How To Understand The Results Of The German Meris Cloud And Water Vapour Product

Transcription

1 Ttel: Project: Doc. No.: MERIS level 3 cloud and water vapour products MAPP MAPP-ATBD-ClWVL3 Issue: 1 Revson: 0 Date: Functon Name Organsaton Sgnature Date Author: Bennartz FUB Preusker FUB Schüller FUB Fscher FUB

2 Datum : Sete : Internal Dstrbuton Name Organsaton Qantty External Dstrbuton Name Organsaton Quantty Change Record Issue Revson Date Changes

3 Datum : Sete : 3 MERIS level 3 cloud and water vapour products R. Bennartz, R. Preusker, L. Schüller, and J. Fscher Insttut für Weltraumwssenschaften Free Unverstät Berln 1 INTRODUCTION Clouds and atmospherc water vapour have sgnfcant mpact on the Earth s radaton budget and on the energy and water cycle. Exact knowledge about cloud parameters and water vapour s therefore prerequste for clmatologcal nvestgatons as well as for numercal weather predcton models e. g. n order to valdate cloud parameterzaton schemes. Most common n clmate research s the ongong Internatonal Satellte Cloud Clmatology Project (ISCCP, e. g. Schffer and Rossow, 1983; Rossow et al., 1991) that provdes global data on a monthly average as well as on a three-hourly bass wth a horzontal resoluton of 50 km and 280 km, respectvely. MERIS wll contrbute to both research areas by provdng estmates of atmospherc water vapour content, cloud top pressure, and cloud optcal depth. For each overpass these parameters wll be retreved wth the full MERIS resoluton of 300 m at nadr. 2 ALGORITHM OVERVIEW Wthn the German MERIS processor monthly and weekly averaged cloud and water vapour products wll be generated. These products are derved from MERIS level 2 cloud optcal thckness, cloud top pressure, and columnar water vapour content. Two dfferent products wll be generated based on these data: 1. A coarse cloud classfcaton solely based on cloud optcal depth and cloud top pressure wll be appled. Smlar to the ISCCP cloud classfcaton (e. g. Rossow et al., 1991) nne dfferent classes of clouds and one cloud-free class wll be dstngushed. Weekly and monthly averages and varances of cloud frequency, cloud optcal depth, and cloud top pressure wll be provded for each class. 2. A second product wll provde data on a dense vertcal grd wthout an explct classfcaton of cloud type. The vertcal grd resoluton wll be 50 hpa where each level 2 data wll be grouped accordng to the derved cloud top pressure. In addton to the cloud products a

4 Datum : Sete : 4 water vapour product wll be provded n the same grd. Ths product conssts of average columnar water vapour content and ts varance above each cloud layer. The horzontal resoluton of both products wll be km 2. Ths resoluton s a compromse between the demand of hgh-resoluton nformaton and numercal and practcal consderatons. Whle the resoluton s stll suffcent to compare even to hgh resoluton numercal weather predcton models, the nvestgaton area s covered by a array of data for each varable and thus the dataset s manageable. Table 1 provdes an overvew of the data derved wthn both level 3 products. Table 1: MERIS Level 3 cloud and water vapour products overvew. The numbers refer to the aforementoned dfferent categores of level 3 products. Name Average Varance No. of Measurements Fractonal cloud cover 1,2 1,2 1,2 Cloud top pressure 1,2 1,2 1,2 Cloud optcal depth 1,2 1,2 1,2 Water vapour content 2 2 2

5 Datum : Sete : 5 3 ALGORITHM DESCRIPTION 1.1 ISCCP-Compatble product Fgure 1 depcts the ISCCP cloud classfcaton. Wthn ISCCP nne dfferent classes of clouds are dentfed accordng to ther optcal depth and cloud top pressure. Although the synoptc classfcaton n dfferent cloud types s somewhat arbtrary and largely depends on the scale on whch the clouds are observed, ts value n global and regonal cloud studes has been demonstrated n varous studes. Fgure 1: ISCCP cloud classfcaton accordng to Rossow et al. (1991). The ISCCP-classfcaton s easy to adapt for MERIS, snce the two parameters on whch the classfcaton s based, are cloud optcal thckness and cloud top pressure, both avalable on MERIS processng level-2. In addton, snce cloud/no-cloud flags are provded, a cloud-free class wll be ntroduced, allowng for the calculaton of cloud-frequency, thus cloud classes

6 Datum : Sete : 6 rangng from 1 to 10. For a gven km 2 box, N vald MERIS-observaton wthn ths box, the followng parameters wll be calculated for each overpass: N N addng class 10 (cloud - free) N TOT TOT = number of observatons n cloud class k, k = 1,..,10 9! = N k = 1 + N 10 = N total number of cloud observatons obvously yelds : (1) (2) (3) C C TOT CTP = N VAR_CTP COD = N VAR_COD / N cloud frequency n class k,k = 1,..,10 TOT / N total cloud cover = MEAN(CTP k = Varance(CTP = MEAN(COD n cloud class k); averagecod n class k k = Varance(COD n cloud class k); averagectp n class k n cloud class k); varance CTP n class k n cloud class k); varance COD n class k (4) (5 ) (6) (7 ) (8) (9) where upper ndces denote the respectve cloud classes dervved from the thresholds n cloud optcal depth (COD) and cloud top pressure (CTP) as gven n Fgure 1 and lower ndces denote MERIS level 2 measurements of cloud propertes (Fscher et al., 1998 a,b). Monthly and weekly averages and standard devatons of varables defned n varables (4) to (7) are calculated as MERIS level-3 products and the cumulatve sum of the varables defned n Equatons (1) and (2) s stored as well. Ths nformaton wll allow to calculate subsequent statstcs on all felds. 1.2 Hgh vertcal resoluton product In addton to the product descrbed n secton 1.1 a hgh vertcal resoluton cloud product wll be derved. Whle no cloud classfcaton s appled, ths product serves to dentfy cloud and atmospherc propertes assocated wth dfferent cloud top heghts. The vertcal resoluton of ths product wll be 50 hpa, startng at 1000 hpa down to 50 hpa, ths leads to 19 cloud top heght classes. For each of these classes the aforementoned parameters (Eq.(1),(2),(4)-(7)) wll be stored and n addton the followng water vapour parameters wll be derved from the level 2 data (Fscher and Bennartz 1998): WVP VAR_WVP = MEAN(WVP k = Varance(WVP n cloud class k); averagewvp n class k n cloud class k); varance WVP n class k (10) (11)

7 Datum : Sete : 7 4 CONCLUSIONS The MERIS level 3 products descrbed n ths document wll allow for detaled clmatologcal nvestgatons of atmospherc propertes, such as frequency and optcal depth of clouds at dfferent alttudes as well as of typcal dstrbutons of water vapour n cloudy and cloud-free atmospheres. In terms of the defnton of dfferent cloud types the dataset wll be compatble wth exstng data products, but t wll be provded wth a much hgher horzontal and vertcal resoluton. Thus, the dataset may not only provde an ndependent source for valdaton and mprovement of exstng datasets, t wll also serve for detaled studes of regonal cloud clmate wth a resoluton better than 5 km. 5 REFERENCES Fscher, J., L. Schüller, and R. Preusker, 1998 (a): Cloud albedo and cloud optcal thckness. Algorthm techncal bass document 2.2. ESA Doc.No.: PO-TN-MEL-GS Fscher, J., R.Preusker, and L. Schüller, 1998 (b): Cloud top pressure. Algorthm techncal bass document 2.3. ESA Doc.No.: PO-TN-MEL-GS Fscher, J. and R. Bennartz, 1998: Retreval of total water vapour content from MERIS measurements. Algorthm techncal bass document 2.4. ESA Doc.No.: PO-TN-MEL-GS Rossow, W.B., L.C. Garder, P-J. Lu and A.W. Walker, "Internatonal Satellte Cloud Clmatology Project (ISCCP) Documentaton of Cloud Data." WMO/TD No. 266 (revsed). World Meteorologcal Organzaton, Geneva, 76 pp. plus three appendces. Schffer, R.A., and W.B. Rossow, "The Internatonal Satellte Cloud Clmatology Project (ISCCP): The Frst Project of the World Clmate Research Programme," Bull.Amer. Meteor. Soc., 64:

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