IPWG, M. Desbois, Megha-Tropiques, Melbourne, October 20061 Status of the Indo-French...

IPWG, M. Desbois, Megha-Tropiques, Melbourne, October 2006 1

Status of the Indo-French Megha-Tropiques satellite project,

perspectives for tropical systems studies

Michel Desbois, Laurence Eymard, Rémy Roca, Nicolas Viltard, Michel Viollier, Michel Capderou

with support of and


Atmospheric energy budget in the intertropical zone and at system scale (radiation, latent heat, …)

Water budget of the systems (including precipitation and water vapor transport)

Conditions of appearance and development of these systems (Surface temperature, water vapor, winds,…)

Life cycle of Mesoscale Convective Complexes in the Tropics (over Oceans and Continents)

Megha-Tropiques scientific objectives


Megha-Tropiques additional objectives

Operational aspects :

assimilation for Cyclones, Monsoons, Mesoscale Convective Systems forecasting. (water vapour and precipitation)

Contribution to climate monitoring :

- Radiative budget (complementary to CERES) - Precipitation (enhanced sampling in the tropics) - Water vapour (tropical sampling)


Frequent sampling of the intertropical zone measuring radiances related to :

- Cloud properties and precipitation

- Water vapor horizontal and vertical distribution

- Outgoing radiative fluxes

Association with operational satellites (geo and leo)

Principles of the Megha-Tropiques Mission


Coverage 23°N to 23°S, repetition time from 3 to 6 times per day

Tropical Orbit (20° inclination)

Wide Swath (altitude > 800 km)

Study of Mesoscale convective systems > 100 km

Surface resolution from 10 to 40 km, depending on the parameter.

Main Mission features


Megha-Tropiques orbit


The three instruments of Megha-Tropiques

•ScaRaB : wide band instrument for inferring longwave and shortwage outgoing fluxes at the top of the atmosphere (cross track scanning, 40 km resolution at nadir)

•Saphir : microwave sounder for water vapour sounding : 6 channels in the WV absoption band at 183.31 GHz. (cross track, 10 km)

•MADRAS : microwave imager for precipitation : channels at 18, 23, 37, 89 and 157 GHz, H and V polarisations. (conical swath, <10 km to 40 km)


Position on the ISRO Platform


1700 km

2300 km

SAPHIR

ScaRaB

MADRAS

Schematic representation of the swath of the 3 MT instruments


Madras characteristicsColumn water vapor : 23 GHz

Sea surface wind :18 GHz

Precipitation, cloud water : 18, 23, 36.5, 89 GHz

Active convective areas, cloud ice : 89, 157 GHz

Two polarisations required for all parameters, except column water vapor (23 GHz)

Precipitation and cloud properties are best obtained from combination of channels. Proper combination requires identical field of views. This is specified for channels 18, 23, 36.5 GHz.

Resolution at 89 GHz : 10 km with adjacent pixels, in order to get complete images of the active convective parts of the systems.

Resolution for lower frequencies will be lower (40 km) with oversampling in order to keep some information on smaller scales

157 GHz channel is an experimental channel intended to provide new information on ice at cloud top, eventually helping for precipitation estimation over land, and for lower layers WV retrieval. It will provide the best space resolution.


MADRAS specifications

Channelno.

Central frequencies(GHz)

Max. Bandwidth(MHz)

Stability of frequency centre (MHz)

NEDT (T°)sensitivity at 300KGoal. Req'd

Absolute calibration (K)(110°K-320°K)

Inter-channel calibration (K)

M1 18.7 ± 100 0.1% 0.5 K 0.7 K ±1 K 0.5 K

M2 23.8 ± 200 0.1% 0.5 K 0.7 K ±1 K 0.5 K

M3 36.5 ± 500 0.1% 0.5 K 0.7 K ±1 K 0.5 K

M4 89 ± 1350 ± 150 1K 1,1 K ±1 K 0.5 K

M5 157 ± 1350 ± 150 2K 2.6 K ±1 K 0.5 K

Channel no.

Frequencies Polarisation Spatial resolution (km)

M1 18.7 GHz H+V 40 ± 10%

M2 23.8 GHz V 40 ± 10%

M3 36.5 GHz H + V 40 ± 10%

M4 89 GHz H + V 10 ± 10%

M5 157 GHz H + V 6 ± 10%


89 GHz channel of AMSU-B


150 GHz channel of AMSU-B


183.31 ± 7 GHz channel of AMSU-B


Monitoring of the Earth Radiation Budget : ScaraB/Megha-Tropiques could partially fill a gap in RB measurements from CERES (Wielicki)

Global Precipitation Mission : Madras/Megha-Tropiques to be associated to

this multi-satellite mission to increase its tropical sampling.

Complementarity to other missions

MT


1) Actions devoted to algorithm developments

- Synthetic and Satellite data bases for testing algorithms and processing procedures of the Megha-Tropiques instrumental package.

- Microwave rain algorithm developments : adaptation and evaluation of TRMM-based algorithms; specific effort on the ice phase.

- Combined geostationary - microwave algorithms.

- Radiative fluxes retrievals at different space-time scales. - Humidity retrievals in the perspective of SAPHIR

Present scientific developments around MT


2) Actions devoted to validation of satellite products

- Precipitation retrieval validation (raingauges and radars) : sampling and scaling effects.

- Validation for other quantities (ice microphysics, others…)

- Different methods for water vapour determination (IR and MW sounders, GPS, … )

- General use of the AMMA campaigns and data bases for satellite validation studies

- Promoting specific campaigns for water vapour, clouds and radiative budget



3) Actions devoted to MT sampling effects

- Use of geostationary observations to simulate the MT sampling of convective systems and their life cycle. To be performed over Africa, India and Brazil.

- Development of a simulator for MT from analyses / satellite observations to analyze the impact of MT orbit on the various fields retrieved (radiative budget, water vapour …)



4) Actions devoted to MT thematic science objectives

- Many actions in the frame of other tropical research programmes (e.g. AMMA, …)

- Opening of a « science team » to the international community expected in 2007 - depending on CNES-ISRO agreements.



Limitations of MT for precipitation determination

Megha-Tropiques was not designed specifically for precipitation measurements

The space resolution at lowest frequencies of Madras is not sufficient for the size of many convective systems

The time sampling is better than for any other microwave instrument, and will be still better when associated to other satellites, but is not sufficient for regions where rainfall occurs in few intensive events.

Sampling of the diurnal cycle is very specific with this orbit, and time cumulations have to be adapted to that sampling

« calibration » through coincident passes with a space radar -as the mother satellite of GPM- will be very occasional.


Proposal for a new mission devoted to tropical rainfall

TRMM Precipitation Radar is presently the best instrument to measure precipitation from space.

It has been proven that its poor time sampling due to the narrow swath can be partially compensated by its association with a proper microwave radiometer. Association with geostationary IR data at 15 minutes intervals is leading to significative improvements in precipitation accumulation estimates (see further presentation on African rainfall during AMMA)

The next generation of precipitation radar for GPM will not be in tropical orbit, and its sampling of the tropical area will be poorer than the TRMM PR

Precipitation issues in the tropics, in the context of climate change, are vital for the people and countries of this area.


Proposal for a new mission devoted to tropical rainfall

It is proposed to study a new tropical mission (Monsoon Precipitation Mission or MPM) carrying a radar and a radiometer.

This mission would be a component of the « virtual constellation » proposed in the context of the CEOS virtual precipitation mission concept, as an extension of GPM

The European company « Alcatel Alenia Space » has already performed preliminary studies of the concept of a new space radar. A radiometer has also to be defined.

International cooperation with concerned tropical countries will be necessary to go further in the studies of this mission.

23IPWG, M. Desbois, Megha-Tropiques, Melbourne, October 2006

– Proposed scanning geometry for radar antenna and radiometer antenna

• Radar across-track scanning– ± 17° off-Nadir angle

(drives the antenna design& rain cell contamination by ground echo at swath edges)

– Around 250 km swath width

• Radiometer conical scanning– 52.8° incidence angle

(typical for radiometer)– around1000 km swath width

Satellite track

Radio

mete

r sw

ath

: 800 t

o 1

000 k

m

Radar

swath

250 k

m

Range resolution

250 m

Footprint

<5 km

Ob

serv

ed

alt

itu

de

ran

ge:

~2

0 k

m

All rights reserved © 2005, Alcatel Alenia Space

Monsoon Precipitation Mission


•Recommended instrument parameters– Orbit altitude : as low as possible

• Recommended lower than 500 km to decrease antenna size and HPA peak power demand

– Around 400 km selected

– Footprint : < 5 km• Drives the antenna size

– ScanningEither• Mechanical scanning

– Either across-track or conical

or• Electronic scanning

– Only across-track



Comparison with other rain radar instruments

Radar Customer Date Orbit Repeatcycle Swath IFOV Rain Rate

(mm/hr)Radiometric

resolutionVertical

resolutionBEST study:Ku-band rain radar CNES 1990-92 400-500 km

28.5° 5 days 100 km <3 km 0.5-60 20% 250 m

Feasibility study:Ku+K-band rain radar ESA 1992-93 500 km

56° 1 day 400 km <4 km 0.5-50 (Ku)0.1-10 (K) 10% 250 m

TRMM-FO phase A:24-GHz rain radar

CNESNASDA 1993-94 420 km

50° 35 days 250 km <3 km 0.15-20 20% 250 m

EGPM* phase A: design& performances of aNadir Pointing Radar inKa-band

ESA 2001-04 SS0 670 km 3 days 3 x 4 km 4 km 0.1-15 <50% 250 m

TRMM-PR:Ku-band rain radar

NASAJAXA

1997still in-orbit

350 km35° ? 215 km 4.3 km 0.5-50 12.5 % 250 m

GPM-PR:Ku-band rain radarKa-band rain radar

NASAJAXA

2011 407 km65° ? 245 km

120 km5 km5 km

0.5-500.2-50

12.5 %12.5 %

250 m250/500 m

ProposedKu-band rain radar

ISRO ?CNES ?

OTHER?TBD ~400 km

TBD° TBD 250 km 5 km 0.2-60 10% 250 m

* European contribution to the Global Precipitation Mission


All rights reserved © 2005, Alcatel Alenia Space


– Joint consolidation phase to be done to refine• Mission specifications with support of scientists

• Interfaces specification between instrument/platform/launcher

• Selection of an instrument concept

• Instrument design and performances

• Subsystem specifications

• Instrument mass/consumption/volume budgets assessment


IPWG, M. Desbois, Megha-Tropiques, Melbourne, October 20061 Status of the Indo-French...

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