This study of winter monsoon clouds follows earlier collaboration with Joanne and Robert Simpson at the University of Virginia. The aim here is simply to explore. Slides by R.H.S. together with Warner's have been used here, sometimes combined in stereo. Synoptic analyses by R.H.S. and Brian Morrison are included. A disturbance propagated southwestward across the South China Sea. It showed marked diurnal and mesoscale variations and was not clearly coherent at synoptic scale. Background information is given in a pair of papers in Mon. Wea. Rev., Aug 1981, 109, by Houze, Geotis, Marks and West (p 1595-1614) and Johnson and Priegnitz (p 1615-28). (back to main page).

1330 local time. Visible satellite image. The track of the aircraft has been added lightly.
(From Sadler, J. C., 1979: Synoptic scale quick-look for Winter Monex - December 1978. Dept. of Meteorology, Univ. of Hawaii)


1330 local time. IR satellite image and aircraft track.


Aircraft track, at 463 hPa (6.1 km), and photo local times (UTC + 7.5 h), locations and directions of view. Click on a time to see the photo.

0857.  3.45 N, 103.6 E, on 130 azimuth.

0927.  4.8 N, 105.3 E, on 135 azimuth. Great number densities of small cumulus are shown in this view, implying strong mixing close to the surface. Reflections of clouds in the sea surface imply that surface winds were not particularly strong. Note that there was strong insolation, so that unstable stratification could have developed near the surface. This is seen in the attached tephigram 1 (0952 local time, 6.0 N, 106.85 E). High clouds of the monsoon vortex under investigation on this day are seen in the distance.

Dropwindsonde 1 of temperature and dew point. Isotherms are at 20, 10, 0 and -10 degrees C.


0955.  6.2 N, 107.05 E, on 035 azimuth.

1017.  7.4 N, 108.15 E, on 135 azimuth. Cloud streets from direction about 050 are seen. The clouds were of no great vertical development; they were overlain by stratus. Penetration of the middle troposphere by cumulus castellanus is seen, with occasional large cumulus. Aloft, stratiform effluent occurred at several levels.

1043.  8.6 N, 109.2 E, on 140 azimuth. Shadows of clouds on the sea surface suggest that surface winds were not very strong. Number densities of small cumulus were quite small, and the clouds lacked vertical development, suggesting suppression. In the distance are a group of mediocris to congestus showing a castellanus appearance symptomatic of a shallow layer of ascent. High stratus suggests deep ascent at greater ranges from the aircraft.

1115.  9.65 N, 111.2 E, on 180 azimuth. Beneath high overcast, several cloud strata were present, and activity near the surface was suppressed. Billows classifiable as cumulus mediocris appeared close to the aircraft. The top of the low stratus layers prominent in this and the next two pictures was near 680 hPa (3.3 km).

1129.  9.5 N, 111.7 E, on 310 azimuth. In stereo, mid-level fragmentary stratus is seen to feature variable thickness and activity. Below it were some suppressed small cumulus, but also some humilis of substantial, slender, vertical development (in the middle of the picture). The upper troposphere appears to have been substantially clear below high overcast.

1158.  7.85 N, 110.2 E, on 340 azimuth. In the foreground, a patch of small cumulus shows a fibrous character. Stereo viewing shows that these clouds were distinctly above the fractus near the surface, and reached close to the mid-level stratus.

1202.  7.6 N, 110.0 E, on 315 azimuth.
A shaft of precipitation from just a shallow layer near flight level. (back)


1253.  6.0 N, 110.1 E, on 075 azimuth.

1259.  6.05 N, 110.5 E, on 290 azimuth. It seems that high overcast may have led to stabilization of air near the surface due to fluxes of long wave radiation. Number densities of small cumulus were variable. Compare this view with that of

1320.  7.3 N, 111.5 E, on 310 azimuth.

1413.  9.8 N, 114.25 E, on 340 azimuth. In stereo, views down to the surface suggest that mixing at low levels was quite vigorous. There was plenty of stratus, and penetration by mediocris into the middle troposphere. Mixing of the low troposphere is seen, but not of the high troposphere below the overcast.

1416.  9.8 N, 114.4 E, on 180 azimuth. This stereo-pair involves a high-resolution color slide by R. H. Simpson (the right-eye view mounted on the left) and a wide-angle black and white photo by Warner as the left-eye view. Stratus occurred just below flight level, through which cumulus, apparently castellanus, penetrated. Chiefly of interest is the island of altocumulus cloud which was yielding precipitation. It was above flight level but below the high overcast.

1426.  9.7 N, 115.2 E, on 345 azimuth. Small cumulus were aligned in streets along about 040, as seen in the left hand member of this composite. They were aligned also across the direction 040, as seen in the right hand picture. The first case may be associated with a shallow layer next to the surface. The second case appears to be related to a shallow stable layer sandwiched between two shallow unstable layers, to form a shallow duct for transverse waves just above the surface. The tephigram outlined below shows the suggested duct at 950 hPa (mid-way between the two lowest isobars). The phenomena were recorded at 1440, 9.45 N, 115.1 E. References include Chimonas and Hines (JGR 1986), and Lindzen (JAS 1974) on Wave-CISK; also an Alberta hailstorm treated by Warner (1976). The convective waves shown on the page

1436.  9.55 N, 115.2 E, on 100 azimuth. Small cumulus were aligned in streets along about 020. The clouds were not slender; relatively stable stratification is indicated also by fragments of stratus. There was penetration into the middle troposphere by mediocris. Aloft there was high overcast.

1445.  9.2 N, 114.9 E, on 185 and on 215 azimuth. Small cumulus aligned from the northeast were fairly numerous. The cumulus and humilis were not slender, so probably the stratification was not maximally unstable. Cumulus mediocris were penetrating into the middle troposphere. Ascent was occurring into the upper troposphere by mid-level cumulus apparently shallow-rooted.

1448.  9.05 N, 114.75 E, on 290 azimuth. With dense high overcast, and stratus layers in the low troposphere, it seems that cumulus activity near the surface was suppressed. Cumulus mediocris to congestus dominated the middle troposphere, with mixing through to flight level. Deep clear areas adjacent to penetrative cumulus suggest deep mesoscale vertical circulations. The slope of the clouds suggests shear at mid-levels towards the north. Near flight levels, the absence of stratus and the slenderness of tall cumulus suggest an absence of stable layers. (Compare the arc shown in

1503.  8.1 N, 114.0 E, on 130 azimuth. This stereo-pair shows a distinct layer of stratus well above small - suppressed - fractus near the surface. Mediocris were penetrating the mid-troposphere. Precipitation was falling close to the aircraft.

1517.  7.25 N, 113.25 E, on 160 and on 220 azimuth. Alignment of small - suppressed - cumulus was from about 060. There were low-level stable layers. Through the mid-troposphere there was ascent in cumulus mediocris, despite stable layers. Congestus - cumulonimbus reached the high troposphere. Deep convection was occurring to the northwest.

1538.  6.7 N, 114.6 E, on 020 azimuth. Cumulus fractus and humilis were aligned along 070. The smallest clouds appear somewhat suppressed, but humilis were quite tall and slender, and no sign of a stable layer is seen between the top of the fractus layer and flight level. Arcs of mediocris are seen directed from about 090, east, with a maximum of easterly wind near the surface, diminishing with height. The most prominent arc circulation seen is of depth several kilometers; it involved a large trailing clear area. Near the aircraft it appears that the stratification was unstable and did not feature any marked stable layer up to flight level. In the distance were cumulonimbus, and fragments of mid-level stratus may be discerned. High overcast was present. (Compare the arc shown in

1551.  6.5 N, 115.2 E, on 270 azimuth. Shadows of small cumulus are seen on the surface, which indicates that surface winds were not very strong. Small cumulus were slender, and there were mediocris present. It appears that the stratification was unstable and did not feature any marked stable layer up to flight level. There was little mid-tropospheric wind shear. In the distance were cumulonimbus. Quite vigorous mixing through the troposphere is indicated, and an absence of strong dynamical forcing. High overcast was present.

Findings

Only a few photographs are shown here. There was great variety. Surface winds changed from very light to 15 m/s within a few tens of kilometers. Given an extra photograph without a time, it would not be possible to place it in context solely from its appearance. Mesoscale features of great complexity and variety were recorded (for instance at 1320 and 1426). It is intended here to cultivate general familiarity.

The large arcs recorded at 1448 and 1538 appear to have been related to arcs described on the page bay.html.

A humid low troposphere appeared often to be capped by stratus at levels near 680 hPa (3.3 km) (see 1115, 1129 and 1158). This corresponds with the moist low troposphere in undisturbed periods of WMONEX shown by Johnson and Priegnitz (1981). Evidence suggestive of a radar bright band was not seen on 29 December. Probably it was of limited extent and concealed from view.

Cumulus mediocris were seen breaking through the middle troposphere, often with a castellanus form. A variety of regimes of both short and long wave radiation is to be inferred.

Brian Mapes wrote in 2001 about "Water's two height scales: The moist adiabat and the radiative troposphere", QJRMS, 127, 2353-66. The former refers to the humid lower troposphere, in which large values of temperature lapse rate occur, with values decreasing through altitude about 8 km. The latter refers to the marked decrease in amounts of water vapor through about 14 km, and concomitant decreases in magnitudes of longwave cooling.

A third height scale of water, 500 m, approximately that of cloud base over tropical oceans, appears to merit recognition. On p 139-40 of his book Clouds and Storms (The Penn State Univ. Press, 1980) F. H. Ludlam explained how there has to be a separation of temperature and dew point of the air adjacent to the ocean in order for heat and moisture to flow upwards off the surface. The air must be unsaturated. Frank Ludlam matched calculated rates of evaporation with measured rates of rainfall. If the air temperature and pressure are 25C and 1000 hPa at the surface, and cloud base is at 500 m, then the relative humidity is 76%.

Over the sea, salt particles act as hygroscopic nuclei and form haze. Andrew F. Bunker and Margaret Chaffee (1969, Tropical Indian Ocean Clouds, East-West Center Press, Honolulu, 193 pp) reported a sharp transition from no haze to haze as aircraft climbed upwards from the surface through levels of relative humidity 80%. In my report Cloud maps for 24 June 1979 over the Arabian Sea: Summer MONEX visibilities much greater at heights below roughly 120 m than above were documented (Warner Dec 1982).

It is remarkable to see from aircraft how quickly one regime of activity near the surface of the tropical oceans gives way to another. Smoothly drawn synoptic charts can appear to be inappropriate, until one remembers that from a violent mesoscale event, it is only the projection upon the hundred kilometer scale that counts. It seems that the small 500 m height scale of water means that many mesoscale events in the boundary layer are of quite small horizontal wavelength, and that amplitudes of local forcings are sometimes enhanced by confinement near the surface. This fact of a small height scale is seen itself to be an important agent of redistributions of energy.

Each radiosonde samples an area in the horizontal to be reckoned in square centimeters, whereas the separation between these instruments is generally hundreds of kilometers. Satellite imagery, and 4DVAR analysis techniques, are used to help generate a representative portrayal of the atmosphere smoothed to the scale of the radiosondes. Here we have been looking at airborne photographs with very great information content. Microwave data likewise have great information content, and areas of horizontal sampling which are nearly ideal for many purposes. AMSU data of many channels - with great information content and good sampling - seem very promising (see Warner 1986 on LEO). It will become possible to see correlations between neighbouring grid squares. (back to main page)