Continental cold fronts in the subtropics and tropics

Cold fronts in the Australian region

The classical models for cold and warm fronts developed in the 1920's by the Bergen School of meteorologists as have survived as important components in the evolution of synoptic-scale low pressure systems in the middle latitudes. However, the reluctance of Australian meteorologists to analyze warm fronts over the Australian continent and unusual features of summertime cold fronts in southeastern Australia cast doubt on the utility of the Norwegian model for extratropical cyclones over the Australian continent.

Recognition of the deficiency of the classical model of cold fronts in the region led to the instigation of a Cold Fronts Research Programme in Australia in the late 70's and early 80's (Smith et al. 1982), with a special focus on the summertime "cool change", a kind of cold front that that brings relief to communities in the southern part of the continent after sometimes many days of scorching hot northerly winds from the continent. Three field experiments were organized as part of the research programme and these together with theoretical and numerical studies that the programme stimulated produced a new conceptual model of the cool change which differed considerably in detail from the classical Norwegian model. Unlike the classical cold front, many summertime cool changes are shallow and completely dry and it was evident that deep turbulent mixing over the strongly heated continent would have an important role in the structure of these fronts. A review of the summertime cool change and a comprehensive list of references thereon is given by Reeder and Smith (1999).

Despite the progress in understanding the structure of the summertime cold front, which may be connected to a parent low over the Southern Ocean, there was still a reluctance by forecasters to analyse cold fronts very far into the interior of the continent. Rather, the surface front was analysed equatorwards as an inland trough line, sometimes extending as far north as the southern half gulf of Carpentaria. Mention the analysis of the German fellow. An interest in cold fronts at subtropical and even tropical latitudes was awakened by attemts to determine the origin of southerly morning glory cloud lines, the existence of which was suggested in data from arrays of microbarometers deployed in the region by Dr. Doug. Christie and colleagues from the Australian National University (Christie et al. 1979) and confirmed on the last day of the first major field experiment to study the morning glory (Smith and Goodfield 1981, Clarke et al. 1981). Like their more frequent northeasterly counterparts, southerly morning glories are now known to be an example par excellence of undular bore-wave disturbances in the lower atmosphere; they are the atmospheric equivalent of undular bores that occur on some tidal rivers. A summary of southerly morning glories observed during field experiments in the early 80's and an analysis of synoptic conditions for each event pointed to the likely role of the inland heat trough as a ridge of high pressure extended eastwards across the continent (Smith et al. 1986).

Field experiments

The findings of Smith et al. (1986) raised fundamental questions concerning the structure and dynamics of the inland trough, the structure and dynamics of subtropical cold fronts, and the mechanism of interaction between an advancing frontal trough and the inland trough. These are important questions vis-á-vis the meteorology of the Australian subtropics, but they had received relatively little study, partly because the routine data base was, and perhaps still is, totally inadequate for this purpose. To help answer these questions, a small pilot field experiment was organized in September 1988 to obtain data on the inland heat trough in central and northern Australia (Smith and Ridley (1988). DISCUSS THE FINDINGS

The Central Australian Fronts Experiment

The Central Australian Fronts Experiment (CAFE) was organized to provide a data set on subtropical cold fronts that could be used to answer some of the basic questions about frontal structure and behaviour and to help confirm or reject predictions obtained from model simulations. The experiment ran from 7 September until 4 October 1991 and documented three cold fronts over central and northeastern Australia in unprecedented detail, with data obtained from a greatly enhanced surface observing network, a boundary-layer wind profiler as well as serial upper air soundings. Data on the surface energy balance were obtained also. The findings of the experiment are reported by Smith et al. (1995).

The structure of dry subtropical cold fronts

A common feature of all six fronts documented during CAFE and pre-CAFE was that they were dry, shallow ( ~ 1 km deep) and moved into a deep ( ~ 4 km) convectively-well-mixed boundary layer. During the night, the well-mixed layer was terminated below by a strong, but shallow radiation inversion. One of the fronts initiated major dust storms across central Australia.

The synoptic environment of these fronts was similar tothat of the summertime "cool-change" of southeastern Australia with frontogenesis occurring in the col region between the two subtropical anticyclones, relatively far from the centre of the parent cyclone. A unique feature of the region is the presence of a heat trough over northeastern Queensland with which the frontal trough eventually merges. Generally, the frontal passage is followed by strong ridging from the west.

The data obtained during the CAFE experiment highlighted the large diurnal variation of frontal structure associated with diabatic processes. The fronts were difficult to locate during the late morning and afternoon when dry convective mixing was at its peak, but developed strong surface signatures in the evening as the convection subsided and a surface-based radiation inversion developed. Moreover, there appeared to be a ubiquitous tendency in the early morning for the formation of non-linear wave-like or bore-like structure at the leading edge of the frontal zone as the inversion strengthened. In each case, as the wave/bore developed, it was observed to propagate ahead of the air mass change on the pre-existing inversion. Such behaviour was exemplified by the data for the first two events during CAFE. In the latter case, the data are unique in providing the first clear evidence of the formation of a southerly morning glory bore-wave in the Gulf of Carpentaria region from a cold front in the south. The passage of a bore brings a strong, but temporary wind surge at the surface accompanied by a sharp pressure jump. These are follows by a series of wind and pressure oscillations with a period of 10-15 min, before the steadier post-frontal airflow is established. There is no air mass change with the passage of the bore, but the vigorous turbulence that accompanies it may lead to a breakdown of any shallow radiation inversion that exists, often causing a rise in surface temperature.

Figure: Surface data from Urandangi for the first frontal passage during the CAFE experiment. The panels show from top to bottom: the temperature in deg. C, the water vapour mixing ratio in gm/kg, the wind speed in m/sec, the wind direction, and the pressure in mb. Note the undular bore-like disturbance that occurs about 0700 Eastern Australian Time (EST). Its passage is marked by sharp jumps in pressure and temperature, a sharp change in wind direction from WNW to SW and a marked freshening of the wind. The temperature rise is a result of the destruction of the shallow nocturnal radiation inversion as the wind freshens. The passage is followed by regular fluctuations in pressure and in wind speed and direction for three quarters of an hour. The passage of the cold front, itself, is indicated by the sharp increase in wind speed and a further backing of the wind about 0815 EST. Following this, the temperature begins to decline steadily, the pressure begins a steady rise and the mixing ratio falls sharply.

A climatological study of cold fronts over central Australia, based on routine data for 1992 and 1993, is currently in progress.
 
 

THE CENTRAL AUSTRALIAN FRONTS EXPERIMENT 1996 (CAFE96)

It is interesting that none of the three CAFE fronts had an obvious surface signature at Alice Springs, but that the initial appearance of a pressure jump occurred in each case within two hundred kilometres to the east of the town. It is not clear whether this is a feature of the local orography, but if so it would have implications for the representatives of data at this station for the analysis of cold fronts.

Unfortunately, the surface network during CAFE was too sparse to enable the formation of a sharp pressure jump in the early evening to be properly documented and it was not adequate to enable the motion of the air mass boundary to be adequately determined. For this reason a third experiment (CAFE96) was organized in 1986 to investigate these features in greater detail. It was carried out in the region between Giles in Western Australia and Burketown in northwestern Queensland from the end of August until early October 1996. It was organized by Monash University, The Australian National University, the University of New South Wales and the University of Munich, Germany, with collaborative support of the Bureau of Meteorology’s Northern Territory Regional Office.

One of the principal findings from CAFE was that the Mount Isa region, the focus for CAFE, is near to the latitude at which non-linear waves are generated and where the fronts are strongly modified by the nocturnal inversion. Consequently, the data there difficult to interpret in relation to the air mass change. For this reason, the focus of CAFE96 was moved southwestward towards Alice Springs.

A primary objective of the CAFE96 experiment was to obtain high resolution surface data on cold fronts as they traverse central Australia in order to document in more detail than has been possible to date the generation of the wave disturbance. A second objective is to better understand the meteorology of central Australia where there are large diurnal effects governing the principal wind-producing systems.

For the experiment, a special network of surface measuring stations was installed in the normally data void region between Yulara and Mount Isa and between Mount Isa and Burketown. These included fourteen automatic weather stations recording wind speed, wind direction, temperature, wet-bulb temperature and pressure. They were interspersed with over thirty high resolution micropressure and temperature recording stations operated by the Australian National University. These recording stations were deployed along a southwest/northeast oriented line between Giles and the lower Cape York Peninsula and also along a southeast/northwest orientated line between Birdsville and the Granites mine in the Tanami Desert.

Numerical simulations

Clarke 1972, 1984

Noonan and Smith 1986, 1988, 1998

Thomsen and Smith 2006, 2008

Theoretical aspects

Christie et al. (1978, 1979), et al. (2007)

Reeder et al. (2013)

References

R. K. Smith, 1988: Waves and bores in the lower atmosphere: the ´morning glory´ and related phenomena. Invited review paper. Earth Sci. Rev., 25, 267-290.

M. J. Reeder and R. K. Smith, 1998: Mesoscale meteorology in the Southern Hemisphere. Chapter 5 of Meteorology of the Southern Hemisphere. Ed. D. J. Karoly and D. Vincent. American Meteorological Society Monograph, No. 49, 201-241.

Subtropical cold fronts (a precursor to at least some southerly morning glories)

Reeder, M. J., R. K. Smith, R. Deslandes, N. J. Tapper and G. A. Mills, 2000: Subtropical fronts observed during the 1996 Central Australian Fronts Experiment. Aust. Meteor. Mag., 49, 181-200.

Smith, R. K., and R. N. Ridley, 1990: Subtropical continental cold fronts. Aust. Met. Mag., 38, 191-200.

Smith, R. K., M. J. Reeder, N. J. Tapper and D. R. Christie,1995: Central Australian cold fronts. Mon. Wea. Rev., 123, 16-38.

Thomsen, G. L., M. J. Reeder, and R. K. Smith, 2009: The diurnal evolution of cold fronts in the Australian subtropics . Quart. J. Roy. Met. Soc., 135, 395-411.

References on the morning glory

Christie, D. R., K. J. Muirhead and A. L. Hales, 1978: On solitary waves in the atmosphere. J. Atmos. Sci., 35, 805-825.

Christie, D. R., K. J. Muirhead and A. L. Hales, 1979: Intrusive density flows in the lower troposphere: a source of atmospheric solitons. J. Geophys. Res., 84, 4959-4970.

Christie, D. R., K. J. Muirhead and R. H. Clarke, 1981: Solitary waves in the lower atmosphere. Nature (in press).

Clarke, R. H., 1965: Horizontal mesoscale vortices in the atmosphere. Aust. Meteor. Mag., 50, 1-25.

Clarke, R. H., 1972: The morning glory: an atmospheric hydraulic jump. J. Appl. Meteor., 11, 304-311.

Clarke, R. H., R. K. Smith and D. G. Reid, 1981: The Morning Glory of the Gulf of Carpentaria: an atmospheric undular bore. Mon. Wea. Rev., 109,1726-1750.

Goler, R. A., and M. J. Reeder. 2004: The generation of the morning glory. J. Atmos. Sci., 61, 1360–1376.

Goler, R., M. J. Reeder, R. K. Smith, H. Richter, S. Arnup, T. Keenan, P. May, and J. Hacker, 2006: Low-level convergence lines over northeastern Australia. I. The North Australian Cloud Line. Mon. Wea. Rev., 134, 3092-3108.

Jackson, G. E., R. K. Smith and T. Spengler, 2002: The prediction of low-level mesoscale convergence lines over northeastern Australia. Aust. Meteor. Mag., 51, 13-23.

Menhofer, A., R. K. Smith, M. J. Reeder and D. R. Christie, 1997: "Morning Glory’ disturbances and the environment in which they propagate. J. Atmos. Sci., 54 , 1712-1725.

Menhofer, A., R. K. Smith, M. J. Reeder and D. R. Christie, 1997: The bore-like character of three morning glories observed during the Central Australian Fronts Experiment. Aust. Met. Mag., 46, 277-285.

Neal, A. B., I. J. Butterworth and K. M. Murphy, 1977a: The morning glory. Weather, 32, 176-183.

Nudelman, I., R. K. Smith, and M. J. Reeder, 2010: A climatology of pressure jumps around the Gulf of Carpentaria. Aust. Meteor. and Ocean. Journl., 60, 91-101.

Noonan, J. A., and R. K. Smith, 1985: Linear and weakly nonlinear internal wave theories applied to ´morning glory´ waves. Geo. Astro. Fluid Dyn., 29, 123-143.

Noonan, J. A., and R. K. Smith, 1986: Sea breeze circulations over Cape York Peninsula and the generation of Gulf of Carpentaria cloud line disturbances. J. Atmos. Sci., 43, 1679-1693.

Noonan, J. A., and R. K. Smith, 1987: The Generation of the North Australian Cloud Line and the ´Morning Glory´. Aust. Met. Mag., 35, 31-45.

Reeder, M. J., D. R. Christie, R. K. Smith and R. Grimshaw, 1995: Nonlinear waves and bores over northern Australia. Bull. Amer. Meteor. Soc., 123, 1165-1171.

Reeder, M. J., R. K. Smith, D. J. Low, J. Taylor, S. J. Arnup, L. Muir, and G. Thomsen. 2013: Diurnal-Forced Convergence Lines in the Australian Tropics Quart. J. Roy Met. Soc. (in early view).

Royal Australian Air Force, 1942: Weather on the Australia Station. R.A.A.F. Publ. No. 252, 2, part 2, 25-26.

Smith, R. K., and J. Goodfield, 1981: The Morning Glory Expedition. Weather, 36, 130-136.

Smith, R. K., N. Crook and G. Roff, 1982: Morning Glory: an extraordinary atmospheric undular bore. Quart. J. Roy. Met. Soc., 198, 937-956.

Smith, R. K., and B. R. Morton, 1984: An observational study of northeasterly ´Morning glory´ wind surges. Aust. Met. Mag., 32, 155-175.

Smith, R. K., and M. A. Page, 1985: ´Morning glory´ wind surges and the Gulf of Carpentaria cloud line of 25-26 October 1984. Aust. Met. Mag., 33, 185-194.

Smith, R. K., M. J. Coughlan and J. Evans-Lopez, 1986: Southerly nocturnal wind surges and bores in northeastern Australia. Mon. Wea. Rev., 114, 1501-1518.

Smith, R. K., and J. A. Noonan, 1998: On the generation of low-level mesoscale convergence lines over northeastern Australia. Mon. Wea. Rev. 126, 167-185.

Smith, R. K., M. J. Reeder,  H. Richter, and P. May, 2006: Low-level convergence lines over northeastern Australia. I. Southerly disturbances. Mon. Wea. Rev., 134, 3109-3124.

Thomsen, G., and R. K. Smith, 2006: Simulations of low-level convergence lines over northeastern Australia. Quart. J. Roy. Met. Soc., 132, 691-707.

Thomsen, G. L., and R. K. Smith, 2008: The importance of the boundary layer parameterization in the prediction of low-level convergence lines. Mon. Wea. Rev. 136, 2173-2185.

Weinzierl, B., R. K. Smith, M. J. Reeder, and G. Jackson, 2007: MesoLAPS predictions of low-level convergence lines over northeastern Australia. Wea. Forecasting, 22, 910-927.

References on related topics

Related theory

Haase, S. P., and R. K. Smith, 1989: The numerical simulation of atmospheric gravity currents. Part I. Neutrally-stable environments. Geo. Astro. Fluid Dyn., 46, 1-33.

Haase, S. P., and R. K. Smith, 1989: The numerical simulation of atmospheric gravity currents. Part II. Environments with stable layers. Geo. Astro. Fluid Dyn., 46, 35-51.

Copyright © Roger Smith, Date 2 June 2014