CLOUD DETECTION AND DISTRIBUTIONS FROM MIPAS INFRA-RED LIMB OBSERVATIONS J. Greenhough, J. J. Remedios, and H. Sembhi EOS, Space Research Centre, Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK; {jg113,jjr8,hs32}@le.ac.uk ABSTRACT Clouds are important for their radiative forcing, control of water vapour, chemical interactions, and effects on trace gas retrievals from remote sensing. Historically, clouds have been observed principally using nadir sounding but more recently limb-sounding instruments have demonstrated their sensitivity particularly to sub-visible cirrus. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), launched on ENVISAT in March 2002, provides unprecedented infra-red emission spectra of clouds at high spectral resolution (0.025 cm 1 unapodised), with vertical sampling of 3 km from 6 to 68 km. Here we show that the cloud index (CI) method of cloud detection, as implemented in the ESA operational processor for MIPAS, is reasonably robust to changes in altitude and water vapour concentrations at 12 km and above in the tropics and mid-latitudes, and in polar winter atmospheres between 15 and 27 km. Using CI, we then present the first cloud frequency distributions from MIPAS. 1. INTRODUCTION Clouds are intrinsic features of a range of weather and climate phenomena from the hydrological cycle to the greenhouse effect and from latent heat exchange to heterogeneous chemistry. Considerable observational challenges are presented by their strong broadband interaction with radiation and by their multi-scale temporal and spatial distributions, demanding a range of active and passive remote-sensing techniques. Here we concentrate on limb-sounding measurements in the infra-red, in which the most pronounced effects arise from conglomerations of particles (or clouds) that can exist as sub-visible ice (cirrus) clouds in the upper troposphere and polar stratospheric clouds (PSCs) in the winter lower stratosphere. Cirrus clouds are important for their radiative transfer, transport of water vapour into the stratosphere and potential chemical interactions, while PSCs are key contributors to polar ozone depletion. Historical infra-red cloud observations via limb sounding have employed radiometers integrating the spectrum between typically 10 and 100 cm 1. For example, seasonal and regional changes in cirrus distributions were analysed by [1] using the Stratospheric Aerosol and Gas Experiment (SAGE) II between 1985 and 1990, finding predominantly sub-visible clouds near the tropopause with up to 40% temporal occurrence in the tropics; this was later confirmed by [2, 3] with data from the Upper Atmospheric Research Satellite (UARS). UARS observations were also suggestive of PSC characteristics and distributions, see for example [4, 5]. Two leading motivations for limb sounding were highlighted by these studies: (i) the vertical resolution and geometry provide unique views of important high-altitude (> 6 km) clouds with small particle sizes (mean radius < 20µm) and (ii) the detection of clouds is crucial for accurate trace gas retrievals in the infra-red. In November 1994 and August 1997, the Cryogenic Infrared Spectrometer and Telescopes for the Atmosphere (CRISTA) provided spectrally-resolved limb measurements that enabled for the first time (i) the distinction of different PSC types [6, 7] and (ii) the detection of clouds, and estimation of their heights and extinction coefficients, using a spectral ratio known as the cloud index (CI) [6, 8, 9]. Here we extend the analyses of [9] to include the dependence of CI on extreme water vapour concentrations for the MIPAS instrument, and present examples of the first MIPAS cloud distributions obtained from CI. 2. THE MIPAS INSTRUMENT MIPAS is an infra-red Fourier transform spectrometer that measures radiation emitted from the Earth s limb. Vertical radiance profiles are obtained by scanning the limb from 6 to 68 km with 3 km spacing in the lower atmosphere, and retrievable pointing errors allow for spectra at other altitudes. Five channels cover the wavenumber range between 685 cm 1 (14.60 µm) and 2410 cm 1 (4.15 µm) at high resolution (0.035 cm 1 apodised). In the current operational processor, temperature and six trace gas profiles are retrieved from the radiance spectra for tangent altitudes above 12 km; extension down to 6 km may be incorporated in future updates. In addition, MIPAS spectra hold information for many other potential Proc. of the 2004 Envisat & ERS Symposium, Salzburg, Austria 6-10 September 2004 (ESA SP-572, April 2005)
products including other trace gases, aerosols and clouds. In the following section we examine the effects of cloud on MIPAS spectra and discuss the current cloud detection method. 3. MIPAS CLOUD SPECTRA AND CLOUD IN- DICES Cloud particles absorb, emit and scatter radiation over a broad range of wavelengths in the infra-red. Fig. 3 shows spectra for no cloud, thin cloud and thick cloud in the field of view. Note (i) the distinct offset increasing with extinction and (ii) the pressure-broadened gas absorption lines from tropospheric radiation scattered into the limb path, carrying information on the location, temperature and mean particle size of the cloud. The offset is measured in microwindow 2 (MW2) by the cloud index (CI) which corrects (to first order) for temperature effects using the CO 2 -dominated microwindow 1 (MW1). CI is given by mean radiance in MW1 / mean radiance in MW2; Fig. 3 shows the MW locations for the MIPAS band A CI, along with the CI values for the plotted spectra. Details of the MWs defining CI in bands A, B and D are listed in Table 1 with their respective threshold values, which are the lowest values (corresponding to thickest clouds see Sect. 3.1) for which accurate gas retrievals are possible from the spectra. The thresholds quoted in the table are currently used in the MIPAS operational processor but are open to revision. 3.1. Cloud indices and water vapour Below altitudes of 8 km in the tropics, H 2 O concentrations typically exceed 2000 ppmv and produce similar spectral offsets to cloud, and the same applies for enhanced H 2 O levels at higher altitudes and latitudes. Hence a single value of CI can represent a broad range of cloud extinctions for varying H 2 O concentrations. In order to examine this behaviour, we construct a set of extinction profiles such that, for tangent altitude h, an extinction of x km 1 refers to a profile having x from h-1 to h+2 km (where the contribution to observations is greatest) and a background aerosol extinction of 10 6 km 1 elsewhere between 0 and 120 km. Profiles of other gases 1 are taken from the MIPAS mean reference atmospheres [10] (equatorial, mid-latitude day/night average, polar winter) with molecular cross-sections from the HITRAN 1996 database [11], and these are input to the RFM 2 (no scattering) to simulate the spectra from which we calculate CI. We use the nominal MIPAS A-band apodised instrument line shape (22/12/2000, version 1.1) and the field of view is approximated by a trapezium of base 4 km and top 2.8 km. For water vapour, we use both the maximum and minimum MIPAS reference profiles [10] 1 CO 2, CH 4, HNO 3, N 2 O, NO 2, O 3, C 2 H 6, F11, F12, F22, N 2 O 5, CLONO 2, O 2 2 Oxford Reference Forward Model, a state-of-the-art line-by-line radiative transfer code [12] for each of the three atmospheres, and the resulting variations of CI with extinction and extreme H 2 O levels are plotted in Fig. 2. It can be seen from Figs. 2 and that if, for example, band A CI<1.8 is used to flag cloud extinctions greater than some threshold given no knowledge of the tropical H 2 O profile, then this threshold is tightly constrained around 10 3 km 1 only for tangent heights of at least 12 km. There is no significant dependence of CI on the H 2 O profile for altitudes of at least 18 km in the tropics and at least 15 km in mid-latitudes, even for the smallest extinctions (clouds or aerosol), but clearly CI is not a robust cloud indicator below 12 km unless H 2 O levels are known. Work is therefore in progress to distinguish cloud from water vapour at low altitudes in the tropics and midlatitudes, which may be significantly affected by future updates to the reference profiles for water vapour. For polar winter atmospheres, Fig. 2(c), we consider altitudes at which PSCs occur and find a significant variation of extinction with both H 2 O and altitude for fixed CI. For example, CI= 1.8 corresponds to extinctions between 10 3 and 2 10 4 km 1 from 15 to 27 km. When in Sect. 4 we present global cloud distributions obtained from a single CI threshold of 2.2 at all altitudes, it is thus important to note that this includes cloud of extinctions potentially as low as 10 4 km 1 at (i) 12 km in the tropics (for maximum H 2 O) and (ii) 27 km in the poles (for minimum H 2 O), whereas in mid-latitudes there is no significant deviation from 10 3 km 1. 3.2. Potential for extinction retrieval If CI is to be utilised for retrieval purposes, the sharp decrease in its sensitivity to extinction coefficients above 10 3 km 1 is clearly a limitation at all latitudes and altitudes (see Fig. 2). Examination of the CI microwindow MW2 in Fig. 3 reveals the change from emission lines in clear skies to both emission and absorption lines in thick cloud. The current CI requires only the mean (offset) radiance and hence ignores this changing asymmetry of the radiance distribution, which is quantified by the third moment known as the skew. In Figs. 2 and (c) we show the behaviour of CI when multiplied by the skew of the radiances in MW2; clearly the skew provides CI with greater sensitivity in the region 10 3 10 2 km 1 ; the effect of skew in the tropics is much smaller and is not plotted. The potential benefits of this factor for extinction retrievals require further investigation, particularly since our simulations exclude scattering which is manifest in the absorption lines and therefore affects the skew. 4. MIPAS CLOUD OBSERVATIONS In this section we demonstrate a most important MIPAS cloud product: spatial frequency distributions, using near real-time level 2 data. The plots in Figs. 3 6 show seasonal mean cloud distribution frequencies for 2003 using a band A CI threshold of 2.2 at a pressure of 146 mb (altitude 13.5 ± 2.5 km) and latitudinal means with
Figure 1. Comparison of MIPAS spectra for clouds of different extinction coefficient in the field of view, characterised by spectral offset. Observations at tangent altitude of 15.7 km at different locations on 5 May 2003. Vertical lines delimit microwindows for calculation of band A CI, whose value for each spectrum is given in the legend. Table 1. Microwindows, thresholds and altitude ranges for CI as utilised in MIPAS operational processor since July 2003. Band MW1 [cm 1 ] MW2 [cm 1 ] Threshold Altitude [km] A 788.2 796.25 832.3 834.4 1.8 10 45 B 1246.3 1249.1 1232.3 1234.4 1.2 10 40 D 1929.0 1935.0 1973.0 1983.0 1.8 12 30 3 km altitude resolution. Features of interest during the northern hemisphere winter of 2003 2 (Fig. 3) are (i) the high occurrence of cloud ( 50% at 146 mb) in the tropics over land and the Pacific, with frequencies decreasing with altitude and extending well into the stratosphere up to 22 km for latitudes below 20 ; (ii) cloud cover over low to mid-latitudes occurs on average with a frequency of 10% at 146 mb; (iii) the existence of Arctic PSCs between 16 and 25 km (occurrence rate <1%). Moving into Spring 2003 (Fig. 4), MIPAS observes (i) mean cloud cover over mid- to high latitudes increasing to 10 20%, (ii) the loss of Arctic PSCs, and (iii) the appearance of Antarctic PSCs at 15 km (occurrence rate <1%). By Summer 2003 (Fig. 5), Antarctic PSCs occur on average over 30% of the time at 146 mb, and extend up to 28 km altitudes, while mean cloud occurrences in mid-latitudes decrease to <10%. A narrowing of the cloud distribution towards the tropics is seen in Autumn 2003 (Fig. 6), leaving largely clear skies in mid-latitudes and far fewer Antarctic PSCs (<10%). 5. SUMMARY MIPAS is observing clouds with high spectral resolution enabling retrieval of microphysical cloud properties. Here we have focused on cloud extinction for which the cloud index CI is used. CI is currently employed in the MIPAS operational processor to flag clouds, that is to detect the presence of cloud with an extinction greater than some threshold for which gas retrievals are inaccurate. The variation of CI with extreme water vapour concentrations and cloud extinction has been simulated in the tropical and mid-latitude troposphere, and in the polar winter stratosphere, in order to quantify the range of extinctions to which an observed CI may correspond. We find that the current value of CI=1.8 corresponds to 10 3±0.2 km 1 in the tropics at 12 km and above, 10 3 km 1 in midlatitudes at 12 km and above, and 10 3.4±0.4 km 1 between 15 and 27 km at the poles. These variations in extinction for a given CI are due partly to different tangent heights, for which one could correct, and partly to uncertainties in the water vapour profile. The reference H 2 O profiles used here require further validation, and subsequent changes may have a significant impact on the altitudes at which CI is useful. A second important factor to consider is the effect of scattering neglected in our simulations; this is also of interest with regard to potential extinction retrievals using CI and the skew of radiances in one of its microwindows. The analyses presented here provide (i) a firm basis for the optimisation of CI thresholding, (ii) preliminary ideas on the feasibility of extinction retrievals, and (iii) encouraging examples of the first MIPAS cloud distributions. ACKNOWLEDGEMENTS J.G. funded by a NERC NOT grant. Data provided by ESA under AO357 (CUTLSOM).
1 1.5 2 log 10 extinction [km 1 ] 2.5 3 3.5 4 9 km min H2O 9 km max H2O 12 km min H2O 12 km max H2O 15 km min H2O 15 km max H2O 18 km all H2O CI=1.8 4.5 5 9 8 7 6 5 CI band A 4 3 2 1 log 10 extinction [km 1 ] 1 2 3 4 9 km min H2O 9 km min H2O x skew 9 km max H2O 9 km max H2O x skew 12 km min H2O 12 km max H2O 15 km all H2O 15 km all H2O x skew CI=1.8 5 15 10 CI band A 5 0 log 10 extinction [km 1 ] 1 2 3 4 5 20 15 km min H2O 15 km min H2O x skew 15 km max H2O 15 km max H2O x skew 27 km min H2O 27 km min H2O x skew 27 km max H2O 27 km max H2O x skew CI=1.8 15 10 CI band A 5 0 Figure 3. Cloud distribution frequencies for Dec Feb 2002-3 using band A CI<2.2 at a pressure of 146 mb (altitude 13.5 ± 2.5 km) and 3 km altitude resolution box 6 11. The 1% closed contour around (-60, 20 km) is an artifact in the data. (c) Figure 2. CI band A, the ratio of the mean radiances in spectral windows 788.2 796.25 cm 1 and 832.3 834.4 cm 1, calculated from the RFM for a range of extinction and water vapour profiles in the tropics, mid-latitudes and (c) polar winters; extinction resolution of 0.25 in log 10 -space.
5 Figure 4. Cloud distribution frequencies for March May 2003 using band A CI<2.2 at a pressure of 146 mb (altitude 13.5 ± 2.5 km) and 3 km altitude resolution box 6 11. The 1% closed contour around (-60, 20 km) is an artifact in the data. Figure 5. Cloud distribution frequencies for June Aug 2003 using band A CI<2.2 at a pressure of 146 mb (altitude 13.5 ± 2.5 km) and 3 km altitude resolution box 6 11.
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