3rd indo-french workshop on megha-tropiques under isro-cnes programme on atmosphere, climate and...

3rd Indo-French Workshop on Megha-Tropiques under ISRO-CNES Programme on Atmosphere, Climate and Oceanography, Ahmedabad, IndiaOctober 17-20, 2005.

Convection, water vapor and radiation over the Indian OceanWill 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)


(From Rajeevan and Srinivasan, Jclim, 2000)

JJAS 1985-1988

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


Data and Methodology

Period of investigation :Julys 1985-1988 and Januarys 1986-1989

ERBE s4g regional monthly mean flux (2.5°x2.5°; monthly mean)LWCRF = Fclear –FtotSWCRF = Sclear –Stot NetCRF = SWCRF+LWCRF

Precipitable water derived from SSM/I (Wentz et al., 1997)

Climatology using the 1988-1999 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


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 heightAveraged BT as an indicator of LW properties


Two key regions:January: the eastern Indian Ocean (EIO) slightly negative net forcing regimeJuly: the Bay of Bengal (BoB) strongly negative net forcing regime

Cloud radiative forcing over the Indian Ocean


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-2Similar Upper Level Cloudiness ?Similar Mesoscale Convective Systems properties & Occurrence ?Similar Clear sky back ground ?


Mesoscale Convective Systems properties

January Eastern Indian OceanTmin (K) Tavg (K) Effective Emissivity

Class 1 207.0 (3.0) 230.2 (0.9) 0.66 (0.02)Class 2 226.0 (0.0) 238.7 (0.7) 0.80 (0.00)Class 3 242.0 (1.7) 249.6 (1.7) 0.89 (0.01)

July Bay of BengalTmin (K) Tavg (K) Effective Emissivity

Class 1 204.3 (3.7) 228.6 (3.0) 0.66 (0.03)Class 2 226.3 (0.5) 238.9 (0.7) 0.80 (0.01)Class 3 243.0 (0.8) 251.3 (0.9) 0.89 (0.01)

eff = Tmin4 / Tb avg of the system

4

SimilarRepartition of Classes Effective emissivity

DifferentULC Fraction: BoB > EiO

39% 35%


Clear Sky Environment : OLR and Precipitable Water

Precipitable Water ~45-50 mmPrecipitable Water ~55-70 mm

Smaller Clear sky OLRIn July than in January

Moister column in July BoB than in January EiO

LWCRF: similar in both seasons & regions ~ 80 Wm-2More ULC in July than JanuarySimilar MCS properties & OccurrenceMoister Clear sky back ground in July than January

277 Wm-2

283 Wm-2

DAMPING EFFECT OF WATER VAPOR ON THE LWCRF


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

PWAT from 48 to 53 mmJuly

LWCRF from 65 to 88 Wm-2

PWAT from 49 to 60 mm


250hPa

850hPa

.….

Relative Humidity (%)

75 1005 10

Idealized radiative transfer computations•Column Radiation Model (Zender et al., 1999)•Temperature profile and surface conditions : ERA40 Mean July 1985-1988 over BoB •Relative humidity in the free troposphere :from 5% to 100% (Pwat ~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


250hPa

850hPa

.….

Relative Humidity (%)

75 1005 10

Idealized radiative transfer computations•Column Radiation Model (Zender et al., 1999)•Temperature profile and surface conditions : ERA40 Mean July 1985-1988 over BoB •Relative humidity in the free troposphere :from 5% to 100% (Pwat ~35 to 75 mm)•“Black” Cloud at 250hPa : fraction from 10 to 70 % per 5%

~3Wm-2

~43Wm-2

~43Wm-2

~40Wm-2

~36Wm-2

~7Wm-2

OBS January ~ 38 Wm-2

July~25 Wm-2



At seasonal scale, over the two key regions, BoB exhibits similar LWCRF to EiO inJanuary together with : More Upper Level Cloudiness ;Similar MCS repartitionSimilar MCS lw radiative propertiesMore 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 ULCconfirms the damping role of clear sky or environmental water vapor

These two observed estimations are in agreement with very idealized radiativetransfer 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


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 (Pwat ~50mm). In the future, the tropical ocean might keep on moistening and we might shift towards a « bay of Bengal » like regime.

3rd indo-french workshop on megha-tropiques under isro-cnes programme on atmosphere, climate and...

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