Rotational Raman scattering in the Earth's atmosphere ... - SRON

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106 Chapter 5 5.5 Information content of the NUV, VIS and NIR spectral window The instrument noise on GOME, GOME-2, SCIAMACHY and OMI measurements is in the order of 0.1% in the NUV, VIS and NIR wavelength ranges. In practice, the differences between modeled and measured reflectivity spectra are a few times larger than this instrument noise level and appear random (e.g. [Joiner et al., 2004, van Diedenhoven et al., 2007] and Chapter 4). The origin of these large residuals can be manifold, e.g. problems in the instrument calibration, errors in the simulation of the measurements, and uncertainties in spectroscopic data such as the O 2 A band cross sections [Rothman et al., 2005] and line-shapes [Yang et al., 2005, Tran et al., 2006]. Due to these possible sources of errors it is useful to study the robustness of the retrieval to random-like biases on top of the instrument noise. Figure 5.3 shows the retrieval sensitivities C pc , C fc , and C τc as a function of a white noise floor η that is added to the instrument noise of 0.1% for the three spectral windows. All curves in this figure were calculated for a fully clouded scene (f c =1). In general, the retrieval sensitivities decrease when η is increased, because the effect of clouds on the signature of the spectral continuum and on the spectrally fine structures becomes more difficult to distinguish from noise. For example, the retrieval sensitivity C pc for the NUV window decreases from C p ≈ 1 at η = 0% to C p ≈ 0.3 at η = 1.5% for a high and optically thick cloud (p c = 500 hPa, τ c = 40). In case of a low and optically thin cloud (p c = 800 hPa, τ c = 10) this loss in retrieval sensitivity is less: C pc is still 0.8 at η = 1.5%. For the VIS window increasing the noise floor has a stronger impact than for the other windows because the cloud sensitive spectral features are weaker in this window. Here, the retrieval sensitivity C pc decreases from C pc =0.6 at η =0% to C pc ≤0.1 for noise floors larger than 1.0% for the high cloud. For low cloud scenarios the sensitivity C pc is significantly higher. This is explained by the quadratic dependence of the O 2 -O 2 absorption cross section on air density, which results in a significantly weaker absorption at high altitudes in the troposphere than closer to the surface. The signature of the O 2 -O 2 absorption features relative to the noise is thus more rapidly lost for high clouds than for low clouds [Acarreta et al., 2004]. This situation is different for the cloud top pressure retrieval from the NIR window, which stays unaffected at noise levels even larger than 1%. The strong O 2 absorption feature dominates the reflectivity spectrum even in the case of very large noise levels. The retrieval sensitivity of cloud fraction shows a similar dependence on noise floor for all three spectral windows. For the VIS and NIR windows the sensitivity remains C fc ≥ 0.9 up to a η = 1.5% for an optically thick cloud, whereas for the optically thin cloud it decreases to about 0.6 at a noise floor of 1.0%. A closer look reveals that the retrieval sensitivity of cloud fraction is somewhat higher in the NUV window than in the VIS and NIR windows. Let us consider the retrieval of cloud optical thickness for the scenario τ c = 40. Here the sensitivity C τc ≈ 0.8 and C τc ≤ 0.5 for the VIS and NIR windows respectively when no noise floor is added to the measurement simulation, whereas for η =0.5% the sensitivity is reduced to C τc ≤0.1. The retrieval of τ c from the NUV window is far less sensitive to noise. Here the retrieval sensitivity is 1 in case of no noise floor and larger than 0.6 for a noise floor of 0.5% for all cloud scenarios that are shown in Fig. 5.3. For a proper interpretation of the retrieval sensitivity one has to be sure that the changes in the
Retrieval of cloud properties from NUV, VIS and NIR 107 Figure 5.3: Retrieval sensitivities (C) to cloud top pressure (p c ), cloud fraction (f c ) and cloud optical thickness (τ c ) in the NUV, VIS and NIR spectral windows as a function of the noise floor (η) that is added to the instrument noise of 0.1%. Results are shown for a fully clouded scene (f c =1) and thus information on the surface albedo is absent. The dotted lines show the retrieval sensitives for an optically thick cloud (τ c =40) and the solid lines for an optically thinner cloud (τ c =10). The lines in gray correspond to a low cloud (p c =800 hPa) and the black lines to a high cloud at (p c =500 hPa), respectively.
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VRIJE UNIVERSITEIT Rotational Raman
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Contents 1 Introduction 1 1.1 Obser
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1 Introduction 1.1 Observing skylig
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Introduction 3 Extracting the wealt
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Introduction 5 and Spurr, 1997, Vou
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Introduction 7 energy: E rot (v,J)
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Introduction 9 Figure 1.4: Probabil
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Introduction 11 scattered radiance
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Introduction 13 approximation in wh
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Introduction 15 scattering have bee
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2 A doubling-adding method to inclu
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A doubling-adding method to include
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40 Chapter 3 nm with a spectral res
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42 Chapter 3 contributions of lower
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44 Chapter 3 if we use the effectiv
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46 Chapter 3 This in turn results i
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48 Chapter 3 This may be shown in a
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50 Chapter 3 The forward and adjoin
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52 Chapter 3 Here, ∆ = diag [1, 1
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54 Chapter 3 Figure 3.3: Relative R
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Page 64 and 65: 58 Chapter 3 3.4.2 Approximation me
Page 66 and 67: 60 Chapter 3 where I ram,app and I
Page 68 and 69: 62 Chapter 3 3.5 Simulation of pola
Page 70 and 71: 64 Chapter 3 Figure 3.10: Relative
Page 72 and 73: 66 Chapter 3 dependence on the vert
Page 74 and 75: 68 Chapter 3 Figure 3.13: Same as F
Page 76 and 77: 70 Chapter 3 3.6 Summary A vector r
Page 78 and 79: 72 Chapter 3 where λ is the wavele
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Page 82 and 83: 76 Chapter 3 3.C Appendix: Evaluati
Page 85 and 86: 4 Accurate modeling of spectral fin
Page 87 and 88: Accurate modeling of spectral fine-
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Page 125 and 126: References Aben, I., F. Helderman,
Page 127 and 128: References 121 Ellery, A., D. Wynn-
Page 129 and 130: References 123 Lacis, A. A., J. Cho
Page 131 and 132: References 125 Schulz, F., K. Stamn
Page 133 and 134: Summary A spectrum of sunlight that
Page 135 and 136: Summary 129 Earth radiance spectrum
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Page 139 and 140: Samenvatting 133 met een vector-mod
Page 141: List of publications Peer-reviewed
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