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

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112 Chapter 5 extracted only from the relative spectral structures of the O 2 A absorption features. Furthermore, cloud fraction can be retrieved with a reasonable sensitivity from NIR of about C fc =0.6−0.7, which means that the cloud fraction is retrieved to a large extend from the relative O 2 A absorption features. For all three windows the optical thickness cannot be retrieved if one of the two scaling factors is applied. The retrieval of surface albedo is also not possible for f c >0.1 due to fitting the two scaling factors (not shown in Fig. 5.6). Thus, Fig. 5.6 demonstrates that especially for the NUV and VIS window the continuum provides essential information on cloud properties. Although the retrieval sensitivities in Fig. 5.6 depend to some extend on the chosen cloud scenario the overall conclusions remain valid for the other cloud scenarios that are used in this study (not shown). 5.6 The synergistic use of spectral windows Figure 5.5 showed that the NIR spectral window provides complementary information on cloud properties to the NUV and VIS spectral windows. The NIR window allows one to retrieve cloud top pressure with a high sensitivity C pc , whereas the NUV and VIS windows provide a higher sensitivity to τ c . A combination of the NUV and NIR or of the VIS and NIR window may improve the retrieval of cloud properties compared to a single window approach. van Diedenhoven et al. [2007] already showed that by combining O 2 A band measurements with measurements in the wavelength range 350–390 nm the sensitivity to cloud parameters can be significantly increased. In the following we investigate the combination of the NUV and NIR and of the VIS and NIR reflectivity spectra to a measurement vector y in Eq. (5.13) and study the retrieval capability of these measurements. The state vector is extended to contain the surface albedo for each spectral window. Figure 5.7 shows the retrieval sensitivities for the combinations of spectral windows. The synergistic use of the NUV and NIR windows allows one to combine the ability to retrieve the three cloud parameters with high sensitivity. As a result, the three cloud parameters can be retrieved with maximum sensitivity C pc , C fc , C τc ≈ 1 for nearly all cloud fractions. In other words one combines the ability of the NIR window to retrieve cloud top pressure for all possible cloud fractions with the ability of the NUV window to retrieve cloud fraction and cloud optical thickness with a high sensitivity. The combination of the VIS and NIR spectral range also improves the cloud retrieval compared to the single window approaches, but here the improvement is significantly less than for the NUV and NIR window combination. Especially the retrieval of cloud optical thickness for thick clouds is not satisfying with sensitivities C τ < 0.5 for cloud fractions smaller than 0.5. We also investigated the combination of the NUV and VIS windows and found no significant improvement for the retrieval of cloud properties compared to the NUV window alone (not shown). Since this spectral combination has no added value, it will not be considered in the remainder of this paper. In addition, the robustness of the fit with respect to random-like measurement biases is enhanced by the combination of spectral windows. Figure 5.8 shows the retrieval sensitivities for cloud parameters as a function of noise floor η. For the combination of the NUV and NIR spectral windows the sensitivities C pc , C fc , and C τc larger than 0.9 in almost all cases. Only for the thick cloud C τc reduces to values between 0.7 and 0.9 for a noise floor η ≥ 1%. A high robustness is also achieved
Retrieval of cloud properties from NUV, VIS and NIR 113 Figure 5.6: Retrieval sensitivities (C) to cloud top pressure (p c ), cloud fraction (f c ) and to cloud optical thickness (τ c ) in the NUV, VIS and NIR spectral windows when in addition to these parameters a power law (solid line) or a factor (dotted line) is fitted to the reflectivity measurement. The retrieval sensitivities are shown as function of cloud fraction for η = 0.1%, p c = 500 hPa and τ c = 40.
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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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56 Chapter 3 biases the simulation
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58 Chapter 3 3.4.2 Approximation me
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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
Page 80 and 81: 74 Chapter 3 where S l is the expan
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 105 and 106: Retrieval of cloud properties from
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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
Page 137 and 138: Samenvatting Het door de aardatmosf
Page 139 and 140: Samenvatting 133 met een vector-mod
Page 141: List of publications Peer-reviewed
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