EARTH RADIATION BUDGET OUTGOING LONGWAVE RADIATION OLR EARTH

EARTH RADIATION BUDGET & OUTGOING LONGWAVE RADIATION (OLR)

EARTH RADIATION BUDGET Why Study About Earth Radiation Budget? Absorption of Solar Radiation and emission of Terrestrial Radiation are largely responsible for general circulation and Earth’s weather & climate

EARTH RADIATION BUDGET l Aim – To discuss the satellite measurements of the constituent radiation quantities which comprise Earth’s radiation budget l First Met measurements from space were made by Suomi radiometer on board Explorer -7 launched on 13 Oct 59

SOLAR CONSTANT l Solar energy reaching the earth l It is the annual average solar irradiance received out side earth’s atmosphere on a surface normal to the incident radiation and at the Earth’s mean distance from the sun l Actual solar irradiance varies by + 3. 4% due eccentricity of earths orbit l Ground based measurements date back to 1913. Need to be corrected for atmospheric effect l Space based measurements available from 1975

HISTORY OF ERB SENSORS l Nimbus 6 & 7 carried ERB – 10 channel sensor designed to measure sun’s radiation in several spectral intervals – 0. 2µm - >50µm l Solar Maximum Mission (SMM)Satellite – ACRIM (Active Cavity Radiometer) – Launched on 16 Feb 80 took measurements until Dec 90 l ERBE sensor was flown on ERBE Satellite & NOAA 9& 10

HISTORY OF ERB SENSORS l l l TRMM – – – PR (precipitation radar) LIS (Lightning Imaging Sensor) CERES (Clouds and the Earth's Radiant Energy System TMI (TRMM Microwave Imager) VIRS – – – ASTER, or Advanced Spaceborne Thermal Emission and Reflection Radiometer; CERES, or Clouds and Earth's Radiant Energy System; MISR, or Multi-angle Imaging Spectroradiometer; MODIS, or Moderate-resolution Imaging Spectroradiometer; and MOPITT, or Measurements of Pollution in the Troposphere. TERRA SORCE (SOlar Radiation and Climate Experiment) Launched January 25, 2003 – SORCE equipped with 4 instruments that measure variations in solar radiation

l The "A-Train" – six satellites flying in close proximity – Aqua, Aura, Cloud. SAT, CALIPSO, PARASOL and, OCO – Glory will join as the seventh in 2008/2009 • Collect data on the properties of aerosols and black carbon in the Earth's atmosphere and climate system • Collect data on solar irradiance for the long-term effects on the Earth climate record

HISTORY OF ERB SENSORS l MEGHA-TROPIQUES – MADRAS(Microwave Analysis and Detection of Rain and Atmospheric Structures): A microwave imager aimed mainly at studying precipitation and clouds properties – SAPHIR(Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie : a 6 channels microwave radiometer for the retrieval of water vapour vertical profiles and horizontal distribution – SCARAB(Scanner for Radiation Budget) a radiometer devoted to the measurement of outgoing radiative fluxes at the top of the atmosphere.

ANNUAL AVERAGE SOLAR CONSTANT Annual average solar constant (W/m 2) as measured by channel 10 C of the Nimbus 7 ERB and by the Solar Maximum Mission ACR 1 M. The dashed line is the annual average Zurich sunspot number. .

ANNUAL AVERAGE SOLAR CONSTANT l Solar constant seems to follow sun spot number l Mean Nimbus-7 solar const measured over sun spot cycle is about 1372 W/m 2 l Mean as measured by Solar Max ACRIM overa period shorter than solar cycle is 1367. 5 W/m 2 l ERBE measurements are apprximately 1365 W/m 2 l Diff attributable to difficulty in calibrating the sensor in lab

TOA RADIATION BUDGET The goal of radiation budget studies is to measure the incoming & outgoing radiation as a function of time & space. Incoming radiation is Irradiance and is given by symbol S(t, , , h), where t is time, is latitude, is longitude & h is height above the earth’s surface l Satellite methods to measure radiation budget quantities depend on the altitude in which one is interested. The two most common altitudes are l – the top of the atmosphere (TOA, often defined for radiation budget purposes as 30 km) – the surface of the Earth

TOA RADIATION BUDGET l Outgoing radiation is Radiant Exitance and is given by the symbol M(t, , , h). Usually we are not interested in the wavelength dependence of these quantities except for the division between shortwave & longwave components. Shortwave radiation, usually defined as radiation with wavelengths < 5 m, is primarily reflected solar radiation. Longwave radiation (> 5 m) is primarily emitted (terrestrial radiation).

TOA RADIATION BUDGET At the top of the atmosphere, the quantities we want to measure are: Ø v v Shortwave radiant exitance (MSW), Longwave radiant exitance (MLW), {Also called OLR} With these quantities we can calculate the albedo: Ø A (t, , h) = M(t, , , h) / ESun

TOA RADIATION BUDGET Ø AND ABSORBED SOLAR RADIATION Eabs(t, , h) = (1 – A) Esun Ø AND THE NET RADIATION Enet(t, , h) = (1 – A) Esun - MLW = Esun - MSW - MLW In these Equations, sun = sun(t, , ) is the cosine of the solar zenith angle, and Esun = Esun(t) is the solar constant Ssun adjusted for distance from the sun, Esun = Ssun(dsun/ dsun(t))2 Where dsun(t) is the earth – sun distance at time t, and dsun is the mean earth-sun distance (1 austronomical unit, au).

RADIATION BUDGET INSTRUMENTS Instruments used for radiation budget studies can be divided into two spectral categories. l – Broadband sensors which attempt to measure spectrally integrated radiation budget quantities e. g (ERBE) sensors – Narrow band sensors which measure only narrow portion of the spectrum: extrapolation of other portions of the spectrum is necessary for radiation budget calculations. E. g AVHRR on the NOAA satellites is a narrowband sensor

RADIATION BUDGET INSTRUMENTS Radiation budget instruments may also be divided into two field-of-view (FOV) categories; l – Wide field-of-view (WFOV) sensors – Narrow-field-of-view (NFOV) sensors. WFOV sensors measure radiation from horizon to horizon. They are often called flat plate sensors because they typically consist of a flat element that measures the irradiance received at the satellite. WFOV sensors make measurements which have been integrated over large areas, typically a circle of radius 2500 -3500 km at the surface. WFOV measurements are useful for global or large-scale studies. l NFOV sensors measure radiation in a narrow cone. AVHRR is a NFOV instrument. l

RADIATION BUDGET A polar-orbiting satellite is a very good platform for measuring the radiation budget. A satellite-mounted instrument makes two observations daily, separated by about 12 h, of every point on earth. These are sufficient to calculate the monthly average radiation budget, which is usually the time scale of interest. l

RADIATION BUDGET l There are some problems, however: -

OLR Calculation l The IR radiance is first corrected for viewing angle. i. e the zenith angle is converted to zero deg zenith angle l Spectral correction is applied to convert narrow band observation to broad band radiant exitance l As per Abel & Gruber(1979), the Flux temperature is statistically related to equivalent black body temperature. Accordingly, radiance L is converted to TBB in 10 -12 m window. Then the flux temp is calculated as : TF = (a+b. TBB)TBB MLW=OLR = σT F 4

OLR Calculated separately for ascending and descending pass l The OLR during day time, OLR during night time separate data sets are available l OLR Atlas prepared l – NOAA data set – Nimbus 7 data set – ERBE data set Comparison of OLR calculated from NOAA & Nimbus-7 ERB showed monthly globally averaged OLR estimates agreed to within about 1 -2 wm-2. Monthly zonally averaged OLR showed rms diff of up to 9 wm-2 l Very useful for Climatic studies l

OBJECTIVE CRITERIA FOR MONSOON ONSET l I Met D ( P K Das: Monsoons by Fein and Stephen 1987) – Beginning with May 10 at least five out of 07 met stations in Kerala should record 24 hr. rainfall amount of 1 mm or more for two consecutive days. The monsoon’s arrival is announced on 2 nd day. l I Met D New Criteria : 2006 – After 10 May, 60% of the available 14 stns (MNC, AMN, TRV, ……. . , MNG) record 2. 5 mm or more for consecutively 2 days, then 2 nd day is the onset day provided the following criteria are also satisfied • Windfield: Depth of westerlies should be up to 600 h. Pa, EQ-10 o and 55 - 80 o E and zonal wind speed in 05 -10 o and 70 - 80 o E should be 15 -20 Kt at 925 h. Pa • OLR: INSAT derived OLR <200 W/m 2 in the box 05 -10 o N and 7075 o E