Equatorial Trough Zone Research Articles

A Multi-satellite Perspective on “Hot Tower” Characteristics in the Equatorial Trough Zone

A Multi-satellite Perspective on “Hot Tower” Characteristics in the Equatorial Trough Zone

From Hot Towers to TRMM: Joanne Simpson and Advances in Tropical Convection Research

Joanne Simpson began contributing to advances in tropical convection about half a century ago. The hot tower hypothesis jointly put forth by Joanne Simpson and Herbert Riehl postulated that deep convective clouds populating the “equatorial trough zone” were responsible for transporting heat from the boundary layer to the upper troposphere. This hypothesis was the beginning of a 50-year quest to describe and understand near-equatorial deep convection. Tropical field experiments in the 1970s [Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) and the Monsoon Experiment (MONEX)] in which Joanne participated documented the mesoscale structure of the convective systems, in particular the deep, stratiform, dynamically active mesoscale clouds that are connected with the hot towers. In the 1980s these new data led to better understanding of how tropical mesoscale convective systems vertically transport heat and momentum. The role of the mesoscale stratiform circulation in this transport was quantified. Tropical field work in the 1990s [especially the Coupled Ocean-Atmosphere Response Experiment (COARE), in which Joanne again participated] showed the importance of a still larger scale of convective organization, the “supercluster.” This larger scale of organization has a middle-level inflow circulation that appears to be an important transporter of momentum. The mesoscale and supercluster scale of organization in tropical convective systems are associated with the stratiform components of the cloud systems. Joint analysis of satellite and radar data from COARE show a complex, possibly chaotic relationship between cloud-top temperature and the size of a stratiform precipitation area. The Tropical Rainfall Measuring Mission (TRMM) satellite, for which Joanne served as project scientist for nearly a decade, is now providing a global census of mesoscale and supercluster-scale organization of tropical convection. The TRMM dataset should therefore provide some closure to the question of the nature of deep convection in the equatorial trough zone.

熱帯外部からの強制と西太平洋における赤道西風の急激な強化(バースト):総観的研究

Based on surface winds at 6-h intervals for four Northern Winters and Springs (1970-73), two cases of strong westerly wind bursts were identified in the core of the equatorial western Pacific (-155°E). One case occurred in early April and another in early May 1972, both prior to the maximum sea surface temperature anomalies along the Peruvian coast during the 1972 ENSO event. During the Northern Spring, as an anomalously strong anticyclone moves rapidly from north-central China to its east coast, the surface wind fields to the southeast of the Philippines respond swiftly, turning from an easterly background to northerly. In the meantime, surface pressure in the far western equatorial Pacific tends to rise. These rapid equatorial resposes are probably due to gravity wave-like motions induced by the pressure-wind imbalance in the midlatitudes. The local pressure increase in the extreme western Pacific enhances the west-to-east pressure gradient in the equatorial trough zone and results in a strong westerly wind acceleration in the core of the equatorial western Pacific. This acceleration is also preceded by a west-to-east displacement of the pressure surge in the equatorial trough zone. The enhanced zonal pressure-gradient force and the associated eastward displacement of the equatorial pressure surge are two critical factors for initiating westerly wind bursts. Westerly wind surges detected primarily from fixed-station data compare favorably with those calculated from ship records of an independent source.

Frontal Influences on Northeast Brazil

Abstract Surface observations for the 10-year period 1961–70 are analyzed for occurrences of fronts in North-east Brazil. Cold fronts, or their remains, are found to enter the Northeast throughout the year. The southern part of the region (interior Bahia) receives much of its precipitation in the period November-February associated with frontal systems. Coastal areas. which receive maximum rainfall during the months of May–July, experience a marked increase in rainfall associated with the approach and passage of cold fronts. Even precipitation events in the extreme northern part of the region (Ceara) sometimes appear to be associated with systems which are cold-frontal in origin. Pressure fluctuations are felt nearly simultaneously throughout the region with greatest amplitudes occurring in the south. These fluctuations are also shown to occur over a fairly broad range of longitude within tropical Brazil and may have a significant effect on the position of the equatorial trough zone over the Atlantic.

Occurrence and structure of the equatorial trough zone in Venezuela

AbstractAircraft traverses were made from north to south across Venezuela during the rainy season of 1972 to determine whether a convectively active equatorial low pressure trough occurred and, if so, its structure, average properties and mechanics. The trough could indeed be located regularly near latitudes 5−7°N flying along the Orinoco River. Near cloud base (920mb) there was a wind shift from east to west and from north to south components. Farther south, winds from southeast were encountered. The low troposphere was warm with a maximum equivalent potential temperature at the axis. At 700mb neither the warm layer nor any wind shift could be found. The structure closely resembled that of the equatorial trough observed in the Garp Atlantic Tropical Experiment of 1974.The traverses largely indicated negative correlation between thermodynamic energy, mainly controlled by water vapour, and buoyancy, given by virtual temperature, on mesoscale distances of 30–50km. Occasional ‘hot tower’ penetrations were encountered. Surface air reached cloud base undiluted. Absolute vorticity was highest at cloud base just north of and at the trough, decreasing sharply to near zero to its south, just where heaviest convection was concentrated. Here also the largest vertical shear of the zonal wind component was located, again in agreement with GATE. The pressure‐height field was calculated from the wind data and indeed yielded a minimum value at the trough axis. To the north, the zonal wind increased with height though temperature decreased northwards. Calculations on the balance of forces with frictional retardation and on the origin of the low‐level warm layer give indications of the mechanics of the structure; the latter appears to be suggestive of a widely applicable equatorial trough model.

Occurrence and structure of the equatorial trough zone in Venezuela

Aircraft traverses were made from north to south across Venezuela during the rainy season of 1972 to determine whether a convectively active equatorial low pressure trough occurred and, if so, its structure, average properties and mechanics. The trough could indeed be located regularly near latitudes 5−7°N flying along the Orinoco River. Near cloud base (920mb) there was a wind shift from east to west and from north to south components. Farther south, winds from southeast were encountered. The low troposphere was warm with a maximum equivalent potential temperature at the axis. At 700mb neither the warm layer nor any wind shift could be found. The structure closely resembled that of the equatorial trough observed in the Garp Atlantic Tropical Experiment of 1974. The traverses largely indicated negative correlation between thermodynamic energy, mainly controlled by water vapour, and buoyancy, given by virtual temperature, on mesoscale distances of 30–50km. Occasional ‘hot tower’ penetrations were encountered. Surface air reached cloud base undiluted. Absolute vorticity was highest at cloud base just north of and at the trough, decreasing sharply to near zero to its south, just where heaviest convection was concentrated. Here also the largest vertical shear of the zonal wind component was located, again in agreement with GATE. The pressure-height field was calculated from the wind data and indeed yielded a minimum value at the trough axis. To the north, the zonal wind increased with height though temperature decreased northwards. Calculations on the balance of forces with frictional retardation and on the origin of the low-level warm layer give indications of the mechanics of the structure; the latter appears to be suggestive of a widely applicable equatorial trough model.

Vertical Distribution of Energy Transfer and Radar Echo Tops in the Equatorial Trough Zone

Vertical Distribution of Energy Transfer and Radar Echo Tops in the Equatorial Trough Zone

The Role of Organized Unsaturated Convective Downdrafts in the Structure and Rapid Decay of an Equatorial Disturbance

Abstract The Line Islands Experiment, conducted on and near Palmyra, Fanning and Christmas Islands during February-April 1967, produced extensive data on disturbances of the equatorial trough zone. One disturbance which passed through the heart of the data network is analyzed in detail. This disturbance intensified rapidly just east of Fanning Island during the night of 31 March–1 April, but satellite observations show that it dissipated rapidly during the daylight hours of 1 April. The convergence-divergence patterns associated with the growth and decay of the disturbance are most intense in the lowest 500 m. Data from serial rawinsonde releases on the islands, combined with research aircraft data, are presented which demonstrate that highly unsaturated downdrafts are produced, first on the convective scale and the mesoscale, and finally becoming organized over the entire 600-km extent of the system. Cumulus development is effectively suppressed in the downdraft air, only being restored after 6–12 hr by ...

Weather and Solar Radiation Reception in the Equatorial Trough

Canton Island (2°49′S, 171°41′W) was selected to study equatorial trough weather conditions and their effects on incoming solar radiation. The chosen study period, July 1957–June 1958, was quite appropriate since the mean position of the trough for this year was about 4° south of its mean annual position and it frequently affected Canton. Cloud distribution, moist layer thickness, precipitable water content and weather conditions in the equatorial trough zone (out to 10° on either side of the trough axis) are discussed. Incoming solar radiation is considered in relation to opaque sky cover since the total sky cover often included an excessively large thin cirriform contribution which did not significantly reduce incoming solar radiation. In the vicinity (within 3°) of the trough axis only one out of five days was affected by disturbance type weather (extensive cloud cover and precipitation). In these disturbance cases, for an overall average, 34.6% of the solar radiation on a horizontal surface at the top of the atmosphere penetrated through to the earth's surface. For fair weather cases 61.3% penetrated through to the earth's surface.

On the Angular Momentum Balance in the Equatorial Trough Zone of the Eastern Hemisphere

A study has been made of the conditions for seasonal balance of zonal angular momentum, during Southern summer, in the troposphere overlying the equatorial (monsoonal) westerlies. The observational data are not adequate to determine the finer details of balance. It is shown that by and large it can be accounted for by zonal pressure gradients and meridional transport of Earth's angular or ?-momentum (a Coriolis effect). Zonal and meridional stresses due to advective and eddy transports of relative zonal or ?-momentum as well as the surface frictional stresses are of considerably smaller magnitude. This confirms for a monsoonal region the conclusions of Palmer (1958), from observations in the North Pacific in June, that quasi-geostrophic equilibrium occurs down to very low latitudes above the planetary friction layer. The high-level easterly jet stream is shown to play an important part in two aspects of balance. DOI: 10.1111/j.2153-3490.1961.tb00066.x

Circulation and Energy Balance in a Tropical Monsoon

Mean monthly meridional and zonal circulations in the region of the Indonesia-Australia summer monsoon and associated transfers of heat energy are investigated using upper air soundings, rainfall and other data from two seasons. Daily sounding data are used to determine also the eddy flux of latent heat. A large increase of meridional circulation strength and energy transfer from November to December on the northern side of the monsoonal trough suggests that establishment of the regime is associated with a comparatively abrupt southward shift and intensification of the Northern Hadley cell in the relevant longitude sector.
 When the monsoon has become established latent heat relase in the region exceeds net export of sensible heat from it by a comparatively small amount. These quantities dominate the remaining ones in the heat balance, viz. net radiative cooling in the troposphere and sensible heat transfer from the ocean. Items in the heat budget are compared with those obtaining during the pre-monsoonal regime in the area and on the winter side of the equatorial trough zone at large, the latter according to RIEHL and MALKUS (1959).
 Gain of sensible heat is also assessed separately for the lower and upper troposphere in terms of gain or loss of internal energy, the work done by horizontal pressure gradient forces and by vertical pressure gradient forces (gain of potential energy). The vertical upward flux of energy across mid-tropospheric level is deduced and the importance of buoyancy in bringing about this flux is indicated.

On the Angular Momentum Balance in the Equatorial Trough Zone of the Eastern Hemisphere

A study has been made of the conditions for seasonal balance of zonal angular momentum, during Southern summer, in the troposphere overlying the equatorial (monsoonal) Westerlies. The observational data are not adequate to determine the finer details of balance. It is shown that by and large it can be accounted for by zonal pressure gradients and meridional transport of Earth’s angular or Ω-momentum (a Coriolis effect). Zonal and meridional stresses due to advective and eddy transports of relative zonal or ū-momentum as well as the surface frictional stresses are of considerably smaller magnitude. This confirms for a monsoonal region the conclusions of Palmer (1958), from observations in the North Pacific in June, that quasi-geostrophic equilibrium occurs down to very low latitudes above the planetary friction layer.
 The high-level easterly jet stream is shown to play an important part in two aspects of balance.