Operational Global Atmospheric Prediction System Research Articles

Abstract In support of the Department of Defense's Gulf War Illness study, the Naval Research Laboratory (NRL) has performed global and mesoscale meteorological reanalyses to provide a quantitative atmospheric characterization of the Persian Gulf region during the period between 15 January and 15 March 1991. This paper presents a description of the mid- to late-winter synoptic conditions, mean statistical scores, and near-surface mean conditions of the Gulf War theater drawn from the 2-month reanalysis. The reanalysis is conducted with the U.S. Navy's operational global and mesoscale analysis and prediction systems: the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS*). The synoptic conditions for the 2-month period can be characterized as fairly typical for the northeast monsoon season, with only one significant precipitation event affecting the Persian Gulf region. A comparison of error statistics to those from other mes...

Various drag mechanisms are currently parametrized in numerical models of the atmosphere. For global models that include the middle atmosphere in particular, these mechanisms profoundly affect weather forecast as well as climate simulation.We have developed an extended-top version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) to include the middle atmosphere by modifying some physical parametrizations and the vertical coordinate. We performed a series of ensemble simulations corresponding to January 2000 for investigating the response of the model to various drag mechanisms, such as mountain drag, orographic gravity wave drag, surface friction drag, and artificial model top drag. Based on the monthly mean fields obtained from the simulations, we first investigate the effect of gravity wave drag due to its direct impact through planetary wave activity as well as indirect impact through induced meridional circulation.We discuss the difficulties in partitioning between the mountain drag due to resolved orography and the gravity wave drag due to unresolved orography, first using conventional diagnostic measures. From analyses of the atmospheric angular momentum budget, we show that various model drag mechanisms when modified interact with one another by redistributing their drag while conserving the total amount. In particular, an overestimation of mountain drag is accompanied by an underestimation of gravity wave drag in the Northern Hemisphere mid-latitudes to conserve the total amount of drag in the model while likely breaking an optimal balance among the mechanisms. Under such a condition, the inclusion of a gravity wave drag parametrization — even if the drag amount itself is reasonable — does not necessarily improve the performance of the model. Diagnosis of this type of imbalance is not clear by conventional monthly mean fields of variables. In this paper, we argue that the budget of atmospheric angular momentum is a useful measure to diagnose impact of such changes in model physics with regard to the partition and balance among drag mechanisms. We also discuss the experimental results that led to the replacement of silhouette orography by mean orography in our model.

An adjoint-based procedure for assessing the impact of observations on the short-range forecast error in numerical weather prediction is described. The method is computationally inexpensive and allows observation impact to be partitioned for any set or subset of observations, by instrument type, observed variable, geographic region, vertical level or other category. The cost function is the difference between measures of 24-h and 30-h global forecast error in the Navy Operational Global Atmospheric Prediction System (NOGAPS) during June and December 2002. Observations are assimilated at 00UTC in the Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS). The largest error reductions in the Northern Hemisphere are produced by rawinsondes, satellite wind data, and aircraft observations. In the Southern Hemisphere, the largest error reductions are produced by Advanced TIROS Operational Vertical Sounder (ATOVS) temperature retrievals, satellite wind data and rawinsondes. Approximately 60% (40%) of global observation impact is attributed to observations below (above) 500 hPa. A significant correlation is found between observation impact and cloud cover at the observation location. Currently, without consideration of moisture observations and moist processes in the forecast model adjoint, the observation impact procedure accounts for about 75% of the actual reduction in 24-h forecast error.

Abstract Forty-six percent of Atlantic tropical storms undergo a process of extratropical transition (ET) in which the storm evolves from a tropical cyclone to a baroclinic system. In this paper, the structural evolution of a base set of 61 Atlantic tropical cyclones that underwent extratropical transition between 1979 and 1993 is examined. Objective indicators for the onset and completion of transition are empirically determined using National Hurricane Center (NHC) best-track data, ECMWF 1.125° × 1.125° reanalyses, and operational NCEP Aviation Model (AVN) and U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) numerical analyses. An independent set of storms from 1998 to 2001 are used to provide a preliminary evaluation of the proposed onset and completion diagnostics. Extratropical transition onset is declared when the storm becomes consistently asymmetric, as measured by the 900–600-hPa thickness asymmetry centered on the storm track. Completion of the ET process is identified using a...

Abstract An objectively defined three-dimensional cyclone phase space is proposed and explored. Cyclone phase is described using the parameters of storm-motion-relative thickness asymmetry (symmetric/nonfrontal versus asymmetric/frontal) and vertical derivative of horizontal height gradient (cold- versus warm-core structure via the thermal wind relationship). A cyclone's life cycle can be analyzed within this phase space, providing substantial insight into the cyclone structural evolution. An objective classification of cyclone phase is possible, unifying the basic structural description of tropical, extratropical, and hybrid cyclones into a continuum. Stereotypical symmetric warm-core (tropical cyclone) and asymmetric cold-core (extratropical cyclone) life cycles are illustrated using 1° Navy Operational Global Atmospheric Prediction System (NOGAPS) operational analyses and 2.5° NCEP–NCAR reanalyses. The transitions between cyclone phases are clearly illustrated within the phase space, including extratro...

Using currently available operational forecast datasets on the tracks and intensities of tropical cyclones over the Pacific Ocean for the years 1998, 1999, and 2000 a multimodel superensemble has been constructed following the earlier work of the authors on the Atlantic hurricanes. The models included here include forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers for Environmental Prediction/Environmental Modeling Center [NCEP/EMC, the Aviation (AVN) and Medium-Range Forecast (MRF) Models], the U.S. Navy [Naval Operational Global Atmospheric Prediction System, (NOGAPS)], the U.K. Met Office (UKMO), and the Japan Meteorological Agency (JMA). The superensemble methodology includes a collective bias estimation from a training phase in which a multiple-regression-based least squares minimization principle for the model forecasts with respect to the observed measures is employed. This is quite different from a simple bias correction, whereby a mean value is simply shifted. These bias estimates are described by separate weights at every 12 h during the forecasts for each of the member models. Superensemble forecasts for track and intensity are then constructed up to 144 h into the future using these weights. Some 100 past forecasts of tropical cyclone days are used to define the training phase for each basin. The findings presented herein show a marked improvement for the tracks and intensities of forecasts from the proposed multimodel superensemble as compared to the forecasts from member models and the ensemble mean. This note includes detailed statistics on the Pacific Ocean tropical cyclone forecasts for the years 1998, 1999, and 2000.

Infrasonic waves can propagate thousands of kilometers in range and sample regions of the atmosphere from the ground up to and including the thermosphere. Conventional infrasound propagation modeling techniques rely on climatological models of mean temperatures and winds to characterize the environment. However, temperature and wind vary over temporal and spatial scales that are not captured by climatological models. Recent work addresses the integration of infrasound propagation models, such as three-dimensional ray tracing, with numerical weather prediction models, such as the Navy Operational Global Atmospheric Prediction System (NOGAPS). Propagation results are computed using both climatological and updated atmospheric characterizations, and comparisons are presented. Implications for global infrasound monitoring are discussed. [Work supported by the Defense Threat Reduction Agency.]

Abstract A set of criteria is developed to identify tropical cyclone (TC) formations in the Navy Operational Global Atmospheric Prediction System (NOGAPS) analyses and forecast fields. Then the NOGAPS forecasts of TC formations from 1997 to 1999 are verified relative to a formation time defined to be the first warning issued by the Joint Typhoon Warning Center. During these three years, the spatial distributions of TC formations were strongly affected by an El Nino–Southern Oscillation event. The successful NOGAPS predictions of formation within a maximum separation threshold of 4° latitude are about 70%–80% for 24-h forecasts, and drop to about 20%–30% for 120-h forecasts. The success rate is higher for formations in the South China Sea and between 160°E and 180° but is generally lower between 120° and 160°E. The composite 850-hPa large-scale flow for the formations between 120° and 160°E is similar to a monsoon confluence region with marked cross-equatorial flow. Therefore, it is concluded that the skil...

Abstract Subtropical boundary layer clouds have a fundamental role on the radiative budget of the atmosphere and on the modulation of the tropical circulations. The development of realistic parameterizations of these clouds in global atmospheric models is a major challenge. Unfortunately, this has been a difficult problem to solve in an acceptable way. In this paper, new and simple parameterization schemes for subtropical clouds are implemented in the U.S. Navy Operational Global Atmospheric Prediction System's (NOGAPS) forecast model. The parameterizations are partially based on large eddy simulation (LES) results and provide a substantial improvement when compared with observations and with the previous scheme. The global distribution of boundary layer clouds and of surface shortwave radiation is more realistic with this new scheme, particularly over the stratocumulus regions. The transition from stratocumulus to cumulus is well captured, the seasonal and diurnal cycles of stratocumulus are realistic, a...

Abstract The method of statistical analysis of wind innovation (observation minus forecast) vectors is extended and applied to the innovation data collected over North America for a 3-month period from the Navy Operational Global Atmospheric Prediction System to estimate the height–wind forecast error correlation and to evaluate the related geostrophy. Both single-level and multilevel analyses are performed. The single-level analysis shows that the geostrophy is well satisfied in the middle troposphere but is not well satisfied in the boundary layer and around the tropopause. The multilevel analysis indicates that the cross correlation between height and tangential wind forecast errors at different vertical levels is not small and thus should not be neglected.

The statistical analysis of innovation (observation minus forecast) vectors is one of the most commonly used techniques for estimating observation and forecast error covariances in large-scale data assimilation. Building on the work of Hollingsworth and Lonnberg, the height innovation data over North America from the Navy Operational Global Atmospheric Prediction System (NOGAPS) are analyzed. The major products of the analysis include (i) observation error variances and vertical correlation functions, (ii) forecast error autocovariances as functions of height and horizontal distance, (iii) their spectra as functions of height and horizontal wavenumber. Applying a multilevel least squares fitting method, which is simpler and more rigorously constrained than that of Hollingsworth and Lonnberg, a full-space covariance function was determined. It was found that removal of the large-scale horizontal component, which has only small variation in the vertical, reduces the nonseparability. The results were compared with those of Hollingsworth and Lonnberg, and show a 20% overall reduction in forecast errors and a 10% overall reduction in observation errors for the NOGAPS data in comparison with the ECMWF global model data 16 yr ago.

Abstract The sensitivity of 2-day Northern Hemisphere extratropical forecast errors to changes in initial conditions, computed daily over a 4-yr period, is documented. The sensitivity is computed using the (dry) adjoint of the Navy Operational Global Atmospheric Prediction System. This diagnostic enables an assessment of where initial errors have had the largest impact on forecast errors and allows for an examination of interannual variations in predictability. Both the forecast error and sensitivity exhibit a large seasonal cycle, with distinct maxima in winter. The monthly mean sensitivity maxima are correlated with regions of baroclinic instability and occur upstream from the forecast error maxima. Interannual variability is also apparent, and results show that the El Nino winter of 1997/98 was anomalously predictable. In contrast, the recent La Nina winters have been relatively unpredictable, especially during January 2000. These sensitivity calculations highlight the significant impact of middle–lowe...

Abstract During the North Pacific Experiment (NORPEX), both the Navy Operational Global Atmospheric Prediction System and the National Centers for Environmental Prediction (NCEP) operational forecast systems found a 48-h forecast degradation over the NORPEX forecast verification region due to the inclusion of a set of NORPEX targeted dropsondes deployed north of Hawaii during 29–30 January 1998. The NCEP three- and four-dimensional varitional data assimilation (3DVAR and 4DVAR) systems are used here to reassess the impact of these dropsonde observations on model predictions. The assimilation of these targeted dropsondes excluding the conventional observations improved the 48-h forecast over the NORPEX forecast verification region. However, the addition of the dropsonde data to an analysis that already contained various conventional observations degraded the 48-h forecast over the NORPEX forecast verification region. In the later case, the dropsonde data still improved and had its largest impact on the for...

As part of the Alaska synthetic aperture radar (SAR) Demonstration Project in 1999 and 2000, wide-swath RADARSAT SAR imagery has been acquired on a regular basis in the Gulf of Alaska and the Bering Sea. During 1998 and 1999, similar data were acquired off the East Coast of the United States as part of the StormWatch Project. The radar cross section measurements from these images were combined with wind direction estimates from the Navy Operational Global Atmospheric Prediction System model to produce high-resolution maps of the surface wind speed. For this study, 2862 SAR image frames were collected and examined. Averaged wind estimates from this data base have been systematically compared with corresponding wind speed estimates from buoy measurements and model predictions, and very good agreement has been found. The standard deviation between the buoy wind speed and the SAR estimates is 1.76 m/s. Details of the SAR wind extraction procedure are discussed, along with implications of the comparisons on the C-band polarization ratio.

Abstract All highly erroneous (>300 n mi or 555 km at 72 h) Navy Operational Global Atmospheric Prediction System (NOGAPS) and U.S. Navy version of the Geophysical Fluid Dynamics Laboratory model (GFDN) tropical cyclone track forecasts in the western North Pacific during 1997 are examined. Error mechanisms that are more related to midlatitude circulations are described in this paper and those errors that predominantly occur while the tropical cyclone (TC) is still in the Tropics are addressed in a companion paper. Responsible error mechanisms are described by conceptual models that are all related to known tropical cyclone motion processes that are being misrepresented in the dynamical models. As in the companion paper, characteristics and symptoms in the forecast tracks and model fields that accompany these frequently recurring error mechanisms are documented and illustrative case studies are presented. Whereas 21 GFDN forecasts were degraded by an improper prediction of a midlatitude system evolution, o...

This article gives the details and results of an investigation into the properties of the temperature and relative humidity errors from the Navy Operational Global Atmospheric Prediction System for a 4-month period from March to June 1998. The spatial covariance data for temperature errors and for relative humidity errors were fit using eight different approximation functions/weighting methods. From these, two were chosen as giving good estimates of the parameters and variances of the prediction and observation errors and were used in further investigations. The vertical correlation between temperature errors at different levels and relative humidity errors at different levels was approximated using a combination of functional fitting and transformation of the pressure levels. The cross covariance between temperature and relative humidity errors at various pressure levels were approximated in two ways: 1) by directly computing and approximating the cross-covariance data, and 2) by approximating variance of the difference of normalized data. The latter led to more consistent results. Figures illustrating the results are included.

Abstract All highly erroneous (>300 n mi or 555 km at 72 h) Navy Operational Global Atmospheric Prediction System (NOGAPS) and U.S. Navy version of the Geophysical Fluid Dynamics Laboratory model (GFDN) tropical cyclone track forecasts in the western North Pacific during 1997 are examined. Responsible error mechanisms are described by conceptual models that are all related to known tropical cyclone motion processes that are being misrepresented in the dynamical models. Error mechanisms that predominantly occur while the tropical cyclone is still in the Tropics are described in this paper, and those errors that are more related to midlatitude circulations are addressed in a companion paper. Of the 69 NOGAPS large-error cases, 39 were attributed to excessive direct cyclone interaction (E-DCI), 12 cases of excessive ridge modification by the tropical cyclone (E-RMT), and 10 cases of excessive reverse trough formation (E-RTF). Of the 50 GFDN large-error cases, 31 were E-DCI, and only two E-RMT and two E-RTF c...

High-density geostationary satellite wind observations have become an important new contributor to the observing network over oceanic regions. During the 1998 North Pacific Experiment (NORPEX), assimilation of these data in the Navy Operational Global Atmospheric Prediction System (NOGAPS) provided substantial improvements in 48-h forecast skill over the northeast Pacific and western North America. The current study shows that the large positive impact of the geostationary satellite winds results mainly from the reduction of analysis errors that project onto the leading singular vectors derived from the linearized forecast model. These errors account for only a small fraction of the total analysis error and, during NORPEX, were confined mostly to the middle and lower troposphere with maxima over the central Pacific. These errors do not necessarily coincide with the locations of the largest analysis errors. Experiments in which the satellite information is retained only at prescribed vertical levels in the analysis confirm that the increments in the middle and lower troposphere account for most of the forecast impact. Implications for the design of future observing systems, including strategies for targeted observing, are discussed. It is argued that the results support the key underlying principles of targeted observing, namely, that the early stages of error growth in most numerical weather forecasts are dominated by a relatively small number of unstable structures, and that preferentially reducing analysis errors that project onto these structures can produce significant improvements in forecast skill.

The U.S. Air Force has a long history of investment in cloud analysis and prediction operations. Their need for accurate cloud cover information has resulted in routine production of global cloud analyses (from their RTNEPH analysis model) and forecasts (using their ADVCLD cloud trajectory forecast model) over many years. With the advancement of global numerical weather prediction technology and resulting forecast accuracy of noncloud meteorological quantities, it is of interest to determine if such technology could be used to benefit cloud cover forecasting. In this paper, a model output statistics approach to diagnosing cloud cover from forecast fields generated by a global numerical weather prediction model is presented. Cloud characteristics information obtained from the RTNEPH cloud analysis supplied the cloud predictands, and forecast fields from the U.S. Navy Operational Global Atmospheric Prediction System global weather prediction model provided the weather variable predictors. RTNEPH layer cloud cover was assigned to three cloud decks (high, middle, and low) based on reported cloud-base altitude, and RTNEPH total cloud cover was used as a separate predictand. Multiple discriminant analysis (MDA) was used to develop the predictand–predictor relationships for each cloud deck and total cloud using 5 days of twice-daily cloud analyses and corresponding forecasts for 30° latitude zones. The consequent relationships were applied to the forecasts fields from the forecast initialized on the day following each 5-day development period to diagnose cloud cover forecasts for the Northern or Southern Hemisphere. In this study, cloud cover forecasts were diagnosed from global NWP model forecasts on hemispheric polar stereographic map projections with a grid spacing of 96 km. The diagnosed cloud forecasts (DCFs) were verified against the RTNEPH analyses for forecast durations of 12–72 h at 12-h intervals. Also verified were 12–48-h cloud cover forecasts (deck and total) from the ADVCLD cloud trajectory model, and from persistence (RTNEPH at initial forecast time). Biases from all three methods were generally small. The DCFs were significantly better than ADVCLD and persistence in all decks and total cloud, at almost all forecast durations in rmse and 20/20 score. ADVCLD scored better in these measures only at 12 h in total cloud, suggesting the possibility of a crossover in superior prediction skill from trajectory to diagnostic method somewhere between 12 and 24 h. DCF better preserved the cloud cover frequency distribution than did ADVCLD. ADVCLD displayed a greater degree of spatial variation inherent in RTNEPH cloud cover than did DCF. Both ADVCLD and DCF visual depictions of hemispheric total cloud cover appeared to provide useful cloud cover forecast information when compared with RTNEPH depictions. The advantages of the diagnosed cloud forecast algorithm presented in this study make it an excellent candidate for operational cloud cover prediction. It is expected that as cloud cover analyses are improved, the trajectory and diagnostic methods will prove complementary with the former more skillful at short-term predictions, and the latter better at long-term forecasts.

Abstract Extratropical transition (ET) in the western North Pacific is defined here in terms of two stages: transformation, in which the tropical cyclone evolves into a baroclinic storm; and reintensification, where the transformed storm then deepens as an extratropical cyclone. In this study, 30 ET cases occurring during 1 June–31 October 1994–98 are reviewed using Navy Operational Global Atmospheric Prediction System analyses; hourly geostationary visible, infrared, and water vapor imagery; and microwave imagery. A brief climatology based on these cases is presented for the transformation stage and the subsequent cyclone characteristics of the reintensification stage. A three-dimensional conceptual model of the transformation stage of ET in the western North Pacific Ocean is proposed that describes how virtually all 30 cases evolved into an incipient, baroclinic low. The three-step evolution of the transformation of Typhoon (TY) David (September 1997) is described as a prototypical example. Four importa...