropical cyclones, and intensification of what was called the non-frontal trough, or the back-bent occlusion, of strong cyclones. It has been believed that the latter kind is characteristic of the strongest surface winds observed in the Northwestern Atlantic. In this paper such a case, resulting in the strongest cyclone landfall on the Norwegian west coast this century, has been investigated from a simulation of a small synoptic-scale (∼ 1000 km), fast-moving extratropical cyclone (∼ 25 m/s−1). It is found that the cyclone evolves as the conceptual frontal model of Shapiro and Keyser (1990), and that the strong winds are developed by a secondary, mesoscale (∼ 500 km) cyclogenesis closely linked to the seclusion process. The time scale of the intensification is 12 h, starting with what is called the seclusion trough at the tip of the back-bent warm front. As the cold air secludes the warm core, the disturbance develops into a separate low, here called the seclusion low. Release of latent heat connected to the back-bent warm front is found to play an important role in forming the seclusion. A part of the generated potential vorticity (PV) remains within the warm air in the seclusion process. Inversion of the low-level PV anomalies results in a low-level jet along the outer side of the seclusion trough. The strong winds are observed when the seclusion trough develops into the seclusion low and the low-level jet becomes parallel to the large scale westerly flow. A positive PV anomaly streamer, formed from a larger scale upper PV anomaly in phase with the surface low, takes part in this stage of the development. DOI: 10.1034/j.1600-0870.1995.00116.x

Tropical Storm Development and Decay: Sensitivity to Surface Boundary Conditions

Hurricane models have rarely been used to investigate the observational fact that tropical disturbances seldom form, develop, or intensify over land. Furthermore, rather ad hoc assumptions have been made when modeling landfall. The general consensus is that energy supplied primarily through surface fluxes is necessary for tropical cyclone development and maintenance. In the past, rather a priori assumptions have been made such as the elimination of surface sensible and latent heat fluxes over land or the reduction of surface land temperature. By incorporating an improved version of the Geophysical Fluid Dynamics Laboratory (GFDL) tropical cyclone model with diurnal radiation and a bulk subsurface layer with explicit prediction of land temperature, a series of experiments was performed to test the sensitivity of surface boundary conditions to tropical cyclone development and decay at landfall. A triply nested version of the GFDL model was used in an idealized setting in which a tropical disturbance, taken from the incipient stage of Gloria (1985), was superposed on a uniform easterly flow of 5 m s−1. A control case was performed for ocean conditions of fixed 302-K SST in which the initial disturbance of about 998 hPa developed to a quasi-steady state of 955 hPa after one day of integration. Using identical atmospheric conditions, a series of experiments was performed in which the underlying land surface was specified with different values of thermal property, roughness, and wetness. By systematically changing the thermal property (i.e., heat capacity and conductivity) of the subsurface from values typical of a mixed-layer ocean to those of land, a progressively weaker tropical system was observed. It was found that the initial disturbance over land failed to intensify below 985 hPa, even when evaporation was specified at the potential rate. The reduction of evaporation over land, caused primarily by the reduction of surface land temperature near the storm core, was responsible for the inability of the tropical disturbance to develop to any large extent. Under land conditions, the known positive feedback between storm surface winds and surface evaporation was severely disrupted. In sensitivity experiments analogous to the all-land cases, a series of landfall simulations were performed in which land conditions were specified for a region of the domain so that a strong mature tropical cyclone similar to the ocean control case encountered land. Again as in the all-land case, the demise of the landfalling storm takes place due to the suppression of the potential evaporation and the associated reduction of surface temperatures beneath the landfalling cyclone. Even when evaporation was prescribed at the potential rate, a realistic rapid filling (36 hPa in 12 h) ensued despite the idealized nature of the simulations. Although not critical for decay, it was found that surface roughness and reduced relative wetness do enhance decay at landfall.

Tropical cyclone frequencies inferred from Gray's yearly genesis parameter: Validation of GCM tropical climates

The Gray Yearly Genesis Parameter (YGP) is an empirical diagnostic tool used to infer regions in which necessary (but not sufficient) conditions exist for tropical cyclone development. This parameter is used here as a measure of the implied tropical cyclone frequency and area of occurrence in climate simulations generated by a General Circulation Model (GCM). In a simulation of the current climate, the CSIRO9 GCM shows reasonable agreement with the original Gray climatology. The YGP for a doubled CO2 simulation is presented.

Laboratory modeling of the early stage of a tropical cyclone.

Laboratory experiments have been carried out on natural convection in a rotating layer of liquid heated from below. A distinguishing feature of these experiments is an excellent thermal insulation of the layer boundaries. As a result, large-scale convection structures form. Laboratory analogs of the early stages of development of tropical cyclones have been obtained.