QUALITY OF MPEF DIVERGENCE PRODUCT AS A TOOL FOR VERY SHORT RANGE FORECASTING OF CONVECTION

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1 QUALITY OF MPEF DIVERGENCE PRODUCT AS A TOOL FOR VERY SHORT RANGE FORECASTING OF CONVECTION C.G. Georgiev 1, P. Santurette 2 1 National Institute of Meteorology and Hydrology, Bulgarian Academy of Sciences Tsarigradsko chaussee 66, 1784 Sofia, Bulgaria 2 Forecast Laboratory, Météo-France, 42, Avenue G. Coriolis, Toulouse Cedex 01, France Abstract EUMETSAT Meteosat Product Extraction Facilities (MPEF) generates the Upper-troposphere Divergence (DIV) product and disseminates it operationally through EUMETCast since From an operational perspective, such a satellite product seems promising since the divergence associated with disturbances in upper-level wind field produces forcing for ascent. This paper presents the first study on the usefulness of the DIV product as a tool for forecasting of intense convection. The quality of the DIV product has been assessed by comparison with NWP model analyses of wind field as well as wind observation data at upper-air. The results show a relation between the DIV product and these reference data that may vary from case to case. A specific colour palette is proposed to allow inferring the information content of the product and to make its suitable for using in support convection forecasting. For assessing the potential of MPEF Divergence product for using in nowcasting applications, 1166 convective cells with cloud top temperature lower than 50 C that developed over Europe and Northern Africa from 15 June to 22 July 2010 were considered. The results show that 76 % of all convective cells initiated at areas of divergence seen by MPEF DIV product and 92 % of the convective cells developed at areas of divergence derived by the MPEF DIV product. This relation can be useful for the purposes of convection nowcasting by considering composite images of the Divergence field derived by MSG and any other parameters from NWP output, as well as IR channel 10.8 µm imagery. INTRODUCTION The DIV product is hourly generated at Meteosat Product Extraction Facilities (MPEF) from Meteosat Second Generation (MSG) channel 6.2 µm Atmospheric Motion Vectors (AMV), EUMETSAT (2005). Schmetz et al. (2005) illustrate that the DIV product derived from 15-minutes MSG data can be used to infer instantaneous divergence field in a case study for convective cloud systems over tropical Africa and show a comparison with ECMWF forecast fields. The authors conclude that the diurnal cycle of upper tropospheric wind field divergence in tropical deep convective systems can be estimated directly from the satellite derived AMVs. They argue that the satellite observed divergence offers a useful diagnostic tool for testing convective parametrisations in atmospheric models and an analysis of longer time series of observed divergence patterns will be undertaken as a basis for climatological studies. The MPEF DIV product seems promising as a tool for very short range forecasting, since the divergence associated with upper-level wind field disturbances produces forcing for ascent that can favour the development of intense convection (see e.g. Santurette and Georgiev, 2005, 2007). In 2009, a project on satellite data interpretation for anticipating deep convective development has initiated by the National Institute of Meteorology and Hydrology (NIMH) of Bulgaria and Meteo-France. The project programme includes research on the quality of the DIV product in mid-latitudes and its usefulness for the operational forecasting. An evaluation of the DIV product quality as a tool in support forecasting of convection has been performed by using NWP model analyses and very short-range forecast fields as well as upper-air wind observations. It was recognized that the validation MPEF DIV product is a difficult task. The following two possible factors may affect the DIV quality and might impose limitations as follows:

2 The interpolation (Barnes, 1964) used as an intermediate procedure has the effect of smoothing out information horizontally. The side effect is that small-scale features are suppressed. The input to the algorithm is all upper-level AMVs, which pass through specific quality control derived by tracking cloud and humidity features in the 6.2 µm WV channel in the layer hpa. This implies that the MPEF DIV values represent an atmospheric layer, rather than an atmospheric level. The MPEF DIV product has been disseminated via EUMETCast since 2008 and there is no any experience in its operational use. The aim of this study was to develop a relevant scheme for its interpretation. The second section of this paper is devoted to the visualisation of the DIV product in image format. The information content of the product as well as its use in combination with relevant NWP fields and other meteorological data are considered in the third section. The fourth section of the paper presents some general ideas on the usefulness of MPEF DIV product from an operational perspective in analysis of convective situations over Europe in June and July MPEF DIV PRODUCT VISUALISATION The first goal of this study was to solve the problem of the visualisation of the MPEF DIV product that can allow the users to infer as much as possible the information content of this product for analysis of upperlevel dynamics. At NIMH of Bulgaria, the visualisation of the DIV product in operational SYNERGIE forecasting system started in Following the principles of the EUMETSAT visualisation template, only divergence values that are clearly positive (representing upper-air divergence) or negative (representing upper-air convergence) are visualised. In this colour table (see Table 1, EUM 2008) the divergence/convergence values in the range [ s s -1 ] are not coloured for that reason. This approach is relevant for studying processes associated with high values of divergence, which can reach s -1 in the tropical convective cloud systems (Schmetz et al., 2005). Values below Values below s -1 and above s -1 are not common and occur rarely over Europe in cases of very strong convergence/divergence. Table 1. Colour table for visualisation of MPEF Divergence product tested at the operational forecasting environment of NIMH of Bulgaria. Range of divergence values (10-6 s -1 ) Colour/RGB (EUM 2008) Colour/RGB (NIMH 2010) below , 0, 0 255, 0, to , 80, , 80, to , 140, , 140, to , 190, , 190, to , 220, , 220, to , 235, , 235, to 0 [none] 248, 248, to +20 [none] 220, 255, to , 235, , 235, to , 220, , 220, to , 190, , 190, to , 140, , 140, to , 80, , 80, 255 above , 0, 255 0, 0, 255 N/A 255, 255, , 255, 255 Fig. 1a shows an image of the DIV product in EUM 2008 palette. The DIV is overlaid by IR 10.8 µm channel image (only cloud top height < -40 C). The areas of cloud top temperatures lower than 50 C are coloured in yellow with the aim to assess the correspondence between upper-level divergence and development of deep moist convection. Our experience on using such an image presentation of the DIV product in EUM 2008 palette shows that considering only values of upper-level divergence greater than s -1 is not a relevant approach for studying convective developments over Europe. As seen in Fig. 1a, deep moist convective cells develop between Northern Spain and Central Europe (e.g. at the brown arrow in Fig. 1b), where the image is not coloured in the EUM 2008 palette. Therefore, such an image presentation of DIV product that does not show areas of weak divergence values in the range [ s s -1 ], may not be useful to indicate the dynamic rate of such synoptic situations.

3 In order to increase the information content of the MPEF DIV images, a new colour palette has been introduced in the operational environment of NIMH of Bulgaria in June 2010 and then in the SYNERGIE system version 4.5 in Météo-France. In the new colour palette, referred to herein after as NIMH 2010, a light green colour is added to show values of little divergence in the range [ s -1 ] that is illustrated in Table 1, third column. Fig. 1b shows the same image as in Fig. 1a with the MPEF DIV, visualised in NIMH 2010 pallet. It is seen that using this new visualisation template allows the user to see that all convective cells, which initiate/develop over Europe on 2 August UTC are located within a large area of upper-level divergence in the range [ s -1 ], as derived by the DIV product. (a) (c) (c) Figure 1. MPEF DIV product, superimposed by IR 10.8 µm channel image (only cloud top height < -40 C) for 2 August 2010 for 1545 UTC (a) visualised in EUM 2008 palette (Table 1) and visualised in NIMH 2010 pallet overlaid by ARPEGE 3 h forecast of CAPE (brown, only > 800 J/kg, interval 400 J/kg) valid for 15 UTC, (c) 0545 UTC, (d) 0745 UTC and (e) 1845 UTC. INFORMATION CONTENT OF MPEF DIV PRODUCT FOR THE PURPOSES OF CONVECTION NOWCASTING It is well known that, even if favourable high-level dynamic conditions are present, severe convective storms are not likely to develop without favourable low-level environment and instability in a deep tropospheric layer. Therefore, for the purposes of convection forecasting the divergence field needs to be analysed in combination with any parameter of instability. On Fig. 1b, the values of Convective Available Potential Energy (CAPE) greater than 800 J/kg (ARPEGE NWP model-derived) are also presented in brown contours. The composite image in Fig. 1b reveals the following aspects of the convective situation, regarding the usefulness of the MPEF DIV product visualised in the NIMH 2010 palette:

4 Convective cells have developed up to -50 C (yellow) cloud top temperature over the area where the NWP model shows tropospheric instability (CAPE > 800 J/kg) and upper-level divergence is present in the satellite DIV product (e.g. at the brown arrow in Fig. 1b). Figs. 1c 1d and 1e show that upper-level divergence has been present over this area for 6 hours before the convective development and it was propagating progressively to the east (see brown arrows). Deep convective cells usually do not develop over the area of strong tropospheric instability and upper-level convergence (e.g. at the red arrow in Fig. 1b). (a) Figure 2. MPEF DIV product for 2 August 2010 at 1145 UTC visualised in pallet NIMH 2010 (Table 1) superimposed by (a) IR 10.8 µm channel image (only cloud top height < -40 C) and upper-air sounding data of wind at 300 hpa at 1200 UTC and ARPEGE analysis wind at 300 hpa at 1200 UTC. Combining MPEF DIV product with information for atmospheric instability can help forecasters to anticipate areas favourable for initiation and development of deep moist convection where the instability and upper-level divergence are coupled. Of course, other elements of the thermodynamic context can be important for producing convection initiation at a specific location. This paper concerns mainly the use of satellite-derived information about the upper-level dynamic context regarding the development of a convective environment. The ARPEGE model CAPE is used in this study only as a conceptual parameter and can not be considered as a real measure of instability, because this CAPE is derived only at the areas where the NWP convective scheme releases instability. For the purposes of nowcasting convection it needs to use any other instability parameters as those derived by using satellite data. Fig. 2 shows the DIV product overlaid by the wind vectors at 300 hpa derived from upper-air sounding observations and NWP analysis. The values of upper-level strong divergence greater than s -1 at the black arrow are due to the jet stream break at the base of an upper-level trough (see Fig. 2b) that is associated with specific area of upper-level subsidence and a dry delta structure in the 6.2 µm channel WV images (see Santurette and Georgiev, 2005). Therefore, although in this case the high NWP modelderived CAPE shows a local area of strong convective instability and upper-level divergence (black arrow in Fig. 1b), deep convection is not likely to develop because of the low height of the dynamical tropopause and upper-level subsidence at the jet break. In the leading part of the trough (at the brown arrow in Fig. 2b) the MPEF DIV product shows upper-level divergence in the range [ s -1 ] that is confirmed by the wind observations at 300 hpa at 12 UTC (Fig. 2a) showing a diffluent flow over this area. However, the divergence/convergence of the wind field are not always due only to diffluence/confluence of the flow. For example in Fig. 2b a confluent flow is present in the ARPEGE 300 hpa wind at the red arrow that is likely to appear downstream an upperlevel ridge in the right jet entrance, which is normally an area of upper-level divergence, confirmed by the MPEF DIV product, showing values in the range [ s s -1 ].

5 (a) E D C (c) (d) Figure 3. Div product, NIMH 2010 palette, 16 August 2010 at (a) 1545 and 1645 UTC. MSG images in 6.2 µm WV channel at (c) 1545 and (d) 1645 UTC. Significant changes of divergence/convergence for an hour appear at D C and E. (a) Figure 4. Div product, NIMH 2010 palette, on 18 June 2010 (a) at 0945 UTC and 1145 UTC superimposed by IR 10.8 µm image (only cloud top < -40 C) and nearest ARPEGE analysis or of heights of constant potential vorticity surface 1.5 PVU (brown, only < 1000 dam) and wind at 300 hpa (only > 30 kt). Our experience shows that areas of divergence (> s -1 ) and convergence (< s -1 ) in the MPEF DIV product may have quite different appearance in consecutive images, derived on hourly basis. Figs. 3a and 3b show the DIV product in a case, where convergence values decrease with s -1 west of location E and increase with s -1 east of location E. In the same time, the divergence has increased up to s -1 at C, while the divergence at D has decreased with s -1. The WV imagery analysis at 1545 and 1645 in Figs. 3c and 3d suggests, that the MPEF DIV product algorithm is sensitive to the structure of the moisture features. The divergence decreasing at D seems related to

6 shrinking of the dry dark zone between the red arrows, while divergence increasing at C seems related to appearance of more uneven moisture/cloud structure of the image at the magenta arrow. A mechanism producing fast changing in the MPEF DIV field can be the upper-level divergence resulting from ascent, associated with strong convective development. This often causes quite different divergence values at such areas of convection in consecutive hourly images. Fig. 4 illustrates this effect (over the Balkans, brown arrow) that appears in the areas of low rate of the upper-level dynamics. Concerning with this as well as because of limitations regarding its generation (see the Introduction), the MPEF DIV product cannot be easily validated by comparing with NWP model wind field or upper-air sounding data at specific level. At the areas of strong upper-level wind the upper-level dynamics dominates in producing divergence or convergence and the convective development does not affect significantly the field of divergence seen by the MPEF DIV product (as over the Central Europe, black arrow in Fig. 4). MPEF DIV PRODUCT AS A TOOL FOR ASSSING FAVOURABLE CONVECTIVE ENVIRONMENT In order to assess the operational usefulness of MPEF DIV product, a study of the relation between the DIV field and deep moist convection has been performed. The convective cells are classified in 8 cases according to the location of their initiation and development regarding the divergence values of the area as derived by MPEF DIV product. The limits of each one of the 8 cases are defined in Table 2. Table 2. Initiation and development of deep convective cells (cloud top brightness temperature < 50 C) regarding the areas of divergence/convergence as derived by the MPEF DIV product on 29 June 2010 and from 16 June to 22 July Convective cells initiating at area of Convective cells developing at area of Divergence, 10-6 s -1 Convergence, 10-6 s -1 Divergence, 10-6 s -1 Convergence, 10-6 s -1 > < 20 Total > < 20 Total 2010 Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 29/06/ /06 22/ % 66.5% 22.5% 1.2% 21.5% 71.0% 7.0% 0.5% Figure 5. MPEF DIV product for 29 June UTC (a) visualised in NIMH 2010 pallet and superimposed by IR 10.8 µm channel image (only cloud top height < -40 C). Six cases according to Table 2 are indicated.

7 In this study, the DIV product, visualised by NIMH 2010 template was superimposed by the deep convective clouds (only cloud top temperature < -40 C, as seen by the IR images in 10.8 µm channel). Fig. 5 shows the composite image over the domain, where the considerations have been performed for an example on 29 June 2006 (see raw 1 of table 2). For the purposes of this study, in Table 2, only convective cells with cloud top brightness temperature colder than -50 C (yellow), which develop at least for two hours are considered. Even in cases of very strong convective development the divergence values over the studied region, derived by the MPEF DIV product, do not exceed s -1. Fig 6 shows such a case, in which intense convection developed in a situation of upper-level potential vorticity (PV) anomalies and a jet stream (at brown arrow in Fig. 6b), which produces strong dynamic forcing for convection (Santurette and Georgiev 2005, 2007). It is also seen in Fig. 6b that convective cells over northern part of the Balkans initiated in the area of a small upper-level divergence [ s -1 0] seen by the DIV product. This confirms the consistency of NIMH 2010 visualisation template for the MPEF DIV product as well as allows, for the purposes of this study, to consider only four levels of divergence/convergence in Table 2 as follows: [> s -1 ], [ s -1 0], [ s -1 ] and [< s -1 ]. (a) (c) (d) Figure 6. Div product, NIMH 2010 palette, superimposed by IR 10.8 µm image (only cloud top < -40 C) on 16 June 2010 (a) at 1045 UTC UTC, (c) 1445 UTC and (d) 1945 UTC superimposed by nearest ARPEGE analysis or 3 h forecast of heights of the constant potential vorticity surface 1.5 PVU (brown, only < 1000 dam) and wind at 300 hpa (only > 55 kt). The results presented in Table 2 show that the initiation and development of deep moist convection over Europe is closely associated with the existence of upper-level divergence as derived by MPEF DIV product. For the period 15 June 22 July 2010 (raw 2 on Table 2) the following figures are obtained:

8 76 % of all convective cells initiated at areas of divergence seen by MPEF DIV product. 92 % of all convective cells developed at areas of divergence, which compared to the 76 % for the first category show that at least in 15 % of the cases the upper-level divergence can be a result of the ascent associated with strong convective development as in the case of Fig. 4. For the same reason (divergence produced by strong convection), the convective cells in Case 3 are 15 % more than the cells in Case 7. These results give us optimism to conclude that the MPEF DIV product provides useful information for assessing areas of upper-level divergence where strong convection preferable initiate. As shown with the consideration of Figs. 1 and 6, this relation can be helpful for the purposes of convection forecasting by considering composite images of the AMV Divergence field and any other parameter from NWP output, as well as MSG 10.9 µm IR imagery. The MPEF DIV product may be useful as a nowcasting tool because in many situations divergence values are present in the images some hours before the initiation of deep convective cells (see the case of Fig. 1) especially in the areas of high rate of the upper-level dynamics, near jet streams where the upper-level divergence significantly contributes to formation of an intense convective environment, as in the case of Fig. 6. CONCLUSION This paper is focused on the potential of nowcasting applications, based on using MPEF Divergence product, MSG image data and NWP output. Such applications may range from possibilities to preconvective identification of potentially favourable environment to a characterisation of the deep moist convective development. The unique opportunities of the MSG 6.2 µm WV and other air mass channels allow the derivation of advanced nowcasting products, which provide information for instability and upperlevel dynamics. Therefore, further studies on utilisation of satellite MPEF DIV product jointly with air mass instability from hyperspectral sounding as MPEF Instability indexes would be of interest. In the area of convective storm nowcasting, a combined use of complementary observation techniques and products is considered to be very beneficial and ensures the optimal use of the satellite data in an operational context. The quick repeat cycle of 5 minutes on Meteosat Rapid Scan would help in the exact determination of the onset of severe convection. Concerning with this, it is recommended EUMETSAT to start generation of DIV product on 15 minutes of half an hourly basis of Meteosat Rapid Scan Services. ACKNOWLEDGEMENTS The authors are grateful to Arthur de Smet from EUMETSAT for the helpful suggestions and comments regarding visualisation and interpretation of MPEF Divergence product. REFERENCES Barnes, S. L., A technique for maximizing details in numerical weather map analysis. J. Appl. Meteor., 3, EUMETSAT (2005). Upper Level Divergence Product Algorithm description. EUM/MET/REP/05/0163. EUMETSAT, Germany, 16 p. Schmetz, J., Borde, R., Holmlund, K., König, M., (2005). Upper tropospheric divergence in tropical convective systems from Meteosat-8. Geophys. Res. Lett., 32, L Santurette, P., Georgiev, C. G., Weather Analysis and Forecasting: Applying Satellite Water Vapor Imagery and Potential Vorticity Analysis. Elsevier Academic Press. ISBN: , 200 pp. Santurette, P., Georgiev, C.G., Water vapour imagery analysis in 7.3µ/6.2µ for diagnosing thermodynamic context of intense convection. Joint 2007 EUMETSAT Meteorol. Sat. Conf. and the 15th AMS Sat. Meteorol. & Oceanogr. Conf., Amsterdam, The Netherlands, September 2007.