Upper tropospheric drying and “breaks” in the Indian summer
monsoonKusuma G Rao, Michel Desbois and Remy Roca
Laboratorie Meteorologie Dynamique, Palaiseau, France………………………………………………………..
Abstract
Most of the Indian summer monsoon rainfall occurs from deep convective cloud systems
embedded within continental Tropical Convergence Zone (TCZ) organized during the active
spells of monsoon1-3. A striking new observation, rapid southward propagation of deep clouds
on diurnal scale travelling 100-1000km in a day over the monsoon region, is made here inhigh resolution pixel observations from the infrared channel(IR) of METEOSAT satellite.
The main focus here is to understand the feedback mechanisms underlying the suddenarrest of the deep convective activity of TCZ leading to "breaks" in the monsoon. For the
first time, based on an analysis of water vapour channel brightness temperatures, we showthat break sets in with substantial large scale drying of the atmosphere observed over thecontinental TCZ in the upper tropospheric layers (600-200hPa), with the area averaged
Upper Tropospheric relative Humidity varying between 40 and 50%, induced by the large
scale subsidence limiting the growth of deep cloud systems to mid tropospheric levels as
revealed in IR brightness temperatures. Break prolongs due to weakening of low level
convergence by almost 50%, as observed.
1. Introduction
Intensification and suppression of deep convective systems embedded within the continental TCZ
dictate the monsoon variability associated with active and break spells (figure 1a) in the Indiansummer monsoon1-3 which affect the predictability of seasonal mean rainfall substantially4. The
mean seasonal rainfall is 85cm and more than 80% of the annual rainfall over a large part of thecountry occurs during the summer monsoon5 . The major difference between strong and weak
monsoon years appears to be in the duration and intensity of breaks or dry spells6,7. It is shown6 thatthe average number of break days is ~23% per year during July-August months in poor monsoon
years whereas it is ~6% in good monsoon years. Therefore it is important to unravel the factorsthat are responsible for the initiation, intensification and suppression of the deep convective
systems of the continental TCZ to understand the dynamic and thermodynamic processes governing
the organized convection
Recent studies8-10 discuss the complex nature of the organization of convection over the
monsoon region into coherent large scale patterns and try to bring out their large scale correlationsin space with vorticity and divergence fields that are invariant over several active and breakmonsoon periods. But still it is not clear as to what extent and how the dynamical andthermodynamical factors determine the deep convective systems organization within the TCZ. Also
it should be noted that the individual active and break spells and the transition from an active to abreak spell are far more complex to understand. Here our chief purpose is to understand what

causes the sudden arrest of convective activity associated with the continental TCZ leading totransition from an active phase to a break phase.
The most important factor governing generations and maintenance of large scale cloudiness is
steady supply of moisture in the boundary layer. It is not sufficient if the atmosphere is moist
enough, the large scale dynamics must be favourable to produce intense low level convergenceinducing large scale lift or ascent pumping moisture upwards up to the levels of buoyant convection
setting the atmosphere to release convective instability and therefore the generation of organized
convection. Thus low level moisture convergence is expected to determine the latent heating of theatmosphere. Earlier investigations reveal that intense time mean convective precipitation is
primarily balanced by low level large scale convergence of moisture11. On the other hand, over
TOGA/COARE region, it has been observed that dry and warm air intrusion above the boundary
layer can inhibit the growth of deep convection by depleting the parcel buoyancy throughentrainment12. Therefore the low level flow carrying the moisture, in turn driven by the heating13
associated with subdued convective activity caused by dry air intrusion above the boundary layer,
may weaken during breaks. An earlier effort14 to model the maintenance of time mean (monthly)
tropical convergence and precipitation by proposing a "minimal" model has shown that thevertically integrated moist static energy gives considerable insight. Also it is shown14 that aminima in "Gross Moist Stability(GMS)'', an integrated measure of the moist static stability, marks
the positions of TCZ. A threshold value, 335 KJ Kg-1, on layer mean moist static energy of thelower troposphere quoted for the existence of TCZ on monthly mean time scales15 need not to be
the same as suggested in the present analysis on the time scales of active and break spells.
We show here that active and break cycles are seen as fluctuations in upper troposphericmoistening and drying of the continental TCZ using METEOSAT pixel data and NCEP16
Reanalysis. Brightness temperatures measured by the IR channel of METEOSAT-5(located over
63o04' E) in the window region of 10.5-12.5 μm averaged over 75-80oE longitudinal belt areplotted in figures 1(a) showing the time-latitude variations. IR channel measures the outgoingradiances from the surface during clear sky conditions or from the cloud top during cloudy
conditions. Figure 1(a) displays the spectacular organization of deep convective systems during theactive spell extending from 10 to 24 June; and the suppressed convection during the break spellfrom 24 June to 4 July for the year 1999. The year 1999 was the normal monsoon year and thecountry received 96% of its long period average rainfall.
2. Active and Break spells

Active and break spells are defined here based on a criterion on the area averaged brightness
temperatures for the region 15-25oN and 75-85oE computed from METEOSAT pixel data available
every three hours over a quarter degree resolution which constrains it to be less (greater) than 270K
continuously for a minimum of four days and above during an active (break) phase. This region ischosen here to construct averages as ideal to assess the monsoon activity after carefully looking at
the longitudinal-latitudinal extent of the deep cloud organization in METEOSAT imageries for theentire monsoon season. India Meteorological department (IMD)17 defined a break in the monsoon
during 20 June to 4 July based on rainfall distribution associated with almost nil rainfall over theentire country and excess near foot hills of Himalayas and over a little region in the south eastern
peninsular India. The present definition of active and break spells based on IR brightness
temperatures is consistent with the earlier ones9,17,18. Thus the periods of the break defined here andby IMD, by two independent methods are consistent as many break days by the two definitions areoverlapping, indicating that the criterion adopted here is quite appropriate. Thus for the year 1999,

we could identify two active spells and one break spell, the periods of these spells are 10-24 June,
9July-10August and 26June-4July respectively. A good percentage of data points satisfy thecriterion on area averaged brightness temperature, being ~90% and 79.2% during the active andbreak spells respectively. Even at pixel level, a good percentage of data points satisfy the criterion
being more than 60% during all the spells.

Figure 1 Time-latitude variations of (a) IR (b) WV brightness temperatures averaged
over 75-80oE longitudinal belt during the active phase (10-24 June,1999) and the breakphase (26 June - 4 July,1999)
We could make several interesting new observations in Figure 1a. During the active phase of
the monsoon, initially the cloud bands of deep convection generate over the equatorial region andsubsequently establish over the continental region by northward propagations, an old observationconsistent with the earlier findings1,2 and recently confirmed9,10. However, it is very striking to notethat deep clouds like to travel southwards within a northward propagating monsoon system. We
could observe these rapid southward propagating deep clouds, for the first time, travelling
100~1000 km in a day on diurnal scale over the monsoon region both over land and ocean in boththe hemispheres, for example, over central parts of India, figure 2a and over the Indian ocean,figure 2b. We notice that the direction of propagation is reversed occasionally in the near equatorial
region, figure 2c and the organization is slightly complicated sometimes(very rare) over the Indian
ocean associated with “x” and “y” patterns as seen around 4S, figure 2b. Deep clouds occur in theevening time(~18hrs) and it is surprising to note they reach highest heights in the night time overthe land region when the monsoon is active and the surface is wet, could be due to the cloud top
cooling and destabilization. The dynamics underlying the southward propagation of deep clouds,
not clear at present, must be playing a significant role in explaining the fluctuations of monsoon
between active and break spells.

Figure 2. Diurnal variations of IR brightness temperatures over (a) the continental parts ofIndia during14-18 June (b) the Indian ocean during 28 June- 2July (c) near equatorial region
16-21 August. Averaged over (a-b) 75-80oE (c) 70-75oE longitudinal belts.

During the break spell, figure 1a, the oceanic ITCZ dominates while the continental TCZ issuppressed, a well known observation1,2. In the present analysis, we could observe the crucial
activity over Bay of Bengal before the northward propagations of oceanic ITCZ takes place. We
notice that, while the continental TCZ is dying with the sudden arrest of convective activity, thecloud activity over the Indian ocean (0o to 5oS) builds up slowly till 27 June and northward
propagations do not occur till 4 July. On the contrary, Bay of Bengal is inactive whenever the
monsoon is inactive. For example during the break spell, an absolute cloud free region in Bay
propagates northwards from 5oS to 20oN(5oN to 20oN) between 27June and 1July(26 and 31June)
seen in time-latitude plots of IR brightness temperatures averaged over 85-90oE(90-95oE). It isinteresting to note that the northward propagations of oceanic ITCZ is preceded by an intense
activity over the Bay from 2 to 4 July with the revival of the monsoon taking place around 6 July
into a fully established active phase from 9 July to 10 August for the second time with deep clouds
extending up to the northeren most latitudes. Whenever the monsoon is active, Bay of Bengal isactive; and as long as the Bay is active, oceanic ITCZ will not intensify. Similar features prevailed
before the monsoon entered the first active phase. Soon after the onset on 25 May(IMD2000)
associated with enhanced convective activity initiated around 5oN and 65-70oE(extending
northwards up to 20oN in 75-80oE belt) and over the entire Bay of Bengal(85-95oE), the monsoon
did not propagate northwards till 10 June. An intense activity over Bay could be noticed from 7
June onwards with deepest clouds on 10-11 June, before the northward propagation of oceanic
ITCZ(figure 1a), probably triggering the first active spell lasting from 10 to 24 June. Thus itappears that the continental TCZ activity is regulated very much by the competition between the
Indian ocean and the Bay of Bengal.
In figure 1a, we observe a spectrum of clouds prevailing during the active spell from very deep
ones to low level clouds. However, the break is not absolutely cloud free, as clouds reaching mid
tropospheric levels and low clouds still persist as indicated by patches of brightness temperatures
lying between 260 and 280, although there is suppression of very deep clouds. Computation of
normalized frequency of occurrence of IRBRT show that very deep clouds (IRBRT < 210oK)shooting tropopause prevail during active spells in 2-4% of the time and in negligible numbers
during the break spell. It is surprising to note that the percentage of occurrences of pixels
representing deep convection (210o < IRBRT < 260oK) are almost exactly tallying during the two
active spells and drastically reduce by almost 40 ~ 70% during the break spell. The number ofpixels representing deep clouds reaching mid tropospheric levels or middle clouds (260 < IRBRT< 270) and low clouds (270 < IRBRT < 280) are comparable between the two active spells and thebreak spell and they are respectively around 12~16% and 14~17%. However, the number of low
clouds (280 < IRBRT < 290) enhance during the break spell by almost three times to about 30%.
Nearly 18% of the area is covered by clear air pixels (IRBRT > 290) during the break spell whereas
it is just around 3% during active spells. However we need to be more cautious while discussing
the types of clouds since the classification based on IR channel alone can be misguiding in thedetection of low clouds. Thin cirrus clouds can be as warm as low clouds owing to their low
emmissivities and can be inadvertently clubbed under low cloud classification. But, the capabilityof WV channel in high cloud retrieval has been shown19,20, especially for the semi transparent thincirrus clouds, based on the argument that they are colder in the WV channel than in the IR channel.
Thus this study necessiates detailed monsoon cloud classification to assess the contribution fromeach of these clouds in moistening and drying of the atmosphere. Since our main emphasis in thepresent study is on deep clouds and these clouds can be readily discriminated from clear sky andlow clouds with the use of IR channel data alone, our results are least biased by not adopting arobust cloud classification scheme
Thus we have made an attempt here to answer the following questions: (1) How moist or dry
the breaks are when compared with the active spells of monsoon? (2) How favourable thedynamics is, in moistening or drying of the atmosphere during the breaks? (3) What is the impact ofmoistening or drying on the atmospheric stability during breaks?
At first, we show that substantial drying occurs in the middle and upper tropospheric layers as
observed by the METEOSAT WV channel during the break spell.
3. Upper tropospheric drying
METEOSAT Water Vapour channel (WV) channel, centered on a maximum water vapour
absorption band around 6.3μm, measures the radiation emmitted by the water vapour in the middle
and upper tropospheric layers (sees down to a pressure level of 600 hpa) and does not show any
surface features as the radiation emmitted by the surface at this wavelength is completely absorbedby the water vapour present in the lower layers adjacent to the surface. Figure 1b shows the timelatitude variation of Water Vapour channel Brightness Temperature(WVBRT) averaged over 7580oE longitudinal belt for the active and break spells. Major observation noted from figure 1b isthat the atmosphere dry all over from near equatorial region (i.e. from ~5oN) to central andnorthern parts of India during the break spell while it is extremely due to deep clouds during theactive spell. Also we can see that the atmosphere is more dry in the immediate viscinity of oceanic
ITCZ when compared with that over central parts of India. Brightness temperatures from IR and
WV channels do not show significant differences during the active spell as deep clouds remaincolder in both. Southward propagations of deep moist clouds are evident in WVBRT distribution
also as seen in figure 1a. Fluctuations in brightness temperature on diurnal scale are still prevailing
during the break spell (figure 1a-b) without any clear preferred direction of propagation. Northward
propagations in the WVBRT from near equatorial oceanic region to continental land region has
been observed, as well as seasaw in moistening and drying of the atmosphere(600-200 hpa)
between these two favoured locations of ITCZ similar to the seasaw in the convective activity seen
earlier1-2 which manifest into active and break cycles.
In order to know how dry the break spell is, we looked into the Upper Tropospheric relative
Humidity (UTH) fields provided by EUMETSAT, a measure of the mean relative humidity of thelayer between 600 and 200 hpa and are provided only for clear sky conditions. The UTH data usedhere is the one averaged from segment grid on to 2o x 2o grid resolution.
3.1 Upper Tropospheric Humidity
Simultaneous day to day variations of area averaged [15-25oN and 75-80oE] UTH and IRBRT
are shown in figure 3 for the break spell. Break spell is clearly identified (from 26June to 4July)infigure 3 with clear sky IR brightness temperatures and low UTH on majority of days varying
between 40 and 50% about the mean value of ~45% with a very few outliers. A good number ofobservations (being ~24 out of 36 in the region) are available everyday during the break to computethe area average UTH indicating that middle and high clouds rarely prevailed as expected andimplies the consistency and robustness of the criterion adopted to define a break spell.
We provide below further evidence on the large scale drying observed over continental TCZ
during breaks by carrying out an analysis of NCEP/NCAR Reanalysis data and show that it iscaused by large scale subsidence occurring over there. Also we discuss the impact of the moisturevariations in the vertical on the stability of the atmosphere.

Figure 3. Simultaneous daily variations of Upper Tropospheric relative Humidity and IR
brightness temperature during the break spell.

4. Large scale subsidence and drying
Figures 4(a-b) show the vertical profiles of specific humidity and relative humidity averaged
over the region [15-25oN, 75-85oE] for the active1, active2 and break spells. We note from figure
4a that the specific humidity does not vary much between the spells at lower levels; whereas, themiddle and upper troposphere are quite dry during the break spell. The difference in specifichumidity, being ~2 g/Kg , is considerable over the mean specific humidity at 7 ~ 8 g/Kg around
600 hpa level. We note from figure 4b that the mean relative humidity lies between 40 and 50 % in
the vertical between 600 and 300 hpa during the break spell, confirming similar variations seen in
UTH discussed before(figure 3). This agreement is very important, because the UTH estimatesderived from METEOSAT WVBRT and the specific and relative humidity from NCEP/NCAR
Reanalysis are based on two completely independent sources of data. Area averaged temperature
profiles show insignificant differences between the active and break spells in the vertical.
It is relevant to mention that NCEP data analysis carried out here revealed that Precipitable
Water (PW), which represents integrated water vapour in the vertical, drastically decreases byalmost 10 mm, that is by nearly 20%, during the break spell from its value ~ 62.5 mm during theactive spells. Furthermore, it is very interesting to note that PW shows a decreasing trend, a day or
two before the break, indicating that it may be a useful precursor to predict breaks in the monsoon;
also we observe that PW builds up in two to three days before the monsoon restored into an active
phase.
Figure 4c describes the vertical profiles of area averaged divergence for the active1, active2
and break spells. From figure 4c, we note that intense convergence prevails at lower levels during
both the active spells and intense divergence at upper levels indicating the strong coupling between
the lower and the upper troposphere. This finding supports the assumption made in an earlier
study14 on this simple vertical structure in regions of intense convection. During the break spell,convergence prevails at both the lower and upper levels indicating the decoupling between the two;low level convergence being substantially weak, by almost 50%, when compared with the active
spells. Weakening of the low level convergence during the break is consistent with an anomalouspressure rise of ~2 mb observed over the continental TCZ reported by earlier studies6,18. The mostimportant observation to be noted from figure 4c is, the "weak divergence'' prevailing between 700
and 300 hpa over the continental TCZ during the break spell, indicating the "large scale
subsidence'' over there causing "drying'' of the atmosphere at these levels, already shown in
METEOSAT measurements displayed in figure 1b. During both the active spells, upward velocities
prevailed throughout the depth of the atmosphere being intense between 700 and 300 hpa; whereas
during the break, very weak upward velocities prevailed at lower levels. It appears that the verticalvelocity during the break spell is driven by the dynamics associated with vorticity and temperatureadvection. Over central parts of India vorticity is cyclonic from the surface up to almost 300hpa
during the active spells, another observation consistent with the vertical velocity field; whereas
during the break, vorticity field is very weak and more confusing.
Thus the analysis carried out here reveal that there is substantial drying of the upper
troposphere induced by large scale subsidence over central and northern parts of India andweakening of the low level convergence over the continental TCZ during the break spell switching
off the “convective trigger” necessary for the generation of convection, inspite of the lower
troposphere being equally moist during all the spells. This large scale subsidence observed overcentral parts of India during breaks may be associated with the sinking limb of the Hadleycirculation3. Finally we discuss the impact of atmospheric moistening and drying on the stability of
the atmosphere.

Figure 4. Vertical profiles of area averaged (a) specific humidity (b) relative humidity (c)divergence for active and break spells.

5. Stability of the atmosphere.
We discuss the stability of the atmosphere here in-terms of` the parameter ‘Gross Moist
Stability' (GMS) proposed by Neelin and Held14, defined as the difference in Mean Moist Static
Energy (MMSE) between the upper and Lower layers. In the vertical, MMSE is low at all levels,
but substantiallyly low between 700 and 500 hpa, during the break spell when compared with theactive spells. Figure 5, shows the daily variations of MMSE averaged for the upper (400-150hpa)
and lower (Surface-400hpa) layers during the active1, active2 and break spells and many
interesting results. We see that MMSE is always higher for the upper atmosphere irrespective ofwhether it is an active or a break spell when compared with that for the lower troposphere
supporting the speculation made by Neelin and Held14. However, MMSE estimates are low for
both the upper and lower layers during the entire break phase when compared with the active
phases. TCZ does not exist during the break spell in spite of MMSE of the lower troposphere overthe break spell being > 338 KJ/Kg , a value much above the threshold value 335 KJ/Kg quoted15 forthe existence of TCZ on monthly mean time scales. This disagreement is not surprising as monthly
means do not signify clear cases of convective or clear air situations. The most important inference
from figure 5 is that GMS increases during the break causing inhibition of deep convection and it isaround 7 KJ/Kg; whereas MMSE of the lower atmosphere enhances and approaches that of theupper atmosphere throught the active2 spell feeding the intense convection continuously supporting
the earlier views14. During the break spell GMS increases mainly because the MMSE of the lower
atmosphere decreases substantiallyly because of the drying observed in the middle layers thus
slowing down the convective processes14,22. Since NCEP humidity is not available above 300hPa,
we assume it to be same as at 300hPa which will not influence our conclusions.

Figure 5. Daily variation of Mean Moist Static Energy averaged for the lower (surface to
400hPa) and upper (400-150hpa) layers during the two active and the break spells

6. Discussion and conclusions
Thus the scenario that emerged from the observations made here on the feedback between the
dynamics and deep cloudiness during the transition from an active to a break phase is as follows.
Monsoon ceases to be active with the sudden arrest of deep convective activity within continental
TCZ and the break sets in with the substantial drying of the atmosphere observed in middle andupper tropospheric layers induced by subsidence, along with the simultaneous building up of
oceanic ITCZ. However during the transition from an active to a break spell, deep clouds reaching
mid tropospheric heights still persist over central parts of India as revealed in the IR brightness
temperature distribution. Thus there is clear indication that while the “convective trigger” is on,
feeding the deep cloud activity during an active spell, the environmental factor that is necessary but
may not be sufficient for the formation of organized convection is absent supressing the convection
leading to break conditions. The coexistence of dry layers in the middle and upper troposphere withthe absence of organized deep convection and presence of middle level clouds during the transitionindicate that dry middle and upper atmosphere must be the environmental factor causing the onsetof break limiting the growth of deep convection to mid tropospheric levels by diluting the parcel
buoyancy through entrainment12. In response to the suppressed convective activity13, the low level
convergence weakens substantiallyly which may reduce or cut off the moisture supply inhibiting
the triggering of convection and hence the break prolongs. Northward propagation of the oceanic
ITCZ and an intense activity over Bay of Bengal precede the revivel of the monsoon for it to enterin to an active phase. Monsoon continues to be active as long as the Bay of Bengal is active. Indian
ocean remains suppressed while the Bay is active. Thus Bay of Bengal, by being active, might
assist the northward propagation of oceanic ITCZ and also keeps the monsoon active. How exactly
the Bay of Bengal operates in regulating the monsoon fluctuations between active and break spells
needs to be explored.
Drying of the atmosphere observed during breaks has major implications on the radiative
energy budget of the atmosphere as loss of infrared energy to space increases nonlinearly withdecrease in relative humidity21. Evaporative and sensible heat fluxes substantiallyly reduce withweak low level flow during breaks through their dependence on wind speed. It is shown by Neelin
and Held14 and Zhang22 that TCZ's are confined to regions of net positive energy convergence inthe troposphere which depends crucially upon the manner in which the water vapour profiles andlow level flow are simulated. The most valuable suggestion here is that for better simulation of
monsoon on intraseasonal time scales, one has to worry more about accurate simulations of verticalprofiles of water vapour in the tropics.

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Acknowledgements
This work is carried out by Kusuma G Rao during her visit to LMD, Palaiseau, France under Indo-French satellite
programme on deputation from Indian Space Research Organization, Bangalore, India. We thank J L Monge for thehelp in understanding the data structure and A Deme for the computer code and discussion.