Determination of Moisture Changes Prior to the Onset of South-West Monsoon Over Kerala Using NOAA/TOVS Satellite Data

Transcription

1 Meteorol. Atmos. Phys. 53, (1994) : Meteorol og)r and Atmospheric Physics 9 Springer-Verlag 1994 Printed in Austria Meteorology and Oceanography Division, Space Applications Centre (ISRO), Ahmedabad, India Determination of Moisture Changes Prior to the Onset of South-West Monsoon Over Kerala Using NOAA/TOVS Satellite Data Baby Simon and Prakash C. Joshi With 13 Figures Received May 25, 1993 Revised November 5, 1993 Summary The temperature and moisture data from TIROS operational vertical sounder (TOVS) are examined to obtain humidity parameters like, mid and upper tropospheric water vapour, and scale height of water vapour. Their usefulness in characterizing the onset of south-west (SW) monsoon over India is studied. The NOAA satellite data (finished product) with a resolution of 2.5 ~ lat/lon are used to obtain these parameters during and prior to the SW monsoon season over selected regions during 1979 to The pentad averaged values in the western Indian Ocean showed an increase in scale height of water vapour and mid-tropospheric moisture (7-5 rob) over about 8 to days prior to the onset over Kerala coast. The association of the moisture flux across the Indian Ocean and the rainfall over Kerala coast has also been examined. Results showed that the gradient of middle level moisture is stronger in the case of rainfall deficit years. 1. Introduction The south-west monsoon is one of the important summer circulations which affects the countries in the Indian subcontinent. The onset of monsoon in the meterological parlance has been associated with the heralding of monsoonal rains over the Kerala coast of the Indian mainland Ananthakrishnan et al. (1967). For more than hundred years the onset dates of monsoon are available. A well analysed statistics of these dates has been presented by Ananthakrishnan et al. (1988). The average onset date is on 1st June, with a standard deviation of eight days. There have been attempts to functionally relate the upper air parameters like temperature, wind and geopotential height in the month of April to onset date (Kung and Sharif, 198, 1982). A sharp rise in upper tropospheric temperature was,observed around the heat low region about a week before the onset of monsoon (Joshi et al., 199). In bringing the monsoonal rains over India the cross equatorial flow of moisture as well as the evaporation over the north Indian Ocean and adjoining seas are known to be equally important (Saha and Bavadekar, 1973; Pisharoty, 1965; Cadet and Reverdin, 1981a, b). These studies suggest that the atmospheric moisture variation studies are important for characterizing the onset of monsoon. Simon and Desai (1986, 1992) and Cadet (1986) have recently studied the moisture variation and evaporation over the seas adjoining India as available from NOAA/TOVS satellite data. The TOVS sensor onboard NOAA polar orbiting satellite has three infrared channels (8.3g, 7.3g and 6.7g) which has the capability of providing three layer moisture data (Smith et al.., 1979). Pathak (1982) has compared the TOVS derived total water vapour and with aircraft dropsonde and the ship-upsonde data in Indian Ocean for the colocated areas during May-July He found a correlation of.98 between the two over oceanic regions. Khalsa and Steiner (1988) have compared the various TOVS products with in-situ measurements. They observed that the TOVS-observed water vapour estimates are within 15% of the

2 1 224 B. Simon and P. C. Joshi in-situ values. The accuracy of the satellite measurement of water vapour decreases in the upper layers. In the present paper we have analysed the TOVS-derived water vapour data and examined it in relation to the onset of monsoon over India. The TOVS data has been recently used in studying various aspects of monsoon circulation (Joshi et al., 1989; Narayanan et al., 1989). The purpose of this paper is to show that the monsoon onset seems to be relatively predictable from NOAA moisture sounding data. Section 2 deals with the source of the data and the methodology of analysis adopted by us. Section 3 describes the results and the conclusions are summarized in Section Data and Methodology The TOVS derived sounding data for temperature and water vapour are now routinely available from NOAA-NESDIS. In the present study daily water vapour data for the months of May and June during the years are used. The data for the years 1983 and 1984 have many data gaps especially during the onset period, in the region of our interest. Hence these are not included in the analysis. Thus essentially five years of data (1979, 198, 1981, 1982 and 1985) have been used by us. In 1985 only data of the month of June was available in the region of interest. We have divided the area of interest into five regions and these are shown in Fig. 1. The cross equatorial flow of Somali jet brings enormous moisture to Indian peninsula and in order to study these regions of cross equatorial flow the whole region was divided into three segments. Also a region where water 25~ 15" - 4 ~ E 5" 6" 7 ~ 8" 9" "E Fig. 1. The five domains over which pentad averages of the parameters were estimated vapour was comparatively low (Arabian sea), and another region where it was higher (Bay of Bengal), were chosen. The analysis is presented as boxaverages of different TOVS-derived parameters. The five regions are: 1. Western Indian Ocean (WIO) 5-65 E -N 2. Central Indian Ocean (CIO) E -N 3. Eastern Indian Ocean (EIO) 77- E -N 4. Western Arabian Sea (WAS) 5-65 E -24 N 5. Bay of Bengal (BB) 85- E -24 N The finished product soundings are available in 2.5 degree lat/lon grid. The TOVS has three channels in water vapour absorption band in the wavelength range 6.5 micrometer to 8.5 micrometer. The response of these channels peak at the surface, ~7mb and ~5mb with broad weighting functions. The weighting functions show the realistic contribution of each layer to the measured radiance at a particular channel (Smith, 1968). Finally the water vapour data is available for three layers (i.e. -7, 7-5 and 5-3 rob). The temperature data are available in 15 layers with a better vertical resolution. In our analysis we have also used scale height of water vapour. Scale height of the water vapour is the height at which it reduces to 1/e of its surface value. It is an important parameter if we have to exponentially extrapolate the humidity, as in the case of satellite data. The NOAA/TOVS data gives only layer-wise humidity. Also variations in (-7rob) and (7-5rob) water vapour changes the scale height of water vapour. The Scale height is determined as follows: The layer mean temperatures (i.e. -85 and 85-7 rob) provided by NOAA satellite sounding data were used (along with surface pressure charts) with hydrostatic approximation to obtain height of various pressure levels. Assuming an exponential density profile we can obtain the scale height of water vapour (Simon et al., 1986, 1992). Water vapour content { V[Z 1} (in lo-7mb layer) f Z(7) = do exp (- z/h)dz (1),J Z(lOOO)

3 - - MAY Determination of Moisture Changes Prior to the Onset of South-West Monsoon 225 Table 1. The Onset of Indian Monsoon over Kerala in the Various Years and the Total Seasonal Rainfall Year Onset date Total seasonal over Kerala rainfall (cm) lth June st June th May st May th May 82.1 Water vapour content { W2} (in 7-5 mb layer) ~ Z(5) = do exp (- z/h)dz (2) Z(7) where H is the scale height of water vapour, do water vapour density, and z the height. Taking the ratio of these two equations do was eliminated, and was solved iteratively to get scale height H. These scale heights of water vapour are averaged pentad wise for the month of May-June and is one of the onset parameter used in this study. Table 1 gives the onset dates over Kerala, (south peninsular India) as declared by India Meteorological Department (IMD). This information is available in the Indian Daily Weather Reports issued by IMD. The total all India seasonal (June- September) rainfall in the corresponding year is also shown. The onset for 1979 was late (llth June), whereas for 1985 it was on 29th May. The year 1982 had the lowest rainfall of 73.9 cm, whereas 198 had the highest rainfall of 88.7 cm. The normal monsoonal rainfall for whole of India is 85 cm. 3. Results and Discussion In the five different domains, all four moisture parameters i.e. water vapour in -7mb, 7-5 rob, 5-3 mbtayer and scale height of water vapour, were analysed during the onset phase. The analyses were made during the months of May and June each year. The results are summarized in Table 2. Table 2 indicates peaking pentad dates of moisture parameters prior to the onset of southwest monsoon, in various domains. Among the parameters, scale height of water vapour, mid and upper tropospheric water vapour are peaking about ten days before the onset over western Indian ocean. However, a similar increase was seen only for three years in the central Indian ocean region. In the eastern Indian Ocean the peaking dates of various parameters were more or less coincident with the onset of monsoon. The highest water vapour scale height (2.7 km) is seen over Bay of Bengal. The peaking of water vapour in the lower level and in the upper levels occur in different pentads. The lower layer water vapour peaking does not show any systematic change in relation to the onset of monsoon. The middle level peaking however shows a good consistency. In the following we describe the variations of eacb parameter for each region in detail. By examining the data it was found that the western Indian Ocean showed very consistent changes in moisture parameters prior to onset of monsoon in comparison to other regions. Hence in the following we discuss more about the western Indian Ocean region. 3.1 Moisture Variation over Western Indian Ocean The pentad averaged values of middle level moisture (7-5 rob), top level moisture (5-3 rob) and scale height of water vapour fields in the Western Indian Ocean (WIO) are shown in Figs. 2, 3, 4 respectively. The variation of these parameters for 5 years during May-June is presented. The curves depict that a peak occurs about 8 to days before the onset of the monsoon over Kerala. The maximum moisture in the middle level is of the order of mm for all the years, while the 11 9 O..E 8 ~E t~ev 7 6 \ A JUNE o~1 t, B2 x 65 Fig. 2. Middle level (7-5 rob) precipitable water vapour over western Indian Ocean for different years

4 226 B. Simon and P. C. Joshi Table 2. Peaking of Moisture Parameters Prior to Onset of South West Monsoon in Different Domains Year Onset date -7 mb 7-5 mb 5-3 mb Scl. HI. of WV M: May WV (ram) WV (ram) WV (mm) (meters) J: June Max Pentad Max Pentad Max Pentad Max Pentad Grid 1:Western Indian Ocean June J.7 1-5J J June M M M May M M M May M M M May M M M Grid 2: CentralIndianOcean June M J M June 41. I-15M J M May M. 2-25M M May M M M May M M M Grid 3: Eastern Indian Ocean June J.3 5-J J June J J M May M M M May M M M May M M M Grid 4: Western Arabian Sea June J J J t98 1 June J J J May M J J May M J J May M M M Grid 5: Bay of Bengal June M M M June J J J May M M M May J SJ J May J J J J M M M M J J M M M J J M M M J J J J M M J M J J (3: 3,1 3' >E ~E 2.4 u 2.7, l D 2.2 ~: 2.1, q 15 2 MAY 25 3 = 5 ~ 15 JUNE g 79 * 8 o 81 " 82 x 85 Fig. 3. Upper level (5-3mb) precipitable water vapour over western Indian Ocean for different years ~ ' I 215 I [ i i 2 i MAY ~ " JUNE = u 79 * 8 o 81 & 82 x 85 Fig. 4. Scale height of water vapour over western Indian Ocean

5 Determination of Moisture Changes Prior to the Onset of South-West Monsoon ~ 2 25N 2 r ~,,. r----,-n i y i ~ IO"E Fig. 5. Precipitable water vapour (ram) at (-7mb), l-5 June pentad, 1979 Fig. 9. Precipitable water vapour (mm) at (5-3 rob), 1-5 June pentad, ~ N 2O - 2C ~ 25~ 2 ---_---3-< ~ Fig. 6. Precipitable water vapour (ram) at (-7rob), 6- June pentad, IO~E Fig.. Precipitable water vapour (mm) at (5-3rob), 6- June pentad, "N 2 OL., / t / I \t I/ L ~ L~ ~ ~ IO"E Fig. 7. Precipitable water vapour (mm) at (7-5 mb), 1-5 June pentad, ~ 2 l ~ Fig. 8. Precipitable water vapour (mm) at (7-5mb), 6- June pentad, t979 mean lbr May-June is of the order of 7 ram. The middle level moisture reaches the peak about 8- days before monsoon onset date, and a 4~o decrease coinciding with the monsoon onset over Kerala. The gradient of maximum to minimum is more in the drought years of 1979, and One of the reasons for the sudden increase of the gradient i' drought years is due to the increased SST. We have also analysed SST fields as derived from NOAA-AVHRR data in the western Indian Ocean during the 5 years. The maximum moisture in the upper level is of the order 3 mm as shown in Fig. 3. During the onset there is approximately 22~ change in this level moisture about 8 days prior to the onset over Kerala Coast. The mean moisture for all these years is about 2.1 ram. The scale height of water vapour (Fig. 4) was 2.5 km about 8 days prior to the onset. This was 25 meters more than the average of May-June put together. The scale height of water vapour falls by 5 meters soon after onset. The maximum moisture at the lower layer is of the order of 37mm. During the onset there is approximately --~3mm rise in the precipitable

6 228 B. Simon and P. C. Joshi water vapour in these levels, which may be within the error limits of estimation. This therefore does not show any remarkable change for onset over Kerala. In summary changes due to onset are taking place significantly first in the middle and upper levels. As a typical example we have analysed water vapour field in the Year The moisture field of surface level, shown in Figs. 5-6 during 1979 onsets shows a tongue from the western Indian Ocean reaching up to the Indian Peninsula which is not as prominent as for middle levels shown in Figs. 7-8, nor as in the upper levels shown in Figs. 9-. On all levels soon after onset the moisture is condensed out as rainfall. 3.2 Moisture Variation over other Regions Central Indian Ocean Maximum middle level moisture was not indicating an appreciable increase prior to the onset of monsoon over Kerala Coast. Out of five years, only three years showed an increase by about 8 days prior to the monsoon onset over Kerala Coast, and in the rest it was showing an increase simultaneously with the onset. This is indicating that cross equatorial flow through the Western Indian Ocean is more important than over central Indian Ocean. The western Indian Ocean middle level moisture is less than in central Indian Ocean as shown in Fig. 11. The moisture tends to increase from the Western Indian Ocean to the Eastern Indian Ocean. The maximum scale height of water vapour in central Indian Ocean was about 2.5 km. This was 2m more than the mean. All the years showed an increase of about 2 m, prior to the onset over peninsular India Eastern Indian Ocean The amount of moisture in the middle level is higher over this region compared to those in the central and western Indian Ocean. This is also in the 5-3 mb layer. The mean value of the moisture in this grid was 9.5 ram Western Arabian Sea Grid This is marked by the lowest water vapour in all the grids in all the three levels. The mean water vapour in the middle level is about 6 mm and at surface level it is 29 ram, when compared to 36 mm in the Bay of Bengal grid. The mean sea surface temperature in the period of May and June over this grid is 3 K and is less than the warm pool grid over the Eastern Indian Ocean. This grid is in proximity with the Somalia region where coastal upwelling takes place during these months. As far as the onset prediction is concerned this grid is of no significance. uj z 5 o--o ONSET DATES OVER KERALA o X--X PEAKING DATES OF PARANETER~ l\ (WATER VAPOUR SCL, HT,7 8diys ~ -5 AND 5-3 mbwater VAPOUR) OVER WESTERN INDIAN OCEAN 12 + X ~: ~"-'-o n ac~ lad 4 > 25 < 9 deys Lays 2( i 1 i i i YEARS -~-GD1 GD 2 --'x--gd3 --O-- GD4 --~-GD5 1S I i I I I Fig. 11. Mean (7-5 mb) water vapour (May-June) over Western Indian Ocean (GD-1), Central Indian Ocean (GD- 2), Eastern Indian Ocean (GD-3), Western Arabian Sea (GD-4), and Bay of Bengal (GD-5) Fig. 12. Dates of maximum value of the pentad averaged parameter (water vapour scale height 7-5 mb and 5-3mb water vapour using NOAA) over western Indian Ocean and onset dates over Kerala

7 Determination of Moisture Changes Prior to the Onset of South-West Monsoon 229 mu,+ { qwo5-o L ) A/~ mm ( qm 5-L} ^~ = I,, 9 I~,~ %.. % :z,_,,>".~.'~, -NO NO~ g ~ ", I >' o>( _. ll'l p. _%. ~ ~ -.. ~.> o I.l.l U.I "",,. \ '+ ZZ N ~ -~ I.ij I.iJ 9 "~ i u- I I ~ 'L% (ira) "llvdniv~,, i ~,,,,= o o o o o -o (ujuj I "]'IVJ-NW~ ww (quj OO5-OOL) A/~ Imw)(quJS-L) A/~. ("'4 ~" (p. r I"'- ~D i.rl -----] I I I I Is" ~ I I Z ~ /_,/ 2.,,," L.l*t l'" ~>.-- "~,-, + / ~" I I II ~ ; l'l el,. I.l.I Z ---,.._~ ~7" :< E, 2(t.,-,! J~ "" _~ "~3 ~ i I ] "r-, e,; I -o --J--L / /." I ~" -.Jo f ~ ",~ Itu~ ~ ' ~ ~ ~.im <'< /,A ~-" Z'7 ~-- I.IJ uj Z ~ ~ ~ tt,l" <:~r.~l...,- -:( s'~ o = ~ / "-','S I.- I.--- I.IJ I.iJ 9,2' r I I.u'!! [ ~," ~" : I ~ ~ ~' i I (::) /_--:'-'<, i i Jr,','.'"" ;-s. i i J i i., i ~ ~Z { B W ) "rlv:#nwi:i ( ww ) llv:jniv~

8 23 B. Simon and P. C. Joshi Bay of Bengal The mean middle level water vapour is about i mm (May-June) and is the highest when compared to all the grids during all the five years studied. The mean scale height of water vapour is 2.6 km. The mean sea surface temperature for all the years is 3 K and is the coldest of all grids during May-June put together. This may be a result of river discharge of cold water from the major rivers like Brahamaputra, Ganges etc., at the head Bay of Bengal, during and prior to the monsoon season as a result of snowmelt, runoff and rainfall. The mean lower tropospheric water vapour is about 36 mm and is the highest throughout the season studied. Dates of maximum value of the pentad averaged parameters (water vapour scale highest, 7-5mb and 5-3mb water vapour using NOAA) over western Indian Ocean and onset dates over Kerala are depicted in the Fig. 12. This clearly indicates that the first signal for the impending onset over Kerala takes place at the Western Indian Ocean about 8- days before the actual onset over Kerala. 3.3 Association of West Coast Rainfall with Changes in the Moisture Field over Western Indian Ocean We have examined the rainfall in the months of May and June in three coastal stations along the west coast in southern peninsular India and its relationship to the'amount of moisture over Western Indian Ocean. The three stations are shown in Fig. 1. The coastal stations selected were Trivandrum on the southern tip of Indian subcontinent, where the monsoon onset takes place and subsequently moves northward in a period of time along the west coast of India. The other coastal stations are Aleppy and Cochin, from south to north in that order. The heralding of monsoonal rainfall over Kerala marks the onset, and hence changes in mid-level water vapour and rainfall are shown simultaneously in Fig. 13. It was found that the rainfall in the pentad averaged around 1-5th and 5-th June, 198, over Trivandrum was around 25 ram, and at Cochin it was around 45mm. This was a good monsoon year with total seasonal rainfall of 88.7 cm in the Indian mainland. The water vapour in the middle level was 9.2 mm over western Indian Ocean and peaked around days before the actual onset over Kerala Coast. In 1982, which was considered to be a bad monsoon year, the rainfall was approximately 55 mm over Trivandrum in the 1-5 June pentad and 35 mm over Aleppy and Cochin. The Western Indian Ocean middle level water vapour peaked around.5 mm about 8 days before the onset, the gradient was sharp. The peaking of parameters has however to be analysed in association with general synoptic situations. Because of some temporary disturbances (like the cyclone in the month of May 1979) the moisture parameters may be high which may not be a signal for advancing monsoon. In all the years studied the maximum middle level water vapour over Western Indian Ocean peaked around 8- days before the onset (maximum rainfall) over Kerala Coast. The middle level water vapour gradient from minimum to maximum was sharp in the two drought years of 1979 and Conclusions It was found that the changes in western Indian Ocean moisture in the middle (7-5 rob), upper level (5-3mb) and scale height of water vapour are forerunners of onset over south peninsular India (Kerala). The moisture and scale height of water vapour increased about 8 days prior to the onset over Kerala. The first signal leading to onset over Kerala is seen in the middle and upper levels rather than in the surface level. In examining the maintenance of onset vortex, Krishnamurti et al. (1981) has also shown that the energy of the horizontal shear flow plays a dominant role. They observed that there was gradual descent of mid-tropospheric cyclone to lower levels and hence increased convergence. The moisture variation around onset time is showing more or less similar behavior. The paradoxical feature that emerges in the average moisture in the middle level in the western Indian Ocean grid is smaller than in the adjacent Central and Eastern Indian Ocean. However changes in this region, which is farthest from the Indian Peninsula, are a forerunner (8 days) of the onset of Indian Monsoon. The average moisture in this grid is of the order of 7 mm whereas in the central and eastern Indian Ocean grids it is about

9 Determination of Moisture Changes Prior to the Onset of South-West Monsoon to mm. The higher moisture over central eastern Indian ocean may be combined effect of the transport from the western Indian Ocean and the increased evaporation over these areas due to higher SST. The amplitude of the middle level moisture over western Indian ocean had a phase lag of 8- days when compared to the maximum pentad rainfall over Kerala Coast The explanation for the moisture changes prior to onset would require more theoretical investigations, along with the circulation features. However for the present it appears that the TOVS data would be useful for monitoring the onset of the South-West monsoon over India. The study points at a feature that modellers might want to check in their general circulation model simulations. Acknowledgments The authors are thankful to Dr. P.S. Desai for useful discussions. References Ananthakrishnan, R., Acharya, U. R., Ramakrishnan, A. R., 1967: On the criteria of declaring the onset of the south west monsoon over Kerala. IMD, Forecasting manual, no iv-ts.i (Available from IMD, New Delhi). Ananthakrishnan, R., 1988: The onse~ of the south west monsoon over Kerala: J. Climate, 8, Cadet, D., Reverdin, G., 1981a: The monsoon over the Indian Ocean during the summer part-i: Mean fields. Mon. Wea. Rev.. 9, Cadet, D., Reverdin, G., 1981b: Water vapour transport over the Indian Ocean during the summer Tellus, 33, Cadet, D., 1986: Fluctuations of precipitable water over the Indian Ocean during the 1979, summer monsoon. Tellus, 38A~ Joshi, P. C., Simon, B., Patra, S.K., 1989: Use of NOAA temperature-sounding data in long-range forecasting of south west monsoon rainfall. Ind. J. Radio and Space Physics, 18, Joshi, P, C., Simon, B,, Desai, P.S., 199: Atmospheric thermal changes over the Indian region prior to the monsoon onset as observed by satellite sounding data. Int. J. Climatol,,, Khalsa, S. J. S., Steiner, E. J., 1988: A TOVS data set for the study of tropical atmosphere. J. Appl. Meteor., 27, Krishnamurti, T. N., Philip, A., Ramanathan, Y., Richard, P., 1981: On the onset vortex of the summer monsoon. Mon. Wea. Rev., 9, Kung, E. C., Sharif, T. A., 198: Regression forecasting of the Indian summer monsoon with the antecedent upper air conditions. J. AppI. Meteor., 19, Kung, E. C., Sharif, T. A., 1982: Long range forecasting of the Indian summer monsoon onset and rainfall with upper air parameters and sea, surface temperature. J. Meteor. Soc. Japan, 6, Narayanan, M.S., Pal, P.K., Joshi, P.C., Ali, M.M., Simon, B., 1989: Observation of mid-tropospheric circulation from water vapour radiances of polar orbiting satellites. Proc. Ind. Acad. Sci., 98(3), Pathak, P. N., 1982: Comparison of sea surface temperature observation from TIROS-N and ships in north Indian ocean during MONEX. Int. J. Remote Sensing, 12, Pisharoty, P. R., 1965: Evaporation from the Arabian Sea and the Indian South West monsoon. Proc. Sym. Met., Results of IIOE, Bombay, (Available from IMD, New Delhi.) Saha, K. R., Bavadekar, S. N., 1973: Water vapour bet and precipitation over the Arabian sea during the northern summer. Quart. J. Roy. Meteor. Soc., 99, Simon, B., Desai, P.S., 1986: Equatorial Indian Ocean evaporation estimates from operational meteorological satellites and some inferences in the context of monsoon onset activity, Bound.-Layer. Meteor., 37, Simon, B., Desai, P. S., 1992: Estimation of heat fluxes over Indian Ocean using satellite data, Physical processes in atmospheric models. New Delhi: Wiley Eastern limited pub., pp Smith, W, L., 1968: An improved method for calculating tropospheric temperature and moisture from satellite radiometer measurement. Mon. Wea. Rev., 96, Smith, W. L., Woolf, H. M., Hyden, C. M., Wark, D.Q., Millin, L. M, M., 1979: The TIROS-N operational vertical Sounder. Bull. Amer. Meteor. Soc., 6, Authors' address: B. Simon and P. C. Joshi, Meteorology and Oceanography Division, Space Applications Centre (ISRO), Ahmedabad-3853, India.