WMONEX   29 December 1978.   Airborne cloud photographs

Charles Warner    2 February 2013

This study of winter monsoon clouds follows earlier collaboration with Joanne and Robert H. Simpson at the University of Virginia. The aim here is simply to explore, to cultivate general familiarity. Slides by R.H.S. together with Warner's have been used here, sometimes combined in stereo. (see also the page on tephigrams) 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). The reader may prefer to go straight to the
    Findings: 29 December 1978
    A third scale height of water over tropical oceans
    Discussion

e29vis
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)

e29ir
1330 local time. IR satellite image and aircraft track.

29www
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.

The contour labelled 335 K refers to equivalent potential temperature at 500-600 hPa derived from dropwindsondes. Within this contour the air was moist (green), with dew point depressions less than about 9 K, rather than 12-18 K in the surroundings.

0857 vis0857
0857.  3.45°N, 103.6° E, on azimuth 130°.
Great number densities of small cumulus are shown in this view, implying strong mixing close to the surface. Vertical development appears to be quite uninhibited by any very low-level stable layer. Reflections of clouds in the sea surface imply that surface winds were not particularly strong, and the cloud forms show a relative absence of low level shear. Some low stratus is seen here, and there was high stratus.

0927 vis0927
0927.  4.8°N, 105.3°E, on azimuth 135°.
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. (bacmap)
e29tef1
See the page on tephigrams for more comments.

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

Cumulus of the northeast monsoon flow are seen, to the northwest of the monsoon vortex under investigation on this day. Large cloud number density suggests quite vigorous heat flux off the surface. The clouds showed little vertical development, and suppression by subsidence may have been occurring. (bacmap)

1017 vis1017
1017.  7.4°N, 108.15°E, on azimuth 135°.
Cloud streets from direction &n1126;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. (bacmap)

1043 vis1043
1043.  8.6°N, 109.2°E, on azimuth 140°.
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. (bacmap)

1115
1115.  9.65°N, 111.2°E, on azimuth 180°.
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). (bacmap)

1129
1129.  9.5°N, 111.7°E, on azimuth 310°.
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, narrow, vertical development (in the middle of the picture). The upper troposphere appears to have been substantially clear below high overcast. (bacmap)

1158
1158.  7.85°N, 110.2°E, on azimuth 340°.
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. (bacmap)

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

1253 vis1253
1253.  6.0°N, 110.1°E, on azimuth 075°.

A line feature may be discerned in the visible satellite image shown at the beginning, which may correspond with these airborne photographs. Here it is unrewarding to try to find close matches between the airborne photography and the satellite imagery. Smallness of many of the clouds shown here suggests shallow depths. (bacmap)

1259 vis1259
1259.  6.05°N, 110.5°E, on azimuth 290°.
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 1426, where two types of organization are seen. Which type of organization is shown here? The answer appears to be organization in lines across low level flow from direction ~010° (rather than in streets aligned with flow near the surface). At larger ranges from the aircraft there was penetration into the middle troposphere. Mediocris and congestus clouds showed considerable shear at levels fairly close to flight level. Concentrated turning of the low-level northeasterly monsoon flow towards that of the high level ridge is indicated. The forms of high cumulus towers indicate quite small shear in the high troposphere. The tephigram below was recorded at 1257 at 6.0°N, 110.4°E. It shows (local) moistening well above the 800 hPa level attributable to the nearby towering cumulus.
e29tef10

   Synoptic charts for layers centered on 950, 750, 550 and 250 hPa, by R. H. Simpson and B. J. Morrison, appear below. The aircraft track is shown on the high level chart. Veering of the wind occurred in the area of interest, between Borneo and Vietnam. (bacmap)

950750 550250

1320
1320.  7.3°N, 111.5°E, on azimuth 310°.
Number densities of small cumulus were small at ranges close to the aircraft. A group of humilis with large number densities occurred at greater range. These narrow humulis were sheared with direction 060, implying a maximum of wind from 060 at levels around 950-900 hPa. Patches of stratus may be seen at flight level and above, with cumulus apparently castellanus accomplishing mixing in the upper middle troposphere. A variety of dynamics is indicated in this photo. Like the preceding sounding the dropwindsonde data shown in this tephigram, for 1320, 7.3°N 111.5°7E, show moisture lofted by convection through a considerable depth. As shown by the photos, there was also much moisture aloft. (bacmap)
e29tef10

1413
1413.  9.8°N, 114.25°E, on azimuth 340°.
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. (bacmap)

1416
1416.  9.8°N, 114.4°E, on azimuth 180°.
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. (bacmap)

1426
1426.  9.7°N, 115.2°E, on azimuth 345°.
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 the two unstable layers, to form a shallow duct for transverse waves just above the surface. The tephigram below shows the suggested duct between about 956 and 937 hPa. [See notes on tephigrams]. The phenomena were recorded at 1440, 9.45°N, 115.1°E. [See Chimonas and Hines (JGR 1986) on ducts, and Lindzen (JAS 1974) on Wave-CISK; also an Alberta hailstorm treated by Warner (1976).] Compare the convective waves shown on the page bay.html. [ (back to 1259) (back to web design)] (bacmap)

e29tef15

1436 vis1436
1436.  9.55°N, 115.2°E, on azimuth 100°.
Small cumulus were aligned in streets along about 020. The clouds were not narrow; relatively stable stratification is indicated also by fragments of stratus. There was penetration into the middle troposphere by mediocris. Aloft there was high overcast. (bacmap)

1445 vis1445
1445.  9.2°N, 114.9°E, on 185 and on azimuth 215.
Small cumulus aligned from the northeast were fairly numerous. The cumulus and humilis were not narrow, 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. (bacmap)

1448 vis1448
1448.  9.05°N, 114.75°E, on azimuth 290°.
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, ith 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 narrowness of tall cumulus suggest an absence of stable layers. (Compare the arc shown in Warner 1982). (bacmap)

1503
1503.  8.1°N, 114.0°E, on azimuth 130°.
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. (bacmap)

1517
1517.  7.25°N, 113.25°E, on azimuth 160° and on 220°.
Alignment of small - suppressed - cumulus was from ~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. (bacmap)

1538 vis1538
1538.  6.7°N, 114.6°E, on azimuth 020°.
Cumulus fractus and humilis were aligned along 070. The smallest clouds appear somewhat suppressed, but humilis were quite tall and narrow, 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 ~090° - east - with a maximum of easterly wind near the surface, diminishing with height. The most prominent arc circulation seen is of depth several kilometres; 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 Warner 1982). The dropwindsonde data recorded at 1538, 6.7°N 114.6°E are shown below.
e29tef18
As shown in the photos, the sounding implies fairly deep convection (see the page on tephigrams). (bacmap)

1551 vis1551
1551.  6.5°N, 115.2°E, on azimuth 270°.
Shadows of small cumulus are seen on the surface, which indicates that surface winds were not very strong. Small cumulus were narrow, 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. (back to start)

Findings: 29 December 1978

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 kilometres. 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).

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. (back) .

A third scale height of water over tropical oceans

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 ~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. Two different characteristics may independently be held to account for this phenomenon:

(1) 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%.)

(2) 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 (Dec 1982), I documented visibilities much greater at heights below roughly 120 m than above 120 m. (back)

Discussion

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 kilometre 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 and information. Nature is chaotic.

Each radiosonde samples an area in the horizontal to be reckoned in square millimetres, whereas the separation between these instruments is generally hundreds of kilometres. Satellite imagery, and clever techniques of analysis, are used to help generate a representative portrayal of the atmosphere smoothed to the horizontal scale of the distribution of radiosondes. Here we have been looking at airborne photographs with very great information content. Microwave data have great information content with areas of horizontal sampling which are nearly ideal for many purposes. (back to main page)