Southbound/northbound traverses on 5 July 1979
Collected here are views from 5 and 7 July 1979 of the depression
observed during the Summer Monsoon Experiment. Published observations include those by
Warner (1984), Warner and Grumm (1984), Michael W. Douglas (Mon. Wea. Rev.,
Aug 1992, 120, 1524-64) and Houze and Churchill (JAS, 15 July 1987, 44, 1845-67).
Warner references are given fully in my curriculum vitae (back to main page).
We have here
Southbound/northbound traverses on 5 July 1979
Southeast of the center on 7 July 1979
Detailed observations on GATE day 261
The Winter Monsoon Experiment, 12 December 1978
Concluding discussion
1009 local time. 92.2E, 17.7N, 483 hPa: Photos shown at B in the 1026
radar map below.
The clouds grew into the mapped location between 1009-26. (back)
1631 local time. On 260
On 080
(back)
Behind the cloud line an absence of strong vertical motions is suggested.
1603 local time. On 270
On 100 (back)
Close to the position of the cloud line, these two views show cumulus of size up to congestus;
there was no stratus seen below flight level.
1601 local time. Radar echoes from the W3PD, as above.
Three short lines identify intense updrafts; the middle one reached 15.7 m/s.
Compare this map with the radar map for 1026 local time, above. (back)
1553 local time. Photo looking N from location D shown above. Small cumulus clouds are shown, symptomatic of unstable stratification
near the surface,
below a forward overhang of dense high overcast. (back)
1549 local time. On 225 (SW)
On 135 (SE) (back)
These views look into the environment into which the cloud line was advancing. There were fluxes off the surface, shown by high
densities of small cumulus. Those seen to the SE, on 135, were slender, symptomatic of unstable stratification and an absence of subsidence.
An absence of marked vertical motions through most of the troposphere is suggested.
Two days after flying southward through the cloud lines of 5 July, the depression was examined at its mature stage on 7 July. Extensive treatment of this day was given by Warner (1984) and Warner and Grumm (1984, WG84). A line of clouds propagating from 255 degrees at 24 m/s was documented. This is shown below.
Three wind barbs have been added to the tephigrams, intended to be represent the winds driving the cloud line. The photo indicates that there was a maximum of wind speed at cloud base, and the speed was probably close to the measured speed of propagation, 24 m/s. The three winds shown are consistent with the measured winds presented by WG84; here it is suggested that a maximum occurred at the leading edge of the cloud line just above the surface.
On 5 July the advancing cloud line was evidently like those documented by Zuidema (2003) and Houze (2004). Through the time of Johnson and Houze (1987, Monsoon Meteorology, Chapter 10, Chang and Krishnamurti, Eds., OUP), until quite recently, we have not realized that this cloud line was closely related to the convection treated by B. E. Mapes, T. T. Warner, Mei Xu and A. J. Negri in the trio of papers published in Mon. Wea. Rev., May 2003, 131, 799-844.
On 7 July the line examined occurred in a location quite similar to the lines of 5 July, but the propagation was eastwards in the strong westerly winds to the southeast of the center. The data implied the occurrence of ducting related to the low-level stable layer seen in drop 11 of 7 July, capping instability close to the surface. (back)
We perceive pulses of convective heating. Detailed GATE observations were described by Warner et al. (1979). The mesoscale arc shown below was one of several moving roughly northwards to northeastwards in convergent low-level flow entering the area of a cluster of cumulonimbus. These formations were observed in visible satellite imagery. Each could be followed for about an hour. The view shown below is from Warner (1978), where details are given from photogrammetry.
Following Warner et al. (1979), an appropriate sounding is shown below:
Behind the arc the air near the surface was clear. This suggests subsidence compensating the strong ascent in the line of cumulus. The extent of the subsidence was several tens of kilometers. Warner et al (1979) found that there was no gust front from cold air near the surface. (back)
One further case will be included here. Warner (1982) described mesoscale features and cloud organization on 10-12 Dec 1978 over the South China Sea during the Winter Monsoon Experiment. [This paper is incorrect in that the data on cloud streets are not inconsistent with inflection point instability (Brown 1970).] The following photograph shows an arc of clouds in the northeast monsoon flow towards the equatorial trough zone at latitude 9.4N.
It was one of many such arcs which populated this flow. The leading edge of this arc - facing away from the camera - is not clearly seen here, so a second photo is shown, which shows a cross-wind line close to or identical with the distant line partly obscured by the wing of the aircraft in the first picture.
Data from a local dropwindsonde are shown in the following tephigram launched at 1651 local time, 8.8N, 114.4E.
On synoptic scale the low-level winds did not in general show gradual changes downstream. Low-level data from the Electra research aircraft on this day showed the presence of isolated speed maxima rather than gradual acceleration. The speed maxima were local. A local speed maximum at cloud base would accompany a horizontal vortex line with an axis somewhere above cloud base. Such a feature is compatible with the photographic evidence.
Convection gradually deepens as winter monsoon flow approaches the equatorial trough (Warner 1982), and a stereo-pair taken further downstream is shown below.
The large dissipating cumulonimbus seen in the middle of the stereo featured an anvil trailing behind it. Streamers of precipitation fell from it; their slopes indicate that the northeast component of the wind decreased with height. While the low-level northeasterly monsoon flow was strong, the flow aloft was light and variable. (back to main page) (back)
A variety of convective pulses has been shown in this study. Among theoretical and modeling treatments, attention is drawn to the work of Emanuel (JAS, 15 Oct 1986, 43, 2183-98), a precursor to section 11.3 of his book Atmospheric Convection (1994). He studied effects of precipitation. Dudhia, Moncrieff and So (QJRMS, Jan 1987, 113, 121-46) presented a simulation with results similar to these GATE and WMONEX studies, prominently featuring a concentrated updraft, and with a surface gust front absent. T. T. Warner, Mapes and Xu (Mon. Wea. Rev., May 2003, 131, 813-29) employed the MM5 model to study the convection they observed in the Panama Bight and vicinity. Discrete propagation over southern Britain was observed by Morcrette, Browning, Blyth, Bozier, Clark, Ladd, Norton and Pavelin, and modelled by Marsham and Parker (QJRMS, Apr 2006 B, 132, 1021-51; 1053-72).
Essential ingredients for the lines appear to be the unstable stratification at low levels, and a wind maximum and stable layer suitable to yield ducting as of a gravity wave. The presence of strong radiative forcing on cloud and haze at altitudes near 500 m leads to stabilization (Mather, McFarlane, Miller and Johnson, 1 Mar 2007, JGR, 112; Pakula and Stephens, 2009, JAS, 66, 62-76). The pulses shown here were not gravity waves: they were powered by energy conversions in cumulus clouds. In view of their propensity to propagate through the marine boundary layer, it seems appropriate to use the name "convective sweepers" for these pulses: they sweep up heat and moisture from the surface. To complete a satisfactory modeling study, fallout and evaporation of precipitation from the cumulus mediocris should be included, friction contributing to reduction of wind speeds near the surface, and probably both long and short wave radiative fluxes. (back to main page)