Morning-Glory Disturbances and the Environment in which They Propagate

1712 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 Morning-Glory Disturbances and the Environment in which They Propagate ANITA MENHOFER Department of Mathematics, University of New South Wales, Sydney, Australia ROGER K. SMITH Meteorological Institute, University of Munich, Munich, Germany MICHAEL J. REEDER* National Center for Atmospheric Research, Boulder, Colorado DOUGLAS R. CHRISTIE Research School of Earth Sciences, Australian National University, Canberra, Australia (Manuscript received 26 June 1996, in final form 2 December 1996) ABSTRACT Results of a field experiment carried out in 1991 to gather upper-air data on the morning-glory environment are presented. The data include daily early morning radiosonde soundings from Burketown in north Queensland, Australia, for a 28-day period during the late dry season, together with pressure, wind, temperature, and humidity data from a number of surface stations in the region. During the experiment, 16 morning glories were recorded. On all but one day, radiosonde soundings were carried out in the pre-morning-glory environment. On 7 days, additional soundings were carried out within an hour or two of the passage of a morning glory. Soundings made on days on which morning glories were generated over Cape York Peninsula but failed to reach Burketown are compared with those on days when morning glories were recorded at Burketown. The comparison shows that the depth and strength of the surface-based inversion did not differ significantly and that the stratification of the almost neutral layer above the stable layer was similar on days with and without morning glories. An examination of the wind profiles is unrevealing and leads the authors to reject the hypothesis that the trapping of wave energy is the key factor that determines the longevity of the disturbances. That the leakiness of the wave-guide is not the only factor in the ability of disturbances to cover large distances from their place of origin is consistent with a numerical study by Noonan and Smith, which suggests that the morning-glory bore-wave system is formed and maintained by mesoscale circulations associated with the sea breezes over Cape York Peninsula. 1. Introduction The Gulf of Carpentaria and Cape York Peninsula region (Fig. 1) of northeastern Australia is distinguished for the regular occurrence there of the morning glory, a spectacular traveling wave-cloud system commonly observed over the southern part of the gulf and adjacent seaboard [see Smith (1988) and Christie (1992) for recent reviews]. The morning glory is accompanied by *Permanent affiliation: Centre for Dynamical Meteorology and Oceanography, Monash University, Victoria, Australia. Corresponding author address: Dr. Anita Menhofer, Department of Mathematics, University of New South Wales, Sydney 2052, Australia. E-mail: anita@alpha.maths.unsw.edu.au sudden wind squalls, intense low-altitude wind shear, and a sharp pressure jump at the surface. The pressure jump is a result of marked vertical displacements of air parcels that are sometimes sufficient to initiate showers or thunderstorms in the wake of the disturbance when the air over the gulf is sufficiently moist. Most morning glories are formed on the western side of the peninsula in the late evening and move subsequently toward the southwest across the gulf. On occasion, these northeasterly morning glories form the southern extension of the north Australian cloud line, a westward moving line of convective cloud that is frequently seen in satellite imagery to extend across the entire gulf (Smith and Page 1985; Drosdowsky and Holland 1987). These phenomena mostly originate from mesoscale circulations associated with sea breezes that develop over the peninsula and adjacent gulf (Clarke et al. 1981; Clarke 1984; Noonan and Smith 1986, 1987, 1997 submitted to J. 1997 American Meteorological Society

1JULY 1997 MENHOFER ET AL. 1713 FIG. 1. Map showing the orography and the data network of CAFE (from Smith et al. 1995). Atmos. Sci). Morning glories that move from the south are observed over the southern gulf region also. It appears that most of these disturbances are associated with frontal systems crossing central Australia (Smith et al. 1986; Smith et al. 1995; Reeder et al. 1995). The first major field experiment to investigate the morning glory was organized in 1979, at which time relatively little was known about the phenomenon (Smith and Goodfield 1981; Clarke et al. 1981). This pioneering experiment was followed by a series of observational studies, the most recent of which was carried out in 1991 as part of the Central Australian Fronts Experiment (CAFE). The principal aim of CAFE was to investigate the structure and dynamics of subtropical cold fronts that affect central Australia, while a related aim was to determine the role of such fronts in the generation of southerly morning glories. A subsidiary experiment was also carried out to obtain upper-air data on the morning glory environment, the results of which are reported here. As well as contributing to the broader aims of CAFE, the subsidiary experiment was designed to investigate the propagation characteristics of the morning-glory waveguide and to test the hypothesis that waveguides that effectively trap wave energy are a necessary prerequisite for the longevity of morning-glory disturbances. Questions concerning the effectiveness of the morning-glory waveguide were raised by Clarke et al. (1981, p. 1739). It was noted that theoretical estimates for the decay rate of weakly nonlinear waves with scales characteristic of morning-glory waves were far too large to

1714 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 account for the observed longevity of morning-glory disturbances. However, these estimates did not consider the vertical wind structure of the morning-glory environment as this was largely unknown, except in the lowest 2 3 km. The question was re-examined by Crook (1986, 1988) through a series of numerical model simulations. He explored the range of mechanisms that could lead to the trapping of wave energy in the lower atmosphere and assessed the efficacy of these in the morning-glory context. The calculations were guided by the meager radiosonde and other sounding data (pilotballoon winds and aircraft temperature measurements) that were available at the time. Prior to the CAFE experiment, there were few data available on the tropospheric wind structure of the premorning-glory environment above 3 km and therefore knowledge of the waveguide structure was incomplete. As a result, the reasons for the longevity of morning glories could not be properly addressed. The present paper presents the results of the subsidiary experiment and explores, inter alia, the factors that contribute to the longevity of morning-glory waves. 2. The data The subsidiary experiment ran for 30 days from 7 September to 6 October 1991. From 9 September onwards, daily radiosonde soundings were carried out at Burketown in north Queensland using a Vaisala Marwinsonde system. With two exceptions, the radiosondes were released around 0430 EST (Australian Eastern Standard Time UTC 10 h) in order to document the vertical structure of the pre-morning-glory environment throughout the troposphere and through part of the lower stratosphere. The two exceptions are 23 and 24 September when the only sonde was released after the passage of the morning glory. On some occasions a second radiosonde was released within an hour or two of a morning-glory passage. Data were obtained also from a number of specially installed surface stations in the region. Digitally recorded 2-min average measurements of pressure, wind, temperature, and humidity at a height of 3 m above the ground were taken at Burketown aerodrome and at Delta Downs Station. Microbarograph measurements were taken at Stirling Station, Maggieville Station, Escott Station (about 20 km west of Burketown), on Mornington Island, and on Sweers Island. (The locations of the various stations are shown in Fig. 1.) Table 1 lists all the morning glories that were observed during the field experiment, a total of 25 during the 30-day period. Of these, 23 formed over the peninsula and two originated to the south of the gulf. Only 14 of those that formed over the peninsula were recorded at Burketown, although both southerly disturbances were recorded there. The disturbances were identified on the peninsula by their accompanying surface pressure jumps recorded at Stirling Station. The southerly disturbances have been described in detail in papers by TABLE 1. Arrival times of morning glories determined from the surface station records at Stirling Station and/or at Burketown aerodrome. The microbarograph station at Stirling Station was not operating before 14 September. The amplitude of the initial pressure jump at the surface is given also for Stirling Station and Burketown, respectively. Detection of pressure jumps at Burketown are marked by b, whereas pressure jumps at Stirling Station that did not reach Burketown are marked by s. CAFE events refer to observed cold front during CAFE (see Smith et al. 1995). On 3 October, the northeasterly and the southerly morning glories (m.g.) interacted over the gulf. This event is discussed in detail by Reeder et al. (1995). Dry refers to disturbances that were unaccompanied by a low roll cloud. Date Time EST Amplitude hpa Location Remarks 07.09.91 0633 0.9 b northeasterly m.g. 08.09.91 0726 1.1 b northeasterly m.g. 09.09.91 0820 0.5 b northeasterly m.g. 10.09.91 0956 0.5 b northeasterly m.g. CAFE event 1 14.09.91 0340 1.2 s northeasterly m.g. 15.09.91 0228/0926 0.6/0.7 b northeasterly m.g. 16.09.91 0307 0.4 s northeasterly m.g. 17.09.91 0312 0638 0.5 1.4 s b northeasterly m.g. southerly m.g. associated with CAFE event 2 19.09.91 0339 1.1 s northeasterly m.g. 22.09.91 0323 0.6 s northeasterly m.g. 23.09.91 0024/0526 0.9/1.3 b northeasterly m.g. dry 24.09.91 2340/0448 1.5/1.5 b northeasterly m.g. dry 25.09.91 2303/0450 0.8/1.2 b northeasterly m.g. dry 26.09.91 0022/0636 0.5/0.8 b northeasterly m.g. dry 27.09.91 0301 1.0 s northeasterly m.g. 28.09.91 0509 0.9 s northeasterly m.g. 30.09.91 0156 1.0 s northeasterly m.g. 01.10.91 0132/0636 1.0/0.9 b northeasterly m.g. dry 02.10.91 0109/0656 1.0/1.1 b northeasterly m.g. CAFE event 3 03.10.91 0204/0745 0455 0.9/0.5 1.1 b b northeasterly m.g. southerly m.g. 04.10.91 0123/0550 1.2/1.0 b northeasterly m.g. dry 05.10.91 0118 1.0 s northeasterly m.g. 06.10.91 0151/0743 1.0/1.0 b northeasterly m.g. Smith et al. (1995) and Reeder et al. (1995). In the present paper, interest is focused primarily on the northeasterly disturbances. The histograms in Fig. 2 show the range of times that morning-glory disturbances were detected at Stirling Station and Burketown. Northeasterly disturbances reached Stirling Station during a 6½-h period between 2300 EST and 0530 EST, the mean being around 0140 EST. At Burketown, the range was between 0430 EST and 1000 EST and the mean was around 0710 EST, including the southerly disturbances. 3. The morning-glory environment a. Basic structure Figure 3 shows time height cross sections of the potential temperature, mixing ratio, and zonal and meridional wind components at Burketown. No sondes were

1JULY 1997 MENHOFER ET AL. 1715 FIG. 2. Histogram showing the times of passage of morning glories at (a) Burketown and (b) Stirling Station. The ordinate marks the number of events occurring within half an hour of the time given on the abscissa. The dark columns in (a) represent the two southerly morning glories. released on 7 and 8 September. The time height cross sections are constructed using the first sounding of each day, which usually commenced at 0430 EST. Hence, the cross sections document changes in the state of the premorning-glory atmosphere only and not the complete evolution during the whole period. The dots below the abscissa mark days on which morning glories were observed at Burketown, while d and s denote dry and southerly disturbances, respectively. The time height cross section of the potential temperature is shown in Fig. 3a. A shallow surface-based stable layer was present on all days in the early morning. The stability and depth of the layer depend on the depth of the sea-breeze flow on the previous day, on the distance that this flow penetrates inland, and on the degree of radiative cooling. Frequently, above this stable layer, a deep well-mixed layer extended to a height of up to 4 km. The well-mixed layer, in which the static stability is small and often close to zero, is topped by a shallow, but strong inversion. This nearly neutral layer is a result of convective turbulent mixing caused by solar heating over the land on the previous day. The tropopause was located at about 16 km and showed little variation during the period. The static stability of the nearly neutral layer shows significant differences from day to day: the first 13 days were characterized by a stronger stability compared with the stability in this layer from 22 September onwards. Both periods include notable exceptions: on 17 September the atmosphere above the surface inversion was closer to the neutral state than on the adjacent days, while from 27 to 30 September the stability was stronger than on adjacent days. Days with a larger stability in the upper layer correspond largely to days on which no morning glories passed over Burketown. The time height cross section of water-vapor mixing ratio is shown in Fig. 3b. Periods of northerly flow are shaded. The mixing ratio below about 3 km was usually higher on days with morning glories. This is consistent with the fact that conditions conducive to the formation of a convergence line over the peninsula are favorable also for the inland penetration of the sea breeze air from the gulf as far south, at least, as Burketown. As can be seen from the diagram, days on which the mixing ratio is high are usually characterized by northerly winds either on the same day or on previous days. Only the increase in mixing ratio on the third day of the experiment is at odds with this observation. On this day, the winds were from the south or southeast above 2 km. On six occasions, dry morning glories, that is, disturbances not accompanied by a roll cloud, were recorded at Burketown (see Table 1). As expected, the near-surface mixing ratio on these days was lower than on other days on which morning glories were observed. The upper winds at Burketown are summarized in the time height isotach cross sections of the zonal and meridional wind components shown in Figs. 3c and 3d. The low-level zonal component was easterly on 11 out of the 15 non-morning-glory days. Low-level easterlies dominated three periods: from ll to 14, from 18 to 22, and from 28 to 30 September. No morning glories were detected at Burketown during these three periods. In contrast, northeasterly morning-glory days were normally characterized by a low-level westerly component of the flow. The exception is the morning glory on 9 September, while the radiosondes on 23 and 24 September were released too late to represent the premorning-glory environment. Thus, northeasterly morning glories generally propagate into an opposing flow. Morning-glory days are not distinguished unambiguously from non-morning-glory days by the southerly component at low levels. On 17 September, the day on which a southerly disturbance propagated from 205, the environment was characterized by weak westerly and relatively strong northerly components (see Fig. 3d). The morning-glory passages were concentrated in two extended periods, the first from 23 to 26 September and the second from 2 to 4 October. Both periods were characterized by deep easterlies throughout most of the nearly neutral layer. Interestingly, a similar feature was found in a recent numerical study of mesoscale circulations in the Gulf region by Noonan and Smith (1997, submitted to J. Atmos. Sci.; hereafter referred to as NS). Above 4 km, westerly winds prevailed over the whole

1716 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 FIG. 3. (a) Contours of potential temperature at Burketown up to a height of 18 km from 9 September to 6 October 1991. The time series is based on the first sounding on each day, usually taken around 0430 EST. The dots along the abscissa mark those days on which morning glories reached Burketown. The d and s mark dry and southerly morning glories, respectively. (b) Contours of mixing ratio at Burketown up to a height of 4 km from 9 September to 6 October. The shaded regions indicate a northerly flow. (c) Contours of the westerly wind component and (d) the southerly wind component up to a height of 18 km. The contour interval is 5 m s 1. The shaded regions indicate negative values, that is, easterly and northerly wind components, respectively. period. The increase in the westerly winds with height was particularly strong between 4 and 6 km from 27 September to 5 October, at which time the mixed-layer capping inversions were stronger than usual also (Fig. 3a). Between 5 and 6 October, the winds above 6 km turned sharply from the northwest to the southeast. A southerly wind prevailed throughout the troposphere from 9 to 14 September also.

1JULY 1997 MENHOFER ET AL. 1717 FIG. 3.(Continued) b. Waveguide characteristics Using data taken during the subsidiary experiment to CAFE, Menhofer et al. (1997) added additional support to the idea that morning glories are internal undular bores propagating on the waveguide provided by the surface-based stable layer and overlying nearly neutral layer. Theoretical considerations suggest that disturbances of this kind can be regarded as a series of amplitude-ordered solitary waves connecting each side of the pressure jump (Christie 1989). The jump itself appears to be associated with a mesoscale convergence line. If the waveguide is leaky, the individual waves radiate energy vertically and their amplitudes at low levels attenuate with time. In the absence of suitable curvature in the vertical profile of the wind normal to the direction of wave propagation, any degree of stratification aloft will allow such upward propagation of

1718 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 wave energy. Although the waves are intrinsically nonlinear, some insight into their propagation characteristics can be obtained using linear theory, which, in the morning-glory context, has been reviewed by Crook (1988). Consider a linear two-dimensional sinusoidal wave propagating in a stably stratified atmosphere. A wave with a horizontal wavelength 2 /k and a vertical wavelength 2 /m has a vertical velocity structure w(z) that is governed by the equation d 2 w/dz 2 m 2 w 0, (1) where m 2 N 2 /(U c) 2 U zz /(U c) k 2 l 2 k 2, (2) N is the Brunt-Väisälä frequency, U(z) is the component of the ambient flow normal to the waves, and c is their horizontal phase speed. The parameter l 2 (z) is called the Scorer parameter. Upward propagation of the wave is restricted when m 2 is negative, that is, when l 2 k 2 (Scorer 1949). A trapped wave requires the dual condition that m 2 0 at low levels, consistent with a propagating wave, and m 2 0 at higher levels in order to suppress the vertical radiation of energy. Ignoring the curvature term in Eq. (2) for the moment, a negative m 2 is achieved for all wavelengths by a neutral stratification (N 0). Thus, a deep nearly neutral layer above a stably stratified surface layer greatly inhibits the vertical radiation of wave energy for all except possibly the longest wavelength components of a disturbance (i.e., those with k N/U ). c. Role of static stability in forming the waveguide We examine now the mean structure of the atmosphere based on the early morning soundings taken at Burketown. Two means are calculated: one for days on which morning glories were observed at Burketown and the other for days on which they were not. The mean virtual potential temperature below 5 km is plotted in Fig. 4a. The solid (dashed) line represents the mean virtual potential temperature calculated for those days on which morning glories were (were not) detected. The dotted line represents the standard deviation from the mean for all morning-glory days. Above 5 km, both profiles are almost identical. Likewise, below about 300 m, both profiles exhibit a very similar strong nocturnal inversion. Between these two heights, however, there exist notable differences. Above the stable layer, the mean profile for morning-glory days is more nearly neutral (d /dz 0) than the mean profile for non-morningglory days. Note that the profile for non-morning-glory days lies well outside the standard deviation of the profile for morning-glory days. In contrast to days without disturbances, the nearly neutral layer on morning-glory days is capped above by an inversion. Nevertheless, the nearly neutral layer on individual non-morning-glory days is generally capped by a weak inversion. The inversions are usually significantly stronger on individual days than in the mean profiles but become smoothed in the mean profile because the position of the inversion varies between 3 and 5 km. The stronger static stability below 5 km observed on non-morning-glory days should allow wave energy to radiate upward more easily than on morning-glory days. Nevertheless, to the extent that the curvature term in the Scorer parameter can be neglected, wave energy can be radiated upwards in both environments. Although the solid line in Fig. 4a is closer to a virtual adiabat than the dashed line, the weak static stability would still enable wave components with a horizontal wavelength of about 10 km to propagate vertically through the upper layer. We examine now whether the difference in the mean thermodynamic structure between morning-glory days and non-morning-glory days is significant. In order to make this assessment, we subdivided the data into two periods. The first period runs from 9 September to 17 September and the second from 24 September to 6 October. This subdivision is chosen because Fig. 3a indicates that the static stability above the surface-based stable layer is stronger during the first period. The soundings of 23 and 24 September are not included because they were not carried out in the pre-morningglory environment. The mean profiles of virtual potential temperature for the morning-glory days in each period are shown in Fig. 4b. It is clear that the mean profile for all morning-glory days (solid line in Fig. 4a) is dominated by the second period (solid line in Fig. 4b), whereas the profile for the first period is much closer to that of non-morning-glory days (dashed line in Fig. 4a). Remarkably, the mean profile for all morning-glory days (as in Fig. 4a) is also similar to the mean profile for the five non-morning-glory days from the second period (Fig. 4c). Thus, to our surprise, the stability of the deep nearly neutral layer seems to be less important for the occurrence of morning glories than has hitherto been supposed. Table 1 shows that, from 14 September onwards, when the microbarograph station at Stirling Station started to operate, ten northeasterly morning glories were observed at Burketown and that nine morning glories observed at Stirling Station failed to reach Burketown. Seven out of the nine disturbances arrived at Stirling Station after 0300 EST (the exceptions being the disturbances on 30 September and on 5 October). Eight out of the ten that reached Burketown passed Stirling Station before 0200 EST (the exceptions being those on 15 September and on 3 October). Thus, it appears that the time at which a northeasterly morning glory arrives at Stirling Station is a factor in determining whether or not a disturbance reaches Burketown. Those northeasterly morning glories that arrive late at Stirling Station are unable to reach Burketown before the waveguide is destroyed by daytime convective mixing. Morning glories that arrive at Stirling Station between 0100 EST and 0300 EST may or may not reach Burketown, de-

1JULY 1997 MENHOFER ET AL. 1719 FIG. 4. (a) Average v (z) for the 11 days on which morning glories passed Burketown (solid line) and for the 15 days without morning glories (dashed line). Soundings on 23 and 24 September are not included since they do not represent the pre-morning-glory state of the environment. The dotted line represents the standard deviation from the mean of morning-glory days. (b) Average v (z) for the periods from 9 September to 17 September (dashed line) and from 25 September to 6 October (solid line). (c) As in (a), but the dashed line represents the 5 non-morning-glory days in the period from 24 September to 6 October. (d) The value v (z) on 30 September (solid line) and on 1 October (dashed line).

1720 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 FIG. 5. Average mixing ratio below 5 km for morning-glory days (solid line) and non-morning-glory days (dashed line). pending on the wave speed and ambient waveguide conditions. The idea that the longevity of morning glories cannot be related to the vertical distribution of the static stability alone is supported by a comparison of the virtual potential temperature profiles on two consecutive days (Fig. 4d). The virtual potential temperature profiles on 30 September and 1 October broadly resemble each other, each possessing a deep nearly neutral layer between surface and upper-layer inversion. On the first of the two days, a disturbance was detected at Stirling Station but decayed north of Burketown. The amplitude of the pressure jump at Stirling Station was 1.0 hpa, and a weak pressure jump (0.25 hpa) was recorded also at Maggieville, but not farther to the southwest. The next day, 1 October, the amplitude of the disturbance was larger (1.0 hpa at Stirling Station, 0.8 hpa at Maggieville, and 0.9 hpa at Burketown) and it reached Burketown. As the profiles are similar on each day, some other mechanism must have prevented the morning glory on 30 September from traveling farther southwest than Maggieville. In the next section we consider the importance of the vertical wind profile in trapping the morning-glory disturbances. Figure 5 compares the mean mixing ratio on morningglory days with that on non-morning-glory days. As shown previously in the time height cross section of mixing ratio (Fig. 3b), the pre-morning-glory environment is distinctly moister at low levels than on non- FIG. 6. (a) As in Fig. 5 but for the westerly wind components below 5 km and (b) the corresponding southerly wind component.

1JULY 1997 MENHOFER ET AL. 1721 morning-glory days. The corresponding mean westerly and southerly wind components (Fig. 6), however, do not provide an explanation for the difference between the mixing ratio profiles. Within the surface-based stable layer, the mean flow was westerly on morning-glory days. Above the stable layer, the mean flow was southeasterly below 4 km and southwesterly above that. However, westerlies or southeasterlies will generally advect dry continental air toward Burketown. On the other hand, the mean flow on non-morning-glory days has a strong northerly component that will transport relatively moist air from the Gulf of Carpentaria over Burketown. Thus it appears that the flow pattern on the preceding night is required to determine the origin of the air mass at 0430 EST. Presumably, the difference between the mean mixing ratio profiles can be attributed to the seabreeze circulation that is usually present on the day before the arrival of a northeasterly morning glory at Burketown. d. Role of vertical shear in forming the waveguide We explore now the role of the vertical wind structure on the strength of the morning-glory waveguide. According to Eq. (2), the Scorer parameter decreases with height if (i) the component of the environmental wind in the direction of the phase propagation U(z) increases with height, and (ii) if the curvature term changes sign from negative in the surface layer to positive aloft. In particular, provided that U c 0 everywhere, a lowlevel jet opposing the propagation of the disturbance satisfies the second condition since the reverse flow above gives a positive curvature term there. Crook (1986, 1988) demonstrated the trapping effect of the low-level westerly jet in the pre-morning-glory environment of northeasterly morning glories in numerical simulations. The propagation speed and direction of all disturbances identified by surface pressure jumps at a minimum of three stations are calculated using the arrival times of the disturbances and the positions of the respective stations. Table 2 lists the values of the propagation speed and direction, and the maximum negative values of U c, together with the height at which the maximum occurs, for all observed disturbances. The wind component U normal to the wave front is less than the propagation speed c at all heights; that is, U c 0 everywhere. Therefore none of the waves have critical levels in the pre-morning-glory environments to help to trap them. On almost all morning-glory days, an opposing lowlevel jet was present with a reverse flow aloft. The latter usually extended up to the height of the mixed-layer capping inversion where the flow component changed sign again, a result of prevailing northwesterly winds in the upper troposphere. A profile of the pre-morningglory flow similar to the one Crook (1988) used in his TABLE 2. Propagation speed and direction of morning glories determined from the surface station records. The last two columns present the maximum negative values of U c in the lowest 9 km along with the height at which they occur. Values on 17 September refer to the southerly morning glory, whereas values on 3 October are for the northeasterly event. Date c (m s 1 ) (deg) Maximum U c Height (m) 10.09.91 10.4 56.0 21.7 7100 14.09.91 9.4 48.5 26.9 8100 15.09.91 8.7 56.8 20.1 9000 17.09.91 15.3 205.0 23.8 300 Southerly 23.09.91 12.6 58.3 33.0 8700 24.09.91 12.1 57.6 29.8 8500 25.09.91 11.5 63.2 35.8 8900 26.09.91 11.0 80.1 36.0 7800 27.09.91 6.8 44.9 22.2 6600 28.09.91 11.8 59.5 34.3 6800 30.09.91 5.2 48.7 23.3 6900 01.10.91 12.8 69.2 29.8 9000 02.10.91 10.9 63.4 29.2 5700 03.10.91 11.3 67.0 26.8 6600 Northeasterly 04.10.91 9.0 64.8 25.9 9000 05.10.91 12.2 61.0 27.4 9000 06.10.91 11.3 73.7 14.9 7300 model simulation was found for the southerly morning glory on 17 September and is shown in Fig. 7a. The square of the Scorer parameter for this day is displayed in Fig. 7b. As the second derivative of U is sensitive to observational noise, the wind profiles used to calculate the Scorer parameter have been smoothed. Heights at which l 2 k 2 support the vertical transmission of wave energy, while wave components with l 2 k 2 are evanescent. The square of the Scorer parameter is large and positive up to about 500 m, relatively small and positive between 500 and 1140 m, and negative in the layer between about 1140 and 1780 m. The large positive value in the stable layer near the surface is associated with both the strong stratification and the large curvature of the wind profile there, the latter giving the larger contribution. The curvature term is important again above the stable layer and is responsible for the large negative values of l 2. In such layers, the vertical propagation of all wave components is suppressed. Hence, the upward propagation of wave energy for the southerly morning glory on 17 September would have been attenuated in the layer between 1.1 and 1.8 km. The loss of wave energy from the waveguide layer would have been inhibited also by the region with negative l 2, which lies between 2.3 and 3.2 km. On all but two occasions, 10 and 15 September, a low-level jet was present in the pre-morning-glory flow, although it was not as pronounced as the example in Fig. 7a. The pre-morning-glory zonal flow and the Scorer parameter for 10 September are shown in Figs. 7c and 7d. Despite the missing low-level westerly component on these two days, l 2 is negative above the stable layer because the wind there is characterized by a similar

1722 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 FIG. 7. (a) The wind component normal to the disturbance front, U(z), in the pre-morning-glory (solid line) environment on 17 September 1991. The morning glory approached Burketown from 205 with a propagation speed, c, of 15.3 m s 1. Positive values mean that U is in the same direction as c. (b) The square of the Scorer parameter for 17 September calculated from the data of the first sounding. (c) As in (a) but for 10 September 1991. The morning glory approached Burketown from 56 with a speed of 10.4 m s 1. (d) As in (b) but for 10 September 1991.

1JULY 1997 MENHOFER ET AL. 1723 curvature to that in the upper layer on 17 September. Hence, a jet in the direction of the disturbance motion above the surface layer results in a decrease with height of the Scorer parameter. The critical wavelength is defined to be the wavelength for which m 2 0. According to Eq. (2), wave components with wavelengths less than the critical wavelength at a particular height are evanescent at that height. Conversely, wave components with wavelengths larger than the critical wavelength radiate energy upwards. A summary of the critical wavelength 2 /l(z) as a function of height for the 16 days on which morning glories were detected over the peninsula is given in Fig. 8. Where the square of the Scorer parameter is equal to zero or negative, the critical wavelength is infinite or purely imaginary and all wavelengths are evanescent. In these regions, the critical wavelength has been set arbitrarily to a value larger than the largest critical wavelength (33 km) calculated when l 2 0. Thus, values of the critical wavelength larger than 40 km contoured in Fig. 8 indicate regions in which all wave components are evanescent. The dots along the abscissa in Fig. 8 mark days on which morning glories were observed at Burketown. Significantly, Fig. 8 does not show any appreciable difference in the critical wavelength between days on which disturbances did or did not reach Burketown. Moreover, the critical wavelength is often smaller than the characteristic wavelength of morning-glory waves, which is between 10 and 20 km. For example, deep layers of maximum value of the critical wavelength were present on day 10 (corresponding to 28 September). On day 3 (corresponding to 15 September), only a thin layer in which the square of the Scorer parameter is negative (i.e., the critical wavelength is imaginary and set to a value 40 km) above 1 km would prevent disturbances with wavelengths larger than 10 km radiating energy vertically. Comparison once again between the structure on 30 September and 1 October, corresponding to days 11 and 12 in Fig. 8, show similar layers with maximum values around a height of 2 km and between 3 and 4 km. The same is true of other consecutive days such as days 14 and 15 (corresponding to 4 and 5 October). Thus, the investigation of the Scorer parameter neither explains the longevity of waves on some days nor their early decay on others. e. Mesoscale forcing Numerical studies (e.g., Noonan and Smith 1987; NS) indicate that the convergence line that initiates the morning-glory disturbance and maintains its borelike structure is formed and maintained itself by the energetic mesoscale circulations associated with the sea breezes over the Cape York Peninsula. In the absence of dissipation, long borelike structures will evolve into an ever-increasing number of solitary wave components that propagate away from the parent disturbance as independent entities. This behavior is modified, however, when energy loss occurs through frictional dissipation or longwave propagation into the upper atmosphere. In this case, the disturbance evolves into a quasi-stationary state that slowly decays with time (e.g., Christie 1989). The numerical studies presented in NS suggest that energy losses in northeasterly morning-glory wave disturbances may be offset by energy gains associated with the evolving mesoscale patterns generated by sea-breeze circulations. This question cannot be addressed with certainty by the numerical calculations to date; for one thing the models have a resolution (20 km) that is too coarse to represent the waves. Moreover, such models are hydrostatic and consequently do not allow for the formation of solitary waves in deep fluids, even though they would be sufficient to allow for bore formation. Another issue is raised by the calculations of NS; these show that, with a moderate (8 m s 1 ) easterly or northeasterly basic flow, a morning-glory convergence line develops over the peninsula each evening and later reaches Burketown. However, for southeasterly flow, convergence lines form on the peninsula but do not extend as far south as Burketown. Thus, the environmental wind direction seems to be a key factor in determining whether a disturbance will occur over the southern gulf coast. In other words, the properties of the waveguide are not the only factor in determining whether a morning glory reaches Burketown. Another factor appears to be the time the disturbance is initiated; those that form late over the peninsula are less likely to reach Burketown before the destruction of the stable layer on which they propagate by renewed convective mixing after sunrise. 4. Summary and conclusions The environment in which morning-glory disturbances propagate has been investigated using a unique dataset obtained during a subsidiary experiment to CAFE in 1991. The quality of the waveguide and its properties were studied to determine whether or not dayto-day variations in its structure could explain why some morning glories were long-lived while others were not. A surface-based nocturnal inversion developed on 27 out of 28 days. The observations showed that the depth of the stable layer and the strength of the inversion did not differ significantly between morning-glory and nonmorning-glory days. It was found that the stratification of the deep almost neutral layer above was similar also on days with and without morning glories. Hence, the neutral layer was not the key factor in the longevity of some morning glories. The typical mixed-layer capping inversion was very weak or nonexistent on 8 days. On only one of these 8 days was a morning glory detected at Burketown. This result suggests that the reflection of wave energy from the lower boundary of the upper-level inversion may be important. The beneficial influence of an upper-level inversion on the stability of nonlinear waves in a sur-

1724 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 FIG. 8. The critical wavelength on the 16 days with morning-glory passages in the southern part of Cape York Peninsula. Where the square of the Scorer parameter is negative, the critical wavelength is set to a value 40 km. Days 1 to 16 correspond to 10, 14, 15, 17, 23 28, 30 September and 1 6 October, respectively. The region of large values below 500 m on 24 September is due to the release of the radiosonde during the passage of the morning glory. The dots along the abscissa mark morning-glory days at Burketown. The contour interval is 10 km. face-based waveguide layer has been demonstrated in a numerical model by Crook (1988). The reflected part of the energy depends on both the height of the inversion and on the vertical wavelength of the disturbance. Since the morning-glory waves are not periodic and some of the wave components may be reflected while others are not, it would be necessary to determine the fraction of energy in each wave. This cannot be easily done with the present dataset and is beyond the scope of this work. The part played by the curvature of the environmental wind in trapping the waves was investigated by calculating the Scorer parameter. A region of negative Scorer parameter that is necessary, but not sufficient, to trap the waves was found on all days on which a morning glory was observed. The Scorer parameter was shown to be much more sensitive to the curvature of the environmental wind than to the static stability above the stable layer, a finding consistent with that of Noonan and Smith (1986) and Crook (1988). The negative curvature above the surface inversion was usually associated with 1) a low-level jet that opposed the propagation of the disturbance within the surface layer, 2) a reverse flow above the jet, and 3) opposing winds at higher levels above the trade-wind inversion. This configuration is similar to that used by Crook (1988) in his numerical modeling study, from which he concluded that an opposing low-level jet represents an effective trapping mechanism for morning-glory disturbances. Two cases examined, however, were without opposing low-level jets, although there were several layers in which the Scorer parameter was negative. The negative sign was attributed to the negative curvature of the wind profile in these layers. Hence, waves can be trapped at low levels by changes in the vertical shear without a low-level jet. However, the degree to which

1JULY 1997 MENHOFER ET AL. 1725 wave energy is trapped depends on the thickness of the layer in which the Scorer parameter is negative. The observations showed that there were no significant differences in the thickness of these layers between morning-glory and non-morning-glory days. Thus, the analysis of the data, together with the results of the numerical study of NS, lead us to reject the idea that the trapping of wave energy is the only factor which determines the longevity of morning glories. Instead, the bore-wave system appears to be formed and maintained by the energetic mesoscale circulations associated with the sea breezes over the Cape York Peninsula. Moreover, the environmental wind direction appears to determine whether a disturbance will occur over the southern gulf coast: a moderate easterly or northeasterly environmental flow supports the development of a convergence line over the peninsula that later reaches Burketown, whereas convergence lines do not extend as far south as Burketown for a southeasterly flow. Another factor in the longevity of morning-glory waves is the time of the formation of the waves over the Cape York Peninsula, that is, whether they reach Burketown before the destruction of the stable layer on which they propagate by convective mixing during the day time. Acknowledgments. We are grateful to the very many people who helped to make the subsidiary experiment to CAFE a success. Special thanks go to the field participants: Morwenna Griffiths from Monash University, Norbert Beier and Heinz Loesslein from the University of Munich, and David Brown from the Australian National University. We thank also the Australian Research Council, the German Research Council, and the Australian Bureau of Meteorology for financial support, and Qantas Airlines for their generous assistance in transporting instruments from Germany to Australia. REFERENCES Christie, D. R., 1989: Long nonlinear waves in the lower atmosphere. J. Atmos. Sci., 46, 1462 1491., 1992: The morning glory of the Gulf of Carpentaria: A paradigm for non-linear waves in the lower atmosphere. Aust. Meteor. Mag., 41, 21 60. Clarke, R. H., 1984: Colliding sea-breezes and the creation of internal atmospheric bore waves: Two-dimensional numerical studies. Aust. Meteor. Mag., 32, 207 226., 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. Crook, N. A., 1986: The effect of ambient stratification and moisture on the motion of atmospheric undular bores. J. Atmos. Sci., 43, 171 181., 1988: Trapping of low-level internal gravity waves. J. Atmos. Sci., 45, 1533 1541. Drosdowsky, W., and G. J. Holland, 1987: North Australian cloud lines. Mon. Wea. Rev., 115, 2645 2659. Menhofer, A., R. K. Smith, M. J. Reeder, and D. R. Christie, 1997: On the structure of morning glories. Aust. Meteor. Mag., in press. 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.,and, 1987: The generation of North Australian cloud lines and the morning glory. Aust. Meteor. Mag., 35, 31 45. Reeder, M. J., D. R. Christie, R. K. Smith, and R. Grimshaw, 1995: Interacting morning glories over northern Australia. Bull. Amer. Meteor. Soc., 76, 1165 1171. Scorer, R. S., 1949: Theory of waves in the lee of mountains. Quart. J. Roy. Meteor. Soc., 75, 41 56. Smith, R. K., 1988: Travelling waves and bores in the lower atmosphere: The morning glory and related phenomena. Earth Sci. Rev., 25, 267 290., and J. Goodfield, 1981: The 1979 morning glory expedition. Weather, 36, 130 136., and M. A. Page, 1985: Morning glory wind surges and the Gulf of Carpentaria cloud line of 25 26 October 1984. Aust. Meteor. Mag., 33, 185 194., M. J. Coughlan, and J. L. Lopez, 1986: Southerly nocturnal wind surges and bores in northeastern Australia. Mon. Wea. Rev., 114, 1501 1518., M. J. Reeder, N. J. Tapper, and D. R. Christie, 1995: Central Australian cold fronts. Mon. Wea. Rev., 123, 16 37.