Use of potential vorticity fields, Meteosat water vapour imagery and pseudo water vapour images for evaluating numerical model behaviour

Transcription

1 Meteorol. Appl. 8, (2001) Use of potential vorticity fields, Meteosat water vapour imagery and pseudo water vapour images for evaluating numerical model behaviour Christo G Georgiev, National Institute of Meteorology and Hydrology, Tsarigradsko shaussee 66, 1784 Sofia, Bulgaria Francisco Martín, Instituto Nacional de Meteorología, Cuidad Universitaria, Camino de las Moreras s/n., Madrid, Spain During November and December 1998, ten cyclonic disturbances over the North Atlantic and Mediterranean area were studied. Features in potential vorticity (PV) fields were tracked in the analyses and short-range forecasts of the Spanish version of the HIRLAM model (HIRLAM-INM) and the ECMWF model. They were superimposed on the corresponding Meteosat water vapour (WV) images with the aim of ascertaining the qualitative relationship between Meteosat WV imagery and PV fields that might be used to assess NWP model behaviour. Two cases predicted incorrectly by HIRLAM-INM were studied in detail: a rapidly deepening depression over the Atlantic (26 27 November 1998) and a cut-off low process over the western Mediterranean (3 4 December 1998). For evaluating HIRLAM-INM model behaviour, absolute vorticity at 2 PVU surface, positive PV anomalies at 500 hpa as well as pseudo WV images were compared with Meteosat WV pictures. Being synthetic products of the numerical model, the pseudo WV images were used to indicate the strength of the relationship between the Meteosat WV imagery and the PV fields, and to indicate whether any mismatches correspond to real NWP model errors. It is demonstrated that the HIRLAM-INM phase error at 3 4 December 1998 was clearly seen in the comparison between Meteosat and pseudo WV images. 1. Introduction The potential for validating model analyses or early forecasts by comparing dynamical fields with water vapour (WV) imagery has recently begun to be investigated. For that purpose, Meteosat WV images have been compared with diagnostic fields such as height of a suitable potential vorticity (PV) surface, e.g. PV = 2 PVU ( m 2 s 1 K kg 1 ) (Mansfield, 1996, 1997), pressure of the 2 PVU surface (Røsting et al., 1996) or absolute vorticity at 300 hpa (Carroll, 1995, 1997). Some other PV fields have also been used and tested for comparison with WV imagery in connection with the problem of validating and verifying numerical weather prediction (NWP) output. (a) Browning & Reynolds (1994) considered the extent of PV anomalies with PV > 2 PVU at 3 km level as well as PV > 1 PVU at the 2 km level. Comparing these PV anomalies in the lower troposphere with WV images showed that the centre of the dark zone on the moisture channel pictures has tracked closely in line with the lowest part of the stratospheric intrusion and has given general support to the validity of the model output. (b) Carroll (1997) referred to the value of absolute vorticity on the 2 PVU surface as a field with which to compare WV imagery; he considered it to show at least as good a correspondence as absolute vorticity on 300 hpa. According to the Status Report on COST-78 Project III.1 (COST-78, 1998), the field most closely related to WV imagery is absolute vorticity on a suitable PV surface, most likely PV = 2 or 1.5 PVU, but with the height of the PV surface also found to be a good match. As the latter field is most easily interpreted, this was the preferred dynamical field, though it was agreed that the physical basis for the relationship of absolute vorticity on a PV surface with the imagery should be clarified. (c) The field of positive PV anomalies (PPVAs) at 500 hpa gives quite a good correspondence to WV image dry slots (Georgiev, 1999). It has also been recommended that using this PV field may be a way of helping to solve the problem of verifying and validating numerical output (Georgiev, 1998, 1999). Recently, pseudo WV images have been derived for use in weather forecasting. These are synthetic pseudo 57

2 C G Georgiev and F Martin water vapour fields that represent the distribution of tropospheric humidity, analysed and predicted by the numerical model, and presented in terms of a Meteosat WV picture. It was confirmed that the best check on NWP analyses and very short period forecasts was by comparison of real and pseudo WV imagery (COST- 78, 1998). However, it was agreed that PV fields should also continue to be compared with the imagery as, when a real error is detected, knowledge of the error in the PV will allow the forecaster to make the best estimate of how to subjectively adjust the subsequent forecast. The aim of this study was to use not only the height/pressure of the 2 PVU surface or absolute vorticity at 300 hpa, but other appropriate PV fields together with pseudo WV imagery for subjectively detecting errors in NWP analyses and very short range forecasts. Three main objectives were set for this paper: (a) To elucidate the problem of the qualitative relationship between Meteosat WV imagery and PV fields that might be used to assess NWP analyses and forecasts. (b) On the basis of case studies, to demonstrate the reliability of using 2 PVU absolute vorticity and PPVAs at 500 hpa for evaluating NWP model behaviour by comparing with Meteosat WV imagery. (c) To show that not only the comparison between real and pseudo WV imagery, as suggested in COST-78 (1998), but also superimposing PV fields and pseudo WV images (derived by using one and the same model) may be a useful way of helping to solve the problem of validating numerical output. In section 2 the method of investigation is described as well as some ideas concerning the use of PV fields and WV imagery. The results on the correspondence between PV fields and dark dry slots on the WV images are presented in section 3. Sections 4 and 5 focus on the problem of validating NWP output by comparing Meteosat WV images, PV fields and pseudo WV images; two case studies are analysed. 2. Methodology In dynamically active regions, especially close to the polar front, there is a relationship between WV images and positive PV anomalies, and when there is a mismatch between the PV and the imagery it can be indicative of a model forecast or analysis error (Mansfield, 1997). However, the agreement between the shape and intensity of the PV anomaly and the WV pattern is complex since various mechanisms of cyclogenesis are possible. Also, the variable WV channel sensitivity is a maximum within the middle and the upper troposphere (see Georgiev, 1999). Forcing of cyclogenesis is usually 58 associated with a positive upper-level PV anomaly indicated by a dry slot in the WV imagery but it can occur at lower levels associated with PV anomalies within the moist air. In rapidly developing cyclones, upper- and lower-level effects tend to couple as the high-level PV maximum overruns the low-level baroclinic zone (Hoskins et al., 1985). Strong upper-level forcing is also observed in the absence of major lowlevel forcing or baroclinic zones. The result may be a relatively weak surface cyclone but the dry intrusion can still produce substantial overrunning and convective weather events (Browning, 1997). On the other hand, the rapid cyclogenesis is not always directly associated with a pre-existing upper-level PV anomaly (Mansfield, 1996). With the aim of ascertaining the qualitative relationship between WV imagery and PV fields, ten cyclonic disturbances (observed during November and December 1998 over the North Atlantic Mediterranean area) were studied at the Spanish Meteorological Service (Instituto Nacional de Meteorología: INM, hereinafter). The origin and development of these depressions were classified in four types: (a) Pre-existing upper-level PV anomaly (six cases). These are processes of cyclogenesis associated with a positive upper-level PV anomaly. In such cases, higher than 1.5 PVU values of PV are observed in the troposphere, which indicates air of stratospheric origin (Hoskins et al., 1985). (b) Frontogenesis reactivation by PV anomaly generation (three cases). These are cases when during the late stages of the wave development a regeneration of PV anomaly is observed (the upper-level PV anomaly becomes greater) and the low-level depression is reactivated. (c) Low-level warm anomaly (two cases). These are disturbances associated with a low-level warm anomaly in environmental conditions of low static stability with values lower than 10 7 J kg 1 Pa 2 at 700 hpa (static stability is defined as (α/θ w ) δθ w /δρ, were α is specific volume, θ w is wet-bulb potential temperature, p is pressure). (d) Non-polar disturbance (one case). Such perturbations are observed in the low latitudes, usually over the North Atlantic below latitudes 45 N as well as over the region of North Africa. These disturbances are important as, when moving through the higher latitudes, they interact with the air of polar origin and deepening process occurs. The classification was made according to the most cyclogenetic-producer relevant in each case. It was also indicated if more than one of these mechanisms for forcing of cyclogenesis has been identified during the evolution of the disturbances. Four of the depressions developed as cut-off lows; in one case there was blocking regime formation as a result of cyclogenesis (see Weldon & Holmes, 1991).

3 Use of PV fields, water vapour imagery and pseudo water vapour images In this study, four PV fields that have previously shown a good relationship with the Meteosat WV channel images are considered. (a) Pressure of the 2 PVU surface (Røsting et al., 1996). (b) Absolute vorticity at the 2 PVU surface (Carroll, 1997; COST-78, 1998). (c) Positive PV anomalies at 400 hpa (Georgiev, 1999). (d) Positive PV anomalies at 500 hpa (Georgiev 1998, 1999). Since the constant PV surface of 2 PVU is chosen to lie in between tropospheric and stratospheric values of PV (Mansfield, 1996), the first two fields might be used in diagnosing development with the aim of reflecting some characteristics of the dynamical tropopause. The 2 PVU surface pressure indicates how deep the air of stratospheric origin has penetrated down into the troposphere in cases of tropopause folding, and this is the same approach as using height of the 2 PVU surface, applied by Mansfield (1996, 1997). The absolute vorticity at the dynamical tropopause is closely related to the vertical motions and humidity in the upper troposphere (see section 4.2). The other two PV fields are associated with the dry air intrusion (see Browning, 1997) to the rear of a developing cyclone. In about 80 % of the studied cases, dry slots on the WV images have been indicated by the contours of PV greater than or equal to 1.2 PVU at 400 hpa. When the darkening process in the WV imagery has not been associated with high PV at 400 hpa (see section 3), it was identified by PPVAs of 0.9 PVU on this level. At 500 hpa, the contours of PV greater than or equal to 0.6 PVU usually indicate the dry slots on the WV images (see also Georgiev, 1999). As well as the 2 PVU pressure, the field of PPVAs at 500 hpa indicates the depth of the stratospheric air penetration into the troposphere, but using a middle pressure level as a reference. Using this PV field, only the lowest parts of the stratospheric intrusion are considered that correspond to the dark zones on the WV imagery, and this is a similar approach to those of Browning & Reynolds (1994). Such a presentation of the PV field is appropriate, considering the nature of the relationship between PV and WV data (Georgiev, 1999), because the level of maximum contribution to the Meteosat WV channel radiance lies close to 500 hpa for extremely dry tropospheric profiles (see Fischer et al., 1981). Although the four PV fields are closely related to the dry stratospheric intrusions in the middle and upper troposphere as seen by Meteosat WV channel, all of them have shown some disadvantages and limitations when used in a forecasting environment. For example, when comparing the 2 PVU pressure with WV imagery, any mismatch might be due to the variable sensitivity of WV channel depending on the pressure (Georgiev, 1999). If comparing WV imagery with only the 2 PVU absolute vorticity there is no indication of the depth of the stratospheric air penetration into the troposphere. That is why, from an operational point of view, it seems to be valuable to use the 2 PVU absolute vorticity together with the PPVAs at 500 hpa because the latter field indicates whether the tropopause folding has reached the middle troposphere. It is not unusual, however, for the field of PV anomalies at 500 hpa to be contaminated by high PV features, which are not connected with tropopause folding and some care is required when using this PV field. For example, PPVAs at 500 hpa may correspond to light shades on the WV image in areas of warm air (see Mansfield, 1997; Georgiev, 1998). In quantitative studies on the correspondence between the 500 hpa PV and WV data, such low tropospheric PV anomalies were eliminated by the application of a 400 hpa threshold approach for definition of the areas of PV anomalies at 500 hpa (Georgiev, 1999). As regards the field of PPVAs at 400 hpa, this level may be very informative but from operational and dynamical points of view it is less relevant than the 300 and 500 hpa levels. None of the PV fields considered in this study are conserved quantities in adiabatic flow, so they are appropriate for comparing with developing dry slots in the imagery (see Mansfield, 1997). The four PV fields were superimposed on the corresponding Meteosat WV imagery to find out the best one for comparing with the WV images during the development of cyclonic disturbances which have shown different kinds of origin and evolution. For that purpose, features in the PV fields were tracked in both the analysis of High Resolution Limited Area Model of INM (HIRLAM-INM) and the T+12 forecast of the European Centre for Medium- Range Weather Forecast (ECMWF) model. It was determined subjectively whether the correspondence between the WV darkening structures and these PV fields were good or if there was any kind of disagreement. Pseudo WV images are derived daily and used in preoperational mode at INM which allows a ready and realistic means of comparing two different data sources. These pseudo WV images are obtained from the full resolution HIRLAM-INM model (with approximately 50 km grid horizontal resolution and 31 hybrid levels in the vertical) data and using the EUMETSAT radiative transfer code (see Schmetz & Turpeinen, 1988) for the 6.3 µm channel of Meteosat. In this study, the 2 PVU absolute vorticity and PPVAs at 500 hpa were superimposed on the corresponding Meteosat WV imagery and pseudo WV images with the aim of monitoring the HIRLAM-INM model s behaviour for early warning of NWP output errors. To assess NWP analyses and forecasts, comparison between the real and pseudo WV images has also been made. 59

4 C G Georgiev and F Martin 3. Correspondence between PV fields and Meteosat WV imagery Features in PV fields were tracked in the analysis and short-range forecast of the HIRLAM-INM and ECMWF models valid for 0000 and 1200 UTC. During the evolution of the ten cyclonic disturbances in this study, 19 tracks of WV images and each one of the considered PV fields from HIRLAM-INM were examined; for the PV fields from ECMWF model 16 such tracks were analysed. The results on the relationship between the PV fields and Meteosat WV imagery are presented in Table 1 for the HIRLAM-INM analysis and in Table 2 for the ECMWF model T+12 forecast. The degree of correspondence is classified in five categories: (a) Good. These are cases where the darkening process is associated with high anomalies in the PV fields (e.g. 2 PVU pressure > 400 hpa, 2 PVU absolute vorticity > s 1, 400 hpa PV > 1.2 PVU, 500 hpa PV > 0.9 PVU); the areas of PV anomalies are mirrored by the dark dry slots on the WV images; the PV maxima fit well the darkest parts of the image. (b) Dislocated. The areas of PV anomalies are not mirrored by the dark dry slots on the WV images or the PV maxima do not fit the darkest parts of the image. In this case, if high PV anomalies are associated with darkening process on the WV image, the correspondence is classified as good but dislocated. (c) Overestimated. High PV anomalies are not associated with darkening process on the WV image. (d) Underestimated. The darkening process occurs on the WV image but is not associated with high PV anomalies. (e) Missing. The darkening process occurs on the WV image but is not associated with any positive PV anomalies. Tables 1 and 2 indicate that the closest relationship appears to be between WV imagery and both the absolute vorticity at 2 PVU surface and the PV anomalies at 500 hpa. Therefore, also taking into account the suggestions in Carroll (1997), COST-78 (1998) and Georgiev (1999), these PV fields might be compared with WV imagery for evaluating NWP model behaviour. This will be demonstrated in sections 4 and 5. According to the results, the PPVAs at 500 hpa give the best field corresponding to dark zones on the WV imagery. Most often, the contours of PV anomalies at 500 hpa are closely mirrored by the dark dry slots and the PV maxima coincide with the darkest parts of the image (see also Georgiev, 1998). The 2 PVU absolute vorticity shows more cases in which the maximum of the PV anomalies is dislocated from the dark zones. Usually, these are cases where the darkest WV dry slots correspond to the highest gradient areas of this PV field. This kind of dislocation is usually observed when WV images are compared with the other two PV fields (PV at 400 hpa or the 2 PVU surface pressure). It is also evident from Table 1 and Table 2 that the PV fields from the T+12 forecast of ECMWF model show a slightly closer relationship to the WV darkening process. In the HIRLAM-INM model analysis, the maximum of the PV anomalies has more often been dislocated from the dark slots on the corresponding WV image. Table 1. Degree of correspondence between Meteosat WV image dry slots and PV fields from the analysis of HIRLAM-INM for 19 tracks of WV images and PV fields PV field Degree of correspondence Good Dislocated Over- Under- Missing estimated estimated Pressure at the 2 PVU surface 6 (31.6%) 13 (68.4%) 1 (5.3%) 10 (52.6%) 0 Absolute vorticity at 2 PVU surface 11 (57.9%) 10 (52.6%) 1 (5.3%) 0 0 PV anomalies at 400 hpa 7 (42.1%) 12 (63.2%) 1 (5.3%) 3 (15.8%) 0 PV anomalies at 500 hpa 11 (57.9%) 8 (42.1%) 1 (5.3%) 0 1 (5.3%) Table 2. Degree of correspondence between Meteosat WV image dry slots and PV fields from the T+12 forecast of ECMWF model for 16 tracks of WV images and PV fields PV field Degree of correspondence Good Dislocated Over- Under- Missing estimated estimated Pressure at the 2 PVU surface 7 (43.8%) 10 (62.5%) 1 (6.2%) 4 (25.0%) 0 Absolute vorticity at 2 PVU surface 8 (50.0%) 8 (50.0%) 1 (6.2%) 1 (6.2%) 0 PV anomalies at 400 hpa 7 (43.8%) 11 (68.8%) 1 (6.2%) 0 0 PV anomalies at 500 hpa 9 (56.2%) 6 (37.5%) 1 (6.2%) 3 (18.7%) 0 60

5 Use of PV fields, water vapour imagery and pseudo water vapour 4. Case study of rapid development over the north-western Atlantic 4.1. Development as simulated in the HIRLAM- INM model Figures 1(a), 1(b) and 1(c) show the evolution of mean sea level pressure (MSLP) on 26 November 1998 when rapid cyclogenesis occurred over the north-western Atlantic. A surface low was moving fast in a north-easterly direction and deepening from 996 to 964 hpa for 24 hours. The HIRLAM-INM forecast was good from a synoptic point of view in terms of the intensity and rate of development. But, by T+24 (Figure 1(d)) the model was in error with respect to the depth of the surface depression the minimum MSLP was overestimated by 2 4 hpa in comparison with the analysis at 0000 UTC on 27 November 1998 (Figure 1(c)). For the purpose of identifying the role of the low-level baroclinic zone and the upper forcing in the deepening of the surface low, various model output fields for 0000 and 1200 UTC on 26 November are shown in Figure 2 and overlaid with Meteosat WV images. In Figures 2(a) and 2(b), the HIRLAM-INM analysis of geopotential height on 500 hpa (white solid lines) is superimposed with the wet-bulb potential temperature (θ w ) at 850 hpa (black dashed lines). These are compared with fields of absolute vorticity at the dynamical tropopause (2 PVU surface) derived by HIRLAM-INM analysis (Figures 2(c) and 2(d)), and T+12 forecast of the ECMWF model (Figures 2(e) and 2(f)). The initial development has been associated (Figure 1(a)) with a low-level warm anomaly in environmental conditions of low static stability at 700 hpa (see section 2). During the next stage, when the surface low deepened by 18 hpa in 12 hours, such a low-level warm anomaly could not be identified (the static stability at 700 hpa is greater than 10 7 J kg 1 Pa 2 as shown by Figures 1(b) and 1(c)). Over that period, the rapid cyclogenesis was largely due to the upper-level PV anomaly associated with the main strip of dry intrusion to the rear of a sharp trough at 500 hpa seen in Figure 2(a). The dry intrusion of interest was related to the dark zone wrapped round the moist/cloud head/hook feature (marked H ) in the WV imagery shown in Figures 3(a) and 3(c). At 12 UTC on 26 November 1998, it is associated with a strong maximum in 2 PVU absolute vorticity, which lies in a low-level baroclinic zone to the rear of the surface wave, as seen by comparing Figures 2(d), 2(b) and 1(b) Evidence for poor HIRLAM-INM model analysis based on Meteosat WV and pseudo WV imagery In this section, the absolute vorticity at 2 PVU surface is compared with Meteosat and pseudo WV images for evaluating model behaviour. Since WV channel radiance is closely correlated with mid- to upper-tropospheric humidity, dark dry regions on the WV imagery are caused by descent and light regions by ascent, which in turn are associated with upper-level convergence and divergence respectively (Carroll, 1995). Following the motion of the air, convergence acts to increase cyclonic vorticity, while divergence reduces it. Carroll (1995) examined a case similar to that on November 1998 and interpreted the situation in terms of a strengthening jet, which has a strip of convergence along its cold side due to isallobaric ageostrophic motion (i.e. resulting from changes in the pressure gradient force at a point). This convergence enhances the cyclonic vorticity associated with the cold side of the jet, and at the same time leads to subsidence and drying out of the upper-tropospheric air. The resulting dry region becomes a marker for the high vorticity air (in the dark slot to the south and south-west of the point H on Figures 3(a) and 2(e)). At the same time, warm advection ahead of the surface depression causes upperlevel divergence, which reduces vorticity, and ascent, which moistens the upper troposphere and lightens the WV imagery. As a result, a light pattern is formed in the WV image associated with the development of a head area of moist air (marked H on Figure 3(a)). As the low deepens, the strength of the low-level thermal advection is increased and the induced ridging process becomes marked. At the same time, the intense ridging tends to decelerate the northward movement of the upper-level disturbance inducing troughing aloft. The positive interaction between upper and lower levels (see also section 4.3), in a low static stability environment, generates an effective self-development. Direct evidences of this process can be clearly seen in the vorticity field at the dynamical tropopause derived by HIRLAM-INM analysis at 1200 UTC on 26 November and shown in Figure 2(d). The ridging is connected with the developing hook-shaped pattern of low vorticity H (shaded light in the WV images of Figures 2(d) and 3(c)) having been caused by warm advection via divergence. The troughing is associated with the dark-shaped intrusion of high vorticity (through T to L in Figures 2(d) and 3(c)) which was originally high shear-vorticity air on the cold side of the jet strengthened by cold advection via convergence. The tendency of ridging aloft has been caught by the ECMWF T+12 forecast valid for 0000 UTC on 26 November resulting in a convex pattern of low vorticity, marked C in Figure 2(e). As seen in the WV image on Figure 3(a), there is a corresponding moist convex feature C, which appears connected with the lightshaded head area H. The ECMWF model forecast at T+12 seems to be better than the HIRLAM-INM analysis at 0000 UTC shown in Figure 2(c), as in the latter field, the tendency for intruding high vorticity air around the moist head H and formation of low vorticity feature in the vicinity of C is not observed. 61

6 C G Georgiev and F Martin Figure 1. Meteosat WV images with superimposed HIRLAM-INM analyses of MSLP (hpa) (white contours) and static stability at 700 hpa (black contours: the solid lines being 10 7 J kg 1 Pa 2 ): (a) 0000 UTC on 26 November, (b) 1200 UTC on 26 November and (c) 0000 UTC on 27 November (d) Meteosat WV image with superimposed MSLP (hpa) at HIRLAM- INM T+24 forecast valid for 0000 UTC on 27 November The labels F and L in (a) and (b) are positioned near to the surface low centre at the same places as in Figures 2 and 3. However, at 1200 UTC both models have analysed/predicted (Figures 2(d) and 2(f)) a reasonable vorticity pattern associated with the moist hookshaped feature (H) and the dry intrusion (through T to L ) seen in the WV image. Comparing pseudo WV images with Meteosat WV imagery and PV fields is another approach in assessing model behaviour. The close agreement between the pseudo WV image (being a synthetic product of HIRLAM-INM) and the model analysis of absolute vorticity at 2 PVU surface for 0000 UTC on 26 November is obvious in Figure 3(b). The area of the highest PV anomalies fits very well a dry (dark) zone in the pseudo WV image. Consequently, the drying/ moistening of the middle (upper) troposphere due to descent/ascent are closely associated with the distribution of absolute vorticity at 2 PVU surface when derived by using the same numerical model. For that reason, since HIRLAM-INM has failed to catch the ascent at location H in Figure 3(a), the corresponding moist-air head does not appear in the pseudo WV image (Figure 3(b)). This disagreement between the pseudo and the Meteosat WV images gives an early warning of poor HIRLAM-INM analysis at the initial stage of the process. In the analysis twelve hours later, the low vorticity hook pattern H in Figure 2(d) corresponds to the cloud/moist hook (H) in the real WV and pseudo WV images (Figures 3(c) and 3(d)). In fact, HIRLAM-INM has been late in capturing this feature in the vorticity field Discussion Carroll (1995) studied a case of a rapidly deepening depression over the Atlantic when the UK Meteorological Office operational limited area model was in error. He showed that the Meteosat WV imagery can be used to visualise the distribution of

7 Use of PV fields, water vapour imagery and pseudo water vapour Figure 2. Meteosat WV images with superimposed various model output fields. HIRLAM-INM analysis of geopotential height (10 m) at 500 hpa (white contours) and wet-bulb potential temperature ( o C) at 850 hpa (black contours, dashed) for (a) 0000 UTC and (b) 1200 UTC on 26 November HIRLAM-INM analysis of absolute vorticity (10 5 s 1 ) at 2 PVU surface for (c) 0000 UTC and (d) 1200 UTC on 26 November T+12 forecast of 2 PVU absolute vorticity (10 5 s 1 ) derived by ECMWF model, valid for (e) 0000 UTC and (f) 1200 UTC on 26 November Also marked: the moist/cloud head/hook (H), the convex moisture and low vorticity pattern (C), the warm ridge cloud pattern (R), the leading (L) and trailing (T) zones of the dry intrusion. The dark area F is close to the surface low centre. Between L and Y appears to be the zone where the dry-intrusion air has overrun air of the warm conveyor belt. 63

8 C G Georgiev and F Martin Figure 3. (a) Meteosat WV image for 0000 UTC on 26 November (b) Pseudo WV image superimposed with the 2 PVU absolute vorticity (10 5 s 1 ), derived by using HIRLAM-INM analysis at 0000 UTC on 26 November (c) Meteosat WV image and (d) Pseudo WV image for 1200 UTC on 26 November Also marked: H, C, F, R, L, T and Y for the same features as in Figure 2. hpa absolute vorticity and also to verify the model forecast. In both cases, November 1998 and those of Carroll (1995), the depressions developed as waves in a cold front cloud band over the Atlantic where a moist/cloud head feature (marked H on Figure 3) is formed in the WV imagery. It is reflected by a ridge in the vorticity pattern, with the developing dark slot matching the strip of shear vorticity along the cold side of the jet (see Carroll, 1995). Obviously, it is not unusual for limited area models to be in error in such synoptic situations, and comparing PV fields, Meteosat WV images and model derived pseudo WV images at analysis and early forecasts can give important warning of inaccuracies in model behaviour. Although the error in HIRLAM-INM forecast of MSLP is small considering the rapid development of the feature over 24 hours (see section 4.1), the case on November 1998 appears to be complex and it calls for more careful description. Figures 2(a) and 2(b) clearly indicate the intrusion of cold air to the rear of a northeast southwest-oriented 64 polar trough leading to frontogenesis, which in turn is associated with an increasing southwards flow and movement of ascending warm air to the north. As a result, a warm ridge has formed that is associated with a convex (in its polar edge) cloud pattern (marked R in Figure 3(c)), which appears isolated from the moist/cloud hook H. At 0000 UTC on 26 November, an elongated dry region (marked F on Figure 3(a)) is located close to the surface low centre in Figure 1(a). Although this pattern is seen (in Figure 2(e)) to be positioned in the vicinity of a local maximum of 2 PVU absolute vorticity, high PV anomalies were not observed near F in any other middle- or high-level PV fields (e.g. Figure 2(c) and the others not presented, that were examined, see section 2). It seems that the initial development took place largely due to low level warm advection (see section 4.1). During the latest stages of cyclogenesis, the upper-level PV anomaly to the rear of the wave approaches the frontal zone and subsequently overlies the lower tropospheric PV anomaly in the region of surface low cen-

9 Use of PV fields, water vapour imagery and pseudo water vapour tre (located near L in Figure 1(b)). It seems that as a result, a narrow feature of medium-grey shades of the WV image between L and Y (LY) in Figure 3(c) has been produced. This is a band with cyclonic curvature located between the dark-shaded strip of dry intrusion to the west and the light-shaded pattern between R and Y (RY). The sharp boundary that separates the two different moisture regimes LY and RY appears to be the zone of the kata-cold front where the dryintrusion air, after overrunning the air of warm conveyor belt (WCB), terminates as an upper cold front and the WCB deepens abruptly (see Browning, 1997). As regards the ana-cold front, most of the dry-intrusion air undercuts an extrusion of WCB air as part of a well-developed transverse circulation which generates a wide rainband behind the surface cold front (SCF) (Browning, 1997). For that reason, at the ana-cold front, the interaction of dry-intrusion air with the high θ w WCB air might not be detectable by WV imagery because it would not produce different enough moisture regimes in the vicinity of the SCF. As shown in Figure 2(b), in accordance with Browning (1997), the leading part of the dry intrusion with low θ w has overrun higher θ w air close to the surface low centre L. Values of about o C for θ w at 850 hpa are associated with medium-grey shades of the feature LY, while in the trailing zone of the dry intrusion (near T on Figure 2(b)) θ w is about 8 o C. It can also be seen in Figure 2(d) that these zones of high and low θ w air at 850 hpa (near L and T ) have been overlaid by 2 PVU absolute vorticity values of s 1. The medium-grey shades of LY, where the low-level frontal zone seems to be overrun, appear to be produced by the layer of moist, high θ w air in the lower middle troposphere with the dry-intrusion air above (see Weldon & Holmes, 1991). Comparison of Figures 1(b) and 2(d) shows that a strong maximum in 2 PVU absolute vorticity lies to the rear of the surface low L. As the upper PV anomaly catches up the low-level feature, the associated cyclonic forcing enhances the low-level circulation, which in turn increases the circulation at upper levels (see Hoskins et. al., 1985; Mansfield, 1996). Similar to a case studied by Mansfield (1996), the reason for this development can also be seen in the position of the wave ahead of the diffluent upper trough, as shown by Figure 2(b). Finally, it is obvious that the main features of the depression as seen in Figure 3(c) (i.e. the moist/cloud head H, the warm ridge cloud system R, and the trailing zone of dry intrusion around T ) correspond to similar patterns of grey shades in Figure 3(d). This quite good agreement between the Meteosat WV picture and the pseudo WV image derived at 1200 UTC analysis on 26 November gives general support to the validity of the model behaviour. At the same time, the interaction of dry-intrusion air with the high θ w air (the medium-grey shades close to the surface cyclone centre L in Figure 3(c)) is not clearly revealed by the pseudo WV field in Figure 3(d). However, this difference appears to be due to the pseudo WV images being derived by the HIRLAM-INM model with around 50 km grid horizontal resolution. According to Browning (1997), in the kata-cold front the dry-intrusion air overruns the SCF and WCB air for a distance that varies from tens of kilometres to perhaps 200 km, which is scarcely above the resolution of the pseudo WV images. 5. Case study of cut-off development over the western Mediterranean 5.1. Development as simulated in HIRLAM-INM model Figure 4(a) shows the field of geopotential height at 1200 UTC on 3 December 1998, when a large cut-off low system was located over the western Mediterranean. It is evident in Figure 4(b) that after 12 hours the high-pressure ridge over the British Isles has distorted and the rear side of the depression has tended to become directly connected to the upper-level northern-latitude flow. As a result, a process of weakening of the cut-off regime was observed, and a vortex redeveloped over the western Mediterranean and began moving to the north-east. The HIRLAM-INM forecasts of the height of the 500 hpa isobaric surface were in error by T+24 and T+12 valid for 1200 UTC on 4 December. Figure 5(a) shows the substantial difference between the T+24 HIRLAM-INM forecast (dashed) and the corresponding analysis (solid) in the region of middlelevel trough (marked B ). It is evident that the model overestimated the 500 hpa isobaric surface height and was in error also with the position of the middle-level low centre Evidence for poor HIRLAM-INM model analysis based on Meteosat WV and pseudo WV imagery Within the cut-off low over south-western Europe at 0000 UTC on 3 December, the correspondence between PV fields and WV imagery is rather poor. In Figures 6(a) and 6(c), the high 2 PVU absolute vorticity and PPVAs at 500 hpa (at location P ) over the Western Mediterranean do not correspond with any WV image dark slot. Such discrepancies are not unusual for large cut-off lows where interleaving of dry and moist air appears to show little relationship to the PV pattern (Mansfield, 1996). Figures 6(b) and 6(d) show both PV fields 24 hours later, when the positive PV anomalies are associated with a much more relevant WV darkening structure. Obviously, the relationship has improved due to an increase in correspondence with time having been caused by the weakening of the cut-off regime (Figure 4(b)). 65

10 C G Georgiev and F Martin Figure 4. HIRLAM-INM analysis of geopotential height (10 m) at 500 hpa superimposed on the corresponding Meteosat WV image for (a) 1200 UTC on 3 December and (b) 0000 UTC on 4 December Figure 5. HIRLAM-INM geopotential height (10 m) of 500 hpa isobaric surface at analysis (solid) superimposed with the corresponding (a) T+24 forecast (dashed) and (b) T+12 forecast (dashed), valid for 1200 UTC on 4 December Also indicated: the region of closed circulation (A) and the position of the redeveloped vortex (B). The important discrepancy in HIRLAM-INM analysis of 2 PVU absolute vorticity at 0000 UTC on 4 December is that the strongest gradient area (usually associated with the driest regions, see section 3) turns out to be to the rear of the dark area of the Meteosat WV image in Figure 6(b). In the other field, Figure 6(d), the PV maximum also appears to the west of the dark zone. However, as shown by Figure 6(f), the area of PPVAs at 500 hpa fits the dry slot of the pseudo WV image in the HIRLAM-INM analysis. Consequently, the darkening process was handled well by the model, but in the model analysis fields the disturbance is located to the west of the real situation seen in the Meteosat WV image on Figure 6(d). The poor correspondence of the PV anomalies with both the real and the pseudo WV images (at location P in Figures 6(c) and 6(e)) on 3 December is due to the nature of the cutoff regime (see Mansfield, 1996). At the same time, the much better agreement of the area of PV anomalies with the pseudo WV image than with the Meteosat WV picture on 4 December is a warning signal for any 66 underestimation of the velocity of movement of the developing wave. The HIRLAM-INM model was in error by T+24 (Figure 5(a)) and by T+12 forecasts (Figure 5(b)) of the 500 hpa isobaric surface height, which was overestimated by m, although the T+12 forecast was better. Moreover, the model forecast was in error in terms of position: in the 500 hpa analysis the low centre turned out to be 5 /4 further to the north-east, while the axis of the middle-level trough was further to the east and rotated 4 /2 clockwise, compared with the model s forecasts at T+24/T+12. This phase error has been seen in the comparison between real and pseudo WV imagery. In the synthetic water vapour fields derived at 0000 UTC on 4 December valid for T+00 (Figure 6(f)) and T+06 (Figure 7(b)), the darkening zone (marked DDD in Figure 7) has a different position from those on the corresponding Meteosat WV images in Figure 6(d) and Figure 7(a), this difference being similar to that between the forecast and analysis

11 Use of PV fields, water vapour imagery and pseudo water vapour Figure 6. Meteosat WV image with superimposed the HIRLAM-INM analysis of 2 PVU absolute vorticity (10 5 s 1 ) for (a) 0000 UTC on 3 December and (b) 0000 UTC on 4 December Meteosat WV image superimposed with the HIRLAM-INM analysis of PV anomalies (10 1 PVU) at 500 hpa for (c) 0000 UTC on 3 December and (d) 0000 UTC on 4 December Pseudo WV image superimposed with the PV anomalies (10 1 PVU) at 500 hpa at HIRLAM-INM analysis for (e) 0000 UTC on 3 December and (f) 0000 UTC on 4 December Also marked: P, F, E, Y, L, R, T, the loosely connected features of PV/vorticity maximum. of the middle-level trough position at 12 UTC on 4 December (Figure 5(b)) Discussion On 3 December, various PV/vorticity features were being advected within the cyclonic circulation of a large-scale cut-off feature. These are rather loosely connected along an axis of maximum vorticity running from the western Mediterranean (see Figure 6(a)), through the point P and north-west Africa to a local maximum F, north-westwards to another maximum E, and then north-eastwards to the area marked Y. On 4 December at 0000 UTC (see Figure 4), the flow has become less blocked on the large scale, the 500 hpa 67

12 C G Georgiev and F Martin Figure 7. Comparison between (a) Meteosat WV image and (b) pseudo WV image derived by HIRLAM-INM T+06 forecast, for 0006 UTC on 4 December Also marked: the darkening zone (DDD) to the rear of the developing wave as well as the region of closed circulation (A) and the redeveloped vortex (B). height gradient has strengthened in the eastern part of the large-scale cyclonic circulation, whereas it has weakened in the western part. As a consequence, a vortex (marked B in Figure 7(a)) associated with the eastern most PV anomaly (marked P in Figure 6(a)) was advected north-eastwards under the influence of strong south-westerlies on its southern flank, while the PV/vorticity features E and F were advected eastwards and southwards respectively. The advection pattern has increased the distance between the features, and thus their isolation from each other. At 1200 UTC on 4 December (see Figure 5(a), solid contours), the westernmost vortex has ended up being reflected by a small circulation (A) in the 500 hpa flow (minimum height contour of 5520 m). At the same time, the redeveloped cyclonic system (B) was located over the Adriatic Sea with minimum 500 hpa height at 5380 m. According to Carroll (1997), a significant proportion of the poor synoptic-scale forecasts by the operational NWP models involves instances of disrupting upper troughs. The details of the disruption process appear to be especially sensitive to initial conditions, and it is not unusual for the models to predict incorrectly how much vorticity (equally potential vorticity) ends up in each part of the disrupted trough (Carroll, 1997). Although the individual PV/vorticity features ( P, E and F ) at 0000 UTC on 3 December were not strongly connected in the large-scale cyclonic circulation (Figure 6(a)), something of a disruption process occurred in the smaller scale. Closed circulation has developed at point A as the vortex B has become more mobile (see Figures 5(a) and 7(a)). It is also evident in Figure 6(b) that the main PV/vorticity feature of interest, marked P in Figure 6(a), has become elongated and consists of loosely connected local maximum patterns ( L, R and T ). Obviously, at 0000 UTC on 4 December, the HIRLAM-INM model failed to analyse correctly the resulting after getting loose 68 PV/vorticity field (on Figures 6(b) and 6(d), the area of maximum PV anomalies does not fit the WV image dark slot). As a result of comparing both Meteosat WV and pseudo WV images with PV fields it was indicated on 4 December at 0000 UTC that the HIRLAM-INM runs had failed to analyse and predict correctly the velocity of movement of the redeveloping cyclonic disturbance. 6. Conclusion In this study, WV images were compared with both the absolute vorticity at 2 PVU surface and the positive PV anomalies at 500 hpa with the aim of indicating any model analysis or forecast errors. It was demonstrated that comparing the 2 PVU absolute vorticity with WV imagery might be used for monitoring upper-level flow evolution. Being the best PV field corresponding to WV imagery dry slots (see section 3), the field of PV anomalies at 500 hpa was compared with Meteosat WV data for monitoring the areas of dry-air intrusion associated with the cyclonic disturbances. The identification of dry intrusions from WV imagery can be used to validate and bogus a NWP model and the bogusing can be quite effective because of the link between (parts of) dry intrusions and high PV (Browning, 1997). Moreover, to help improve the interpretation of dynamical fields derived from the HIRLAM-INM model and to help solve the problem of validating NWP output, pseudo WV images were used. Usually, the distribution of the water vapour in the middle and upper troposphere seen on a Meteosat WV image gives quite a good view of the synoptic-scale motion field. In such cases the features of PV fields associated with cyclonic disturbances correspond well to patterns on the WV imagery and this relationship might be used for detecting errors in NWP output. However, this agreement varies with the location and time as well as with

13 Use of PV fields, water vapour imagery and pseudo water vapour the synoptic situation (see, for example, Mansfield, 1996; Georgiev, 1999), and not all mismatches correspond to real errors (Mansfield, 1997). Being synthetic products of the NWP model, pseudo WV images are used in this study to indicate the strength of the relationship between the Meteosat WV imagery and the PV fields, and to indicate whether any mismatches correspond to real NWP model errors. A typical case of incorrect model output is when model-derived PV fields show good correspondence with the pseudo WV image and, at the same time, there is any disagreement between the Meteosat WV image and these PV fields as well as between the Meteosat and pseudo WV image (see sections 4.2 and 5.2). In this way, it was demonstrated that the HIRLAM-INM model phase error at 3 4 December 1998 has been clearly seen in the comparison between real WV and pseudo WV images. At 0000 UTC on 04 December the darkening zone in the T+00 and T+06 pseudo WV images has a different position compared with its location in the corresponding Meteosat WV pictures. This difference is similar to that between the forecast and analysis trough position at 500 hpa on 4 December at 1200 UTC. Finally, as a consequence of this study it is suggested that the best evaluation of model behaviour is obtained by comparing WV imagery with more than just the diagnostic fields that are easily interpreted in the forecast environment. Although absolute vorticity at the 2 PVU surface and the PPVAs at 500 hpa have not been preferred in assessing NWP output (see Carroll 1995, 1997; Mansfield 1996, 1997; Røsting et al., 1996), they are the fields most closely related to WV imagery (see section 3; Carroll, 1997; Georgiev, 1999). At the same time, the use of these PV fields can give important warning of model output errors (sections 4 and 5) in synoptic situations where poor synoptic-scale forecasts by the operational NWP models are possible (see also Carroll 1995, 1997). Moreover, a quantitative relationship between PPVAs at 500 hpa and Meteosat WV image dry slots for a case study of cyclonic developments over the Mediterranean has been reported (Georgiev, 1999). The quantification of the relation between WV data and PV could increase the potential for use of WV images in automatic detection of model discrepancies. Furthermore, although pseudo WV imagery is not used by the operational forecasters in the daily routine at INM to any great extent, obviously it is a useful tool for indicating errors in NWP output. For that purpose, comparison not only between real and pseudo WV imagery, as suggested in COST-78 (1998), but also between PV fields and pseudo WV images (see sections 4 and 5) might be used. Acknowledgements This study was performed at the Instituto Nacional de Meteorología, Madrid and supported by the Spanish Government and WMO (a fellowship awarded to the first author). The authors are grateful to Ricardo Riosalido and Fermín Elizaga for the valuable discussions and helpful suggestions as well as to Miguel Angel Martinez for providing the pseudo WV images. Special thanks are due to anonymous reviewers of this paper. References Browning, K. A. & Reynolds, R. (1994). Diagnostic study of a narrow cold-frontal rainband and severe winds associated with a stratospheric intrusion. Q. J. R. Meteorol. Soc., 120: Browning, K. A. (1997). The dry intrusion perspective of extra-tropical cyclone development. Meteorol. Appl., 4: Carroll, E. B. (1995). Diagnosis of a rapidly deepening depression: 16/17 January Meteorol. Appl., 2: Carroll, E. B. (1997). Poorly forecast trough disruption shown in water vapour images. Meteorol. Appl., 4: COST-78, (1998). 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