Brief Report on Extremely Heavy Rainfall Events over Tamil Nadu (1-15 Jan 2021) 1. Introduction Northeast monsoon (NEM) season comprising of the months from October to December is the major rainfall season for Tamilnadu & Puduchery& Karaikkal. The normal rainfall for the subdivision for this period is 44cm. About 45% of the annual rainfall of this subdivision (about 60% for coastal regions) is accounted by the rainfall during this period. Though the NEM seasonal rainfall normally end by the last week of December, there were years in which NEM rainfall spill into January of the next year. In 2021, the NEM rainfall extended into the month of January with 133.9mm till 15th January against the normal of 11.4mm for the period with 1074% departure from normal . The salient features of this event are discussed below. The subdivision has experienced a wet spell of about 11 days during the first fortnight of January 2021, especially during 5-15 Jan 2021. If the cumulative rainfall for twenty four hours is considered , there had been two spells of peak rainfall activity; one during 5-7 January in the districts of Chennai, Kanchipuram, Chengalpattu, Ariyalur, Tiruvarur, Coimbatore, Cuddalore, Dharmapuri, Dindigul and Kallakurchi and another during 11-13 Jan in the extreme southern districts of Thanjavur, Tirunivali and Toothukudi. Figures 1 to Figure 4 shows the weekly rainfall map for the week ending on 30th December 2020, 6 January 2021, 13th January 2021 and 20th January 2021 for the country as a whole. It is observed that, during the week ending on 30th December 2020, the percentage departure of realized rainfall over the subdivision had been -91%. However during the week ending on 6th, 13th and 20th January 2021, it has become 807%, 1036 and 727 % respectively which accounted the intense rainfall activity during the first fortnight of the month experienced by the sub division. The realized weekly rainfall of Tamil Nadu, Puducherry & Karaikkal had been 1.0 , 39.9, 76.1 and 28.9 mms against the normal of 10.7, 4.4, 6.7 and 3.5 mms for the week ending on 30th Dec 2020, 6th, 13th and 20th January 2021 respectively. Figure 1: Weekly rainfall ending on 30th December 2020 Figure 2: Weekly rainfall ending on 6th January 2021 Figure 3: Weekly rainfall ending on 13th January 2021 Figure 4: Weekly rainfall ending on 20th January 2021 The district wise rainfall distribution for the subdivision during 1-15, January 2021 is given in Table 1. It is observed that all the districts have received large excess rainfall during the period. Table 1: District rainfall for Tamil Nadu& Puducherry & Karaikkal,1-15 Jan 2021 Winter Monsoon Rainfall From 01.01.2021 to 15.01.2021 District ACTUAL NORMAL( Departure(%) (mm) mm) ARIYALUR 209.2 16.9 1138 CHENGALPATTU 164.3 11.5 1329 CHENNAI 194.4 17.0 1043 COIMBATORE 95.4 6.9 1282 CUDDALORE 281.6 21.5 1210 DHARMAPURI 56.3 4.2 1240 DINDIGUL 177.1 11.5 1440 ERODE 58.7 5.0 1074 KALLAKURICHI 105.0 14.9 605 KANCHEEPURAM 100.7 10.0 907 KANYAKUMARI 92.4 10.4 788 KARAIKAL 283.5 35.6 696 KARUR 108.3 6.0 1706 KRISHNAGIRI 40.1 3.4 1081 MADURAI 129.5 8.4 1442 NAGAPATTINAM 290.8 37.8 669 NAMAKKAL 60.6 5.8 944 NILGIRIS 124.4 12.8 872 PERAMBALUR 153.6 13.7 1021 PUDUCHERRY 174.1 13.8 1162 PUDUKKOTTAI 183.0 12.6 1353 RAMANATHAPURAM 222.4 18.3 1115 RANIPET 44.0 7.2 511 SALEM 53.0 5.8 814 SIVAGANGA 166.1 9.5 1648 THANJAVUR 259.5 20.4 1172 THENI 105.4 10.6 894 THENKASI 120.3 15.1 696 TIRUNELVELI 354.4 26.4 1242 TIRUPATHUR 16.7 2.7 519 TIRUPPUR 102.8 4.9 1999 TIRUVALLUR 127.5 13.4 851 TIRUVANNAMALAI 52.7 9.4 460 TIRUVARUR 354.5 27.6 1184 TOOTHUKUDI 151.9 12.3 1135 TRICHY 130.9 8.9 1370 VELLORE 46.2 5.2 788 VILLUPURAM 108.7 13.8 688 VIRUDHUNAGAR 112.0 11.6 866 SUBDIVISION RAINFALL 133.9 11.4 1074 During the NEM -2020 periods, the subdivision received 6% above normal rainfall (Figure 5) Figure 5: Rainfall over Tamil Nadu, Puducherry & Karaikkal during NEM2020 However, during the short period of 1-15 Jan 2021 , the subdivision received 133.9 mm against the normal of 11.4mm which is about 1074% departure from normal ( Figure 6 ) . Figure 6: Rainfall over Tamil Nadu, Puducherry & Karaikkal during 1-15 Jan 2021 The daily rainfall for the period from 1-16 January 2021 for Tamil Nadu, Puducherry & Karaikkal are given in Annexure I. 2. Main Synoptic Features Generally the main synoptic systems during NEM season are Low pressure systems like depressions, cyclones and westward moving easterly waves. It is noted that during 1-15 January 2021, the frequent westward moving easterly waves in the form of troughs and cyclonic circulations caused large excess rainfall as mentioned earlier. Figure7 & Figure 8 show that the easterlies were stronger and the perceptible water content were more than normal over south peninsular region during 1-15, Jan 2021. Figure 7: Zonal wind Composite Anomally (850 hPa) , 1-15 Jan 2021 Figure 8: Precipitable Water Content Composite Anomally, 1-15 Jan 2021 The synoptic features during the period from 1 to 5 January, 6 to 10 January and 11 to 15 January along with rainfall distribution are discussed below. 1 to 5 January 2021 Moderate easterlies/north-easterlies prevailed over the region in lower tropospheric levels. During this period the sub division received 25.5 mm against the normal of 3.0mm rainfall which is about 748% above normal. About 7 stations received heavy rainfall and significant no. of stations received rainfall between 2 to 4cm. The district rainfall distribution is shown in Figure 9. 3. Figure 9: District rainfall Distribution, 1-5 January 2021 6 to 10 Jan. During this period, there were low level easterlies with embedded trough extending up to 1.5 km above mean sea level from south Sri Lanka coast to north Coastal Andhra Pradesh which moved westwards across Comorin area to Southeast Arabian sea. Also a cyclonic circulation lay over Equatorial Indian Ocean and adjoining central parts of South Bay of Bengal and extended upto 3.1 km above mean sea level. At the end of this period the sub divisional seasonal rainfall increased to 751% of the normal. About 14 stations received very heavy rainfall, 25 stations received heavy rainfall and significant no. of stations received rainfall between 2 to 4cm. The district rainfall distribution at the end of this period is shown in Figure 10. Figure 10: District rainfall Distribution, 6-10 January 2021 11 to 15 Jan. During this period, a cyclonic circulation extending upto 3.1 km above mean sea level lay over Equatorial Indian Ocean & adjoining central parts of South Bay of Bengal which moved progressively westwards towards Southeast Arabian Sea across Comorin and Maldives area and neighborhood. During this period, about two stations received extremely heavy Rainfall on 13.1.2021 (Parangipettai (Cuddalore District): 28, Sethiyathope (Cuddalore District): 21). 12 stations received very heavy rainfall, 50 incidences of heavy rainfall and significant no. of stations received rainfall between 2 to 4cm .The district rainfall distributions for the period is shown in Figure 11. Figure 11: District rainfall Distribution, 11-15 January 2021 In addition to the above mentioned intense rainfall occurrences, fairly widespread to widespread rainfall activity with rainfall amounts ranging from 1 cm to 5 cm occurred over several districts. It is to be mentioned that the normal rainfall for the period itself are very low and are of the order of few mm for these districts thus making the percentage departure of the realized rainfall very high. 3. Impact on Agriculture The above mentioned large excess rainfall during the first fortnight of January 2021 in Tamil Nadu caused damages to various crops especially the lowland rice in Cauvery Deltaic areas and also the pulse crops in southern districts of the State. Most of the crops, especially Samba rice crop at harvesting stage in Thiruvarur, Thirunelveli, Nagapattinum, Cuddalore, Thanjavur, Ramanathapuram, Ariyalur, Pudukkottai, Dindigul, Sivagangai, Chengalpattu, Tuticorin and Perambalur districts got submerged in rain water and the farmers could not harvest the crop. The grains got germinated in the field itself, leading to huge loss for the farming community. Maize crop in these districts also got affected and the quality of harvested maize grins is also reduced. In some of the fields, even maize grain got germinated hence cannot be used even for poultry industry or as fodder. Waterlogging and lodging of crops at maturity stage also have been observed in many fields. In Karaikkal region (Puducherry) around 4500 hectares of Paddy are under cultivation in this Rabi season which is nearing harvesting stage. These crops were damaged due to weight of panicle, wind velocity and standing water. In southern districts like Ramanathapuram, Tuticorin, Virudhunagar and Thirunelveli, pulse crops like black gram have been affected at harvesting stage. Drainage facilities could not be provided due to continuous rainfall and overflow of all channels. Here, the soil is also of clay texture where agricultural operations are difficult to carry out at rainy conditions. Continuous rainfall situation hampered agricultural operations including harvesting operations. The Agricultural Advisory Service bulletins issued by concerned Agro Met Field Units (AMFUs) and District Agro Meteorological Units (DAMUs) also indicate that, drainage facilities and postponing of agricultural operations were advised to the farmers during the period.