3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Convection, water vapor and radiation over the Indian Ocean Will the tropical oceans turn into Bay of Bengal ? Rémy Roca, Michel Viollier, Laurence Picon and Michel Desbois Laboratoire de Météorologie Dynamique, Paris and Palaiseau, France (from Trenberth et al., 2005)

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, (From Rajeevan and Srinivasan, Jclim, 2000) JJAS Strong net negative forcing over the Indian Monsoon region and over the Bay of Bengal (BoB) This unique regime is attributed to the high upper level and thick cloud fraction over the region with respect to other tropical regions Further investigating this unique regime by looking at the MCS and environmental water vapor Cloud radiative forcing over the Indian monsoon region

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Data and Methodology Period of investigation : Julys and Januarys ERBE s4g regional monthly mean flux (2.5°x2.5°; monthly mean) LWCRF = Fclear –Ftot SWCRF = Sclear –Stot NetCRF = SWCRF+LWCRF Precipitable water derived from SSM/I (Wentz et al., 1997) Climatology using the period INSAT-1B Infrared imagery (11km- 2 to 8 images /day) Upper Level Cloudiness in % over 2.5° grid computing the fraction of pixels Tb < 255K Mesoscale Convective Systems Segmentation of the imagery using clustering technique

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Detect And Spread (Boer and Ramanathan, 1997) Multiple threshold in the infrared imagery Very deep CS (Tmin < 220K) Convective Debris (Tmin > 235K) Class 2 INSAT - IR Mesoscale Convective Systems Characterization Class 3 Class 1 Deep CS (220K<Tmin<235K) (Roca and Ramanathan, 2000) Min temperature as an indicator of height Averaged BT as an indicator of LW properties

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Two key regions: January: the eastern Indian Ocean (EIO) slightly negative net forcing regime July: the Bay of Bengal (BoB)strongly negative net forcing regime Cloud radiative forcing over the Indian Ocean

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, SWCRF: greater in July BoB than in January EiO (-115 Wm-2 vs -95 Wm-2) NETCRF: more negative (-35Wm -2 ) in July BoB than January EiO (-15Wm -2 ) Cloud radiative forcing over the Indian ocean LWCRF: similar in both seasons & regions ~ 80 Wm-2 Similar Upper Level Cloudiness ? Similar Mesoscale Convective Systems properties & Occurrence ? Similar Clear sky back ground ?

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Mesoscale Convective Systems properties January Eastern Indian Ocean Tmin (K)Tavg (K) Effective Emissivity Class (3.0) (0.9) 0.66 (0.02) Class (0.0) (0.7) 0.80 (0.00) Class (1.7) (1.7) 0.89 (0.01) July Bay of Bengal Tmin (K) Tavg (K) Effective Emissivity Class (3.7) (3.0) 0.66 (0.03) Class (0.5) (0.7) 0.80 (0.01) Class (0.8) (0.9) 0.89 (0.01)  eff =  T min 4 /  T b avg of the system 4 Similar Repartition of Classes Effective emissivity Different ULC Fraction: BoB > EiO 39% 35%

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Clear Sky Environment : OLR and Precipitable Water Precipitable Water ~45-50 mmPrecipitable Water ~55-70 mm Smaller Clear sky OLR In July than in January Moister column in July BoB than in January EiO LWCRF: similar in both seasons & regions ~ 80 Wm-2 More ULC in July than January Similar MCS properties & Occurrence Moister Clear sky back ground in July than January 277 Wm Wm -2 DAMPING EFFECT OF WATER VAPOR ON THE LWCRF

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, Regional scale : dependence upon ULC ULC bin of 10% width from 15 to 55% ; All 4 years are accumulated ULC From 25% to 55% January LWCRF from 60 to 98 Wm -2 P WAT from 48 to 53 mm July LWCRF from 65 to 88 Wm -2 P WAT from 49 to 60 mm

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, hPa 850hPa.…. Relative Humidity (%) Idealized radiative transfer computations Column Radiation Model (Zender et al., 1999) Temperature profile and surface conditions : ERA40 Mean July over BoB Relative humidity in the free troposphere :from 5% to 100% (P wat ~35 to 75 mm) “Black” Cloud at 250hPa : fraction from 10 to 70 % per 5% * January * July  ~7Wm -2 ~3Wm -2 OBS January~July~80Wm -2

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, hPa 850hPa.…. Relative Humidity (%) Idealized radiative transfer computations Column Radiation Model (Zender et al., 1999) Temperature profile and surface conditions : ERA40 Mean July over BoB Relative humidity in the free troposphere :from 5% to 100% (P wat ~35 to 75 mm) “Black” Cloud at 250hPa : fraction from 10 to 70 % per 5% ~3Wm -2 ~43Wm -2 ~40Wm -2 ~36Wm -2 ~7Wm -2 OBS January ~ 38 Wm -2 July~25 Wm -2

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, At seasonal scale, over the two key regions, BoB exhibits similar LWCRF to EiO in January together with : More Upper Level Cloudiness ; Similar MCS repartition Similar MCS lw radiative properties More Precipitable Water; The water vapor damping effect on the LWCRF in July seems to explain the obs. At regional scale, over the whole Ocean, the dependence of LWCRF upon ULC confirms the damping role of clear sky or environmental water vapor These two observed estimations are in agreement with very idealized radiative transfer simulations ruling out a large bias in the data sets This study suggests that the strongly negative net CRF regime of Bay of Bengal is not only due to unique shortwave and albedo effects but also to the heavy water vapor loading that damped the long wave crf that cannot compensate the strongly negative shortwave crf. Conclusions

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, India October 17-20, If anything this study reaffirms the need to look at the water vapor together with the cloud radiative properties to understand the LWCRF and hence its net forcing. Speculations… (from Trenberth et al., 2005) The « near cancellation » regime in the warm pool is happening in a relatively dry environment (P wat ~50mm). In the future, the tropical ocean might keep on moistening and we might shift towards a « bay of Bengal » like regime.