Cloud Oxygen Pressure Algorithm for POLDER-2

Transcription

1 Cloud Oxygen ressure Algorithm for OLDER-2 1/7 Cloud Oxygen ressure Algorithm for OLDER-2 Aim of the : Determination of cloud gen pressure from arent pressure by removing the contribution. Date of the document: April 2003 (revised) Author: C. Vanbauce Laboratoire d Optique Atmosphérique UMR CNRS Université des Sciences et Technologies de Lille, Villeneuve d Ascq Cedex (France) [email protected] Content: 1. INTRODUCTION 2. ALGORITHM DESCRITION 3. OUTUT ARAMETERS 4. EXECTED RESULTS 5. REFERENCES Development of the OLDER Earth radiation budget, water vapor, and clouds s results from a joint effort of Laboratoire d Optique Atmosphérique (LOA), Laboratoire des Sciences du Climat et de l Environnement (LSCE) and Laboratoire de Météorologie dynamique (LMD). It has been supported by CNES (Centre National d Etudes Spatiales), CNRS (Centre National de la Recherche Scientifique) and Région Nord-as de Calais.

2 Cloud Oxygen ressure Algorithm for OLDER-2 2/7 1. INTRODUCTION The aim of this is to determine the cloud gen pressure from the arent pressure by removing the contribution. In a first step, the ground reflectivity effect is neglected. An arent pressure is inferred by assuming that the atmosphere behaves as a pure absorbing medium overlying a perfect cloud reflector located at pressure (Vanbauce et al., 1998). is determined from differential absorption between the radiances measured in the channels centered at 763 and 765 nm respectively (Buriez et al., 1997). More details can be found in the Apparent Oxygen ressure Algorithm for OLDER-2. Because of the effects of reflection and multiple scattering inside the cloud, the arent pressure is almost always higher than the cloud top pressure. It is even higher than the cloud base pressure when a lot of photons reach the before being reflected back to space, that is in the case of a thin cloud layer above a bright. Cloud Oxygen pressure is determined from arent pressure by removing the contribution when necessary. This correction is made using the cloud spherical albedo inferred from the Spectral Albedo and Cloud Optical Thickness Algorithm. 2. ALGORITHM DESCRITION The Cloud Oxygen ressure Algorithm is lied to each cloudy super-pixel. When the cloud spherical albedo S is not determined (over sea ice or snow covered land s for example) the cloud gen pressure is not calculated. A schematic description of the is reported in Fig.1. In the following, all operations are made viewing direction by viewing direction. In a first step, the arent pressures ( ), the absorption-corrected reflectances at 765 nm (R 765 ) and the cloud spherical albedoes (S) are averaged over the cloudy pixels for which the cloud spherical albedo is greater than a threshold value S o = Thick clouds In the case of thick clouds, is generally situated between the cloud top level and the cloud base one. For superpixels over ice-free ocean we consider that a correction is not necessary because the sea- albedo is weak and only cloudy pixels for which the cloud spherical albedo is larger than 0.3 have been selected. It is also the case for superpixels over snow-free continental s with great enough cloud spherical albedo (S> 0.75). In these two cases, the cloud gen pressure is simply equal to the arent pressure. For all other cases, further calculations are needed to obtain. The different cases are resumed in Table 1. ocean land S 0.3 < S < 0.75 S a b a Table 1: Description of the different cases treated in the Cloud Oxygen ressure Algorithm. a: =, b: = f(, r, )

3 Cloud Oxygen ressure Algorithm for OLDER-2 3/ Thin clouds In the case of a thin cloud layer, a lot of photons can reach the before being reflected back to space. In this case, the arent pressure can be outside the cloud layer limits (arent pressure larger than cloud base pressure). To remove this contribution a method has already been proposed by Buriez et al. (1997). Let the corrected cloud pressure be defined as the arent pressure that would be observed if the reflectivity was equal to zero. Because the gen A-band correspond to strong absorption lines, the gen transmission T O2 can be treated in first roximation by means of a random band model (Goody, 1964): T O exp( C m ) (1) 2 where m is the air-mass factor and C a constant depending on spectroscopic data. Schematically, this transmission can be decomposed in a term corresponding to the light directly reflected by the cloud and a term corresponding to the light reflected after reaching the (Fig. 2): exp ( C m ) r.exp ( C m ) (1 r).exp ( C m M [ ] ) (2) where r is the fraction of photons directly reflected by the cloud, M is the effective air-mass factor corresponding to the mean photon path between the cloud and the and the pressure. 0 Figure 2: Schematic representation of radiation transfer through a cloudy atmosphere (Buriez et al., 1997). Because is not directly deducible from Eq. (2), we use a simplified method. Assuming that the effective air-mass factor M is equal to m, Eq. (2) can be rewrite in exp ( C m ) exp ( C m ). 1 r. exp ( C m( )) 1 (3)

4 Cloud Oxygen ressure Algorithm for OLDER-2 4/7 Considering that the transmission between the cloud and the is a small corrective term Eq. (3) can be roximated as: exp ( C m ) exp ( C m ). 1 r C m ( ) (4) using again this roximation, we find which is equal to exp ( C m ) exp ( C m ). exp ( r C m( ) ) (5) exp ( C m ) exp ( C m r ( ) ) (6) which can finally be rewritten as (7) [ ( r 1) ]/ r. The fraction of photons directly reflected by the cloud, r, is calculated using r = R o 765 / R 765 where R 765 is the reflectance measured by OLDER at 765 nm after correction for gaseous absorption and R o 765 is the reflectance that would be measured if in addition the was black. is obtained from the ECMWF (European Center for Medium range Weather Forecasts) analysis. In practice, R o 765 is computed by using the cloud spherical albedo determined from OLDER measurements at 670 nm and look-up tables (LUTs) of calculated reflectances at 765 nm Look up tables The LUTs used to compute R o 765 are similar to those used in the Spectral Albedo and Cloud Optical Thickness Algorithm: The top-of-atmosphere bidirectional reflectances are calculated by using the plane-parallel radiative transfer model developed by de Haan et al. (1987). These tables are built at 765 nm, with an atmosphere model overlaying a black and for: - 2 cloud types (ice and liquid water) - 20 values of cloud spherical albedo (which is a one-to-one function of the cloud optical thickness) - 33 values of cosine of solar zenith angle - 28 values of viewing zenith angle - 37 values of relative azimuth angle The atmosphere model and the cloud types are those used over land s in the Spectral Albedo and Cloud Optical Thickness Algorithm : the atmosphere is only composed of molecules (no aerosol) ; liquid water clouds are composed of droplets with effective radius of 9 m and ice clouds are composed of inhomogeneous hexagonal monocrystals (IHM) as described in C.-Labonnote et al. (2000) Averaging over the directions The last step of the is the averaging over the viewing directions i. The averaging of all the calculated values (i) is weighted by the percentage of photons directly reflected by the cloud r(i) and by the cloud cover CC(i). The associated angular standard deviation is also calculated.

5 Cloud Oxygen ressure Algorithm for OLDER-2 5/7 Cloud detection CC 0 Cloud optical thickness S Apparent gen pressure For every viewing direction i CC(i) 0 Averaging over the cloudy pixels with S > S o R 765 Gaseous absorption correction Surface conditions Further calculations needed? no yes Geometry conditions = LUTs R o 765 hase Cloud thermodynamic phase fraction of photons directly reflected : R r R o ( r 1) r Averaging over the viewing directions Figure 1: Scheme of the Cloud Oxygen ressure Algorithm 3. OUTUT ARAMETERS Concerning the gen pressure, only non-directional parameters are delivered in the ERB, WV & clouds products: - the mean cloud gen pressure - the Angular standard Deviation AD( )

6 Cloud Oxygen ressure Algorithm for OLDER-2 6/7 4. EXECTED RESULTS From comparisons between OLDER-1 cloud gen pressure and ARM/MMCR (Clothiaux et al., 2000) cloud boundaries pressures, ears to indicate the cloud middle pressure rather than the cloud top pressure as shown in Fig. 3 (Vanbauce et al., 2003). Figure 3: Location of ADEOS1-OLDER corrected gen pressure in relation with ARM/MMCR cloud boundaries pressures, as a function of cloud geometric thickness for the 37 selected cases. In this figure, the y-axis pressures have been linearly rescaled as follows: cloud tops are fixed to 0 and cloud bases are fixed to 1. Circles represent mono-layered clouds and triangles multi-layered clouds. White symbols are for ice clouds while liquid clouds ear in black. OLDER data are means and standard deviations over 3*3 pixels around the SG Central Facility and over all viewing directions. 5. REFERENCES Buriez, J. C., Vanbauce, C., arol, F., Goloub,., Herman, M., Bonnel, B., Fouquart, Y., Couvert,. and Sèze, G., 1997: Cloud detection and derivation of cloud properties from OLDER, Int. J. Remote Sensing, 18, C.-Labonnote, L., Brogniez, G., Gayet, J. F., Doutriaux-Boucher, M. and Buriez, J. C., 2000: Modeling of light scattering in cirrus clouds with inhomogeneous hexagonal monocrystals. Comparison with in-situ and ADEOS-OLDER measurements, Geophys. Res. Lett., 27, Clothiaux, E. E., Ackerman, T.., Mace, G. G., Moran, K.., Marchand, R. T., Miller, M. A. and Martner, B. E., 2000: Objective determination of cloud heights and radar reflectivities using a combination of active remote sensors at the ARM CART sites, J. Appl. Meteor., 39, Goody, R. M., 1964: Atmospheric Radiation I. Theoretical Basis, Oxford: Clarendon ress.

7 Cloud Oxygen ressure Algorithm for OLDER-2 7/7 de Haan, J. F., Bosma. B. and Hovenier J. W., 1987: The adding method for multiple scattering computations of polarized light, Astron. Astrophys., 183, Vanbauce, C., Buriez, J. C., arol, F., Bonnel, B., Sèze, G. and Couvert,., 1998: Apparent pressure derived from ADEOS-OLDER observations in the gen A-band over ocean, Geophys. Res. Lett., 25, Vanbauce, C., Cadet, B. and Marchand, R.T., Comparison of OLDER arent and corrected gen pressure to ARM/MMCR cloud boundary pressures, Geophys. Res. Lett., 30(5), 1212, doi: /2002GL016449, 2003.