Improved diagnosis of low-level cloud from MSG SEVIRI data for assimilation into Met Office limited area models

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1 Improved diagnosis of low-level cloud from MSG SEVIRI data for assimilation into Met Office limited area models Peter N. Francis, James A. Hocking & Roger W. Saunders Met Office, Exeter, U.K. Abstract The latest Met Office cloud-top height retrieval using Meteosat Second Generation (MSG) SEVIRI data is described, and the impact of some proposed changes is investigated. For one particular case study, comparisons with coincident radiosonde data show that, for low-cloud situations, the resulting cloud-top height values are systematically and significantly improved with the updated scheme. INTRODUCTION Correct diagnosis of the initialised low-level cloud field is crucial to the performance of limited-area NWP models. Errors in the determination of low-cloud coverage, or of its height, can lead to significant deficiencies in the analysed structure of the model boundary layer, resulting in too much or too little boundary layer cloud in the initial state, and errors in the time-evolution of the cloud through the forecast period. These in turn can cause serious inaccuracies in important forecast quantities such as precipitation, near-surface temperature and visibility. The Met Office limited-area models (i.e. the North Atlantic/European and UK models) currently make use of MSG-derived cloud-cover and cloud-top height (CTH) fields as part of their cloud analysis, using algorithms developed at the Met Office. This paper presents the latest results of some ongoing work which aims to improve the accuracy of the CTH derivation, particularly for low-level clouds. DERIVATION OF CLOUD-TOP HEIGHT FROM SEVIRI DATA A number of different schemes are used at the Met Office to derive the CTH from the SEVIRI infrared channels (see Saunders et al., 2006). The default method is a minimum residual technique (Eyre and Menzel, 1989), which works best for mid- and high-level clouds, but for low-level cloud the so-called Stable Layers method (Moseley, 2003) is chosen, due to the infrared channels' reduced ability to sense accurately the height of such clouds. The Stable Layers method uses the IR10.8 channel radiance and attempts to find the best level for the cloud-top to obtain agreement between the observed and simulated radiance. The cost function used in this scheme currently also includes the constraint that no cloud can form at the base of an unstable layer. In this case, the model background (i.e. the forecast from 6 hours previously) is clearly a very important constraint in determining the derived cloud-top height. However, problems can arise when the model background is poor for example, when the boundary layer-capping temperature inversion is not captured accurately by the model. In these cases, it is very easy for automated CTH schemes to place the cloud-top at the wrong level e.g. in the dry layer above the inversion which can lead to serious shortcomings in the subsequent model evolution. It is important therefore to improve the techniques used to derive CTH, in order to make optimum use of both the satellite and the model background data.

2 RECENT CHANGES INTRODUCED FOR TESTING Two changes have been introduced into the processing software. The first of these involves updating the fast radiative transfer model RTTOV (Saunders et al., 1999) used to calculate the simulated radiances from the model background, with RTTOV-9 ( nwpsaf/rtm/rtm_rttov9.html) replacing the RTTOV-7 version currently being used by the scheme. The advantage of this is that, whereas RTTOV-7 uses a fixed vertical grid of 43 levels, RTTOV-9 allows us to compute the radiative transfer on a user-defined vertical grid. This means that we can carry out all our calculations on the model vertical grid (38 levels currently for the NAE model), and removes the need to carry out vertical interpolations between the different vertical grids. The second change involves an improvement to the way the cost function is calculated within the Stable Layers scheme. In the existing scheme, an initial check is made to ensure that the stability of each model layer is greater than a prescribed threshold (-5 K km -1 ) before proceeeding to the rest of the algorithm, but this information is not itself otherwise used in the calculation of the cost function. In the updated scheme being tested here, the amount by which the stability exceeds the threshold is now also used in the derivation of the cost function, such that layers whose lapse rates only just exceed the threshold are penalised heavily, the additional cost decreasing linearly with increasing lapse rate and reaching zero when a value of +7 K km-1 is reached. AN EXAMPLE OF THE IMPACT OF THESE CHANGES Below is an example of the impact of these changes on a CTH image, in this case using data from 00Z on 26th October The result of the old processing is shown in Figure 1, with the image from the updated scheme shown in Figure 2. Of particular interest is the large area of low-level cloud over the UK, the North Sea and NW Europe, and the fact that there is generally a systematic increase in the derived CTH values when using the updated processing. Note also the area of very low cloud (red Figure 1: Cloud-top height image derived from SEVIRI data for the slot ending at 00 UTC on 26th October 2007, for an area surrounding the British Isles. The fast model used in this case was RTTOV-7, and the existing Stable Layers algorithm was also used (see text for more details).

3 and orange shades) diagnosed over the Low Countries, the extent of which has been reduced by the new processing. Figure 2: Cloud-top height image derived from SEVIRI data for the slot ending at 00 UTC on 26th October 2007, for an area surrounding the British Isles. In this case, the RTTOV-9 fast model was used, together with the updated Stable Layers algorithm (see text for more details). ANALYSIS OF RESULTS FOR 26 TH OCTOBER Z CASE STUDY Here, we look at some specific pixels for this case, in order to investigate the reasons why the scheme has produced different cloud-top heigh t values, and to compare with coincident radiosonde ascents. o o The first case corresponds to the midnight ascent at Ekofisk in the northern North Sea (56.5 N, 3.2 E). Figure 3(a) shows the sonde temperature (solid line) and dew-point (dotted line) profiles, indicating that the cloud-top height was around 892 hpa in this case. Figure 3(b) represents the retrieval using the old processing scheme: the black solid line is the background model temperature profile, the red line is this same profile interpolated onto the standard RTTOV 43 levels, and the green line is the calculated profile of overcast top-of-atmosphere (TOA) brightness temperature effectively the model temperature at each level corrected to account for the atmospheric absorption and emission between that level and the top of the atmosphere. We see that the interpolation onto the RTTOV levels has caused the minimum of the overcast brightness temperature profile (and in this case the one that agrees best with the measured brightness temperature) to lie around 10 hpa closer to the surface than the inversion height suggested by the model profile, resulting in a derived cloud-top pressure of around 919 hpa. Figure 3(c) represents the retrieval using the new processing, with the green line again showing the overcast top-of-atmosphere brightness temperature profile (now calculated directly on the model levels by RTTOV-9). The resulting cloud-top pressure is around 910 hpa, still not perfect when compared with the sonde ascent, but nevertheless a significant improvement over the previous version of the scheme.

4 (a) (b) (c) Figure 3: (a) Radiosonde temperature (solid line) and dew-point (dotted line) profiles for Ekofisk (56.5 o N, 3.2 o E) at 00 UTC on 26th October (b) Retrieval using the existing processing scheme the black solid line is the background model temperature profile, the red line is this same profile interpolated onto the standard RTTOV 43 levels, and the green line is the calculated profile of overcast top-of-atmosphere (TOA) brightness temperature (BT). The dotted line is the model dew-point profile. (c) Retrieval using the new processing the black solid line is again the model temperature profile, the dotted line is the model dew-point profile, and the green line shows the overcast top-ofatmosphere BT profile (now calculated directly on the model levels by RTTOV-9). o o The Emden (53.4 N, 7.2 E) ascent in Figure 4(a) shows a cloud-top at around 924 hpa, whereas the retrieval using the old processing scheme in Figure 4(b) comes up with a value of around 953 hpa, i.e. it puts the cloud-top around 250 m too low. The retrieval using the new processing in Figure 4(c) puts the cloud-top around 939 hpa. This corresponds to an improvement in the CTH of around 120 m (i.e. the CTH error is halved), and is again due almost entirely to the removal of the need to interpolate between the different vertical grids.

5 (a) (b) (c) The Herstmonceux (50.9 N, 0.3 E) ascent in Figure 5(a) shows a cloud-top at around 873 hpa, with the retrieval from the old processing scheme in Figure 5(b) returning a value of around 920 hpa, i.e. an error in CTH of around 425 m. The retrieval using the new processing in Figure 5(c) puts the cloud- top around 870 hpa, very close to the true value. In this case, the improvement is not only due to the introduction of RTTOV-9, but also due to the change in the cost function even though the model s Figure 4: (a) Radiosonde temperature (solid line) and dew-point (dotted line) profiles for Emden (53.4 o N, 7.2 o E) at 00 UTC on 26th October (b) Retrieval using the existing processing scheme the black solid line is the background model temperature profile, the red line is this same profile interpolated onto the standard RTTOV 43 levels, and the green line is the calculated profile of overcast top-of-atmosphere (TOA) brightness temperature (BT). The dotted line is the model dew-point profile. (c) Retrieval using the new processing the black solid line is again the model temperature profile, the dotted line is the model dew-point profile, and the green line shows the overcast top-ofatmosphere BT profile (now calculated directly on the model levels by RTTOV-9). o o

6 temperature inversion is much weaker than shown by the sonde ascent, the new scheme has now successfully identified the base of the model inversion as the best place to position the cloud-top. (a) (b) (c) Figure 5: (a) Radiosonde temperature (solid line) and dew-point (dotted line) profiles for Herstmonceux (50.9 o N, 0.3 o E) at 00 UTC on 26th October (b) Retrieval using the existing processing scheme the black solid line is the background model temperature profile, the red line is this same profile interpolated onto the standard RTTOV 43 levels, and the green line is the calculated profile of overcast top-of-atmosphere (TOA) brightness temperature (BT). The dotted line is the model dew-point profile. (c) Retrieval using the new processing the black solid line is again the model temperature profile, the dotted line is the model dew-point profile, and the green line shows the overcast top-ofatmosphere BT profile (now calculated directly on the model levels by RTTOV-9). As a final example, we show the Essen (51.4 o N, 7.0 o E) ascent in Figure 6(a) compared with the CTH retrievals from the old and new versions of the CTH scheme (Figures 6(b) and 6(c) respectively). The

7 sonde shows the cloud-top at around 909 hpa, whereas the old processing puts the cloud-top much closer to the surface, at around 1007 hpa. This is because the model does not capture the strong inversion at around 900 hpa at all well, and this inversion is also smoothed out by the vertical interpolation. However, we see that even the new scheme has failed to improve matters significantly in this case although the inversion around 900 hpa is now better represented by the RTTOV-9 calculations, the presence of a small but significant model inversion near the surface has caused the scheme to place the cloud-top at around 999 hpa, still significantly in error. (a) (b) (c) Figure 6: (a) Radiosonde temperature (solid line) and dew-point (dotted line) profiles for Essen (51.4 o N, 7.0 o E) at 00 UTC on 26th October (b) Retrieval using the existing processing scheme the black solid line is the background model temperature profile, the red line is this same profile interpolated onto the standard RTTOV 43 levels, and the green line is the calculated profile of overcast top-of-atmosphere (TOA) brightness temperature (BT). The dotted line is the model dew-point profile. (c) Retrieval using the new processing the black solid line is again the model temperature profile, the dotted line is the model dew-point profile, and the green line shows the overcast top-ofatmosphere BT profile (now calculated directly on the model levels by RTTOV-9).

8 CONCLUSIONS AND FURTHER WORK We have shown the impact of changes to the Met Office s SEVIRI cloud-top height scheme, including the introduction of the RTTOV-9 fast model into the processing. Comparisons with coincident radiosonde data have shown that, for low-cloud situations, the resulting CTH values are systematically and significantly improved with the updated scheme. This is the case for the 26th October 2007 data shown here, and also for several other case studies not presented here. However, there are still instances (e.g. the Essen ascent in Figure 6 and the persistence of sizeable areas of very low diagnosed cloud over the Low Countries in the image in Figure 2) where the new scheme still gives CTH values which are significantly in error. More work is planned on further improvements to the CTH scheme, and also on making improvements to the assimilation of cloud information into the forecast models (see Taylor et al., 2008), which should help us produce better low-cloud forecasts in the future. REFERENCES Eyre, J.R. and W.P. Menzel, Retrieval of cloud parameters from satellite sounder data: a simulation study. J. Appl. Meteorol., 28, Moseley, S., 2003: Changes to the Nimrod cloud top height diagnosis. Met Office Forecasting Research Technical Report no Saunders, R.W., M. Matricardi and P. Brunel, 1999: An improved fast radiative transfer model for assimilation of satellite radiance observations. Quart. J. Roy. Meteorol. Soc., 125, Saunders, R.W., R.A. Francis, P.N. Francis, J. Crawford, A.J. Smith, I.D. Brown, R.B.E. Taylor, M. Forsythe, M. Doutriaux-Boucher and S.C. Millington, The exploitation of Meteosat Second Generation Data in the Met Office. Proceedings of the 2006 EUMETSAT Meteorological Satellite Conference, Helsinki, Finland. Taylor, R.B.E., R. Renshaw, R.W. Saunders and P.N. Francis, Assimilation of SEVIRI cloud-top parameters in the regional Met Office forecast model. Proceedings of the 2008 EUMETSAT Meteorological Satellite Conference, Darmstadt, Germany.