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This book is designed as an introductory course in Tropical Meteorology for the graduate or advanced level undergraduate student. The material within can be covered in a one-semester course program. The text starts from the global scale-view of the Tropics, addressing the zonally symmetric and asymmetric features of the tropical circulation. It then goes on to progressively smaller spatial and time scales – from the El Niño Southern Oscillation and the Asian Monsoon, down to tropical waves, hurricanes, sea breezes, and tropical squall lines. The emphasis in most chapters is on the observational aspects of the phenomenon in question, the theories regarding its nature and maintenance, and the approaches to its numerical modeling. The concept of scale interactions is also presented as a way of gaining insight into the generation and redistribution of energy for the maintenance of oscillations of a variety of spatial and temporal scales.



Chapter 1. The Zonally Averaged Tropical Circulation

We shall begin this text with a zonal-time averaged representation of the tropical atmosphere. These are height (or pressure)-time diagrams that illustrate several simplified climatological features of the tropical atmosphere such as the zonal wind, temperature, moisture (specific humidity) and the mean meridional circulation, i.e., the Hadley cell.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 2. Zonally Asymmetric Features of the Tropics

This chapter covers the zonally asymmetric climatology of the tropics. Seasonal or monthly mean weather maps are particularly important in the tropics. The following example illustrates this point. Over the North American continent, the January mean sea level isobar map consists of a large continental anticyclone extending southwards from the Arctic and Canada towards the southern United States. This monthly mean pattern does not reveal any of the migrating polar front cyclones that cause much of the weather there during the winter season. The reason for this is that climatological charts in middle latitudes do not display the transient disturbances. A similar exercise carried out over the global tropical belt shows that daily as well as monthly mean charts both carry much the same information. The subtropical highs, the equatorial troughs, the monsoon troughs, the trades of the two hemispheres are common in both the daily and the monthly mean charts. Another way of expressing this is that climatological means carry much of the variance of the total motion field in the tropics. Thus an understanding of the maintenance of the time averaged zonally asymmetric features of the tropics is important.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 3. The Intertropical Convergence Zone

The Intertropical Convergence Zone, or ITCZ, is a belt of wind convergence and associated convection encircling the globe in the near-equatorial region. In a zonally averaged view, it is located at the equatorward edge – the rising branch – of the Hadley cell. The ITCZ is characterized by the low-level wind convergence, low sea level pressure, intense convection and associated cloudiness. The zonally averaged ITCZ is not stationary, but slowly migrates from south of the equator during the northern hemisphere winter to north of the equator during the northern hemisphere summer, following the Sun.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 4. Heat Induced Circulation

This chapter dwells on the importance of diabatic heating in the deep tropics that acts as a forcing on some of the large-scale circulation features in the tropical and sub-tropical latitudes. First, we begin with the analytical solutions of a simple linearized shallow water model equations following Gill (1980). This work although an idealized modeling study, is highly instructive in understanding the circulation features such as the east–west (Walker) circulation, meridional (Hadley) circulation and easterlies associated with the trade winds. This idealized atmospheric modeling study is often referred as the “Gill” atmosphere.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 5. Monsoons

The monsoons of the world are defined through the seasonal reversal of large-scale surface winds. The best place to see an illustration of this phenomenon is in the domain of the Indian Monsoon where the southwesterly surface winds of the summer monsoon get replaced by the northeasterly surface winds of the winter monsoon, as shown in Fig. 5.1.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 6. Tropical Waves and Tropical Depressions

Tropical waves can be very difficult to locate on synoptic maps, especially in the western Pacific and in the Southern Hemisphere. Such waves, referred to as tropical waves, easterly waves, African waves, etc., are perturbations of the tropical easterlies that propagate westward, at a phase speed of roughly 5–7° longitude/day, sometimes accompanied by clouds and precipitation and pressure changes. Tropical waves can be identified in satellite imagery. The amount of precipitation that a small island in the path of the wave receives is not always proportional to the strength of the wave, as evidenced by the amount of cloudiness seen in the satellite imagery. This is because the amount of cloudiness and weather associated with the wave can change quickly and because rainfall tends to be organized into mesoscale systems which may or may not move directly over the island weather station. Tropical waves moving off the African coast into the eastern Atlantic usually have little convection due to the relatively cold waters and the dry air flowing off the Sahara Desert to the north of the wave. These waves may be tracked across the eastern Atlantic in satellite imagery by following an “inverted-V” cloud pattern, as pointed out by Frank (1968). The inverted-V pattern indicates a perturbation in the wind field possessing a weak vorticity maximum.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 7. Madden Julian Oscillation

The Madden Julian Oscillation, often abbreviated as MJO, is a major feature of the tropical circulation. It manifests as quasi-periodic fluctuations in the sea level pressure and wind structure, and consequently – in the sea surface temperature, convection and rainfall. The time scale of the phenomenon is, on average, about 30–60 days. It was first discovered by Madden and Julian (1971) as they studied the zonal winds and the sea level pressure in a 10-year long record of tropical data. The MJO time scales carry a significant proportion of the atmospheric variance in the tropics.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 8. Scale Interactions

The tropical atmospheric circulation contains a number of spatial and temporal scales of motion. In terms of temporal scales – or frequency domain – these range from scales of a few seconds (for turbulent motions), through semi-diurnal and diurnal scales, synoptic (2–7 day) scales, semi-biweekly scales, Madden-Julian oscillation scales (30–60 days), annual cycle, ENSO, PDO, etc. In terms of spatial scales – or wave number domain – the scales range from a few mm for turbulent processes, few 100m for cloud-scale motions, few 100km for tropical disturbances, few 1,000km for the African easterly waves, and so on.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 9. El Niño and Southern Oscillation

The Southern Oscillation is a sea level pressure oscillation over the equatorial latitudes between the Eastern Pacific and the Indian Ocean. El Niño is a phenomenon that is characterized by the occurrence of warmer than normal sea surface temperatures over the Central and Eastern Equatorial Pacific Ocean. These two phenomena are intimately interwoven, so much so that they are usually considered together, under the name of ENSO, or El Niño-Southern Oscillation. The Southern Oscillation has a time scale of roughly 4–6 years. Within that period warm SST anomalies over the Central and Eastern Pacific Ocean (El Niño) are followed by cold SST anomalies (La Nina). This chapter is devoted to the observational, theoretical and modeling aspects of ENSO.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 10. Diabatic Potential Vorticity Over the Global Tropics

Most readers are familiar with the basic concepts of conservation of absolute vorticity and potential vorticity. The conservation of absolute vorticity is generally used in a two-dimensional context, where the effects of heat sources and sinks, divergence, vertical motion and friction are ignored and the vertical component of the vorticity, i.e.,

$$ \nabla \times \mathbf{V}+f $$

is conserved following a parcel. The conservation of potential vorticity, on the other hand, is properly applied to three-dimensional motions of parcels on isentropic surfaces, following the seminal work of Ertel. Here again the heat sources and sinks and the friction terms are ignored. The effects of divergence and vertical motion, however, are retained. In a very simplistic sense, we can use the following two approximate equations:

$$ \frac{{d{\zeta_a}}}{dt }=-{\zeta_a}\nabla \cdot \mathbf{V} $$

$$ \frac{{d{\varGamma_d}}}{dt }={\varGamma_d}\nabla \cdot \mathbf{V} $$

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 11. Tropical Cloud Ensembles

As one proceeds from the west coast of a continent, such as California in North America or Morocco in Northwest Africa, towards the near-equatorial oceanic ITCZ, one notes the following transition in the predominant species of tropical clouds: coastal stratus, stratocumulus, fair weather cumulus, towering cumulus and cumulonimbus. This is a typical scenario over the Pacific and Atlantic Oceans of the two hemispheres. The Asian Monsoon carries some of its own cloud features over the Indian Ocean. Figure 11.1 is a collage of cloud types and typical rainfall distributions over the tropics during the northern summer. This identifies precipitation features, such as those from the ITCZ, typhoon, monsoon, and near coastal phenomena. The precipitation illustrated here was estimated from microwave radiances received by the TRMM satellite. A plethora of clouds types abound in the tropics. Dynamics, physics, and microphysics are important interrelated scientific areas for these clouds’ life cycles. Modeling of the life cycle of individual clouds and cloud ensembles and representation of the effects of unresolved clouds in large scale environment are areas of importance for tropical meteorology. The ocean, land surface, and planetary boundary layer large scale wind systems and thermal and humidity stratification have a large control over the nature of evolving clouds.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 12. Tropical Boundary Layer

In this chapter we shall present a brief outline of the tropical boundary layer. This is the region of the atmosphere where the air/sea and air/land interactions take place and the surface fluxes of heat, momentum and moisture are generated and vertically distributed. Its structure is very important for studies of tropical convection and tropical disturbances, especially hurricanes, the ITCZ, waves and low-level jets, all of which require a detailed knowledge of the lowest 1km of the atmosphere.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 13. Radiative Forcing

Radiative transfer is very important for tropical/subtropical meteorology. The subtropical highs occupy a large part of the subtropics, the temperature does not change much here from 1 day to the next inspite of the fact that descending air is continually warmed up by the adiabatic descent. That warming is offset by radiative cooling. Accurate modeling of radiative cooling in the presence of lower tropospheric stratus and strato-cumulus is an important issue for the tropics. Dust and aerosols abound in Asia and Africa and well stretched into the Atlantic and their rates of warming/cooling by radiative effects are a central issue for the modeling of the monsoon and hurricanes. Immediately below the cloud base long wave radiative flux convergence often leads to a warming by the long wave radiative process. Those are also important modeling issues. The clear weather shallow startocumulus carry a field of long wave cloud top cooling that dictates the low temperatures of the base of the trade wind inversion that also needs understanding of cloud radiative transfer processes. Various regions of the troposphere of the tropics show radiative stabilization or destabilization that are important for the thermodynamical/dynamical interactions and require understanding of physical processes. The entire seat of diurnal change in the atmosphere relates to the zenith angle of the sun and the understanding of diurnal change requires a detailed interpretation of radiative transfer. The heat budget of the land surface carries the impacts of short and long wave radiation at the Earth’s surface. These are areas where a considerable amount of research is needed to fully understand the role of land surface on the atmospheric processed. Even the Sea Surface Temperatures show a diurnal variability of the order of 0.5 C/day, that too has important implications for tropical buoyancy and clouds. There are major tropical circulations such as the Hadley and East West circulations that require understanding and careful modeling of radiative convective thermal balances.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 14. Dry and Moist Static Stability

Nearly 80 % of the soundings over the tropical oceans contain a lower tropospheric inversion. In the regions of the trade winds this is often referred to as a trade wind inversion. The strongest inversions are found over the eastern regions of the oceans. Convection, clouds, and turbulent mixing constantly work to erode them. Evidently, however, there are strong restoring forces that counteract the eroding effects and act to maintain the inversion.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 15. Hurricane Observations

The terms “hurricane”, “typhoon”, and “tropical cyclone” reflect only a difference in geographical location, but not of underlying physical principles. For this reason, when geographical considerations are not relevant, the above terms will be used more or less interchangeably throughout this book. The unique vertical structure of a hurricane can best be seen from a few vertical cross-sections that portray its wind, temperature, equivalent potential temperature, and clouds. Some of the classical illustrations of wind and temperature have come from an analysis of Hurricane Inez of 1966 by Hawkins and Imbembo (1976). Figure 15.1 is a cross-section across the hurricane that was produced using multiple aircraft reconnaissance data sets. The hurricane reconnaissance aircraft provided flight level winds and a center fix that enabled the construction of this inner core structure of the strong winds in a hurricane.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 16. Genesis, Tracks, and Intensification of Hurricanes

A global map showing over 150 years of hurricane/typhoon/tropical cyclone tracks is presented in Fig. 16.1. The starting points of these tracks are the genesis locations in the different ocean basins. These genesis locations pertain to different months and seasons. The Northern Atlantic and Pacific Oceans’ genesis locations pertain mostly to the months of June through October. The Northern Indian Ocean locations refer mostly to the pre- and post summer and winter monsoon months, i.e., April, May, November and December. In the Southern Indian Ocean and the South Pacific (close to Australia) the months of genesis are December and January. The South Pacific east of Australia encounters a few typhoons each year during the southern summer months of November, December, and January.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 17. Modeling and Forecasting of Hurricanes

Both global and mesoscale models have provided useful information about the structure, motion and life cycle of hurricanes. The global models have horizontal resolution of roughly 80 km in the tropics, with some 25 vertical levels. The mesoscale models currently have horizontal resolution of much less than 10 km, with upwards of 50 vertical levels. These models have built in a full array of physical parameterizations, such as surface and planetary boundary layer physics (for transfer processes), cumulus parameterization, large scale condensation, radiative transfer with effect of clouds, water vapor, carbon dioxide and ozone, surface energy balance, air-sea interactions, inclusion of topography and sub-grid scale diffusive processes.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 18. Sea Breeze and Diurnal Change Over the Tropics

The sea breeze phenomenon is very striking over many parts of the tropics since it is known to produce cooling associated with afternoon showers that occur with regularity on most undisturbed days. Figure 18.1 from a classical diagram of van Bemmelen (1922) illustrates the time evolution of the sea breeze (on shore winds) in Batavia (now Jakarta). The Batavia sea breeze time section shows that it is a shallow circulation essentially confined to the lowest 3 km. The intensity of the upper land breeze is roughly half that of the sea breeze. The land breeze (off shore winds) during the early morning is much less intense by comparison. Extensive observational studies of sea breezes have been conducted by Hsu (1970), Flohn (1965) and many others. Hsu (1970) portrayed a schematic evolution of the land/sea breeze phenomenon based on observations in the Gulf Coast of Texas. Figure 18.2 illustrates this evolution, the diagram being self explanatory. Here the horizontal and vertical extent of the wind system is enclosed within a heavy solid elliptical curve.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra

Chapter 19. Tropical Squall Lines and Mesoscale Convective Systems

Passage of tropical squall lines is a common phenomenon in the rain areas of the tropics. These systems form over the land areas and even continue over the ocean for several days. The meso/beta scale (20–200 km) is a common scale of organization of convection within what are called the Meso Convective Systems (MCS). These normally carry long anvil clouds that can extend for several 100 km. MCS carry both stratiform and convective rains.

T. N. Krishnamurti, Lydia Stefanova, Vasubandhu Misra


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