CHAPTER IV DYNAMICS OF OROGRAPHIC RAINFALL Page Summary 132 4 .1 In troduc tion 133 4.2 Region of our study 135 4.2.1 Rainfall distributions in these r eg ions 138 4.3 The model of orographic rainfall 145 4.3.1 Model for vertical velocity 146 4.3.1.1 Solution for Western Ghats 148 4.3.2 Numerical evaluation 157 4.4 Computation of rainfall 158 4.4.1 Modification for unsaturation 160 4.5 Downwind drift of precipitation 163 4.6 Discussion and results 164 4.7 Conclusion 188 132 CHAPTER IV DYNAMICS OF OROGRAPHIC RAINFALL SUMMARY In this chapter, an attempt has been made to study the dynamic influence of orography on rainfall distribution. Some sections in the Western Ghats and in the Khasi and Jaintia hills in India have been considered for this purpose. A two-dimensional model for orographic rainfall was deve­ loped by Sarker (1)66, 1967) and applied by him along the Western Ghats and later applied by De (1973) for the Khasi-Jaintia hills. The same model has been applied here for a number of cases in the Western Ghats and the Khasi-Jaintia hills. Vertical velocities have been computed by the model. Rainfall has been computed from the vertical velocities using the principal of conservation of mass and moisture. The computed rainfall distributions agree well with the observed rainfall distribution both for the Western Ghats and the Khasi and Jaintia hills during the active monsoon situations. The peak in rainfall distribution is seen to be purely an orographic effect. However, the performance of the model is not good for weak monsoon cases when the atmosphere is not saturated. Some modifica­ tions have been made in the model to take into account unsaturated conditions. The results of such cases have been presented. 133 CHAPTER IV DYNAMICS OF OROGRAPHIC RAINFALL 4 . 1 Introduction : The rainfall of a place is believed to be influenced by the presence of orographic barriers. The effect of topography is fairly well known at least in qualitative term in the sense that rainfall increases with height and is greater on the slopes facing the prevailing wind than on the lee slopes. The precipitation occurs due to lifting of saturated air as a result of perturbations caused in it by the orography. However, the precipitation in mountainous regions may also be due to several other causes viz., horizontal convergences and convective instability. Qualitative evaluation of rainfall in mountainous regions is a complex problem as it involves different physical and dynamical aspects of the process on different scales. First,there are synoptic scale factors which determine the characteristics of the airstream which crosses the mountain, its speed, direction, thermal stability and moisture content. Second aspect is the microphysics of the rain and cloud forming process. Third aspect is the dynamics of the airflow over and around the hill which determines the distribution of vertical velocity and the displacement of the airstream at each level and along the entire path. 134 These important aspects of the problem have been studied by several investigators. Mention may be made of the studies by Douglas and Glasspoole (1947), Ludlam (1955, 1956) and Sawyer (1956); although the list is not quite ex­ haustive. An empirical model for computation of orographic rainfall is also available in the Hydrometeorological Report No.36, U.S. Weather Bureau (l96l). The monsoon rainfall over the Western Ghats and in Assam particularly over the Khasi and Jaintia hills is believed to be strongly orographic in character. During the southwest monsoon, the windward slopes of the mountains almost in the direct path of monsoon winds, get copious rainfall and heavy clouding, the rainfall decreasing on the lee ward side. For the first time Sarker (1966, 1967) proposed a sound dynamical model of orographic rainfall based on the theories of airflow for the Western Ghats. The model gives the amount of rainfall due to lifting caused by orographic forcing and also accounts for the variation of rainfall along the slopes. The agreement between the observed and computed rainfall during monsoon season over the Western Ghats is remarkable. This model was successfully applied by De (1973) for the Khasi and Jaintia hills in northeast India. We apply the same model for a number of cases in some sections of the Western Ghats and the Khasi-Jaintia hills. 13 5 This model is based on the assumption that the atmosphere is saturated. We found that there is good agree­ ment between observed and computed rainfall during active monsoon cases when the atmosphere is saturated. However, during weak monsoon cases the performance of the model is not satisfactory. Accordingly we made some modifications in the model to take into account unsaturation during weak monsoon cases. Some results are also presented for a few cases of this type. 4.2 Region of our study : We shall be considering orographic rainfall in the Western Ghats and the Khasi-Jaintia hills. The topography is shown in Fig. 4.1 or two sections (marked by rectangles) of the Western Ghats and in Fig. 4.2 for the Khasi and Jaintia hills and its neighbourhood (marked 2). The Western Ghats run approximately in the north-south direction with average height 1 km. The prevailing wind during southwest monsoon season is westerly in the lower levels and easterly in the higher levels. The Assam hills run approximately in a north-south direction with a westward extension along the Khasi and Jaintia hills; the height of the hills varies between 1-2 km. The prevailing wind over the Khasi and Jaintia hills is southerly. Hence in case of the Western Ghats as well as the Khasi and Jaintia hills, the prevailing wind is perpendicular to the barrier. 136 IBOMAi¥J.n j\ ! <; \ KHANMlA^~-».VADGAON^ "' -^POONA Fig. 4-1 137 1 BRAHMPUTRA VALLEY 5 KUMONRANGE a NORTH BURMA HILLS 2 KHASI a JAINTIA HILLS 6 IRRAWADY VALLEY 3 MANIPUR & NAGA HILLS 7 YUNNAN PLATEAU 4 CHINDWIN VALLEY 8 GREAT SNOWY MOUNTAINS >3 kmI 2-3 kmll-2 krr. [5CO m IzOOrr. Fig. 4-2 138 4.2.1 Rainfall distribution in these regions : Before proceeding to the actual model of orographic rainfall it is of interest to examine the observed features of rainfall over the Khasi and Jaintia hills and the Western Ghats in a broad sense. The normal rainfall daring southwest monsoon (June to September) is found to increase from south to north as we proceed along a profile from the plains of the Khasi and Jaintia hills on the westward slopes (Fig. 4.3(a)). Rainfall is maximum over Cherrapunji (25 15'N and 91 44'E) which is situated on the windward side at about 20 km from the crest of the hill. Rainfall decreases sharply between Cherra­ punji and Mawphlang which is at a distance of 20 km only. Further northwards Shillong and Gauhati have much lesser amount of rainfall. The rainfall distribution along two sections of the Western Ghats eastward from the coast is shown in Fig. 4.3(b) and (c). The strong orographic effect is also seen here. The rainfall of Khandala and Agumbe on the windward side are the highest with low lee side values over Pune and Chikmagalur. The rainfall distributions during different months of the southwest monsoon for three sections are shown in Figs. 4.4, 4.5 and 4.6. Strong orographic effects are noticed in all these months over Agartala - Gauhati section of the Khasi and Jaintia hills (Fig. 4.4) and Mangalore - Chikmagalur section 139 o _c/> I- o if LiJ C/) o < a 5 1 2 « • _. -3 < « tt < z i= x < ? £ < a: 3 < E ~o cc " I ft:w d^ J r1> ^> CD w<t d^ -U JLxd. -tx-- <^ <r 5* c I O< ^< wd oOX w00 CuD 5 < < cT V f/1 f 1 _) O -1 -1 X CL a <t ^ 00 o oo o g; sz 5 < CD < fO o> OOO 0 0 OOO 0 0 O 00 en ^r CM ~ y3awaid3S 01 3Nnr 1H9I3H (oio) HVJNIVy 1VN0SV3S 140 Western Ghats ( Bombay - Pune Section) B- BOMBAY, Pe-PEN, R - ROHA, K-KHANDALA, L-LONAVLA, V-VAOGAON, P-PUNE . 600 E 400k *•• a. a» CO 200 k c I Km 40 50 DISTANCE (Km) Fig, 4-3 (b) 141 < o < < o ce a. < a. cc CD o a. CO i CO •*: < < cc i < CD o JQ X E CO XL < _J cc < cc < _J UJ < CO I co CD I I CD z: < i_ cc o CO o I CO cc c UJ z> O o cr 2! i 2! X o UJ O _J CD o < X o ID UJ 2: _J < < < i i CD < I CO (uiQ) ITVdNIViJ 1VN0SV3S J.H9I3H 142 KHASI - JAINTIA HILLS ( AGARTALA-GAUHATI SECTION) JUNE JULY AUGUST SEPTEMBER AGARTALA. KAILASHAHAR. SILCHAR. CHERRAPUNJI. MAWPHLANG. SHILLONG. GAUHATI. -160 -160 -140 -120 100 -80 -60 -4-0 -20 HORIZONTAL DISTANCE (km) Fig. 4-4 143 WESTERN GHATS (BOMBAY-PUNE SECTION) OO-J HORIZONTAL ..DISTANCE (km) Fig.4-5 144 WESTERN GHATS ( MANGALORE-AGUMBE SECTION ) JUNE - JULY AUGUST SEPTEMBER M :MANGALORE. Mu :MULKI. KA iKARKAL. A iAGUMBE. S :SRINGERI. KO :KOPPA. B :BALEHONNUR. N :NARSIMHARAJPURA. CH :CHIKMAGALUR. -100 -80 + 60 HORIZONTAL DISTANCE (km) Fig. 4-6 145 (Fig. 4.6) of the Western Ghats. However, in case of Bombay - Pune section (Fig. 4.5) of the Western Ghats#orographic effects are more prominent in the month of July and August. 4.3 The model of orographic rainfall : The model of orographic rainfall consists of two parts. First, a model of airflow to compute the vertical velocities due to orographic forcing and a second part, a physical model to compute the rainfall caused due to lifting of saturated or partially saturated airmass at different vertical levels.