An Evaluation of Model Performance During the 1995 Hurricane Season

Home , Hurricane Felix (1995), Hurricane Iris

MAY 1998 SURGI ET AL. 1287

Improvement of the NCEP Global Model over the Tropics: An Evaluation of Model Performance during the 1995 Hurricane Season

NAOMI SURGI Tropical Prediction Center/National Hurricane Center, Miami, Florida

HUA-LU PAN AND STEPHEN J. LORD Environmental Modeling Center/NCEP, Washington, D.C. (Manuscript received 14 October 1996, in ®nal form 13 May 1997)

ABSTRACT An evaluation of the performance of the National Centers for Environmental Prediction Medium-Range Forecast Model was made for the large-scale tropical forecasts and hurricane track forecasts during the 1995 hurricane season. The assessment of the model was based on changes to the deep convection and planetary boundary layer parameterizations to determine their impact on some of the model de®ciencies identi®ed during previous hurricane seasons. Some of the de®ciencies in the hurricane forecasts included a weakening of the storm circulation with time that seriously degraded the track forecasts. In the larger-scale forecasts, an upper- level easterly wind bias was identi®ed in association with the failure of the model to maintain the midoceanic upper-tropical upper-tropospheric trough. An overall modest improvement is shown in the large-scale upper-level tropical winds from root-mean-square- error calculations. Within a diagnostic framework, an improved simulation of the midoceanic tropical trough has contributed to a better forecast of the upper-level westerly ¯ow. In the hurricane forecasts, enhanced diabatic heating in the model vortex has signi®cantly improved the vertical structure of the forecast storm. This is shown to contribute to a substantial improvement in the track forecasts.

1. Introduction impact of all potential upgrades on model performance for applications over the extratropics and Tropics rang- An ongoing and important task of the Environmental ing from the larger-scale forecasts to forecasts involving Modeling Center (EMC), as an integral part of the Na- smaller-scale phenomena over more speci®c areas of tional Centers for Environmental Prediction (NCEP), is interest. to provide increasingly skillful numerical guidance to A number of papers documenting the Medium-Range meet the various national and international demands for Forecast Model (MRF) analysis±forecast system up- improved weather forecasts. Toward this end, a con- grades, assessing the overall model performance, and certed effort at the EMC is directed at developing, test- evaluating model behavior in terms of systematic error ing, and evaluating potential upgrades to the global anal- studies has been carried out by Kanamitsu (1989), Kan- ysis±forecast system as a part of the worldwide effort amitsu et al. (1991), Derber et al. (1991), Caplan and of all major forecast centers to ensure the steady ad- White (1989), White and Caplan (1991), and Caplan et vancement in operational numerical weather prediction. al. (1993, 1997). This effort involves an ongoing development of more During the 1994 hurricane season several systematic sophisticated methodologies for data assimilation tech- de®ciencies in the Aviation (AVN) tropical forecasts niques to best maximize the use of observations that are were identi®ed by Surgi (1994). Some of the more now routinely being made available by both regional and global state-of-the-art observing platforms, as well prominent errors in the hurricane forecasts included a as to continue a high degree of concentration on im- left or westward track bias that was particularly con- proving model representation of physics. Commensurate spicuous for recurving storms such as Hurricanes Chris with this effort is a continual critical assessment of the and Florence. It was also noted that the forecasts con- sistently displayed a vertical decoupling of the storm vortex after 24 h that led to a serious degradation of the forecasts with time. The vortex decoupling was Corresponding author address: Dr. Naomi Surgi, Tropical Predic- found to be related to the failure of the model to maintain tion Center, NOAA/NWS/NHC, 11691 SW 17th St. Miami, FL a vigorous and deep circulation. In the larger-scale ¯ow, 33165-2149. a mid- to upper-level easterly bias, which was previously

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TABLE 1. Hurricane guidance models. physics in the boundary layer and in the deep convection AVN Aviation Run of the Medium-Range Forecast Model schemes. In the parameterization of deep convection, BAMD Beta Advection Model deep the simpli®ed Arakawa±Schubert scheme of Pan and BAMM Beta Advection Model medium Wu (1994) has been further modi®ed by a change to BAMS Beta Advection Model shallow the closure assumption, which has led to an enhance- CLIPER Climatology and persistence track model GFDL NOAA's Geophysical Fluid Dynamics Laboratory ment in the convective heating in disturbed regions and Hurricane Model a decrease in convective heating in suppressed regions. MRF NCEP Medium-Range Global Forecast Model The original closure of the scheme is an adjustment of NHC90 Statistical dynamical model (A90L is late version) the cloud work function to a set of climatological values VBAR V. Ooyama's barotropical model [cloud depth dependent, calculated from a variety of observations as described by Lord and Arakawa (1980)] over a timescale of 1 h. The cloud work function is a identi®ed by White and Caplan (1991) for the global measure of conditional instability of a column of air. Tropics, was shown to be large over the Caribbean. This Conditional instablitiy characterizes the atmospheric proved to be problematic for intensity forecasts since stability over the tropical oceans so that a lack of fa- the prediction of easterly ¯ow is favorable to storm vorable forcing such as large-scale subsidence can in- genesis and intensi®cation. Fitzpatrick et al. (1995) ex- hibit convection and prevent the model atmosphere from plored this bias in relation to a de®ciency in the AVN returning to neutral conditions. Alternatively, in dis- to properly maintain the tropical upper-level midoceanic turbed regions, where strong convergence and rising trough and associated westerly ¯ow. motion often exist, when the convection scheme adjusts During October 1995 an improved version of the the soundings to the climatological cloud work func- NCEP global model was implemented operationally. tions, the atmosphere will remain conditionally unstable This occurred after extensive testing and evaluation of even when the parameterized heating stops. This can the various proposed changes to the global analysis± lead to large-scale supersaturation in the forecast and forecast system and after assessing their impact on the the heating will primarily be in the lower troposphere. overall predictive skill of the model. The National Hur- The new closure change allows the atmosphere to adjust ricane Center (NHC) in collaboration with EMC ac- to more neutral conditions when the cloud base rising tively participated in this assessment. The performance motion gets stronger. This change has generally led to of the model was evaluated with relation to the model enhanced gradients of convective precipitation that re- forecasts in the Tropics, and more speci®cally, the hur- sults from larger maxima due to the adjustment toward ricane forecasts, to determine if the proposed upgrades more neutral conditions. made a positive impact to reduce any of the above cited The changes in the planetary boundary layer (PBL) model de®ciencies. The NHC relies on NCEP for nu- diffusion scheme has been reported in Hong and Pan merical guidance support as an integral part of the over- (1996) and will be only brie¯y described here. The ma- all hurricane forecasting process at NHC. This guidance jor change is the use of a bulk Richardson number of is provided by the AVN 72-h hurricane track forecasts the entire boundary layer to specify the coef®cients of as described by Lord (1991). Additionally, the overall diffusivity in place of a local Richardson number. While quality of the global model system is extremely im- the scheme was designed to improve the diurnally vary- portant since it provides the initial ®elds for other hur- ing PBL over land, it also improves the maintenance of ricane forecast models used by NHCÐthat is, the GFDL the oceanic PBL. One important result of the new Hurricane Prediction System, NHC90, VICBAR, and scheme is the stronger mixing of the moisture in the the BAM models. A list of acronyms de®ning these PBL as described in Hong and Pan (1996) resulting in models in given in Table 1 and further details on model weaker vertical moisture gradients in the PBL. The ef- characteristics are provided in Aberson and DeMaria fect of this change is the replenishment of the PBL (1994). Thus, this study was quite relevant to the NHC's moisture after a convective event or the restoration of forecast interests. the conditional instability. This has had a signi®cant The 1995 MRF upgrades are described in section 2, in¯uence on the convection parameterization in the as is the methodology used for this study. The results model by increasing the moisture supply to the free showing the impact of the upgrades on the large-scale atmosphere. In fact, the two changes to the deep con- tropical ¯ow and for the hurricane forecasts are de- vection and the boundary layer have contributed nearly scribed in section 3. The conclusions are presented in equally in magnitude in the enhancement of the surface section 4. latent heat ¯uxes and the deep convective heating. Also, in addition to the above-mentioned changes, the 2. Design of model evaluation statistical interpolation system (SSI) analysis scheme was modi®ed to directly incorporate the use of satellite a. Changes to the MRF radiance data. This change was shown to have a very The changes that were implemented in the model that positive impact on the forecasts in the extratropics (most are most relevant to this study were made to the model notably in the Southern Hemisphere) as described by

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Derber and Wu (1997). This study does not include forecast sensitivity to this change since all the forecasts presented here were rerun from the new global data assimilation system (GDAS) with the incorporation of the radiance data. b. Design of experiments During the summer of 1995, for approximately two months (6 June±28 July), two versions of the global model were run in parallel. For the purpose of this study, the operational modelÐthat is, without the 1995 changesÐwill be referred to as the MRF,while the model version including the upgrades will be referred to as the MRX. As a preliminary study, these parallel runs were evaluated daily during this period with a general emphasis on examining the ¯ow over the tropical Atlantic and eastern Paci®c basins. The preliminary ®ndings suggested that the MRX was a ``moister'' model than the operational MRF with a more realistic distribution of precipitation and moisture near the intertropical convergence zone (ITCZ) as qualitatively determined from satellite imagery. Also, the tropical convective systems appeared to be more sharply de®ned in the MRX with more concentrated regions of vorticity and convective precipitation. Based on those encouraging ®ndings, the MRF and the MRX were rerun for the months of August and September 1995 at 0000 UTC to determine what impact the new physics might have on the hurricane forecasts and on improving some model de®ciencies mentioned above as noted for the 1994 hurricane season. The forecast ®elds from these runs were archived at a 1Њ horizontal resolution. Out of the six storms that formed during this period, several case studies were chosen to closely examine the details of the forecasts. The cases selected were those where the MRX had clearly shown FIG. 1. (a) Root-mean-square error of 200-mb wind of 72-h forecast ensemble between 10Њ±35ЊN and 140Њ±20ЊW. The top plots are for an improvement over the MRF forecast. Moreover, the the total (vector) wind error; the bottom plots are for the zonal wind forecast scenarios that we selected were ones that proved component error. The solid lines are the MRF; the dashed lines are dif®cult for other models to forecast, for example, the MRX. Units are meters per second. (b) As in (a) but for 200-mb NHC90, BAMS, VICBAR, and the GFDL model. total wind over the tropical Atlantic, Caribbean, and eastern Gulf of Mexico (90Њ±50ЊW).

3. Impact of model upgradesÐMRF versus MRX a. Large-scale tropical forecasts and for the entire Northern Hemisphere. Predictive skill is measured relative to the analyses. Results are pre- To provide an objective basis to evaluate the overall sented for August only since there was quantitively little performance of the MRF and the MRX, root-mean- difference between the two periods. Figures 1a and 1b square (rms) errors for an ensemble of 72-h forecasts show the results of these calculations. The solid lines were calculated over a two-week period in August and are the results from the MRF; the dashed lines are from for another two-week period in September. These cal- the MRX. culations were carried out for the tropical and subtrop- Figure 1a shows the 200-mb rms vector wind errors ical Atlantic and eastern Paci®c Oceans (10Њ±35ЊN, 20Њ± (top plots) and zonal wind errors (bottom plots) between 140ЊW, Fig. 1a), for a smaller area over the tropical 140Њ and 20ЊW. The vector wind errors show an overall Atlantic including the Caribbean and eastern Gulf of modest improvement in the MRX 72-h forecasts as in- Mexico (Fig. 1b) between 90Њ and 50ЊW (where the dicated by the area and time mean value (area averaged upper-level wind errors have been systemically large) over this domain over the 14-day ensembles) of 8.86 m

Unauthenticated | Downloaded 10/01/21 11:33 AM UTC 1290 MONTHLY WEATHER REVIEW VOLUME 126 sϪ1 errors compared to 9.25 m sϪ1 from the MRF av- Largely, this is due to a historical lack of consistent data eraged. Although the errors from both runs show the over the tropical oceans that has limited our progress same trend over certain days in the ®rst half of the in providing an adequate description of the details of forecast period, the variability of the 72-h error is some- the physical and dynamical interactions within the trop- what reduced in the MRX over the second half of the ical systems and with the larger-scale environment for forecast period. The variability of the 72-h rms error a variety of storm situations that impact storm motion for the 500-mb temperature is also reduced in the MRX and intensity. While a detailed treatment of the numer- but over the entire forecast period (not shown). The ical prediction problem is well beyond the scope of this contribution of the 72-h zonal wind error at 200 mb paper, Krishnamurti et al. (1993) provide insight into (bottom plots) is similar to the total vector wind error. the recent advances of hurricane forecasting with very There is a modest improvement in the MRX average high resolution models; and Elsberry (1995) provides a rms error values; however, a decrease in the variability comprehensive description on the more general role of of the error is shown throughout the forecast period. the dynamical modeling effort in the advancement of Figure 1c shows the result of this calculation for the tropical storm prediction. vector wind over the Caribbean including the Gulf of The NHC makes use of a hierarchy of models ranging Mexico. Although the magnitudes of the errors are larg- from purely statistical modelsÐthat is, CLIPER (cli- er in this region than we would like to see in both the matology and persistence) to mixed statistical-dynam- MRF and the MRX, the mean rms error of the MRX is ical models, NHC90 to barotropic models, VICBAR to reduced to 9.30 m sϪ1 from 10.09 m sϪ1 from the MRF. multilevel fully dynamical models with state-of-the-art To assess the broader impact of the upgrades on the physics, and the AVN and the GFDL model. The reason predictive skill of the MRX, from the same ensemble for maintaining a number of models operationally is of 72-h forecasts, the rms error of the 500-mb heights purely a pragmatic one: some models perform better were calculated for the Northern Hemisphere (not under some situations than others. Hurricanes interact- shown). In general, there is a moderate overall improve- ing with midlatitude systems are usually handled better ment in the MRX 72-h error and the area and time by the baroclinic models as are storms near land where average errors are reduced in the MRX to 22.14 gpm real-time data can be optimally used. Aberson and from 22.82 gpm in the MRF, which are consistent with DeMaria (1994) provide an assessment of VICBAR per- the above results. The details of the error statistics for formance and describe scenarios in which barotropic the global and extratropical forecasts are described in dynamics are adequate for accurate track predictions. Pan et al. (1997). Most dynamical models perform better for well-devel- Although the objective errors were encouraging for oped rather than weaker storms and most models per- the upper-level winds, disappointing results were ob- form better with well-developed storms and strong rath- tained for the 850-mb tropical winds. At low levels, the er than weak steering currents. The reasons why one MRX performed worse at 72 h than the MRF over the model outperforms another for a given storm are, how- entire Tropics. The low-level tropical wind error is one ever, not always that clear. And less clear are the in- that has long plagued the NCEP global model, and other consistencies that seem to randomly occur in the pre- global models (Surgi 1989). One possible source for dictions between one model forecast period and the next, this model bias on larger scales of motion could be perhaps partly re¯ective of the various states of pre- related to an improper treatment of the shallow non- dictability of the larger-scale atmosphere. (Certainly the precipitating clouds that form along the trade wind in- hurricane forecasters who rely on model forecasts for version that are important transporters of heat, moisture, guidance every 12 h are keenly sensitive to this prob- and momentum into the deep Tropics. The low-level lem.) wind error is an area of active investigation. Trying to isolate and determine exact causes for particular ``failed'' or ``successful'' forecasts in a global dynamical model is a very dif®cult and daunting task. b. Tropical storm/hurricane forecasts It is dif®cult due to the strong nonlinear interactions The ``hurricane forecasting problem'' to date remains between the boundary layer and deep convection pa- one of the most dif®cult and challenging forecast prob- rameterization schemes and between these physics and lems in numerical weather prediction. This is true for dynamics of the model. This is true in trying to isolate track forecasting in spite of a signi®cant increase in causes for model errors in general. Determining the best numerical forecast guidance skill over the past two de- method in which to identify the source of the errors in cades and most conspicuously for intensity forecasts that another important aspect of this problem. A recent con- have shown little or no skill in our operational model tribution to the study of both mean and transient sys- forecasts (DeMaria and Kaplan 1997). One of the rea- tematic model errors is described in Kanamitsu and Saha sons for the limited progress in numerical prediction of (1995). Moreover, this dif®culty escalates tremendously tropical cyclones is due to a fundamental lack of un- for the hurricane forecast problem due to all the com- derstanding of the details of the scale interaction prob- plexities discussed above. In this study, we make use lem governing these systems in the real atmosphere. of our knowledge of the systematic model biases and

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FIG. 2. Numerical track guidance received at the National Hurricane Center for Hurricane Felix, initialized at 0000 UTC 16 August 1995. The observed track is denoted by tropical storm/hurricane symbols. FIG. 3. The MRF and MRX model forecast tracks for Hurricane Felix initialized at 0000 UTC 16 August 1995. The observed track is denoted by tropical storm/hurricane symbols. model behavior in particular forecast scenarios identi- ®ed during the 1994 hurricane season (described in section one), to evaluate the 1995 model upgrades in the to a much improved track forecast in the MRX, the MRF convective and boundary layer parameterizations within and MRX forecasts of the hurricane and of the larger-scale the context of those de®ciencies. steering ¯ow were examined. The results are presented for three forecast scenarios Figures 4a±d show the 500-mb ¯ow for the 36- and in which the upgrades dramatically improved upon the 72-h forecasts for the MRF and MRX forecasts. At 36 hurricane forecasts from the operational model. Also, h, just prior to recurvature, Felix is located at 36ЊN, from those cases, we selected scenarios deemed to be 75ЊW and is a stronger vortex in the MRX (Fig. 4b) more ``dif®cult'' forecastsÐthat is, recurving storms than in the MRF (Fig. 4a) as denoted by the 15 m sϪ1 and/or weak steering currents. These scenarios were typ- isotach over the eastern semicircle of the storm in the ically where the other NHC model guidance showed MRX. Also, from the difference ®eld of the 500-mb decreased forecast skill as well. ¯ow for this forecast periodÐthat is, the MRF subtracted from the MRX (not shown)Ðalthough the magnitude of the difference of the vortex is not that large at this 1) HURRICANE FELIX level (5±10 m sϪ1), it is signi®cant that the isotach dif- An illustration of a ``dif®cult'' forecast period is ference pattern is essentially wrapped around the vortex shown in Fig. 2. These were the various track model showing a more circular hurricane wind pattern in the forecasts received at NHC for Hurricane Felix initialized MRX with the maximum difference over the eastern on 0000 UTC 16 August 1995 before recurvature. The semicircle of the storm. observed track of Felix is denoted by the tropical storm/ With respect to the synoptic-scale ¯ow, a high pres- hurricane symbols. What is notable here is the disparity sure area was building over the Ohio Valley with as- in the forecasts between the models that predict landfall sociated northerly ¯ow strengthening along the Atlantic and the models that predict recurvature. This caused coast. This feature is shown in the 36-h forecasts of serious concern among hurricane forecasters who were both the MRF and the MRX; however, the forecast of issuing warnings along the coastline during this time. the MRX suggests a greater buildup of the high east- In the next forecast period, 12 h later, all the models ward, with the northerly part of the circulation reaching forecasted recurvature. the coastline. In fact, it was the very strong northerlies Figure 3 shows a comparison of the MRF and MRX that developed along the coastline in association with track forecasts for Hurricane Felix, initialized for the same the high building northward into Canada that became forecast period as above, at 0000 UTC 16 August. The an important steering in¯uence that kept Felix well off- MRX shows a clear recurvature of the storm (although shore. A major difference is seen in the simulations of about 220 km too far to the west) followed by an accel- this feature in the MRF and MRX 72-h forecasts as eration toward the east. The MRF, however, essentially shown in Figs. 4c and 4d, respectively. A much stronger stalls the storm off Cape Hatteras. That is a scenario usu- northerly component of the ¯ow parallels the coastline ally associated with weakening storms and/or weak steer- in the MRX with a stronger northward building of the ing currents. To help determine the factors that contributed ridge. This is not the case in the MRF simulation, which

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FIG. 4. The 36-h forecast of 500-mb ¯ow from (a) the MRF model and (b) the MRX model initialized at 0000 UTC 16 August 1995. The solid lines are streamlines. The dashed lines are isotachs. Units are meters per second. Contour interval is 5 m sϪ1. shows a markedly weaker synoptic ¯ow where the syn- throughout the depth of the troposphere. The MRF with- optic- and storm-scale waves are hardly distinguishable. in 36 h shows a weakened and shallow circulation with With respect to the simulation of Felix, there is also a the strongest vorticity becoming concentrated in the stronger midlevel storm circulation in the MRX as in- lower levels. By 72 h, a marked vertical decoupling of dicated by the 20 m sϪ1 isotach, and although the stream- the storm in the MRF is shown at mid- to upper levels line pattern does not display a well-developed closed as the vorticity becomes diffuse and the area of maxi- circulation, it is nevertheless a marked improvement mum rising motion (0.5 Pa sϪ1) is seen to be well dis- over the elongated trough impinging onto the coastline placed from the center of the vortex (Fig. 5c). This is that represents the storm in the MRF.To further examine to be compared to the MRX 72-h forecast (Fig. 5d), the simulations of the vortex in greater detail, vertical which develops a very strong vertically coherent cir- cross sections through the storm were made at the storm culation with maximum rising motion of 2.4 Pa sϪ1. locations shown in Figs. 4a±d to assess the impact of To try to gain some insight on the role that the changes the new physics on maintaining the vertical structure of to the physics had on the apparent structural disparities Felix. shown in the above ®gures, the latent heating from the A vertical cross section through the storm of the vor- deep convection was examined. Figure 6 shows a com- ticity and vertical velocity at 36 h and 72 h is shown parison of the vertical pro®les of deep convective heat- in Figs. 5a±d. At 36 h, the MRX maintains a deep and ing through the center of Felix for the 48-h forecasts strong vortex with a vigorous vertical circulation (as for the MRF and the MRX. Not only is the magnitude depicted by the contours of vertical velocity) extending of maximum heating increased in the MRX by about 5Њ

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FIG.4.(Continued) As in (a) and (b) but for the 72-h forecast from (c) the MRF and (d) the MRX. dayϪ1 in the upper troposphere, but the increased heating which were found to be signi®cantly enhanced in the is spread over a greater depth of the column. The main- MRX. At 36 h, the maximum surface latent heat ¯ux tenance of the strong pro®le of condensational heating over the eastern hemisphere of Felix produced by the is essential in maintaining the vertical structural integ- MRX was 325 W mϪ2, compared to approximately 210 rity of the storm and thus the storm circulation through- WmϪ2 for the MRF (not shown). Also, the horizontal out the forecast. Analogously, a storm weakens in the gradients of the ¯uxes were much larger in the MRX. real atmosphere when it encounters strong vertical shear At 72 h, as shown in Figs. 7a and 7b, the very large by causing a disruption of the latent heating associated ¯uxes in the MRX (ϳ700 W mϪ2) are quite realistically with the deep convection. It is crucial for a developing indicative of a strongly disturbed surface layer in as- storm that the upper-level latent heat release remain sociation with the passage of a hurricane, whereas the above the low-level circulation center. By 72 h (not MRF ¯uxes do not at all indicate the presence of a storm. shown), the MRF convective heating weakens substan- The results presented above strongly suggest that a tially at the mid- and upper levels (about 50% of the positive feedback between the boundary layer and the heating at 36 h), which is consistent with the decoupling deep convection serve to strengthen the vertical struc- of the vortex shown in Fig. 5a, while the strong con- ture of the storm by enhancing the vertical distribution vective heating is maintained throughout the forecast of the heating. In the simpli®ed Arakawa±Schubert period in the MRX. The increased heating is a result of scheme, the condition at the cloud base depends strongly the new closure assumption described in section 2a. on the moisture structure in the PBL. The rising parcel The change to the PBL was also examined via the can lose buoyancy when it encounters dry subcloud lay- energy supplied to the storm from the surface ¯uxes, ers. When convection is initiated, the drying associated

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FIG. 5. Vertical cross section of vorticity (ϫ105 sϪ1, shaded) and vertical velocity (Pa sϪ1, dashed) for 36-h forecast of Hurricane Felix from (a) the MRF and (b) the MRX.

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FIG.5.(Continued) As in (a) and (b) but for 72-h forecast from (c) the MRF and (d) the MRX.

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FIG. 6. Vertical pro®le of deep convective heating (K dayϪ1) for Hurricane Felix 48-h forecasts from the MRF (solid line) and the MRX (dashed line). with the subsidence reduces the buoyancy and stabilizes westward motion still exists in the earlier forecast pe- the atmosphere. A PBL scheme that vigorously mixes riods of Felix. It is not, however, any worse than the the moisture from the surface to the top of the PBL can MRF forecasts. The westward bias appears early in the restore the buoyancy and maintain the convection. In forecast period and is closely related to the bogussing fact, a set of sensitivity experiments were made to in- problem as described by Lord et al. (1996, manuscript vestigate the individual contributions of the deep con- submitted to Mon. Wea. Rev.). vection and PBL to the surface ¯uxes and convective heating by only including one of the changes at a time. 2) HURRICANE IRIS These results (not shown) indicated that each contributed nearly equally in magnitude to the enhancement of Another interesting forecast improvement was for the surface ¯uxes and to the convective heating, which Hurricane Iris initialized at 0000 UTC 24 August, 6 h is a good indicator of an improved interaction between after being upgraded to a hurricane. This was a dif®cult the convection and the PBL parameterization schemes. forecast period due to the complex interaction between The general result of the improved physics is to sim- Iris and Hurricane Humberto that was strengthening ulate a stronger and improved storm structure that con- about 1390 km to the east. The relatively close position tributes to an improved track forecast. Additionally, in of Humberto was in¯uential in causing Iris to make a this simulation, we found an improvement in the fore- rather abrupt turn toward the west southwest from a cast of the amplitude of the larger-scale ¯ow that also previous northwest heading. In fact, all of the other in¯uences the recurvature of Felix. Here, it is suggested numerical guidance showed dif®culty in changing from that the improved forecast of recurvature is a result of the northward heading. Further dif®culty arose in the an improved simulation of both these features. More- AVN forecasts for this storm since the bogussing system over, this also suggests the subtle but important role of was inoperative during this period that also negatively diabatic heating in providing energy for the maintenance affected the other models initialized by the NCEP of the larger-scale circulations. GDAS. Finally, with respect to the westward track bias, al- Figure 8 shows the MRF and MRX tracks corre- though an improved track simulation was shown for this sponding to this period. Although the MRX did not forecast scenario in the MRX, a general tendency for correctly forecast the more southerly track component,

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FIG. 8. The MRF and MRX model forecast tracks for Hurricane Iris initialized at 0000 UTC 24 August 1995. The observed track is denoted by tropical storm/hurricane symbols.

A clear signature of Iris is shown in both forecasts of the vorticity but the vertical circulation in the MRX forecast is somewhat stronger. What is strikingly dif- ferent in the MRX is the clear spinup of Humberto at 50ЊW. Again, this spinup is without the bene®t of an arti®cially induced initial vortex and arises solely from the ability of the model to generate storm-scale vorticity via the strong interactions of the model physical and dynamical processes. Finally at 72 h, the MRF (Fig. 11a) has lost the signatures of all the storms while the MRX (Fig. 11b) correctly forecasts not only Iris (at 15.1ЊN, 61.4ЊW) but Humberto (at 25ЊN, 50ЊW, slightly FIG. 7. Surface latent heat ¯ux (W mϪ2) for 72-h forecasts for north and east of observed position), Karen (15.6ЊN, Hurricane Felix forecasts from (a) the MRF and (b) the MRX. 35.4ЊW), and to some extent what was to become Hur- ricane Luis, although the position of Luis is too far west. it did correctly forecast the turn toward the west in the 3) TROPICAL STORM PABLO ®rst 12 h. Figures 9a and 9b show the 12-h forecasts of Iris initialized at 0000 UTC 24 August for the MRF Last, we present results that addressed the de®ciency and MRX, respectively. At this time Iris was located at in the model to properly simulate the mid-oceanic trop- 14.8ЊN, 55.1ЊW and was a 64-kt hurricane. The main ical upper-level trough over the tropical Atlantic that difference between the MRF and the MRX is a stronger affected the intensity forecasts for Tropical Storm Pablo. and more distinct circulation of not only Iris but also Due the inability of the model to forecast the very strong for Humberto, which was a 90-kt hurricane at this time westerly shear associated with the southward extension (located at 15.4ЊN, 42.6ЊW) in the MRX forecast. Also, of the trough into the deep Tropics, no hint was provided the position of Humberto is too far north in the MRF. in the global model forecast of the rapid weakening of At the midlevels the MRF begins to diffuse the vorticity Pablo upon encountering the westerlies within three and develops an elongated trough. This is readily ap- days after being upgraded to storm status. At 0000 UTC parent by 36 h and is shown in Figs. 9c and 9d in a 5 October, Pablo was a depression located at 8.4ЊN, comparison of the 500-mb ¯ows. The MRX, on the other 32.8ЊW or about 3100 km east of the Lesser Antilles. hand maintains the circulations of Iris and Humberto At this time, Pablo was located in easterly ¯ow, which and evidence is also seen in the eastern Atlantic of the was associated with an upper-level anticyclone located spinup of what was to become Tropical Storm Karen over the eastern Atlantic. This scenario supported fur- (located at 11ЊN, 27ЊW). Figures 10a and 10b show a ther strengthening from a depression to tropical storm vertical cross section of the vorticity and vertical motion intensity. Within 48 h, the anticyclone was pushed to along the latitudes of the storms for the 36-h forecast the south as the upper-level trough deepened over the from the MRF (Fig. 10a) and from the MRX (Fig. 10b). tropical mid±Atlantic Ocean pushing southward and ex-

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FIG. 9. The 12-h forecast ®elds of vorticity (ϫ105 sϪ1, shaded) and streamlines (solid lines) at 850 mb for Hurricane Iris at 0000 UTC 24 August 1995 from (a) the MRF and (b) the MRX. The letters ``I'' and ``H'' mark the location of Iris and Humberto. tending westward with time. Figures 12a and 12b show the 72-h forecasts. The cyclonic circulation, with the the 48-h forecasts of this feature for the MRF and MRX, center located at approximately 18ЊN, 52ЊW, is indica- respectively. The position of Pablo is indicated in the tive of the deeper trough in the MRX forecast. Also note ®gures by the dots. The verifying analysis for 7 October that Pablo is located between the 10 and 15 m sϪ1 is- is presented in Fig. 12c. The important difference be- otachs of the westerly wind difference between the two tween the forecasts is the more southward extension of models, highlighting the area of the stronger MRX west- the trough in the MRX, essentially bringing the westerlies. It was 18 h later that Pablo dissipated at 57.5ЊW erlies over the circulation of Pablo. In the MRF, Pablo due to the very strong westerly vertical shear associated remains under northeasterlies associated with the poor with the trough. Although we did not run Pablo in par- forecast of the deepening of the trough and the building allel systems to explicitly examine the differences in the of a ridge over the northeastern Caribbean toward the forecasts of the storm in detail, we are con®dent based east. Comparing the MRX 48-h forecast (Fig. 12b) with on the above result for the large-scale ¯ow that an im- the verifying analysis (Fig. 12c), the entire con®guration provement in the intensity forecast would have been of the trough is extremely well simulated with the axis obtained. extending northwestward. Figures 12d and 12e show the 72-h 200-mb ¯ow for c. Impact of MRX upgrades on the NHC model MRF and MRX forecasts with the verifying analysis on veri®cation system 8 October shown in Fig. 12f. Again, the MRX provides an excellent forecast of a well-developed trough with Since it became operational two years ago, the GFDL associated northwesterly ¯ow over Pablo that corre- hurricane forecast model (Kurihara et al. 1995) has pro- sponds well with the analysis of this feature. To illustrate vided the best track veri®cation scores over any other the magnitude of the difference between the forecasts, model. The model makes use of the NCEP global model Fig. 12g shows the MRF subtracted from the MRX for analysis during its initialization process. Therefore, to

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FIG.9.(Continued) As in (a) and (b) but for 36-h 500-mb forecast from (c) the MRF and (d) the MRX. determine the impact of all of the upgrades on the GFDL other operational models that rely on the global model track forecasts, the storms of August and September for initializationÐfor example, BAM, NHC90, VIC- were rerun for the GFDL model from the new global BAR, as well as the GFDL modelÐthe veri®cation skill model system. Figure 13 shows the skill of the model of all the models is shown for the 1995 and 1996 hur- relative to CLIPER as a function of forecast time. The ricane seasons in Figs. 14a and 14b, respectively. All GFDL is the version of the model run from the oper- model skill is relative to CLIPER. ational system without the upgrades. The GFDX is the It is shown that the AVN at 48 h has increased skill forecast run from the global system with the upgrades. by approximately 10% relative to CLIPER from the The number of cases indicates the number of runs at 1995 season to the 1996 season (cf. 0.12 in Fig. 14a to 12, 24, 36, 48, and 72 h for the hurricanes that occurred 0.22 in Fig. 14b) For the 72-h forecasts, the skill has between 1 August and 30 September 1995. As indicated increased by about 25% between the 1995 season and from Fig. 13, the GFDX shows an improvement in the the 1996 season (cf. 0.05 in Fig. 14a to 0.30 in Fig. track forecasts compared to the GFDL at all forecast 14b). Most noticable here is not only is there a sub- times. From these statistics, roughly a 20% reduction stantial gain in skill of the global model after 36 h, but in the track error was found for these cases. These results that there is a complete reversal in the trend of the skill were statistically signi®cant at all time levels (Bender after 48 h. In the 1995 hurricane season a marked loss 1996, personal communication). The track error at 36 of skill occurred for the AVN after 48 h (this was also h has been reduced from 189 to 152 km; at 48 h from true for the simpler models). This deterioration in skill 236 to 199 km; and at 72 h from 368 to 326 km. has been eliminated in the global model. We are con- Finally, to provide an overall context to assess the ®dent that this improvement is a direct consequence of global model upgrades by comparing its performance the improved physics that dramatically impacted the for the entire 1995 hurricane season (without the up- storm circulation and environment after 36 h as shown grades) with the 1996 hurricane season (with the up- in the previous sections. grades), and to further assess their impact on all the The veri®cation scores for the eastern Pac®c for the

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FIG. 10. Vertical cross section of vorticity (ϫ105 sϪ1, shaded) and vertical velocity (Pa sϪ1, dashed) for 36-h forecast of Hurricane Iris initialized at 0000 UTC 24 August 1995 from (a) the MRF and (b) the MRX.

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FIG. 11. The 72-h forecast of 850-mb vorticity (ϫ105 sϪ1, shaded) and streamlines (solid lines) for Hurricane Iris from (a) the MRF and (b) the MRX. The letters I, H, K, and L mark the location of Iris, Humberto, Karen, and Luis, respectively. global model were also calculated (not shown). From izatons in the NCEP global model. We have examined these results the AVN also showed substantial improve- the impact on the large-scale tropical forecasts and on ment for the 1996 season and showed comparable skill hurricane forecasts during the latter half of the 1995 to the BAMS models, whereas in the 1995 season the hurricane season. Our diagnostics were based on known AVN showed no skill even relative to CLIPER. de®ciencies in forecasting tropical systems during the Additionally, the GFDL model shows a 15% increase 1994 hurricane season. in the 48-h forecast skill between the seasons (cf. 0.35 A modest overall improvement in the 72-h 200-mb in 1995 to 0.5 in 1996) and an approximately 18% im- rms vector wind error was found in the MRX across provement in skill at 72 h (cf. 0.35 in 1995 to 0.53 in the tropical/subtropical Atlantic and eastern Paci®c 1996), which is comparable to the preliminary results Oceans. This improvement was consistent with the low- for the test cases of the 1995 season mentioned above. er rms errors for the 500-mb height ®elds over the Furthermore, all of the other models show an increase Northern Hemisphere. In particular, the upper-level in skill during the 1996 season. Although the barotropic wind errors over the Caribbean, which had been iden- model still loses skill after 48 h, the skill at 48 h has ti®ed as a problem area for hurricane intensity forecasts, increased 5% relative to CLIPER between the seasons; were reduced. Forecasts of the low-level tropical ¯ow, NHC90 has 5% increased skill at 48 h and reverses a however, were degraded with the new model upgrades. negative trend from the previous season out to 72 h and Trying to identify the source of this error will be the the BAMS models show about a 5% increase in skill. focus of future investigations. Changes in the deep convective and boundary layer parameterizations have increased convective activity 4. Conclusions and diabating heating for tropical disturbances, which This study has evaluated the impact of recent changes has led to stronger vertical coupling and an improved to the deep convective and boundary layer parameter- vertical structure of the forecast model vortex. Improved

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FIG. 12. The 48-h forecast 200-mb streamlines for Tropical Storm Pablo initialized at 0000 UTC 5 October 1995 from (a) the MRF and (b) the MRX. The solid dot represents the position of Pablo. (c) The verifying analysis for the 200-mb ¯ow valid for 0000 UTC 7 October 1995. vertical vortex structure contributed to an improved pattern over the eastern part of the United States also track forecast for Hurricane Felix and ameliorated a contributed toward improving the recurvature in the problem noted in earlier 1994 cases. Additionally, a track forecast. stronger amplitude in the forecast of the synoptic wave The MRX generated and maintained stronger circula-

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FIG. 12. (Continued) As in (a) and (b) but for the 72-h forecast from (d) the MRF and (e) the MRX. (f) The verifying analysis for the 200-mb ¯ow valid for 0000 UTC 8 October 1995. tions for Hurricanes Iris and Humberto, and indicated the maintain a stronger vortex has led to improved track fore- regions of cyclogenesis for Karen and Luis. Generally, the casts, the leftward track bias has not been totally elimi- improved model appears to develop realistic storm vor- nated. This bias may be related to the storm initialization ticity via strong interactions of the model physics and problem and is being investigated (Lord et al. 1996, manu- dynamics. Although the model's ability to generate and script submitted to Mon. Wea. Rev.).

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FIG. 12. (Continued) The difference ®eld between the MRF and the MRX of the 200-mb 72-h forecast ¯ow. The solid lines are the streamlines. The dashed lines are the isotachs. Units are meters per second. Contour interval is 5 m sϪ1. The shading highlights the isotach ®eld.

Additionally, it was shown that the MRX provided a de®ciency, which we suggest, is re¯ective of the more much improved simulation of the midoceanic tropical global upper-level easterly bias problem that the NCEP upper-level trough. The maintenance of the trough was global model and other global models have been in- important in the intensity forecasts for Tropical Storm vestigating over a period of years. As a possible con- Pablo because of the associated westerly shear. This tributor to the planetary-scale problem, we plan to ad- model de®ciency is one that had been documented over dress the maintenance of the upper-level troughs within the Caribbean in the previous year. It is however, a the framework of the transfer of energy between the scales of motion via the divergent and rotational circulations. The positive impact of the new model physics was also shown for the August and September 1995 hurricane forecasts from the GFDL hurricane model. This resulted in an overall 20% increase in skill in the GFDL hurricane track model forecasts. Furthermore from the veri®cation scores for the NHC numerical guidance models comparing their performance for the 1995 hurricane season with the 1996 hurricane season, improved forecast skill was shown for all of the model guidance used at NHC. These results showed an increase in skill between 1995 to 1996 from 10% to 25% for the global model for the 36±72-h forecasts. There was also substantial improvement in the performance of the global model over the eastern Paci®c wherein the previous season the model showed no skill at all in this basin. Fur- FIG. 13. The skill of the GFDL vs the GFDX hurricane forecast model for hurricane/storm tracks from 1 August±September 1995. thermore, a 15%±18% improvement was shown in the Forecast skill is relative to CLIPER. GFDL model with the other models increasing between

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REFERENCES Aberson, S. D., and M. DeMaria, 1994: Veri®cation of a nested barotropic hurricane track forecast model (VICBAR). Mon. Wea. Rev., 122, 2804±2815. Caplan, P. M., and G. H. White, 1989: Performance of the National Meteorological Center's medium-range model. Wea. Forecast- ing, 4, 392±400. , , and J. G. Jiing, 1993: Skill of the NMC global model in the Tropics. Preprints, 13th Conf. on Weather Analysis and Fore- casting, Vienna, VA, Amer. Meteor. Soc., 225±228. , J. Derber, W. Gemmill, S. Y. Hong, H. L. Pan, and D. Parrish, 1997: Changes to the 1995 NCEP operational medium-range forecast model analysis/forecast system. Wea. Forecasting, 12, 581±594. DeMaria, M., and J. Kaplan, 1997: An operational evaluation of a statistical hurricane intensity prediction scheme (SHIPS). Pre- prints, 22d Conf. on Hurricanes and Tropical Meteorology. Fort Collins, CO, Amer. Meteor. Soc., 280±281. Derber, J. C., and W.-S. Wu, 1998: The use of TOVS cloud-cleared radiances in the NCEP SSI analysis system. Mon. Wea. Rev., in press. , D. F. Parrish, and S. J. Lord, 1991: The new global operational analysis system at the National Meteorological Center. Wea. Forecasting, 6, 538±547. Elsberry, R. L., 1995: Recent advancements in dynamical tropical cyclone track predictions. J. Meteor. Atmos. Phys., 56, 81±99. Fitzpatrick, P. J., J. A. Knaff, C. W. Landsea, and S. V. Finley, 1995: Documentation of a systematic bias in the aviation model's forecast of the Atlantic tropical upper-tropospheric trough: Impli- cations for tropical cyclone forecasting. Wea. Forecasting, 10, 433±446. Hong, S.-Y., and H.-L. Pan, 1996: Nonlocal boundary layer vertical diffusion in a medium-range forecast model. Mon. Wea. Rev., 124, 2322±2339. Kanamitsu, M., 1989: Description of the NMC Global Data Assim- ilation and Forecast System. Wea. Forecasting, 4, 335±342. , and S. Saha, 1995: Spectral budget analysis of the short-range forecast error of the NMC Medium-Range Forecast Model. Mon. Wea. Rev., 123, 1834±1850. , and Coauthors, 1991: Recent changes implemented into the global forecast system at NMC. Wea. Forecasting, 6, 425±435. FIG. 14. Forecast skill of NHC numerical guidance for (a) the 1995 Krishnamurti, T. N., and H. S. Bedi, K. S. Yap, and D. Oosterhof, hurricane season and (b) the 1996 hurricane season. Model skill is 1993: Hurricane forecasts in the FSU Models. Adv. Atmos. Sci., relative to CLIPER. 10, 121±131. Kurihara, Y., M. A. Bender, R. E. Tuleya, and R. J. Ross, 1995: Improvements in the GFDL hurricane prediction system. Mon. Wea. Rev., 123, 2791±2801. Lord, S. J., 1991: A bogussing system for vortex circulations in the 5% and 15% in performance skill as well. The improve- National Meteorological Center Global Forecast Model. Proc. ment in the forecast performance of the global model 19th Conf. on Hurricanes and Tropical Meteorology, Miami, and in the other models that are impacted by changes FL, Amer. Meteor. Soc., 328±330. , and A. Arakawa, 1980: Interaction of a cumulus cloud ensemble to the global model, we believe, is due to the improve- with large-scale environment, Part II. J. Atmos. Sci., 37, 2677± ments to the deep convection and boundary layer model 2692. physics. Pan, H.-L., and W.-S. Wu, 1994: Implementing a mass ¯ux convection parameterization package for the NMC Medium-Range Forecast Model. Preprints, 10th Conf. on Numerical Weather Prediction, Acknowledgments. The authors would like to thank Portland, OR, Amer. Meteor Soc., 96±98. Ron McPherson, Eugenia Kalnay, and Robert C. Sheets Surgi, N., 1989: Systematic errors of the FSU Global Spectral Model. Mon. Wea. Rev., 117, 1751±1766. for encouraging and supporting this collaboration. , 1994: Performance of the AVN model for the 1994 hurricane Thanks are also due to John Manobiano for a very thor- season. Proc. 49th Interdepartmental Hurricane Conf., Wash- ough and helpful review of the manuscript and to the ington, DC, Of®ce of the Federal Coordinator, A-13. other reviewers who helped in improving this work. White, G. H., and P. M. Caplan, 1991: Systematic performance of the NMC Medium-Range Model. Preprints, Ninth Conf. on Nu- Much appreciation is expressed to Joan David for her merical Weather Predition, Denver, CO, Amer. Meteor. Soc., expert drafting of some of the diagrams. 806±809.

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