COMBINED USE OF MSG IMAGES, PRODUCTS AND OTHER DATA FOR OPERATIONAL FORECASTING AT AEMET-SPAIN: APPLICATIONS TO FOG AND IN-FLIGHT AVIATION

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1 COMBINED USE OF MSG IMAGES, PRODUCTS AND OTHER DATA FOR OPERATIONAL FORECASTING AT AEMET-SPAIN: APPLICATIONS TO FOG AND IN-FLIGHT AVIATION José Miguel Fernández-Serdán ATAP/AEMet, c/leonardo Prieto Castro 8, Madrid, Spain Abstract For many years forecasters mostly used METEOSAT images, IR, VIS, and WV. MSG is providing new channels, and many products are now available to the meteorological community, as those in the Satellite Application Facility in support to Nowcasting (SAFNWC) 1. But in order to have these actually used, it is important to both, propose them in an attractive way and be able to determine adequate field-oriented displays and products. Some are described in this paper, part or to be part of the intranet pages for access to real time data and products from any AEMet location. A real-time visualisation combining Cloud Type (CT) product, IR window channel, and in-situ observations, has proven to be useful to monitor and nowcast fog coverage. Night-time, and at twilight when CT performs worse, channel-differences are also displayed. As the vertical description provided by CT is too coarse, the use of Cloud Top Temperature and Height (CTTH) SAFNWC product and terrain elevation is being checked to refine description of the layer of potential fog. Observation of phenomena affecting in-flight aviation, as icing, is a challenge: no or no distribution of aircraft reports over Europe, no products as cloud water phase by now proposed by the SAFNWC. At AEMet a well known product developed for GOES (ICECAP), has been adapted to MSG and is displayed to aviation forecasters, with addition of CTTH, CT and NWP information. The product is kept close to ICECAP (checked where both satellites overlap, in the Atlantic), which is available in the web with coincident observations over the USA: this has allowed to set-in some indirect, provisional, subjective verification method. APPLICATIONS TO THE MONITORING (AND NOWCASTING) OF FOG OR REDUCED VISIBILITY MSGlow-twl : Display of channels and channel-differences for day/night and night/day transitions: Use of satellite information is more critical in twilight conditions. Derived cloud products fail to detect part of the low cloudiness for one or more MSG time-slots, despite improvement in version 1.4/2008 of SAFNWC cloud products. Day-time visible channels (HRV as the most interesting) are useful but few illuminated. The well known IR3.9-IR10.8 channel difference, displayed night-time as a complement to CTlow (see next), can be useful despite affected by some reflected sunlight. MSGlow-twl, proposed around twilight to ensure continuity in fog monitoring, shows simultaneously 4 panels: 2 enhanced channel differences (IR3.9-IR10.8, IR8.7-IR10.8), VIS06 and sun-normalised VIS0.6. Sequence of last 4 images (one hour) with superimposed METAR observations. Low cloudiness in IR8.7-IR10.8 channel difference is known to give similar response (even though less intense) than IR3.9-IR10.8 nigh-time, and as IR8.7 is pure thermal channel, it ensures day/night continuity. Nevertheless, some (bare) soils give response similar to low cloud, varying along the year,

2 sometimes confusing (e.g. in the N. Plateau of Spain in winter, where and when thick fog is far from rare). Figure 1: MSGlow-twl display, 17/08/08 5:15z. Left to right and up to down: IR3.9 dif. + METAR most recent observations (locations with an available observation in green, present weather in yellow, and visibility but only plotted where less than 3km, not observed here); IR8.7 dif.; VIS0.6 and VIS0.6 norm. Low cloudiness covers the northern coastal regions, yet at this time only partially detected at 3.9 difference ; 8.7 difference slightly noisy but useful in this summer example (after rainy spring and early summer, no or almost no confusing bare-soil signatures, that are more common in winter). CTlow visualisation combining Cloud Type (CT) product, IR window channel, and in-situ observations: Of the SAFNWC CT (PGE02) product, only very low, low and broken cloudiness classes are displayed, as these should include any potential fog. Elsewhere, the IR10.8 channel image is shown, as it gives information on synoptic and surrounding cloudiness context, ground cooling, etc. IR10.8 also modulates colour intensity in the selected CT classes, as this is sometimes useful for space and time considerations on cloud (or fog) thickness or cloud top heating. Other features in the display (in the intranet page) are: overlay of updated relevant in-situ observations (SYNOP, METAR), to confirm the existence of fog at some point and for coverage considerations at local scale. And a sequence that could be animated of the last 4 images (one hour), to help discriminating moving/non moving cloudiness and for details in the evolution of candidate cloudiness (e.g. dissipating edges).

3 CTlow has proven useful to monitor and nowcast fog coverage. But some limitations come from the CT product itself: poor vertical resolution and inversions not resolved, ground elevation not considered, etc. And of course, nothing is known about cloud base (information is just for its top). Figure 2: CTlow product for 1/07/08 at 7:45z. IR10.8 image for CT classes separately: very low (reddish tone), low (yellowish), broken (bluish), other (black and white). Low and very low cloudiness, pre-frontal (northern coast) or related to the front, is seen in CTlow product, but without much detail, see also comments to fig. 4. CTTHlow product, complementary to CTlow, with Cloud Top Temperature and Height (CTTH), and ground elevation information: In order to overcome CT weaknesses due to poor vertical resolution, and better manage ground elevation, a product using the geo-potential in SAFNWC CTTH (PGE03) product, and an elevation database, has been developed and is under testing. The database is at an initial resolution around 1km and is pre-processed to derive mean, maximum, and gap (maximum-minimum) elevation at the basic 3km resolution of MSG and of course CTTH. CTTHlow is the difference [CTTH - maximum elevation 3x3km 2 ], shown in colour in the range to 1000m (cold colours for negative or around 0 values, white around 200m, warm colours other positive ): in most cases (except for very thick fog, or deep valleys), the cloud presumed to be really fog, locates around the central values of the range (, green, white, yellow ). Of course there is a dependence on local topography, for example, a thin layer of radiation fog will have slightly positive values (white, yellow) over a fairly flat terrain, but increasingly negative as the elevation gap increases, as CTTHlow is referred to the maximum elevation. And topography has an influence in the occurrence, persistence, evolution, etc., of fog. Thus, additional mouse-driven roaming windows, for the mean and gap elevation, see figures, are part of the CTTHlow display.

4 Figure 3: CTTHlow product display, same time of fig. 2. Product is [CTTH m ax. elev.], colour scale as indicated, IR10.8 where no cloud or above 1000m ; and broken cloudiness in product CT (maroon) for which CTTH is not known. Right side boxes are for mean topography (up, in usual colours) and gap elevation (down, same 0 or negative colourscale part as for CTTHlow), both centred at the location indicated by the small squared empty box over product (here, north of Portugal). It is also shown the html/java frame with interactive -option buttons, the same or similar to already used for intranet operational display of CTlow and MSGlow_twl, proposed for CTTHlow still experimental. CTTHlow is routinely computed and checked on an experimental basis since spring 2008, and looks promising but more work on product characterisation is needed before operational use. Other display features are, by now, proposed as for CTlow: Filling with IR10.8 data, overlaid observations, and one hour sequences. Note some limitations in CTTH itself: it sometimes reflects processing done at segment resolution (somewhere rectangular aspect) and (intrinsically) the NWP model resolution. Some very low cloud pixels with erroneous CTTH_z = 0m, so are not colour enhanced in the display.

5 Figure 4: as in fig. 3, but buttons zoom ( Acercar ) and SYNOP/SHIP are activated (visibility in red if less than or equal to 1km - properly fog, in yellow from 1 to 3km; other criteria as for METAR in fig. 1). Fig. 2 to 4: frontal cloudiness reaches NW corner of the Iberian Peninsula. In CTTHlow (fig. 2 and 3), fog is likely to be found when cloud-tops slightly above, around or even below elevation maxima, i. e. very small patches in the north and mostly in the frontal cloudiness, in inner valleys (see elevation information), almost reaching the coast in its northern edge, where SYNOP observations (some 2h old anyway) confirmed the existence of fog. Figure 5: 6/08/08 at 6:30z. CTlow and SYNOP observations (left, colours as in fig. 2, SYNOP information as in fig. 4), CTTHlow (right, colour scale as in fig. 3). Fog more probable (in green, quite low cloud top) in SW of France (notice the grey hole due to bad CTTH assignment = 0m) and inner Spain (Ebro valley). Probability is lesser in coastal provinces of Spain and over sea (warm colours, higher cloud top).

6 Figure 6: 19/07/08 at 8:00z. CTlow (left), CTTHlow (right), SW of France. Despite the (2h old) observation, and the broad very low-level CT cloudiness, holes and the CTTHlow aspect suggest a rather elevated, dissipating pattern of (maybe, partly confirmed in fact by previous sequences) earlier fog, or St layer. AN APPLICATION TO THE MONITORING (AND NOWCASTING) OF ICING MSGicing product of cloud icing potential, and visualisation, with Cloud Type (CT) and Cloud Top Temperature and Height (CTTH) SAFNWC products and NWP information: Super-cooled liquid water cloud droplets could be harmful to lightweight aviation, when accumulating ice on aircraft surfaces. Icing forecast techniques using NWP fields are well known, but direct observations are difficult to collect (and are presently unavailable at AEMet over our region). A satellite-derived icing diagnostic could be useful for nowcasting purposes. Feasibility is prove n, given the sensitivity to various meteorological cloud-top parameters (temperature, water phase, liquid water path, effective radius), ingredients of icing, in channels similar to SEVIRI (Minnis, 2004, MSG products including icing but not in real time are shown in the NASA-Langley web page 2 ). These microphysical cloud properties must be computed in a quite complex chain, as could be the SAFNWC, unfortunately not or not yet providing but the cloud-top temperature. In AEMet instead, an existing, much more simple product developed by NOAA/NESDIS for GOES, ICECAP (Ellrod, 2007), has been adapted to MSG. Temperatures in CTTH product are used instead of IR10.8 channel for the temperature range check (-1º to -29º). Presently, thresholds for the IR3.9 channel-difference (the basic liquid-water discriminator), are: IR10.8-IR3.9 > 4.5º as night-time condition. IR3.9-IR10.8 > 5.5º as day-time condition. Either, in partly illuminated images. No semitransparent (ice) cloudiness check and removal is applied in MSGicing as is in ICECAP, since it could hide a lower (icing) cloud layer, but CT product detailed information on cirrus or semi transparency is displayed. Basic MSGicing display is CTTH flight-level for potentially icing cloud tops, plus fairly too cold (< -30º) cloudiness. Buttons for interactive display of complementary information are available: < -12º cloudiness, semitransparent clouds, NWP information (isotherms 0º and -12º, liquid water content in range 0º to -12º at different levels), other satellite data. It is computed hourly and a 5h sequence is available to ensure continuity (e.g. through day and night transitions).

7 Figure 7: 16/07/08 at 8:45. MSGicing product in flight-level (vertical colour scale) with some options activated: cloud tops colder than -12º (in grey), opaque, plus CT semi transparent cloudiness (3 blue classes, the darker the more transparent, plus daytime- Ci with other cloudiness underneath, turquoise). Isolines for NWP icing product at flightlevels 130 (yellow) and 180 (green), HIRLAM model, 0:00z run valid at 9:00z. Interpretation: most of the icing is certainly hidden below great part of the <-12º large cloudiness. But not over south of France and Gulf of Lion, semi transparent Ci/Cs for the most, as are spots in Spain and the SE image corner, where icing is less probable. NWP fields support icing over the Mediterranean, not in Africa nor in left to right cloudiness top of the image, where its possible presence has in consequence to be taken in mind. In theory, MSGicing tops masked by <-12º but not by <29º opaque cloudiness (part of the basic display not shown here as it is hidden by the <-12º overlay, but it was in fact the case in the frontal edge north of France, and in other smaller zones), would not represent significant icing since cloud reaches very cold temperatures, thus important presence of ice nuclei, known to strongly reduce cloud super-cooled content. Multilevel cloudiness (e.g. left-upper corner) is similar to opaque (but this class only available day-time, this cloudiness could have been confused with other types night-time, making the diagnostic more subject to error). 0º and -12º isotherms (not displayed) could provide some information on icing thickness. It has to be reminded that only potential icing at cloud top level is known, not actual depth, and just outside hidden (rather opaque or multilayer) too-cold tops; nor the icing intensity. And as no PIREP observations are by now available at AEMet, the product is not validated, or quite indirectly, see hereafter. MSGicing is kept close to GOES ICECAP: an ICECAP clone, and MSGicing, are being computed for comparison purposes in an MSG and GOES overlapping region over the Atlantic. And, PIREP observations being part of the display in ICECAP web page over the USA 3, some subjective, short verification was tried (2 periods of 30 days) and will certainly we repeated, more detailed, in the next future. Results by now (see also Ellrod, 2007), deemed also of some validity for Spain, were:

8 More than one half of the cases (and almost 2/3 in the warm season) of observed icing were correctly diagnosed by ICECAP. While only around 10% missed but the rest were doubtful (too cold cloudiness for MSGicing). The false alarm ratio was found to be large, more than 1/3 of cases. But observations of no icing are scarce, likely only transmitted when favourable conditions were expected. FINAL CONSIDERATIONS User oriented applications, as shown for fog and icing monitoring purposes are needed, but also new products and improvement in existing ones, for the SAFNWC products to be found fully useful in nowcasting. Other data, mainly observations, are needed for integration in the applications and also for validation and verification of products. Actions are started at AEMet to get more observations on visibility, and icing reports. In the fog applications, the replacement or complement at twilight of the IR8.7 channel difference (sometimes problematic) by the (improved) CT or CTlow product, to get a more complete overview, is being studied. As MSGicing product is too simple, and other products of interest are now possible from MSG, suggestions for new ones have been addressed to the SAFNWC on the occasion of its 2008 Survey. In the meantime, are also studied at AEMet the use of NIR1.6 channel daytime, and RGB adequate displays. REFERENCES Ellrod, G.P. and A.A. Bailey, 2007: Assessment of aircraft icing potential and maximum icing altitude from geostationary meteorological satellite data. Weather and Forecasting, 22, February 2007, pp Minnis, P., L. Nguyen, W. Smith Jr., J.J. Murray, R. Palikonda, M. Khaiyer, D.A. Spangenberg, P.W. Heck and Q.Z. Trepte, 2005: Near real-time satellite cloud products for nowcasting applications. Proceedings of WWRP Symposium on Nowcasting & Very Short Range Forecasting, Toulouse, France, 5-9 September SAFNWC web page: 2. NASA-Langley products web page: 3. NOAA/NESDIS ICECAP web page: