Simultaneous Presence of 30 and 60 days ISO modes in Indian Summer Monsoon Observed from TRMM Merged Rainfall Data M S Narayanan National Atmospheric Research Laboratory, Gadanki Shivani Shah, C.M. Kishtawal, V. Sathiyamoorthy Meteorology & Oceanography Group (MOG), SAC M. Rajeevan India Meteorological Department, Pune R. H. Kriplani Indian Institute of Tropical Meteorology, Pune 3 rd ISRO – CNES joint Workshop on Megha Tropiques 19 th October 2005

 Instantaneous Rainfall – over oceans and Land 19, 22, 37, 85 GHz  Less utility – because of poor temporal sampling (3-4 per day)  Rainfall highly variable in space and time Megha – Tropiques – MADRAS MW sensors

Products from TRMM Project – (Predecessor to Megha Tropiques)  24 hr accumulated daily rainfall at 1° x 1° produced in combination with IR / Vis data from Geostationary satellite - 3B42 version 5 (global) from 1998 to 2003 (now version 6)  3B43 monthly rainfall

Merged Rainfall from MW (TRMM), Meteosat (IR) & Gauge Rain-rate : CTT Threshold, Texture based C/S Separation Rain-rate : MW Emission/ Scattering algorithms Tuning of Vis/IR Rain algorithm Using MW and gauge rain Tuning Parameters for Merger Rainfall Observations from Gauge Network TRMMTRMM Meteosat Merged  Such data set for TRMM available from 1998 to date  Also GPCP 1DD from 1996 to date

OBJECTIVES  Validation of daily TRMM merged rainfall over Indian land mass, and  Studying Monsoon ISO oscillations in rainfall

grids  TRMM data for 1998 to 2003

Frequency Distribution

(b) : Grids in which IMD analysis shows 1 mm or more rain and 3B42 has no rain (c) : Average percentage of days (all six years – monsoon months) in which 3B42 shows no rain, for cases when IMD shows 1 mm or more rain (a) : Grids in which 3B42 shows 1 mm or more rain and IMD analysis shows no rain

2001 y = x CC = 0.33 y = x CC = 0.54 IMD (a) Individual days (b) Seasonal mean

15 July IMD15 July 2001 – 3B42Difference – 15 July 2001 July 2001 Seasonal rd July 2001

Seasonal variance in daily rainfall in 3B42 and IMD for 2001 and 2002

3  x 3  5  x 5  1  x 1  Box y = x CC = 0.63 y = x CC = 0.83 y = x CC = 0.87

MINICOY PORTBLAIR Comparison over the oceans ( 5 day running mean) – Minicoy and Port Blair y = x cc = 0.69 y = x cc = 0.72

For studying Intra Seasonal Periodicities

IMD - daily3B42 - daily IMD – 5 day running mean3B42 – 5 day running mean Time series of all India daily and running mean pentad rainfall 2001

YearObserved3B All India Seasonal Rainfall (cm) YearJuneJulyAugustSeptember IMD3B42IMD3B42IMD3B42IMD3B Monthly All India average rainfall

1 June – 30 September Time series of IMD and 3B42 rain at all India scale for 2003 Rain Rate

IMD – All India TRMM – All India ModeMode ModeMode Wavelet showing various modes in IMD and 3B42 rainfall

PowerPower Year PowerPower Contribution of 10 – 20 day mode Contribution of day mode Contribution of 10 – 20 day and 30 – 60 day modes in different years

IMD – All India TRMM – All India ModeMode ModeMode Wavelet showing various modes in IMD and 3B42 rainfall

IMD – All India TRMM – All India Significance of the various periodicities at 95 % confidence Global Wavelet Variance (mm 2 ) Global Wavelet Variance (mm 2 )

1998

2001

2003

Rainfall over the oceans

Buoy TMI DS Bay of Bengal Period (Day) Oceanic Winds

Buoy TMI DS Arabian Sea Period (Day) SST

Water Vapour over the oceans

Global Wavelet Variance (m/s 2 )

Hovmollor plot showing propagation characteristics of the ~ 30 and ~ 60 day modes : for rainfall and NCEP 850 mb winds for the year 2001

2003 : No clear propagation for wind or rainfall 1998 : Northward for 30 day for both wind and rainfall Overall : Inconclusive – but both parameters show similar trend in both modes

GPCP 3B42 1° x 1° deg Version -5 3B ° x 0.25° deg Version - 6 IMD Mean July

Conclusions & Future Work * 3B42 limitation in picking variability & high rainfall * Differences in Specific boxes IR estimates also to be improved for merged rainfall Comparison with version 6 and GPCP 1DD * Useful for studying ISO / IAV * Simultaneous ~ 30 and ~ 60 days modes seen in some years * Linkage with Interannual Variations