Near-Surface Satellite Wind Observations of Hurricanes and Their Impact on ECMWF Model Analyses and Forecasts

1274 MONTHLY WEATHER REVIEW VOLUME 126

M. TOMASSINI,D.LEMEUR, AND R. W. SAUNDERS European Centre for Medium-Range Weather Forecasts, Shin®eld Park, Reading, Berkshire, United Kingdom (Manuscript received 4 September 1996, in ®nal form 6 May 1997)

ABSTRACT During August±September 1995 new near-surface wind datasets over the tropical Atlantic from both the ERS- 1 scatterometer and Meteosat satellites were available at the European Centre for Medium-Range Weather Forecasts. At this time there was an unusually high number of hurricanes present in the tropical Atlantic and so the impact of these data on analyzing and forecasting the main cyclones was investigated. Assimilation experiments using a new variational scheme, with the ERS-1 winds, showed clear improvements both in the analyses and short-range forecasts, compared with the optimal interpolation scheme without these data. For example, the forecast positions for Hurricane Iris were reduced by almost 50% when the scatterometer data was included. For Hurricane Luis the improvement was for a higher percentage of cases when the model identi®ed the cyclone in the 24- and 48-h forecasts. For the 72-h forecasts 80% of the reported cyclones were detected compared with only 33% for the analyses without ERS-1 data. The impact of the Meteosat lower-tropospheric cloud motion winds was found to be small due to lack of coverage in the vicinity of the center of the hurricanes at this time. The impact of one pro®le from a ship in the vicinity of Hurricane Luis just before its approach to the Caribbean Islands was clearly demonstrated by large improvements to both analyses with and without the scatterometer winds.

1. Introduction vious optimal interpolation scheme and 3D-Var for over 60 TC cases showed an overall improvement in short- Accurate forecasts of tropical cyclone (TC) positions range (Ͻ96 h) forecasts (A. Lanziger 1996, personal and intensities are vital for countries affected by their communication). However, to date the lack of conven- associated severe weather. Several operational global tional data and satellite observations (Reed et al. 1988) forecast centers (e.g., U.K. Meteorological Of®ce, U.S. over the tropical oceans hampers signi®cant improve- National Weather Service) routinely disseminate TC ments in the model analyses in the vicinity of TCs. This forecasts to national meteorological services. The fore- has led to ``bogussing'' techniques being developed by casting of TCs by global numerical weather prediction some centers (Serrano and Unden 1994; Heming et al. (NWP) models has improved recently through a better 1995), which derive synthetic observations for assimi- representation of the physical processes in the models lation into the analysis to improve the position of the and higher model resolutions allowing the TCs to be TC. better represented (Shun 1992). The current operational The current resolution of the ECMWF T213 model forecast model at the European Centre for Medium- does not allow the possibility of resolving the ®nescale Range Weather Forecasts (ECMWF) resolves half- structure and physical processes of the TC center. How- wavelengths to about 90 km (referred to as T213), but ever, the model can analyze the large-scale ¯ow in which there are plans to improve this within the next few years the TC exists, giving indications of its intensity and allowing more of the structure of a TC to be resolved. position. Since no bogussing is performed, good data Another factor leading to improvements is a better def- coverage is essential for accurate analysis. In recent inition of the initial state of the atmosphere through years several new sources of satellite data have become improved data assimilation systems. ECMWF has re- available in near-real time on the Global Telecommu- cently implemented a three-dimensional variational as- nication System that potentially could improve the anal- similation (3D-Var) system (Courtier et al. 1993) and a ysis of TCs. First, a satellite dataset that has been as- subjective study comparing the forecast skill of the pre- similated operationally at ECMWF from 30 January 1996 is the microwave scatterometer surface winds from the European Space Agency polar-orbiting satellites Corresponding author address: Dr. Roger W. Saunders, ECMWF, ERS-1 and ERS-2. These data are unaffected signi®- Shin®eld Park, Reading, Berkshire RG2 9AX, United Kingdom. cantly by cloud and precipitation allowing the sea sur- E-mail: [email protected] face winds in the vicinity of a TC to be inferred. Their

᭧ 1998 American Meteorological Society

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC MAY 1998 TOMASSINI ET AL. 1275 only drawback is the relatively narrow swath width (450 b. Data assimilation km) and hence limited coverage of the ERS instruments although this will be improved for future instruments. A major change in the ECMWF data assimilation In spite of this there have been positive impacts in the system was made at the end of January 1996, when the short-range forecast demonstrated over the Southern optimal interpolation (OI) method was replaced by a Hemisphere oceans and northern Paci®c Ocean due to new 3D-Var analysis scheme. In 3D-Var the observa- the inclusion of scatterometer data in the analysis (Gaf- tions are combined with the model ®rst guess (6-h forecast) through the minimization of a cost function (Court- fard and Roquet 1995). Second, low-level geostationary ier et al. 1993). Compared with OI, one major advantage cloud motion winds (CMW) can have a better coverage of 3D-Var is that it can handle observations that are if the higher-resolution visible channel images are nonlinearly related to the analyzed quantities, for ex- tracked during daylight. From June 1994 until Novem- ample, it can directly assimilate radiance measurements ber 1995 the European Space Operations Centre pro- from satellites. 3D-Var includes coupling between mass duced such a product from the Meteosat visible channel and wind ®elds through the correlation in background imagery in an attempt to improve the CMW coverage errors of mass and wind. At the time of these experi- over the Atlantic and southwest Indian Oceans (Otten- ments, however, this multivariate formulation was used bacher et al. 1996). only in the extratropics, so both the 3D-Var and OI The 1995 hurricane (i.e., Atlantic TCs) season was schemes were univariate in the Tropics. characterized by an unusually high level of activity. At The data used for the geopotential height and wind least 19 tropical cyclone events were identi®ed between ®eld analyses come from in situ observations from sur- June and the end of October, generally developing off face stations, aircraft, radiosondes, and drifting buoys the West African coast and crossing the tropical region and from remotely sensed data such as CMW from var- of the North Atlantic. Among these, 11 were classi®ed ious geostationary satellites and radiances from TOVS as hurricanes. This paper gives some examples during (TIROS Operational Vertical Sounder). The temperature this period of the impact of the scatterometer near-sur- analysis increments are derived from the geopotential face winds combined with the new variational assimi- height increments assuming hydrostatic equilibrium. lation for improving the analysis of hurricanes in the The analysis of speci®c humidity relies on radiosondes ECMWF forecast system and also their impact on the and TOVS radiances. Low-level CMW and TOVS data short-range forecasts. The impact of the Meteosat low- are used only over oceanic areas. The OI and 3D-Var level CMWs was also investigated. systems include the same observations, with two ex- ceptions: TOVS data are assimilated in OI as retrieved pro®les (Eyre et al. 1993) and in 3D-Var as radiances 2. Characteristics of the ECMWF forecast system (Andersson et al. 1994); second, the ERS scatterometer data are not assimilated in OI. The scatterometer data a. The model are assimilated in 3D-Var directly using the retrieved ambiguous wind vectors with the most appropriate wind The ECMWF forecast system consists of two com- vector being selected during the minimization process ponents: a general circulation model and a data assim- itself (see below). ilation system. The general circulation model [see Rit- chie et al. (1995) for details] is a spectral, primitive equations model with a horizontal truncation of T213 3. Satellite surface wind observations and 31 vertical levels, using a semi-Lagrangian time Conventional observations on the West African coast integration with a time step of 15 min. The spectral help identify the easterly waves that could evolve into model truncation of T213 corresponds to a smallest re- tropical storms over the Atlantic Ocean. Data coverage solved half-wavelength of a little over 90 km and the is usually rather poor as the disturbance moves across computational grid has a resolution of about 60 km. The the Atlantic, where only sparse wind soundings from model includes a comprehensive set of physical para- occasional ships are available. The consistent coverage meterizations, representing processes such as convec- offered by satellites over the ocean can be essential for tion, clouds, radiation, friction, and diffusion. The cloud improving TC monitoring. An example of this is given parameterization scheme allows the main cloud related in Fig. 1a, where a Meteosat visible image at 1200 UTC processes to be treated in a consistent manner by fore- 31 August 1995 reveals four tropical storms present at casting both cloud fraction and cloud water/ice content this time. The yellow vectors are scatterometer surface with prognostic equations. The model can generate in- winds derived from two swaths of the ERS-1 satellite, tense cyclonic systems, but its ability to represent the at 1245 and 1445 UTC, respectively; the green and red structure of hurricanes is limited by resolution. The mo- vectors are, respectively, visible and infrared Meteosat mentum transport in the convection scheme (Tiedtke CMW below 700 hPa. Note the CMW data from the 1989) prevents the surface pressure values from reach- GOES-East satellite were not available at this time but ing unrealistically low values for this model resolution. the TOVS radiances were always used if a NOAA-12 or

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC 1276 MONTHLY WEATHER REVIEW VOLUME 126

FIG. 1. (a) Data coverage at 1200 UTC 31 August 1995, of satellite wind observations. ERS-1 scatterometer surface winds (yellow vectors), Meteosat VIS (green) and IR (red) CMW below 700 hPa are overlaid on a Meteosat visible image. (b) The coverage from the conventional observations over the same area between 700 hPa and the surface.

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC MAY 1998 TOMASSINI ET AL. 1277

NOAA-14 overpass occurred during the analysis period. from the background ®eld and the surrounding obser- The conventional wind reports, shown separately in Fig. vations carried by the cost function, together with the 1b, were rather sparse, and almost absent from the large corresponding physical constraints. The scatterometer area relevant for the formation of TCs. Thus the poten- winds are propagated in the model ®elds through the tial bene®t of the new satellite datasets, particularly the error structure functions of the JB term, both in vertical scatterometer winds as shown in Fig. 1a, is clear. and horizontal directions, and in terms of mass as well The scatterometers on board the ERS-1 and ERS-2 as windÐwith the restriction that the 3D-Var scheme satellites use three antennas, pointing in azimuths of 45Њ, was actually univariate in the Tropics at the time of the 90Њ, and 135Њ with respect to the satellite ground track. assimilation experiments considered in this study. These antennas measure the microwave backscatter Before being passed to 3D-Var, the ERS data are pro- from the sea surface in the C band (5.3 GHz) at a 50- cessed and quality controlled within a preprocessing km resolution within a 450-km-wide swath related to a procedure called PRESCAT (Stoffelen and Anderson grid with a mesh of 25 km in the across and along track 1997). They are also sampled every 100 km to match directions. Wind vectors are retrieved from the measured the model resolution without introducing horizontal cor- microwave backscatter triplets obtained in that grid by relation. In addition a speed bias, correction is applied inverting an empirical model (Stoffelen and Anderson to remove the underestimation of high winds produced 1997) expressing the backscatter coef®cient as a func- by the transfer function, CMOD4 (Gaffard and Roquet tion of the surface wind speed and its direction with 1995). This bias correction, derived from collocations respect to the viewing azimuth. Since the wind-driven with buoy data, was crucial to make good use of scat- waves responsible for backscattering show little asym- terometer data in the 3D-Var system. Before it was ap- metry when viewed in the upwind or downwind direc- plied, assimilation experiments showed a systematic tions, the inversion results in a directional ambiguity, tendency to ``®ll'' deep midlatitudes lows because of with two possible wind vectors of similar amplitudes too low wind speeds inferred from the scatterometer and roughly opposite directions. This ambiguity needs measurements. a priori information to be removed. ERS scatterometer data have been used operationally The 3D-Var analysis scheme relies on the minimi- in the ECMWF 3D-Var system since its implementation zation of a cost function J, in January 1996, starting with ERS-1 and switching to ERS-2 after the decommissioning of ERS-1 in June J J J J , (1) ϭ B ϩ O ϩ C 1996. The orbit of the satellite is such that the swaths where JB and JO are quadratic terms measuring the dis- (see Fig. 1a) are separated by about 100 min in time tance from the model state to its background estimate and 2500 km in distance at the equator, and roughly

(®rst guess) and the observations, respectively, and JC cover the same geographic area around local noon and is a penalty term imposing physical constraints (es- midnight. It should be noted that this data coverage is pecially the absence of fast-growing gravity waves). For sometimes reduced, especially in coastal areas where scatterometer data, it has been shown to be more suc- only two antennas are often available, or in places where cessful to specify JO in terms of wind vector rather than the synthetic aperture radar is operated in image mode, backscatter measurements (Stoffelen and Anderson which requires the scatterometer to be switched off. 1997). The observation errors are close to Gaussian in However, none of these cases occurred in the region and wind components, whereas the high nonlinearity of the period studied, and the data availability was remarkably transfer function complicates the speci®cation of the stable. observation errors in the backscatter domain. ERS scatterometer data are assimilated in the 3D-Var 4. Results of assimilation experiments system as pairs of ambiguous winds, through a two- minima cost function of the following form: a. The study period JJ J scatt ϭ 12 (2) The study period was from the 24 August to 8 Sep- O pp1/p (J 12ϩ J ) tember 1995. Four major tropical disturbances occurred over the tropical Atlantic in this period: Humberto, Iris, with Karen, and Luis. Humberto and Iris had already reached (u Ϫ u)(22␷ Ϫ ␷) the stage of mature hurricanes at the beginning of the J ϭϩii, (3) i du22 d␷ assimilation period. Karen only developed to tropical storm intensity, while Luis was one of the most dev- where u and ␷ are the analyzed 10-m wind components; astating hurricanes of the year causing much damage ui and ␷ i (i ϭ 1, 2) the scatterometer observations; du on the Leeward Islands. Karen and Luis could be fol- and d␷ the associated measurement errors, assumed to lowed from their initial development in the eastern At- be2msϪ1; and p an empirical exponent set to 4. The lantic, just to the west of Africa. Reports on the position ambiguity is thus removed implicitly during the mini- and intensity of the cyclones from the U.S. National mization process, taking into account all the information Weather Service (National Hurricane Center, Miami)

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC 1278 MONTHLY WEATHER REVIEW VOLUME 126

FIG. 2. Time sequence of analyzed wind and relative vorticity ®eld at 850 hPa from 24 August to 1200 UTC 8 September 1995, for OI (CON) versus SCAT/3D. The relative vorticity contour interval is 5 ϫ 10Ϫ5 sϪ1. The ®rst letter of the assigned name of each event is also plotted to aid identi®cation (when in brackets the position was not available from the NHC): HÐHumberto, KÐKaren, IÐIris, and LÐ Luis. provided the best estimate of the hurricane positions for the new 3D-Var analysis scheme at T106 resolution the evaluation of the performance of the analyses and (half-wavelength of 180 km) and scatterometer data forecasts of the ECMWF model. showed clearly that this con®guration of the assimilation system was performing better than the OI analysis (also at T106 resolution) without scatterometer data. Exper- b. Description of the assimilation experiments iments at T106 resolution using low-level CMW from The impact of the scatterometer and Meteosat winds Meteosat derived from both infrared and visible channel on the analyses and forecasts was studied in a series of data indicated little impact from these data because of data assimilation experiments. Initial experiments using their limited coverage for the period and area of interest.

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC MAY 1998 TOMASSINI ET AL. 1279

FIG.2.(Continued)

Given the positive impact of 3D-Var with scattero- 3D-Var analysis to the improved performance, two 3D- meter data at T106 resolution, it was decided to assess Var experiments were rerun with and without the scat- the impact of a SCAT/3D experiment at the operational terometer data. (i.e., T213) resolution. This experiment was started on In the SCAT/3D case, given the orbit pattern of the 24 August 1995 and ran for 16 days. The results were ERS-1 satellite and the range of longitudes under con- compared to those of the operational assimilation run, sideration, only four swaths were available on most which used the OI assimilation scheme with no scat- days, with two swaths in the analysis period centered terometer winds. This control run, denoted hereafter as on 1200 UTC (descending tracks) and two centered on CON, had the same model physical parameterizations 0000 UTC (ascending tracks). With their 450-km width as the SCAT/3D experiment. Finally, to investigate the and nearly 2500-km separation distance at the equator, relative contributions of the scatterometer data and the these swaths when present led to a data coverage ratio

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC 1280 MONTHLY WEATHER REVIEW VOLUME 126

FIG.2.(Continued)

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC MAY 1998 TOMASSINI ET AL. 1281

TABLE 1. Comparison of SCAT/3D and CON analysis position vorticity ®elds at 850 hPa at 1200 UTC for SCAT/3D errors during the period 24 August±8 September 1995. The number and CON over the tropical Atlantic as shown in Fig. 2. of cases recognized in each experiment is also compared with the Ϫ5 Ϫ1 number of reported positions. In the last column is given the number The contour of the vorticity ®eld starts at 5 ϫ 10 s , of improved SCAT/3D analyses in terms of position error. to make the interpretation of the maps easier and to be consistent with the threshold method used for the lo- No. of cases No. cation of the hurricanes. Position error (km) impr. SCAT/ Report- SCAT/ The initial development of the cyclones is generally SCAT/3D CON 3D CON ed 3D well described in both analyses. The formation of Karen Humberto 223 218 6 6 6 3 and Luis can be clearly recognized in the high relative Iris 71 181 10 10 10 7 vorticity values around 10ЊN, 20ЊW on 24 and 27 Au- Karen 102 131 5 4 5 3 gust, respectively. The capability of the model to form Luis 90 156 11 11 11 9 these type of disturbances, using synoptic observations All 111 173 32 31 32 22 and CMW, has already been demonstrated (Reed et al. 1988). The main problem is the conservation of the system over the sea. After leaving the African coast, of the order of 20%. However, their in¯uence was in- easterly waves frequently weaken when they encounter creased to almost 40% of the area of interest if one takes colder water in the mid-Atlantic. In the CON analysis into account the 250-km correlation distances of the the signature of the TC is systematically weaker than error structure functions used in the analysis scheme. in the SCAT/3D analysis. This is particularly evident in the case of Hurricane Luis between 31 August and 3 September. Only a weak circulation is present in CON, c. Impact on the analysis whereas SCAT/3D exhibits a more realistic system with During the study period the National Hurricane Cen- more sustained winds (see Fig. 2) and better positioning ter reported the position of the four tropical cyclones at of the storm center (the mean position error for these 4 1200 UTC for a total of 32 cases. The location of the days is 64 km for SCAT/3D compared with 212 km for cyclone in the analysis was estimated from the latitude CON). The case of Karen on 30 August is another ex- and longitude of the maximum in the relative vorticity ample, with the storm being missed by the CON analysis ®eld at 850 hPa as proposed by Reed et al. (1988). while SCAT/3D has its intensity correct and a position Recent work on the ability of the ECMWF reanalysis error of 83 km. ®elds to describe TCs showed that TC tracking is more The later stages of the hurricane development (i.e., accurate when performed following vorticity maxima intensi®cation while approaching the Caribbean Islands) rather than pressure minima (E. Serrano 1996, personal were well described in both the CON and SCAT/3D communication). A threshold of 5 ϫ 10Ϫ5 sϪ1 was re- analyses. For example, the high relative vorticity values quired for the relative vorticity maxima values to be of Iris at the beginning of the study period and of Luis considered representative of a strong circulation and be- from 4 September onward were well represented as low this value the TC was considered as missed by the shown in Fig. 2. The main difference between the two analysis. Only the cases when the TC was located equa- experiments is in the location of the storms, with torward of 30ЊN were considered. The results are in¯u- SCAT/3D being more accurate. For example, the mean enced by the resolution of the model itself, which is position error for Hurricane Iris from 28 to 31 August approximately 90 km. Table 1 shows for each identi®ed is 49 km in SCAT/3D, an improvement of 200 km with TC the mean difference between the analyzed position respect to CON. and the reported position, the number of cases success- In summary SCAT/3D improved the analysis of the fully detected as a TC for each experiment, and the three storms: Iris, Karen, and Luis, while it was neutral number of cases improved by the SCAT/3D assimila- for Hurricane Humberto (which had already reached a tion. mature stage of development at the beginning of the In the SCAT/3D analysis all the reported TC positions assimilation period). The higher performance of were detected, while one, for Karen on 30 August 1995, SCAT/3D with respect to CON is a combined effect was missed by CON. Out of the 32 cases considered, of the change from the OI to the 3D-Var assimilation 22 had a better position in SCAT/3D. More signi®cantly scheme and of the use of scatterometer winds (see be- the mean position error was reduced from 173 to 111 low). Figure 3 is an example, showing the scattero- km. The difference in the means was found to be sig- meter swath covering the storm Karen 0000 UTC 31 ni®cant at the 95% con®dence level. As the uncertainty August (a) and the SCAT/3D ®rst guess (FG) (b) and in the TC position is limited by the model resolution of analyzed (c) wind ®eld at 850 hPa valid at the same 90 km, the mean value achieved by SCAT/3D is close time. The analysis minus FG increments arise from the to the maximum precision achievable with this model lowest model level in correspondence with the scat- resolution. terometer swaths and are then transferred to the wind To study the day-to-day model performance, it is use- ®eld at 850 hPa through the analysis vertical structure ful to refer to the time evolution of the wind and relative functions. The same increments were not present in the

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC 1282 MONTHLY WEATHER REVIEW VOLUME 126

TABLE 2. Comparison of SCAT/3D and CON analysis position errors during the 24-h forecast. The number of cases recognized in each experiment is also compared with the number of reported positions. In the last column is given the number of improved SCAT/3D analyses in terms of position error.

No. of cases No. Position error (km) impr. SCAT/ Report- SCAT/ SCAT/3D CON 3D CON ed 3D Humberto 267 288 3 4 5 2 Iris 157 252 8 9 9 5 Luis 135 156 11 10 11 5 All 161 216 22 23 25 12

Caribbean Island stations and aircraft in this region re- sulted in a more accurate description of the hurricane in CON and hence small analysis differences between the two experiments from this point on. Note that at the end of the period the model gave values of 977 hPa for the minimum surface pressure compared with 942 hPa reported due to the limited resolution.

d. Impact on the forecast The impact of the scatterometer data with 3D-Var was investigated in terms of forecasts by repeating the maximum vorticity study with the 24-, 48-, and 72-h forecasts with the same method employed for the analysis. The results are shown in Tables 2, 3 and 4 and are limited to the cases of Humberto, Iris, and Luis, which were the only ones to actually become hurricanes. Note that the forecasts for the ®rst days were not available (as SCAT/3D analyses were not computed before the experiment period) resulting in a lower number of cases than for the analyses. Cases are considered missed by FIG. 3. Surface winds measured by the scatterometer (a) for storm Karen at 0000 UTC 31 August. First guess (b) and analyzed (c) wind the forecast when the maximum vorticity is lower than Ϫ5 Ϫ1 ®eld and relative vorticity at 850 hPa of SCAT/3D valid at the same the threshold of 5 ϫ 10 s (as for the analysis) and time. The relative vorticity contour interval is 2.5 ϫ 10Ϫ5 sϪ1. also when the location of the reported and forecast maxima are more than 400 km apart. These ``missed'' cases must be taken into account when evaluating the forecast performance. The mean 24-h forecast error is reduced CON analysis (not shown), which missed the storm at from the 216 km of CON to 161 km of SCAT/3D, with that time. A case showing positive impact of the conventional data is that of Hurricane Luis at 1200 UTC 4 September. TABLE 3. Comparison of SCAT/3D and CON analysis position The circulation at 850 hPa was intensi®ed to the east errors during the 48-h forecast. The number of cases recognized in of the Caribbean Islands in both SCAT/3D and CON each experiment is also compared with the number of reported po- and the reason for this is the modi®cation of the analyses sitions. In the last column is given the number of improved SCAT/3D by a critical SHIP/TEMP observation at 19.4ЊN, 57.9ЊW analyses in terms of position error. reporting a wind speed of 32 and 37 m sϪ1 at 850 and No. of cases No. 820 hPa, respectively, compared with the CON and Position error (km) impr. SCAT/3D ®rst guesses of 13 and 20 m sϪ1, respectively. SCAT/ Report- SCAT/ This pro®le was from a vessel participating in the Au- SCAT/3D CON 3D CON ed 3D tomated Shipborne Aerological Programme. The assim- Humberto 190 347 1 2 4 1 ilation of this single observation was especially bene- Iris 157 216 5 4 8 3 ®cial for CON, giving considerable increments to its Luis 192 150 11 8 11 7 analysis. Subsequently the better data coverage from the All 182 197 17 14 23 11

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC MAY 1998 TOMASSINI ET AL. 1283

TABLE 4. Comparison of SCAT/3D and CON analysis position of the system every day from 30 August, either around errors during the 72-h forecast. The number of cases recognized in 0000 or 1200 UTC, contributes to reduce the SCAT/3D each experiment is also compared with the number of reported positions. In the last column is given the number of improved SCAT/3D position error to about 90 km or less. A similar shift to analyses in terms of position error. that found in the analyses appears in the CON 48-h forecasts valid for 30 August±1 SeptemberÐthat is, 48 No. of cases No. h later, and is greatly reduced in the SCAT/3D case as Position error (km) impr. SCAT/ Report- SCAT/ illustrated in Fig. 4 with the forecast from 30 August. SCAT/3D CON 3D CON ed 3D The analyses of SCAT/3D and CON shown for veri®- Humberto 385 Ð 3 0 3 3 cation indicate a better agreement between the forecast Iris 182 257 4 3 7 2 and analyzed ®elds for the former. Luis 264 190 10 4 11 6 With Hurricane Luis, the impact of SCAT/3D is seen All 266 219 17 7 21 11 more in terms of intensity than position. Indeed, the cyclone center is fairly well located in the CON analyses throughout the period, but the low pressure and the associated wind speeds are signi®cantly underestimated just one more case missed by SCAT/3D. A similar im- as the system strengthens between 1 and 5 September provement is found also in the 48-h forecast, from 197 before reaching the Caribbean Islands. As before, scat- to 182 km, with more positions detected in the SCAT/3D terometer swaths crossed or passed very close to the forecast. Finally, a most striking improvement is found center every day from 29 August to 3 September, al- in the 72-h forecast. At this range the SCAT/3D ex- ternately around 0000 and 1200 UTC, and improved the periment is able to forecast 17 TC positions (80% of analyses having a positive impact on the forecasts. The the total reports) with an error of 266 km compared with bene®ts of SCAT/3D are greatest for the analysis period only seven cases detected in CON (the large difference from 1 to 3 September (see Fig. 2) when the lack of in number of cases means the position errors in Table conventional data is the most severe and virtually causes 4 cannot be compared except for Iris). For all forecast CON to miss the development of Luis. The analyzed ranges examined, SCAT/3D improves the position error minimum surface pressure remains lower by 2±3 hPa of half of the total cases reported. with SCAT/3D than CON every day, and the 48-h fore- The forecast impact is almost neutral for Humberto cast pressure also remains lower. The results are more (same as the analysis impact), while it is very positive obvious for wind speed, as shown in Fig. 5: the for Iris and Luis. For Iris, as the number of cases de- SCAT/3D forecasts display systematically much stron- tected by the two experiments only differ by a maximum ger winds close to the center, although the vortex is of one, the forecast errors can be compared. SCAT/3D roughly located at the same place in both cases. Even misses one case, the forecast from the ®rst day of the when the operational analysis has captured the cyclone assimilation period, but its reduction in the 1-day fore- correctly on 4 September and produces more realistic cast error is almost 50%. For Luis the improvement is wind speeds, the SCAT/3D winds remain stronger at for a higher percentage of cases where SCAT/3D is able shorter distances from the center. to forecast all the reported positions in the 24- and 48- h ranges and misses only one in the 72-h forecast (cor- e. Relative impact of the 3D-Var analysis and the responding to seven for CON). scatterometer winds For forecasts beyond 72 h the impact of the improved analyses was lost as forecast model errors became more Two additional assimilation experiments were per- important in the prediction of the position and strength formed at a later stage of the study to check the re- of the hurricanes. spective roles of the scatterometer observations and the As the SCAT/3D experiment improved the analysis 3D-Var analysis scheme in the above results. As it was of Hurricanes Iris and Luis, case studies were used to not possible to recreate identical conditions to the 1995 illustrate the impact on the corresponding forecasts. The preoperational tests, the 3D-Var assimilation system was analyses and 48-h forecasts from the SCAT/3D and rerun over the ®rst part of the study period with the CON experiments were examined individually and com- latest version of the ECMWF model and 3D-Var at T213 pared for the most signi®cant phases of both events. The resolution, successively with and without scatterometer study focused on the surface wind and pressure ®elds. data. For Hurricane Iris, the CON analyses exhibit an er- Although the use of a slightly different version of roneous eastward shift of 180±360 km in the position 3D-Var made the comparison with the previous of the low pressure center from 28 to 31 August, which SCAT/3D results dif®cult, the study of the analyzed does not occur in the SCAT/3D analyses using scatter- cyclone center positions from the maximum of vorticity ometer data. A study of the forecasts shows that the at 850 hPa clearly con®rmed the positive impact of the improvement is retained by the model. A swath covering scatterometer data on the analysis, and hence on the the storm center in the 1200 UTC analysis period from forecast. The mean position error was again signi®cantly 28 August, followed by new swaths crossing the center reduced in the experiment including scatterometer data,

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC 1284 MONTHLY WEATHER REVIEW VOLUME 126

FIG. 4. CON (a) and SCAT/3D (b) 48-h forecasts of Hurricane Iris and their respective verifying analyses (c) and (d) for surface winds (arrows) and pressure (isobars) at 1200 UTC 1 September 1995. The reported cyclone center is marked by the black square.

and the cyclone center was analyzed systematically, the conventional and other satellite observations avail- whereas it was virtually missed for Luis in the 3D-Var able on the Global Telecommunication System. experiment with no scatterometer data (for the few days The scatterometer surface winds used in the opera- included in the rerun, i.e, 1±2 September). In the latter tional 3D-Var assimilation system clearly showed in- case the improvements could be related to the analysis stances where the position of the hurricanes had been increments observed along the ERS-1 tracks as the ex- improved by the inclusion of these data in the analysis. ample showed with Karen previously (Fig. 3). However, In addition, the short-range forecast positions were also a positive contribution from the 3D-Var analysis scheme improved. The ERS scatterometer has a relatively nar- was also evident, especially in the case of Iris where row swath so a pass over a tropical cyclone is not always the mean position error in the absence of scatterometer obtained in one day. However, the next generation of data was reduced to 157 km from 181 km obtained for scatterometers (e.g., NSCAT on ADEOS, ASCAT on the OI scheme in CON. These new results con®rm the METOP) will provide better coverage and increase the bene®ts seen in the SCAT/3D experiment over CON likelihood of providing surface wind vectors regularly were a combination of both the scatterometer winds and over tropical cyclones. The use of the 3D-Var assimi- the 3D-Var assimilation. lation system clearly provides a consistent way of se- lecting the correct wind vector from the two ambiguous vectors. It is important to emphasize that in addition to 5. Conclusions the use of the scatterometer winds some of the bene®ts New datasets of satellite near-surface wind obser- come because the 3D-Var assimilation system made bet- vations of TCs have been used during the August±Sep- ter use of all available observations to improve the anal- tember 1995 period over the tropical Atlantic to inves- ysis in the Tropics. The optimal use of the new scat- tigate their impact on improving the analysis of the hur- terometer data with improved coverage could eliminate ricanes occurring at that time. Several experiments were the need for bogussing techniques to be applied to NWP carried out, with the new data sources being assimilated analyses. into the ECMWF forecast model ®elds along with all The impact of the lower-tropospheric CMW from Me-

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC MAY 1998 TOMASSINI ET AL. 1285

FIG. 5. CON (a) and SCAT/3D (b) analyses on 3 September 1995 of Hurricane Luis and their corresponding 48-h forecasts (c) and (d) for surface winds (arrows) and pressure (isobars) at 1200 UTC 5 September 1995. The reported cyclone center is marked by the black square.

teosat (both the operational infrared low-level winds and for the useful comments from A. Hollingsworth at the high-resolution visible winds) was for this period ECMWF and two anonymous reviewers. not signi®cant on the analysis. This was due to the relatively poor coverage of the Meteosat CMW as in most cases they were only available around the periphery of REFERENCES the hurricanes, making it dif®cult for the assimilation Andersson E., J. Pailleux, J.-N. Thepaut, J. R. Eyre, A. P. McNally, system to improve the hurricane position and magnitude G. A. Kelly, and P.Courtier, 1994: Use of cloud cleared radiances of the winds. Recently, high-resolution datasets from in three/four-dimensional variational data assimilation. Quart. J. the new generation GOES satellites, not available op- Roy. Meteor. Soc., 120, 627±653. erationally at this time, have demonstrated signi®cant Courtier, P.,and Coauthors, 1993: Variational assimilation at ECMWF. ECMWF Research Dept. Tech. Memo. 194, 84 pp. [Available impacts in the NWP models (Velden et al. 1998). Sat- from ECMWF Librarian, Reading, Berkshire RG2 9AX, United ellite operators are encouraged to develop techniques to Kingdom.] produce more low-level CMW in the vicinity of tropical Eyre, J. R. G. Kelly, A. P. McNally, E. Andersson, and A. Persson, cyclones. 1993: Assimilation of TOVS radiances through one-dimensional variational analysis. Quart. J. Roy. Meteor. Soc., 119, 1427± Finally, the importance of radiosonde observations is 1463. also clearly shown in this study by the rapid modi®- Gaffard, C., and H. Roquet, 1995: Impact of the ERS-1 scatterometer cation of the analyses of Hurricane Luis on 4 September wind data on the ECMWF 3D-VAR assimilation system. by one key ASAP wind pro®le observation from a ship ECMWF Research Department Tech. Memo. 217, 21 pp. [Avail- able from ECMWF Librarian, Reading, Berkshire RG2 9AX, to the east of the Caribbean Islands. United Kingdom.] Heming, J. T., J. C. L. Chan, and A. M. Radford, 1995: A new scheme Acknowledgments. This work was carried out as part for the initialisation of tropical cyclones in the UK Meteorolog- ical Of®ce global model. Meteor. Appl., 2, 171±184. of the EUMETSAT fellowship program and European Ottenbacher, A., M. Tomassini, K. Holmlund, and J. Schmetz, 1996: Space Agency Contract 11699/95/NL/CN for monitor- Low-level cloud motion winds from Meteosat high-resolution ing ERS scatterometer winds. The authors are grateful imagery. Wea. Forecasting, 12, 175±184.

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC 1286 MONTHLY WEATHER REVIEW VOLUME 126

Reed, R. A., A. Hollingsworth, W. A. Heckley, and F. Delsol, 1988: 1990±91. ECMWF Research Dept. Tech. Memo. 184, 32 pp. An evaluation of the ECMWF operational system analyzing and [Available from ECMWF Librarian, Reading, Berkshire RG2 forecasting tropical easterly wave disturbances over Africa and 9AX, United Kingdom.] the tropical Atlantic. Mon. Wea. Rev., 116, 824±865. Stoffelen, A. and D. Anderson, 1997: Ambiguity removal and assim- Ritchie, H., C. Temperton, A. Simmons, M. Hortal, T. Davies, D. ilation of scatterometer data. Quart. J. Roy. Meteor. Soc., 123, Dent, and M. Hamrud 1995: Implementation of the semi-La- 491±518. grangian method in a high-resolution version of the ECMWF Tiedtke, M. 1989: A comprehensive mass ¯ux scheme for cumulus forecast model. Mon. Wea. Rev., 123, 489±514. parameterization in large scale models. Mon. Wea. Rev., 117, Serrano, E., and P. Unden, 1994: Evaluation of a tropical cyclone 1779±1800. bogusing method in data assimilation and forecasting. Mon. Wea. Velden, C., T. Olander, and S. Wanzon, 1998: The impact of multi- Rev., 122, 1523±1547. spectral GOES-8 wind information on Atlantic tropical cyclone Shun, C. M., 1992: Performance of the ECMWF model in tropical track forecasts in 1995. Part I: Dataset methodology, description cyclone track forecasting over the western North Paci®c during and case analysis. Mon. Wea. Rev., 126, 1202±1218.

Unauthenticated | Downloaded 10/02/21 08:27 AM UTC