Kerala is the land of Monsoons. In fact the first burst of the southwest monsoon over the Indian subcontinent takes place over Kerala. The location of Cochin is highly significant to understand and the timely prediction of the variability and characteristics features of monsoon onset, intensification, break and withdrawal phases, are highly relevant to the proper planning for agriculture, hydro-electrical power generation, economy and industrial sector of Kerala.The timely onset, optimum duration and reasonable strength of the southwest monsoon are of vital importance to the life and economy not only of our Country but the entire South Asian region.
SCIENTIFIC OBJECTIVES for ST Radar network in India:
The scientific objectives of the proposed ST Radar network are the following:
I) To study the dynamics of the tropical tropopause on scale ranging from meso to synoptic including the influence of the extra tropical lower atmosphere:
The tropical tropopause has become the focus of great scientific interest spurred largely by a realization that tropics hold the key to the variability of global climate. The tropopause which demarcates the troposphere and stratosphere denotes the transition between the regions of convective-radiative equilibrium and the region of pure radiative equilibrium. Its most basic characteristic is a change in the static stability and the associated change in vertical mixing time. The mean. Heat balance of the troposphere is considered to be between convective heating (through the sensible heat flux from the surface and latent heat released during precipitation I by convective clouds) and cooling by radiation from water vapour. Thus, radiative cooling is an important constraint on convection (Hartman and Larson, 2002). This implies that the most active convection will be limited to the altitude range where radiative cooling is efficient. The peak in detrainment of convection occurs at the level of maximum mass divergence. This detrainment and divergence (maximum) occur well below the tropopause as defined by the level of minimum temperature (cold point) and do not appear to be caused in any direct way by lapse rate change (Hartman and Larson, 2002). The level of maximum divergence can be taken to represent tropopause level corresponding to top of convection. Although most convection is generally thought to detrain below150mb level, it can penetrate to higher levels and sometimes even above the level of cold point in temperature especially during strong deep convection events. The fact that temperatures in the lower
and middle (tropical) troposphere are governed mainly by convection is borne out by the lapse rates which are close to moist adiabatic. The lapse rates deviate from that of the moist adiabatic in the upper troposphere (Sherwood et al 2003). In fact, simple models suggested that the cold point in the temperature profile is actually a stratospheric feature which does not depend upon convection, owning its existence mainly to the photochemical production of ozone near l00 hPa level (Kirk-Davidoff et al 1999; Thuburn and Craig 2002). Cn2 the basis of the simple picture of tropopause that it represents the cap imposed on troposphere convection by the stably stratified inversion layer of the stratosphere the potential temperature of the air at tropopause is expected to be roughly equal to the equivalent potential temperature of the boundary layer air (Reid and Gage 1981). However, the tropopause (defined by minimum temperature) potential temperature is nearly always higher than the boundary layer air equivalent potential temperature. This additional heating is higher in northern winter and smaller in summer (Reid and Gage 1996). This additional heating can come from i) episodic Rossby-wave activity in the Northern hemisphere (from extra tropical region) injecting ozone rich stratospheric air directly along isentropic surfaces into the tropical upper troposphere and ii) stronger tropical troposphere convective activity leading to increased penetration of troposphere air into lower stratosphere and entraining ozone-rich stratospheric air into the upper troposphere as it settles down to its equilibrium level (Reid and Gage 1996). Which of these two is dominant and in which season are questions that remain unanswered? At any rate, it appears that the large ‘scale suction pump’ driven by extra tropical wave activity and strong troposphere convection are the two dominant processes playing key role in governing the tropical tropopause. It is clear that the tropical tropopause is not a fixed material surface but is a region of transition from convective to radiative equilibrium (Holton et al 1995). Gettleman and Foster (2002) suggested that the tropical tropopause can be considered as a layer of transition between the convectively dominated troposphere and the radiatively controlled stratosphere.
A complete knowledge of the tropical tropopause characteristics is essential to the understanding of the physical processes in the tropical tropopause region and the STE. Part of the difficulty for this is in making observations at the required spatial and temporal scales. The Stratosphere Troposphere (ST) radar provides vertical wind data from which temperature can be obtained with good altitude-resolution giving a means to study the tropical tropopause characteristics.
The tropopause itself can be identified by different ways. These are i) the lapse rate tropopause ii) the cold point tropopause iii) level of top of all convective outflow. The first two can be determined from the temperature profiles obtained from the radar. The level of top of convective outflow can be determined from the horizontal divergence profile obtained from the radar (Satheesan and Krishna Murthy 2005). In fact, the radar provides a reliable means of determining the level of top of convective outflow and has many advantages over the indirect method from satellite radiance data (Satheesan and Krishna Murthy 2005).
The tropical tropopause dynamics and consequently the STE process exhibit strong regional variation. This arises basically due to different strengths of convection and land-sea contrasts. The tropical tropopause dynamics and STE have strong implications to the transport of minor species including radiatively active ones and hence to global change. Thus, it is imperative that concerted and well-coordinated studies are carried out on these over the Indian sub-continent. The proposed network of ST radars will be the most appropriate means for these studies as it can provide the required information on different temporal and spatial scales. Further, the network enable study of convection itself on different scales making use of the unique capability of vertical wind measurement and the derived parameters of horizontal divergence and vorticity.
II) To study stratosphere-troposphere exchange phenomenon:
The dynamical, chemical and radiative couplings between the stratosphere and troposphere are among the important processes
playing a crucial role in the global change. The transport of the minor chemical species, natural and anthropogenic, between the tropical troposphere and stratosphere is of special significance. The anthropogenic species transported from the troposphere to the stratosphere initiate much of the stratospheric ozone chemistry. The downward transport from the stratosphere constitutes the main removal mechanism for many stratospheric species and represents a significant input of ozone and other reactive species into the troposphere chemical system.
The contribution of Stratosphere to Tropospheric ozone remains a subject of intense debate because industrialized countries would rather throw the blame on stratospheric input. At least in the southern Hemisphere, upto 45% of tropospheric ozone may be of stratospheric origin while in the Northern Hemisphere, the stratospheric contribution could be as low as 30% but shows an increasing trend. This subject assumes a great significance in the present context in India as India gets blamed for increased Methane (a green-house gas) from its paddy fields. Tropospheric ozone is a precursor to the Hydroxyl radical which can oxidize methane and carbon monoxide. As the tropics contain plenty of water vapour as well as the UV radiation less than 31Onm (because of low ozone content in the tropical stratosphere) which can photolyse ozone, plenty of OH radicals are produced here to destroy the locally produced Methane.
The altitude capacity of the proposed Radars should be at least 20lan for a focused study of the Stratosphere-Troposphere exchange phenomena. The stratospheretroposphere exchange (STE) can, in turn, influence the radiative flux balance in the troposphere and stratosphere. Thus, STE can have a very important role in the radiative forcing of the global change. An important factor in the STE processes is the vertical transport scales in the troposphere and stratosphere. The vertical transport of air and chemical species through the depth of the troposphere can occur on time scales as short as a few hours via moist convection which is strong in the tropics and on time scales of days via baroclinic eddy transport in extra tropics. On the other hand, the vertical transport through a similar altitude range in the stratosphere
can take a year or more in the stratosphere. The vertical transport must be accompanied by radiative heating or cooling. The difference between the vertical transport scales in the stratosphere and troposphere is part of what lies behind the rapid increase in ozone mixing ratio and the rapid decrease in water vapour mixing ratio with altitude just above the tropopause.
The very low temperatures of the tropical tropopause are responsible for the very low water vapour mixing ratios in the lower stratosphere thru a process called ‘freeze drying’ in which the air passing thru the tropical tropopause has its water vapour mixing ratio reduced to its ice saturation value at or near the tropopause.
III) To study the tropical convection from micro to synoptic scales:
Strong convection aided by water vapour prevails in the tropical troposphere and plays an important role in mixing. It occurs on a wide range of scales (spatial and temporal) and the high resolution vertical velocity measurement capability (in addition to the horizontal wind components) of the radars provides an important and direct means of studying convection. Horizontal divergence and vorticity can be derived from the radar data for studying the convection. The proposed network of radars enables studies on convection from mesoscale to synoptic scale.
IV) To study the precipitating systems and Radar Bright Band:
Studies on the radar bright band, a strong enhancement in reflectivity at around the 0°C isotherm level, in the precipitating atmosphere are of major interest to the radar community. Though observations of the radar bright band began a few decades back, still there is a need to examine the bright band carefully, because it is usually more intense than predicted. The attenuation caused by the bright band at X band and higher frequencies is a serious obstacle for communications systems. The melting layer is also of interest to meteorologists in understanding cloud physics and dynamics and radiation and heating processes. Thus characterizing the radar bright band (bright band height, thickness, their variability, etc.) at several
locations is highly essential. For instance, the cooling rate of the atmosphere is inversely proportional to the thickness of the radar bright band. Information on the radar bright band can be obtained from a variety of platforms, starting from ground-based radars to space borne radars like Tropical Rainfall Measuring Mission (TRMM). The TRMM provides the bright band information directly as one of its products. Unfortunately, the overpass frequency of TRMM is limited and one needs to pool observations over a long period to get meaningful statistics. Moreover, we may miss some of the specific events, which are of interest to the scientific community. On the other hand, location- specific radar measurements can be used to study the mesoscale structure of radar bright band. ST Radars are perhaps the most useful tools to study the radar bright band because of the following reasons.
1. The attenuation at these frequencies is negligible compared to microwave radars.
2. ST radars provide very high temporal (few tens of sec.) and height resolution (few tens of meters) measurements of the radar bright band.
3. The ST Radars provide Eulerian type of measurements ie., the atmospheric parameters measured at different heights at almost the same time.
4. On many occasions, they provide both precipitation as well as background atmospheric information which enable us to study both dynamics and microphysics of rain systems.
V) To study the turbulence in the troposphere and lower stratosphere in the tropics and extra tropics:
The radar provides a unique capability of estimating the turbulence parameters like the rate of dissipation of the turbulent kinetic energy. This can be done i) from the radar signal strength (reflectivity) ii) from the Doppler spectral width of the echoes and iii) from the vertical wind data directly (see Narayana Rao et al 2001; Satheesan and Krishna Murthy 2002). Turbulent mixing is an important process of minor species transport which has implications to the radiation budget 9involving water vapour and ozone for example). As turbulence in the troposphere is expected to vary with latitude, it is essential to carry out turbulence parameter estimation at different locations spread in latitude in order to understand the minor species transport on different scales and their influence on radiation budget. This again can be facilitated by the proposed network of radars.
VI) To study the characteristics of the short and medium scale gravity waves in relation to their sources and their influence on the large scale atmospheric dynamics:
The importance of gravity waves in the middle and upper atmospheric dynamics stems from their capability to carry vertically momentum and energy from their source region (mainly troposphere) to higher altitudes, to contribute to turbulence and to influence the mean circulation and thermal structure of the middle atmosphere (e.g., Lindzen, 1982; Holton, 1983). However, our understanding of the characteristics of gravity waves in relation to their sources, their climatology, spectral behavior and the processes and interactions that constrain the wave amplitudes that contribute for turbulence and mixing is very little especially in the low latitudes (tropics).
Gravity waves are generally classified into three groups, namely the large scale with horizontal scales of a few thousand kms, the medium scale with horizontal scales of a few hundred kms and the short scale with horizontal scales of few tens of kms. The large scale gravity waves propagate essentially horizontally and have well defined sources in the magnetic disturbances at high latitudes. These are fairly well documented and understood. The medium and short scale gravity waves propagate inclined to the horizontal and have their sources mainly in jet streams (wind shears), strong convection and orography. These waves are indeed responsible for influencing the middle atmospheric circulation carrying vertically the momentum and energy fluxes, turbulence and upper atmospheric phenomena such as equatorial spread F and Counter electro jet (e.g., Krishna Murthy, 1998).
The gravity waves (medium and short scale) as they propagate to higher altitudes from their source region grow in amplitude in the absence of any dissipation to conserve the wave energy compensating for the decrease of density with height. As they attain greater amplitudes, the wave fields become unstable. This unstable condition generates sufficient turbulence to prevent further growth in amplitude (i.e.) the wave is said attain saturation. This phenomenon, called wave breaking, is responsible for the wave-mean flow interactions affecting the mean flow (circulation) and the thermal structure (e.g., Lindzen, 1981; Fritts and Alexander, 2003). The shorter vertical wavelength gravity waves attain saturation at lower altitudes compared to the longer ones. Thru the wavemean flow interaction, gravity waves accelerate the mean flow contributing to the equatorial stratospheric QB0 and the middle atmospheric semiannual Oscillations.
The most important sources of the gravity waves include orography, convection and wind shear. These source strengths are different indifferent longitude zones and are highly region specific. Further, a source like convection is highly intermittent (spatially and temporally). It is well known that strong zonal wind shears prevail in the tipper troposphere. The tropical easterly jet (TEJ) occurs in the south-west monsoon season (June-September) which is active in the South East Asian region. The TEJ exhibits strong zonal wind shears with significant day to day variability. The short scale gravity waves are expected to show increased variance at latitudes <30 (Fritts and Alexander 2003) due to increased strength of the sources like convection and wind shears.
From the above, it is clear that there is a need for a comprehensive coordinated study of gravity waves at low latitudes quantifying their characteristics and influences on the middle atmospheric dynamics, turbulence and upper atmospheric effects and relating these to their sources. There have been a few studies on gravity waves in the low latitude region (e.g. Prabhakaran Nayar and Sreelatha, 2003) on estimation of momentum fluxes. However, these studies don’t link (with the gravity wave sources. As the gravity waves propagate inclined to the horizontal, their effects at higher altitudes will be felt away from their sources. The proposed network of radars can yield the desired information on direction of propagation of the waves leading to an understanding of the effects of gravity waves linked to their sources. Hitherto such studies have not been possible because of the non-availability of a proper network of radars. In the proposed network, ST radar stations forming triangles of different sizes can be selected so that characteristics of gravity waves of different scales can be studied. For example, Trivandrum, Kochi and Tirunelveli constitute a triangle for small scale gravity waves; Ahmedabad, Kharagpur and Trivandrum constitute a triangle for the study of medium scale gravity waves. Different combinations of stations can be selected forming different triangles so that source regions of gravity waves can be identified.
VII) To study the latitudinal and seasonal variation of equatorial waves and their influence on large scale atmospheric dynamics:
The equatorial east ward propagating Kelvin and the westward propagating Rossby Gravity (RG) waves contribute to the generation of the QBO in the equatorial stratosphere through wave mean flow interaction process in addition to the gravity waves. In fact, the relative contribution of the gravity waves and the equatorial waves remains still to be assessed.
The equatorial waves have their sources in the latent heat releases by cumulus convective formations in the troposphere. It is known that the ITCZ (Inter Tropical Convergence Zone) which is a zone of strong convection and hence a potential source of the equatorial waves, has a seasonal latitudinal excursion different in different longitude zones. It undergoes the maximum seasonal excursion in the south west monsoon region. Thus the latitudinal dependence of the strength of the equatorial waves (within the tropics) can be expected to be different in different longitude zones. In India concerted experimental campaigns have been conducted using roclcetsondes, ballooonsondes and the radar and lidar at Gadanki. However all these campaigns could address mainly the estimation of the vertical fluxes of the horizontal momentum of these waves (see review by Sasi et al 2005). The latitudinal dependence of the strength of the equatorial waves still remains to be studied and the proposed radar network can provide the necessary information for this.
VIII) To study the Monsoon and the winter westerly wind systems:
The monsoons are the major meteorological phenomena which control the weather and climate over the Indian sub-continent and a comprehensive study of the monsoon wind system is vital not only from the vast applications point of view but also from the science point of view. The one parameter measured by the ST radar which is not available from any other measurements is the vertical wind, a crucial parameter for the understanding of meteorological phenomena including the monsoon. The proposed network with help to address various aspects of monsoon and also the winter westerly Wind flow.
a) Mean Wind Flow:
The principal mean wind flows over the Indian sub-continent are due to the Southwest (SW) monsoon system during June to September, Northeast (NE) monsoon system over peninsular India during October to December and the Westerly wind system over North India during December, January and February. During the SW monsoon, westerlies in the lower troposphere and strong easterlies (tropical easterly jet) in the upper troposphere prevail over peninsular India. The proposed ST radar stations of Trivandurm, Kochi, Pune and Visakhapatnam as well as Port Blair provide adequate coverage to delineate and monitor the SW monsoon westerlies and easterlies as well as the low level winds during the NE monsoon wind. One other important feature of the monsoon wind system is the monsoon trough in the lower and middle troposphere extending from Pakistan to the head of Bay of Bengal. The proposed ST radar stations at Kharagpur/Kolkata, Visakhapatnam, Pune, Allahabad and Silchar are very well suited to delineate the monsoon trough as well as its north-south movement. The proposed ST Radar stations at Allahabad and Kolkata will help to delineate this westerly wind system as well as the east ward – moving westerly waves and western disturbances which bring winter rain I snow to north India.
b) Monsoon transient disturbances:
Much of monsoon rainfall is associated with monsoon synoptic weather systems like monsoon lows and depressions. The ST radar stations at Port Blair, Kolkata, Visakhapatnam, Pune and Allahabad besides those in the peninsular India are essential for delineating and tracking these monsoon disturbances which are crucial for monsoon rain fall. The peninsular ST radar stations as well as Port Blair are also 1:1seful in tracking westward moving upper tropospheric easterly waves and rain producing disturbances during the north east monsoon.
c) Monsoon Variability:
Monsoon winds, cloudiness and rainfall also exhibit variability on different time scales.
(i) Intra-seasonal 30-50 day oscillations
On this time scale the monsoon wind system cloudiness, rainfall slowly shifts northwards. The proposed ST radar network will be useful in delineating this intraseasonal variability.
(ii) Inter-annual variability:
As is well known there is large year to year variability of monsoon rainfall and wind system. There are years of excess monsoon and drought years. This variability is known to have some association with ENSO (El Nine So1:1thern Oscillation). The ST radar network will provide valuable data needed for this. There is also monsoon variability on decadal and century time scales. The continual availability of this ST radar network wind data would be useful in studying this variability.
d) In Pre monsoon (March- May) and Post m0nsoon (October- December) seasons the peninsular ST radar stations as well. as Kharagpur will be useful in tracking tropical cyclones that form over the Bay of Bengal and Arabian sea and, move to India.
IX) To provide wind information for initializing the numerical weather forecast models and to provide information on turbulence and momentum fluxes for proper inclusion of sub grid scale dynamics in the models:
The wind information from a properly chosen radar network can provide data for initialization of numerical forecast models. As already mentioned, radars provide vertical wind data with high altitude resolution. The profile of vertical wind is indicative of latent heat associated with convection and would be useful in parameterizing the heating and momentum fluxes generated by clouds required in the models. The vorticity and horizontal divergence profiles derivable from the radar data will be extremely useful in the numerical models. Further, the small scale (turbulent) fluxes provided by radars can be extremely important in certain dynamical situations and thus in improving sub grid scale motions in numerical models. The momentum fluxes that can be derived from radars have a great potential in improving the parameterization of small scale dynamics used in the forecast models. A properly chosen sub grid array embedded in a larger array (of radars) would serve the above requirements. The proposed network fulfills these requirements.
X) To study the dynamics of the Upper Mesosphere and Lower Thermosphere
The dynamical phenomena of the UMLT (80-120Km) region including wave breaking hold the key for the understanding of the thermal structure of the region and the influence of the wave disturbances originating from the troposphere on the dynamics of the upper atmosphere as they pass through the UMLT region. As the ST radars in the proposed network will have the capability of antenna beam swinging, they can be employed to receive the meteor echoes and hence can be used to derive wind information in the UMLT region enabling studies on dynamics of the UMLT region.
MAJOR AREAS OF INVESTIGATION USING THE ST RADAR FACILITY AT COCHIN
(i) Studies on the characteristics of the onset, intensification, break and withdrawal phases of Indian summer monsoon and its dynamics
(ii) Comprehensive understanding of the causative mechanism for the monsoon variability for improved long term prediction
(iii) Low level jet and its association with monsoon activity
(iv) Tropical Esterly Jetstream in the upper troposphere and its linkage with monsoon activity
(v) Monitor, analyze and modeling of Tropical mesoscale convective systems
(vi) Orographic Forcing by the Western Ghats and the Generation of Wave Activity
(vii) Investigation of the characteristics of Extreme Rainfall Events and its synoptic and mesoscale features
(viii) Role of Stratospheric QBO on Tropospheric Circulation
(ix) Moisture transport from troposphere to stratosphere and understand the Stratosphere Troposphere Exchange processes
(x) Role of tropical tropopause on the interaction between upper troposphere and lower stratosphere
(xi) Characteristics of orographically induced gravity waves and their role in Stratosphere troposphere coupling
(xii) Formation of offshore vortices and heavy rainfall events.