MARITIME CONTINENT THUNDERSTORM EXPERIMENT (MCTEX)

Overview

Contributors:

T.Keenan¹, G.Holland¹,S.Rutledge²,J.Simpson³,J.Wilson⁴, M. Moncrieff⁴, R. Carbone⁴, B. Sanderson¹, N.Tapper⁵

 Bureau of Meteorology Research Centre  (BMRC)
 Colorado State University  (CSU)
 Goddard Space Flight Center  (GSFC)
 National Center for Atmospheric Research  (NCAR)
 Monash University(MU)

MCTEX OBJECTIVES

Basic Objective:

To improve knowledge of the dynamics and interaction of the physical processes involved in the organisation and lifecycle of tropical island convection over the Maritime Continent and the role of this convection in the atmospheric energy and moisture balance.Component Scientific Objectives

1. Examine the lifecycle of convection from the initiation process through mesoscale organisation of the deep precipitating system;
2. Quantify microphysical processes in convective and stratiform regions with emphasis on the relative importance of coalescence and ice processes;
3. Study cloud electrification mechanisms, especially couplings between ice phase precipitation and the occurrence of strong electric fields;
4. Provide an improved basis for cloud parameterisations in numerical models;
5. Document the radiative properties of convectively-generated cirrus;
6. Quantify the water budget of island thunderstorms;
7. Improve algorithms for rainfall estimation from ground-based and spaceborne sensors;
8. Develop improved short-range forecasting techniques for convective systems;
9. Investigate the effects of the surrounding ocean on island thunderstorm development and the response of the ocean to convective activity.

  MCTEX is thus aimed at investigating mesoscale systems over tropical islands of O(100 km), which are strongly influenced by coastal effects and diurnal effects. The full convective lifecycle will be examined, together with coupling among microphysics, radiation and storm dynamics and their effects on larger scales. The domain of MCTEX makes it particularly attractive for numerical modelling studies of mesoscale systems. MCTEX will provide direct benefits to short-term forecasting of convection through its observational and numerical modelling studies.

The MCTEX experimental program will be undertaken from 13 November 1995 to 10 December 1995 over the Tiwi Islands, 50 km north of Darwin, Australia(12.50S, 131.0E). MCTEX will be a multi-national program with funding contributed by a combination of agencies.

Rationale for MCTEX

Almost two-thirds of the global precipitation occurs in a variety of mesoscale convective and stratiform cloud systems located equatorward of 30 degrees (Sellers, 1969). Latent heat release and radiative heating by these clouds are important driving mechanisms for the general circulation and atmospheric teleconnections, and variations in the distribution of tropical clouds are of importance to climate change (Rasmusson and Arkin, 1985).

To further our understanding of tropical convection and its impact upon the environment, additional knowledge is required about the physical and dynamical processes responsible for the convection, its mesoscale organisation and precipitation processes, and the scale-interactions. With good quality observations, observational and modelling approaches can properly study the evolution of convective systems and their impact on the environment. Observations of oceanic convection in the tropics were obtained during the Global Atmospheric Research Program(GARP) Atlantic Tropical Experiment GATE, the Tropical Ocean-Global Atmosphere(TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) (Webster and Lucas, 1992), and the Equatorial Mesoscale Experiment (EMEX) (Webster and Houze, 1991) but basic observations of island systems in the atmospheric "boiler box" of the Maritime Continent are lacking.

MCTEX offers the opportunity to undertake a study of processes relevant to the lifecycle and mesoscale organisation of tropical convection within one of the major areas of global latent heat release. Storms observed over the Tiwi Islands to the north of Darwin, Australia are regular in occurrence, fixed geographically and exhibit a regular lifecycle ranging from shallow non-precipitating convection to the evolution and decay of amesoscale squall-like system. These islands thus provide a diurnally forced atmospheric laboratory for the study of processes associated with island convection. MCTEX aims to observe and study the dynamical and physical processes involved with the complete convective lifecycle, including: initiation, development of deep precipitating and electrically active convection, the subsequent evolution and decay of organised mesoscale systems and the impact of these systems upon the atmospheric and oceanic environment.

MCTEX will form the basis for studies related to the remote retrieval of rainfall measurements from space by providing information on rainfall production and the vertical distribution of hydrometeors during the entire convective lifecycle of the island thunderstorms. The target storms are extremely active electrically(flash rates of 60 /min are common) and thus studies of the evolution of the electrical field and lightning production will be part of the program.

Relevance of MCTEX to other Studies of Convection

The study of the physical processes and dynamics of tropical convective rain producing systems requires investigation of synoptic, mesoscale, convective and microscale processes. For example, mesoscale convective systems produce extensive stratiform anvils, which persist throughout the tropics, may have rapid interactions with larger scales (Holland, 1994), and play a fundamental role in the heating of the large scale environment (Houze, 1989).

The dynamics of these mesoscale systems is coupled to environmental effects including buoyancy, wind-shear and fluxes of latent and sensible heat from the underlying surface. TOGA COARE and CEPEX(1993) have studied the processes occurring over the open ocean, and other experiments, such as VIHMEX (Miller and Betts, 1977), COPT81(Sommeria and Testud, 1984) and DUNDEE (Rutledge, et al., 1992) have examined convective systems in continental locations.

MCTEX aims to investigate a different mode of convection than studied in the previous tropical programs. Satellite imagery of the Maritime Continent region indicates regular diurnal forcing of convection over a wide area (Holland and Keenan, 1979), which are present regardless of the phase of the Madden-Julian Oscillation(Madden and Julian, 1971) and which, collectively, are of similar scale and energetics to the oceanic supercluster systems. For instance, anvils produced over New Guinea and Northern Australia spread and merge from individual mesoscale systems into extensive cloud decks. These mesoscale systems over the tropical maritime continent region forced by island influences are also of importance and fundamentally different from similar-scale oceanic systems (Keenan and Carbone, 1992). Oceanic convection is characterised by weak updraughts ( 20 m/s) and a well-developed mixed-phase region resulting in copious production of lightning. The differences in convective structure imply differences in vertical heating profiles which are particularly significant for cumulus parameterisation. MCTEX will focus on these thunderstorms complementing the previous studies of tropical oceanic and continental convection.

Our interests are commensurate with the primary aim of the Global Energy and Water-cycle Experiment (GEWEX) Cloud System Study (GCSS) in which the formulation of improved parameterisations for cloud processes in General Circulation Models (GCMs) is a main aim.

The GEWEX Scientific Steering Group has endorsed the principle of GCSS collaboration in new field experiments provided that (a) adequate existing sources of field data are unavailable, and (b) the new data sets will meet the needs of the GCSS modelling activities which centre on the cloud resolving model (CRM) approach. The GCSS/MCTEX collaborative study satisfies both criteria being a modelling study and an observational validation of convection in a tropical environment dominated by island-coastal influences.

In particular, the GCSS Working Group 4: Precipitating Convective Cloud Systems role in MCTEX centres on the physics of mixed-phase, cirrus-producing deep convection in the tropics on diurnal time scales. Emphasis will be on a) the cloud system life cycle especially the evolution of transport properties; b) microphysical, radiative, and dynamical interactions including the identification of critical issues in the parameterisation of microphysical processes in CRMs and GCMs; c) mesoscale and large-scale effects of organised convection; and d) a CRM intercomparison project incorporating initial conditions from selected MCTEX data sets, supplementing some from TOGA COARE.

The observed diurnal cycle of deep convection in the tropics (Gray and Jacobson, 1977,: Mapes and Houze, 1993), as well as the diurnal modulation of large mesoscale systems, indicates that radiation must have a significant interaction with convection. Hu and Randall (1994) have shown that a simple radiative-convective model with interacting parameterisation schemes for radiation, convection, and surface fluxes produces a realistic Madden-Julian Oscillation. Further, that oscillation disappears from the model when the link between the convection and radiation is severed.

Understanding the linkage between tropical convection and radiation is one of the primary reasons why the Atmospheric Radiation Measurement (ARM) program of the US Department of Energy (DOE) has identified the Tropical Western Pacific (TWP) as one of its focal sites. While one component of the TWP regional program is aimed at making long-term measurements of surface radiation and cloud properties, another component is focussed on detailed field studies of convection. MCTEX is ideally designed for this second component. The concentrated measurements of atmospheric thermodynamics and cloud properties over the convective lifecycle proposed for MCTEX can be augmented by the radiation and remote sensing observations of the ARM program to provide a data set that can be used in investigations of the interaction between convection and radiation. The TWP program of ARM has recognised the obvious scientific synergism between ARM and MCTEX, as well as the financial benefits of the collaboration, and has warmly endorsed the participation of ARM in the MCTEX campaign (Ackerman et al., 1993).

The vertical distribution of hydrometeors in storms is relevant to the problem of remote retrieval of rainfall from space. The Tropical Rainfall Measuring Mission (TRMM)(Simpson et al. 1988) has already devoted considerable effort investigating rainfall retrieval problems associated with a range of rain producing systems including oceanic systems observed in GATE and TOGA COARE. MCTEX will act as a focus for testing algorithms over the maritime continent by providing valuable and unprecedented data relevant to this problem.

Williams et al.(1990) propose that island and tropical continental thunderstorms, rather than oceanic convection, are dominant in establishing the global electrical circuit. However, the physical origin of the electrical structure and the role of cloud microphysics in electrification are poorly understood. MCTEX will increase our understanding of these processes especially the coupling between ice nucleation and strong electric field occurrence.

Convective Lifecycle

The experimental program will focus on convection over the Tiwi Islands which are relatively flat and extend east-west approximately 150 km and north-south 50 km. The islands are relatively uninhabitated, although townships, roads and the necessary infrastructure to support the field program are available. The islands are controlled by the Tiwi Land Council and permits are required for access to the islands. The thunderstorms occur during monsoon break flow characterised by deep subtropical origin easterly flow with a moderate maximum of approximately 10 m/s near 3 km height. CAPE can be considerable and is typically near 1500 J/Kg. A typical pre-storm sounding shows a trade wind sounding with the characteristic mid-level dry region maintained by subsidence. During the experimental period island thunderstorms are detected on average between 65-90% of days (Keenan et al, 1990)

A typical lifecycle consists of initial widespread shallow convection over the islands starting in the mid-morning and then concentrating along the sea breeze front. Surface fluxes warm, mix and moisten the subcloud layer sufficiently to support the initial cloud growth. Initial precipitating systems are short- lived and shallow with warm rain processes dominant. Deeper convection is forced along the sea breeze fronts and merger of cloud systems follows especially in regions of sea breeze convergence. This merger process results typically in the evolution of an east-west oriented mesoscale system aligned parallel to the low-level shear. With the merger, explosive growth occurs with vertical velocities estimated to be in excess of 40 m/s and echo tops reaching to 20 km in height.

Following the merger process and down-draught production, a reorientation of the convective system occurs such that it typically becomes aligned north-south, i.e. perpendicular to the low-level environmental shear. The system then typically moves to the west having the characteristics of a squall like system with a trailing stratiform region. The whole process from first precipitating system to decay lasts approximately eight hours with the first precipitating system being detected about midday.

The MCTEX Experimental and Research Programs

The MCTEX experimental program will be undertaken from 13 November 1995 to 10 December 1995. The observational network planned for MCTEX is summarised in Fig. 1. MCTEX will employ the BMRC polarised radar as the primary observational platform to define the structure and evolution of the convection. These observations will be supplemented with the Flinders Institute for Atmospheric and Marine Science (FIAMS) Cessna 340A for low-level missions aimed at defining the structure and evolution of planetary boundary layer circulations and near surface turbulent fluxes. An autonomous aerosonde will also be deployed to define the planetary boundary layer circulations over longer time periods.

A Monash University (MU) rawinsonde station is planned for deployment on the islands undertaking frequent soundings providing a basis for numerical simulations and assessing the impact of the storms upon the environment. The BMRC 5 cm polarimetric radar will be located on the islands to obtain high resolution radar data pertaining to convective initiation mechanisms, rainfall distribution, cloud microphysics/electrification and the vertical distribution of hydrometeors. This multiparameter radar will be used to sense remotely cloud microphysical and precipitation processes in the island convection which is essentially impenetrable by research aircraft. The Bureau of Meteorology Research Centre (BMRC) high resolution network of raingauges ( 25 gauges at 2 km intervals covering a 10 km by 10 km area) will be deployed over the islands for ground truth rainfall measurement. In addition, the BMRC mesonet of fifteen automatic weather stations will enable mesoscale surface analysis and additional direct rainfall measurement over the islands.

Surface energy budget measurements are planned at several island locations by MU and will be linked with extensive radiation measurements at one island site deployed as part of the ARM TWP. An approximate north-south line of BMRC, National Oceanic and Atmospheric Administration (NOAA) and Environmental Technology Laboratory (ETL) 920 MHz wind profilers will be deployed to further define the low-level wind circulations with the middle island station employing Radio Acoustic Sounding System (RASS) to define the thermodynamic modification of the sea breeze convergence zone.

Ancillary observations include the island deployment of a CSIRO Lidar, the Massachusetts University millimetric wavelength cloud radar for study of cirrus anvil structure, and the Kyushu University (KU) balloon borne particle image sensor to measure hydrometeor distribution within the convection. It is planned to undertake electric field measurements from the Electra and on- island electric field measurements using National Aeronautics and Space Administration (NASA)/Marshall Space Flight Center (MSFC) field mills. An LLP network will be implemented by NASA/MSFC with coverage extending over both islands.

Oceanographic measurements will be conducted for approximately two weeks from the Commmowealth Scientific and Industrial Research Organisation (CSIRO) RV Franklin.

The observational network is well suited to meet the MCTEX objective of convective lifecycle examination. The high predicability of the island thunderstorm occurrence and the close proximity to Darwin ensure that as far as possible, mission aims especially with aircraft operations will be met because of the negligible ferry times and the dimensions of the convective system. At least ten Intensive Operational Periods (IOPs) are planned during MCTEX. An IOP will be a day of aircraft and detailed rawinsonde operations and will be decided upon during the experimental period based on forecast information obtained at the MCTEX Operations Centre which will be located in Darwin at the Northern Territory Regional Office of the Bureau of Meteorology. This will give the MCTEX investigators access to all meteorological products available through the Bureau of Meteorology.

Coupled with the experimental period will be modelling and diagnostic studies relevant to the objectives summarised above. These studies will focus on simulations of the lifecycle both before and after the experimental period with emphasis on initiation mechanisms, interaction of the cold pool with the environment, evolution of convective cells and mesoscale circulations, evolution of momentum, heat and moisture fluxes and their scale dependence, temporal evolution of the water budget on scales O (100 km) and the effects of microphysics and radiation on the dynamics of organised convection.

Other areas of interest for modelling studies include testing of simple (archetypal) dynamical models of cloud systems as produced by Moncrieff (1992) in representing observed fluxes of momentum; upper-tropospheric processes and transports associated with the interaction of microphysics and radiation, and large scale effects of convection, i.e. the sub-gridscale effects on heat, moisture and momentum. Modelling of the storms will also be done at various institutions enabling direct intercomparison of individual strength and weaknesses as required by the GCSS Working Group 4 Model Intercomparison Project. Within the ARM program, the combined dataset will be used to evaluate the performance of parameterisation schemes for convective clouds currently used in climate models. MCTEX facilities will provide the kinematic structure of the lifecycle of convection, the initiation mechanisms, and mesoscale momentum fluxes etc. The study of the interaction of microphysics and radiation, one of the major uncertainties of climate modelling, will be well advanced by observations from the polarised radar, cloud radars and soundings. In-situ microphysical and radiation measurements from the DOE/ARM will be particularly valuable.

MCTEX Participants

MCTEX Science Steering Committee

Co-Chairmen: T.Keenan (BMRC), S. Rutledge (Colorado State University-(CSU). Members:R. Carbone (NCAR), M. Moncrieff (NCAR), J.Simpson (NASA Goddard Space Flight Center-GSFC), G. Holland (BMRC), W. Frank (PSU), J. Wilson (NCAR), T. Ackerman(PSU), J. Hacker(FIAMS),K.Gage(NOAA), B. Sanderson (BMRC)

MCTEX Logistical Coordinators

D. Jasper (BMRC), P. Spyers-Duran (NCAR), P. Taylor (NCAR)

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