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Estimating Longwave Net Radiation at Sea Surface from the Special Sensor Microwave/Imager (SSM/I)

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Estimating Longwave Net Radiation at Sea Surface from the Special Sensor Microwave/Imager (SSM/I) JULY 1997 LIU ET AL. 919 Estimating Longwave Net Radiation at Sea Surface from the Special Sensor Microwave/Imager (SSM/I) QUANHUA LIU,CLEMENS SIMMER, AND EBERHARD RUPRECHT Institute for Marine Sciences, Kiel, Germany (Manuscript received 18 March 1996, in ®nal form 24 December 1996) ABSTRACT A neural network is used to calculate the longwave net radiation (Lnet) at the sea surface from measurements of the Special Sensor Microwave/Imager (SSM/I). The neural network applied in this study is able to account largely for the nonlinearity between Lnet and the satellite-measured brightness temperatures (TB). The algorithm can be applied for instantaneous measurements over oceanic regions with the area extent of satellite passive microwave observations (30±60 km in diameter). Comparing with a linear regression method the neural network 22 reduces the standard error for Lnet from17to5Wm when applied to model results. For clear-sky cases, a 22 good agreement with an error of less than5Wm for Lnet between calculations from SSM/I observations and pyrgeometer measurements on the German research vessel Poseidon during the International Cirrus Experiment (ICE) 1989 is obtained. For cloudy cases, the comparison is problematic due to the inhomogenities of clouds and the low and different spatial resolutions of the SSM/I data. Global monthly mean values of Lnet for October 1989 are computed and compared to other sources. Differences are observed among the climatological values from previous studies by H.-J. Isemer and L. Hasse, the climatological values from R. Lindau and L. Hasse, the values of W. L. Darnell et al., and results from this study. Some structures of Lnet are similar for results from W. L. Darnell et al. and the present authors. The differences between both results are generally less than 15 W 22 m . Over the North Atlantic Ocean the authors found a poleward increase for Lnet, which is contrary to the results of H.-J. Isemer and L. Hasse. 1. Introduction accuracy depends also on cloud types. In addition, few measurements of sea surface temperature, air temper- Longwave net radiation over oceanic areas is an im- ature and humidity of the boundary layer, and of cloud portant component of the surface energy budget, which coverage are available over oceanic areas. Therefore, determines the interaction between ocean and atmo- calculating Lnet from satellite observations becomes an sphere. Bulk formulas are often applied (e.g., Anderson interesting topic. Gupta (1989) uses the International 1952) to calculate the longwave net radiation at the sea Satellite Cloud Climatology Project (ISCCP) dataset to surface from measured sea surface temperature, air tem- determine L . Zhi and Harshvardhan (1993) calculate perature, humidity of the boundary layer, and cloud cov- net Lnet from a combination of general circulation model erage. Gilman and Garrett (1994) discussed eight bulk cloud radiative forcing ®elds, cloud radiative forcing at formulas and found that the resulting annual mean dif- the top of the atmosphere from ERBE (Earth Radiation ference of L over the Mediterranean Sea can be up to net Budget Experiment), TOVS (TIROS Operational Ver- 20Wm22. They give also the uncertainty for the long- tical Sounder) pro®les, and sea surface temperatures on term averaged estimate of ¯uxes at the sea surface by ISCCP C1 tapes. Smith and Woolf (1983) performed a using the bulk formulas. The uncertainty is 5 W m22 linear regression analysis on 1200 radiosonde pro®les for the solar radiation, 10 W m22 for longwave radiation, to derive a relationship between the net surface long- 15Wm22 for the latent ¯ux, and 4 W m22 for the wave radiation and the infrared radiances computed for sensible heat ¯ux, which leads to an error for the total the VISSR (Visible Infrared Spin Scan Radiometer) At- net ¯ux of about 20 W m22. The reported standard error mospheric Sounder (VAS) channels on the Geostation- for Lnet from bulk formulas compared to direct methods is 10±15 W m22 for clear sky (Fung et al. 1984); for ary Operational Environmental Satellites (GOES). They 22 cloudy atmospheres the error becomes larger and the obtained a standard error of 9.7 W m for clear and 21.4 W m22 for cloudy atmospheres. The large error in the latter case is due to the fact that surface net longwave radiation and the satellite-measured infrared radiances Corresponding author address: Dr. Quanhua Liu, Institute for Ma- for cloudy atmospheres are largely decoupled (Schmetz rine Sciences, DuÈsternbrooker Weg 20, 24105 Kiel, Germany. 1989). E-mail: [email protected] Measurements from passive microwave radiometers q1997 American Meteorological Society Unauthenticated | Downloaded 09/29/21 10:05 PM UTC 920 JOURNAL OF APPLIED METEOROLOGY VOLUME 36 provide a better way to calculate Lnet for both clear and in the microwave spectral range, where the radiance has homogeneous cloudy atmospheres because most clouds been substituted by the brightness temperature using are semitransparent and detectable in the microwave Rayleigh±Jean's approximation. Here, t is the total u range. The satellite-measured upwelling radiances over transmittance of the atmosphere. The quantitiesIIR and d u d oceanic areas contain information about the downwell- IIR (TB and TB ) are the upwelling and downwelling ing radiances due to the high microwave re¯ectivity of radiances (brightness temperatures). The subscripts v the sea surface. It will be shown that the downwelling and h denote the vertical and the horizontal polariza- microwave radiance is linearly related to the polariza- tions, respectively. Here, «v and «h are the vertical and tion, that is, the difference between vertically and hor- the horizontal components of the microwave sea surface izontally polarized radiances. The downwelling micro- emissivity, respectively. The downwelling longwave ra- d wave radiances are highly correlated with Lnet because diationFIR is calculated as both depend mainly on temperature, water vapor, and 1 cloud liquid water. In section 2, we describe the theory Fdd52pmIdm. (3d) for calculating the net longwave radiation over oceanic IR E IR 0 areas. In section 3, a neural network is introduced. In d section 4 an algorithm is developed with a training da- The downwelling radianceIIR contributes very little taset, and the algorithm is veri®ed with simulations us- to the measurements of satellite-based infrared radi- ing independent radiosonde data. In section 5, compar- ometers because the thermal infrared emissivity of the sea surface «IR is close to 1. This has been demonstrated isons are carried out between the surface measured Lnet and the retrievals using SSM/I measurements. In section by Ramanathan (1986), who compared the outgoing 6, we illustrate and discuss the global distribution of net longwave radiative ¯ux at the top of the atmosphere longwave radiation over oceanic areas for October 1989 with the net longwave surface ¯ux for selected regions and we compare our results with other published esti- from simulations with the National Center for Atmo- mates. spheric Research (NCAR) general circulation model. The correlation between both ¯uxes is very poor. Be- cause of the low microwave emissivity of the sea surface 2. Theory (about 0.5), microwave radiometers on satellites mea- The longwave net radiation over oceanic areas can sure a mixed information containing contributions from be written as the sea surface, the upwelling brightness temperature TBu, and the downwelling brightness temperature TBd. 4 d d Lnet 5 «IR (sTFs2 IR), (1) Here TB can be extracted by subtracting (3c) from (3b) where «IR is the longwave emissivity of the sea surface TB 2 TB d vh and its value is commonly assumed to be 0.98 (e.g., TB 5 Ts 2 , (4) tMV(«2«vh) Gilman and Carret 1994). The quantities s,Ts, and d FIR are the Stefan±Boltzmann constant, the sea surface where «v and «h are a function of the sea surface rough- temperature, and the downwelling longwave radiation, ness caused by wind and the sea surface temperature. respectively. The total transmittance of the atmosphere tMV is mainly In the thermal infrared spectral range, scattering ef- a function of the total precipitable water, the cloud liquid fects can be neglected for most applications. The scat- water path, and precipitation (Greenwald et al. 1993; tering effects can also be neglected for nonprecipitating Petty 1994; Weng and Grody 1994; Wentz 1992). The and light to moderate raining clouds in the microwave sea surface wind, sea surface temperature, total water spectrum at low frequencies. The equation describing vapor, cloud liquid path, and rainwater can be deter- the monochromatic radiative transfer through the plane- mined from measurements of spaceborne microwave ra- parallel atmosphere reads (Liou 1980): diometers (Goodberlet et al. 1990; Crewell et al. 1991; Bauer and Schluessel 1993; Liu and Curry 1992). Thus, dI(d, m) m 5 I(d, m) 2 B(T), (2) with the polarization Q (Q 5 TBv 2 TBh) directly ob- dd servable and all other parameters in (4) affecting mi- crowave radiation in general, information about TBd is with I the radiance, m 5 cos(u), u the zenith angle, B(T) also contained in the microwave signal at satellite al- the Planck function, T the temperature, and d the optical titudes. depth. The solution of (2) at the top of the atmosphere The question is, how closely do the downwelling can be written by brightness temperatures relate to the downwelling ther- ud IIR 5 tIR «IR B(Ts) 11IIIRtIR(1 2 «IR) IR (3a) mal infrared ¯uxes. To investigate this relation, a set of oceanic radiosonde pro®les were used as input to ra- in the thermal infrared spectral range, and diative transfer models (see section 4). Table 1 shows ud d d TBv 5 TB 1 tMW[«vsT 1 (1 2« v)TB ] (3b) the correlation coef®cients betweenFIR and TB for the four Special Sensor Microwave/Imager (SSM/I) fre- ud TBh 5 TB 1 tMW[«hsT 1 (1 2« h)TB ] (3c) quencies.
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