Cloud Top Height Retrieval From MIPAS Jane Hurley, Anu Dudhia, Graham Ewen, Don Grainger Atmospheric, Oceanic and Planetary Physics, University of Oxford MIPAS is an infrared limb-sounding Michelson interferometer onboard the ENVISAT satellite. At low tangent heights, clouds are frequently detected in the field of view (FOV) and, when retrieving profiles of atmospheric composition, cloud-contaminated spectra are usually excluded. However, clouds themselves are of great interest scientifically, playing an important role in the Earth’s radiation budget. The radiative effects of clouds depend upon both their micro- and macro-physical properties, such as cloud top height, cloud depth, particle number density and effective radius. Here we present results of an investigation into the retrieving cloud top height (C top ) from MIPAS spectra. In this preliminary study we have used simple models of clouds in the infrared to effectively assume thick, flat clouds to retrieve the cloud top height with a high vertical resolution of ± 0.25 km. MODELS AND DETECTION METHODS Colour Index (CI) CI work on the principle of radiance ratios between two microwindows which respond differently to cloud. Colour index CI = L MW1 / L MW2. Cloud presence is determined by setting a threshold, below which it is said to be cloudy and above which it is said to be cloud-free. A band: MW cm -1 and MW cm -1 threshold of 1.8. (Spang et al. 2004) Planck Approximation of Cloud Top Height (PACT) Assume the cloud can be modelled as a blackbody and neglect the radiance contributed by the atmosphere. RFM Iterative Approximation of Cloud Top Height (RIACT) Assume cloud can be modelled as a column of aerosol with volume extinction coefficient β ext = 1 km -1 starting at the Earth’s surface and extending homogeneously upwards to C top. ABSTRACT CASE STUDY These methods for the retrieval of C top were applied to a set of Level 1B MIPAS data from 1-8 August Fig. 1 shows the C top reported by the CI Method and Fig. 2 compares the C top s resulting from the three methods. The three methods give corresponding C top s with minimal scatter, but the PACT and RIACT Methods give C top s within 0.25 km of each other, implying a finer resolved C top. Fig. 1: C top s reported by CI Method. Fig. 2: Comparison of C top results. Looking at an individual case at 17:22:03 on 4 August 2003, the CI Method flags cloud at km. Fig. 3 shows the PACT and RIACT Methods’ results of km and km respectively. Furthermore, the RIACT simulated spectrum does a fairly good job of modelling the measured spectrum, with a mean difference between the two of 18 nW/cm 2 sr cm -1 (noise ~ 50 nW/cm 2 sr cm -1 ). Fig. 3: PACT Method Retrieval (left), RIACT Method Retrieval (right) CONCLUSIONS AND FURTHER WORK This preliminary study confirms that C top can be successfully retrieved by modelling clouds as having blackbody-like properties, either by estimating with the Planck function (PACT Method) or by taking β ext =1 km -1 (RIACT Method). Both methods yield C top s that are well in keeping with the CI C top, but that are almost entirely self-consistent, indicating that the PACT and RIACT Methods give an improved retrieval of C top with greatly increased vertical resolution.. The success of these simple models suggests that other parameters of interest could be retrieved if further degrees of freedom were introduced into the simple retrievals presented. Future work includes the retrieval of other parameters, like cloud top temperature, and comparison with EUMetSat meteorological products. These results for C top compare well with EUMetSat’s SEVIRI infrared image over the Indian Ocean taken at 18:00 on 4 August 2003, as shown in Fig. 4. These results clearly indicate the presence of thick cloud in the area of the case study, as highlighted by a dark circle. A comparison with EUMetSat’s SEVI cloud top height meteorological product will be carried out. Fig. 5: Sample EUMetSat SEVI cloud top height map EUMetSat. Identify cloudy spectrum by CI Method At this tangent height, partition FOV vertically into 40 divisions z i, at a resolution of 0.1 km Interpolate temperature at each height partition T(z i ) using corresponding L2 temperature profile. Assume brightness temperature T Bi of a cloud at z i is T(z i ) Calculate radiance emitted by blackbody at each z i by evaluating Planck function B at T Bi in microwindow MW ( cm -1, most transparent region of A band) Each z i is a possible C top. Integrate over the cloud-filled portion of the FOV to get the total radiation emitted by the cloud: L mod (z i ) = ∑ i j=0 B(T Bi, z j ) w j / ∑ 40 j=0 w j Calculate mean radiance of measurements in chosen MW (L meas ) and compare this value to those modelled at each possible C top,. When L mod (z i ) ≈ L meas the C top has been found as z i. Identify cloudy spectrum by CI Method At this tangent height, partition FOV vertically into 0.25 km vertically separated levels. Each of these levels is a possible C top. Use Reference Forward Model (RFM) (Dudhia 2005) to simulate radiance emitted in FOV in the cm -1 microwindow. Compare RMS error for RFM runs at each possible C top : the height for which RMS error is minimized is the C top. REFERENCES Dudhia, Anu, ``Reference Forward Model Software User's Manual'', Spang, R. et al., ``Colour Indices for the Detection and Differentiation of Cloud Types in Infra-red Limb Emission Spectra'', Advances in Space Research, 33, Fig. 4: EUMetSat SEVIRI infrared image EUMetSat.