Showing papers in "Monthly Weather Review in 1995"

Three-Dimensional Kinematic and Microphysical Evolution of Florida Cumulonimbus. Part II: Frequency Distributions of Vertical Velocity, Reflectivity, and Differential Reflectivity

Abstract: High-resolution radar data collected in Florida during the Convection and Precipitation/Electrification Experiment are used to elucidate the microphysical and kinematic processes occurring during the transition of a multicellular storm from convective to stratiform stages. A statistical technique is employed to examine the evolving properties of the ensemble small-scale variability of radar reflectivity, vertical velocity, and differential reflectivity over the entire storm. Differential radar reflectivity data indicate that the precipitation at upper levels was nearly glaciated early in the storm's lifetime. Dual-Doppler radar data show that throughout the storm's lifetime both updrafts and down-drafts were present at all altitudes and that most of the volume of the radar echo contained vertical velocities incapable of supporting precipitation-size particles. Thus, the ensemble microphysical properties of the storm were increasingly dominated by particles falling in an environment of weak vertic...

The Seasonal Cycle over the Tropical Pacific in Coupled Ocean Atmosphere General Circulation Models

Abstract: The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model. The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others...

Feature Tracking on the Unit Sphere

Abstract: Techniques used in a previous study of the objective identification and tracking of meteorological features in model data are extended to the unit sphere. An alternative feature detection scheme is described based on cubic interpolation for the sphere and local maximization. The extension of the tracking technique, used in the previous study, to the unit sphere is described. An example of the application of these techniques to a global relative vorticity field from a model integration are presented and discussed.

On-line Estimation of Error Covariance Parameters for Atmospheric Data Assimilation

Abstract: A simple scheme is presented for on-line estimation of covariance parameters in statistical data assimilation systems. The scheme is based on a maximum-likelihood approach in which estimates are produced on the basis of a single batch of simultaneous observations. Simple-sample covariance estimation is reasonable as long as the number of available observations exceeds the number of tunable parameters by two or three orders of magnitude. Not much is known at present about model error associated with actual forecast systems. Our scheme can be used to estimate some important statistical model error parameters such as regionally averaged variances or characteristic correlation length scales. The advantage of the single-sample approach is that it does not rely on any assumptions about the temporal behavior of the covariance parameters: time-dependent parameter estimates can be continuously adjusted on the basis of current observations. This is of practical importance since it is likely to be the case that both model error and observation error strongly depend on the actual state of the atmosphere. The single-sample estimation scheme can be incorporated into any four-dimensional statistical data assimilation system that involves explicit calculation of forecast error covariances, including optimal interpolation (OI) and the simplified Kalman filter (SKF). The computational cost of the scheme is high but not prohibitive; on-line estimation of one or two covariance parameters in each analysis box of an operational bozed-OI system is currently feasible. A number of numerical experiments performed with an adaptive SKF and an adaptive version of OI, using a linear two-dimensional shallow-water model and artificially generated model error are described. The performance of the nonadaptive versions of these methods turns out to depend rather strongly on correct specification of model error parameters. These parameters are estimated under a variety of conditions, including uniformly distributed model error and time-dependent model error statistics.

Integration of the fully elastic equations cast in the hydrostatic pressure terrain-following coordinate in the framework of the ARPEGE/Aladin NWP system

Abstract: ARPEGE/Aladin is a limited-area 3D primitive equation model, which belongs to the integrated NWP ARPEGE/IFS system. Like its global counterpart, the limited-area version has a spectral representation of variables in the horizontal but uses double-Fourier series instead of the classical spherical harmonies, in the manner introduced by Machenhauer and Haugen. Following the suggestion of Laprise, a nonhydrostatic version of ARPEGE/Aladin has been developed using hydrostatic pressure as an independent variable. The dynamics employ the fully elastic Euler equations of motion, orography being introduced via a terrain-following hybrid coordinate. A semi-implicit scheme has been formulated to control both acoustic and gravity waves. The discrete linear operators appear to have the same form as in the hydrostatic dynamics, except an additional one representing the vertical part of the Laplacian operator. To keep an elegant elimination, it was necessary to modify the approximation of logarithmic thicknesse...

Improvements in the GFDL Hurricane Prediction System

Abstract: The hurricane model initialization scheme developed at GFDL was modified to improve the representation of the environmental fields in the initial condition. The filter domain defining the extent of the tropical cyclone in the global analysis is determined from the distribution of the low-level disturbance winds. The shape of the domain is generally not circular in order to minimize the removal of important nonhurricane features near the storm region. An optimum interpolation technique is used to determine the environmental fields within the filter domain. Outside of the domain, the environmental fields are identical to the original global analysis. The generation process of the realistic and model-compatible vortex has also undergone some minor modifications so that reasonable vortices are produced for various data conditions. The upgraded hurricane prediction system was tested for a number of cases and compared against the previous version and yielded an overall improvement in the forecasts of s...

Numerical Integration of the Shallow-Water Equations on a Twisted Icosahedral Grid. Part I: Basic Design and Results of Tests

Abstract: The streamfunction-velocity potential form of shallow-water equations, implemented on a spherical geodesic grid, offers an attractive solution to many of the problems associated with fluid-flow simulations in a spherical geometry. Here construction of a new type of spherical geodesic grid is outlined, and discretization of the equations is explained. The model is subjected to the NCAR suite of seven test cases for shallow-water models.

An Ocean Analysis System for Seasonal to Interannual Climate Studies

Abstract: A dynamical model-based ocean analysis system has been implemented at the National Meteorological Center (NMC). This is used to provide retrospective and routine weekly analyses for the Pacific and Atlantic Oceans. Retrospective analyses have been performed for the period mid-1982 to mid-1993. The analyses are used for diagnostics of past climatic variability, real-time climate monitoring, and as initial conditions for coupled multiseason forecasts. The assimilation system is based on optimal interpolation objective analysis solved using an equivalent variational formulation. Analysis errors are estimated by comparisons to independent datasets such as temperature data from moorings and sea level information from tide gauges. In the near equatorial zone rms errors in thermocline depth are of order of 6–15 m. Comparisons of sea level estimates from the reanalyses with the records from tide gauges indicate that the rms sea level errors for monthly analysis are of the order of 0.04–0.09 m. For the we...

Implementation of the Semi-Lagrangian Method in a High-Resolution Version of the ECMWF Forecast Model

Abstract: In this article the implementation of the semi-Lagrangian method in a high-resolution version of the ECMWF forecast model is examined. Novel aspects include the application of the semi-Lagrangian scheme to a global model using the ECMWF hybrid coordinate in the vertical and its use in a baroclinic spectral model in conjunction with a reduced Gaussian grid in the horizontal. The former Eulerian vorticity-divergence formulation is first converted into a momentum-equation formulation that is considerably more economical, thanks in part to the incorporation of Legendre transform efficiencies that were previously demonstrated for the shallow-water equations. The semi-Lagrangian formulation is presented in detail, together with a discussion of computational aspects that are relevant for executing the high-resolution model efficiently on a modestly parallel supercomputer. The impact of formulation changes is assessed via numerical experiments on a set of 12 independent cases. In particular it is shown t...

Environmental Conditions Associated with Amazonian Squall Lines: A Case Study

Abstract: The environmental conditions associated with squall lines (SL) that were observed during the period of 13 April–13 May 1987 (GTE/ABLE-2B) originating at the northern coast of South America and propagating over the Amazon Basin are documented. The SL observed on 5–7 May are examined in more detail. The SL days had in common a stronger and deeper low-level jet when compared with the days without SL. Two possible explanations are found for the intensification of the low-level jet: propagating easterly waves in the tropical Atlantic, which eventually reach Manaus, and localized heat sources in the western Amazon. Both were observed on 5–6 May. It is suggested that numerical simulations should be performed to unravel the relative importance of each large-scale mechanism.

An explicit cloud physics parameterization for operational numerical weather prediction

Abstract: In anticipation of computers that will be able to run weather forecasting models on very fine grids fast enough for real-time purposes, an algorithm for representing water phase change and precipitation processes was developed. The design criteria guiding this development are sufficiency (i.e., providing the required services), computational efficiency, and compatibility within four-dimensional data assimilation systems. The implication of the last criterion is that the model's moisture variables need to be consistent with observable or inferable cloud properties. The algorithm is compared with a well-documented research microphysics algorithm in terms of computing efficiency and agreement with observations. The weather forecasting model runs much faster using the new package instead of the research algorithm. The agreement between the new algorithm and the research algorithm is much better than the agreement between the observations and the results from either algorithm, which suggests that erro...

A Climatology of Cyclogenesis for the Southern Hemisphere

Abstract: This study surveys life cycle characteristics of cyclones in the Southern Hemisphere (SH). An objective technique for locating instances of cyclone formation, intensification, maturation, and decay is applied to a 7-year dataset of SH cyclone positions, obtained from an automated finding and tracking scheme based on twice-daily ECMWF analyses during 1980–86. Geostrophic vorticity ζg at 1000 hPa is used to locate cyclones and determine their intensity. Cyclones form and develop within middle latitudes and migrate eastward and poleward during their lives. Preferred genesis and cyclogenesis regions include eastern coasts of Australia and South America in winter, and leeward of the Andes and near the baroclinic zones associated with the principal upper-tropospheric jet streams year round. Over open water, winter cyclones tend to form and intensify near zones of strongest SST gradient. Rapid cyclogenesis is most frequent east of South America, southeast of Africa, south of Australia, and near New Zeal...

Distortion Representation of Forecast Errors

Abstract: Forecast error is decomposed into three components, termed displacement error, amplitude error, mid residual error, respectively. Displacement error measures how much of the forecast error can be accounted for by moving the forecast to best fit the analysis. Amplitude error measures how much of the forecast error can be accounted for by changing the amplitude of the displaced forecast to best fit the analysis. The combination of a displacement and an amplification is called a distortion. The part of the forecast error unaccounted for by the distortion is called the residual error. The distortion must be large scale, in line with the basic premise that forecast errors are best described by reference to large-scale meteorological features. A general mathematical formalism for defining distortions and decomposing forecast errors into distortion and residual errors is formulated. The distortion representation of forecast errors should prove useful for describing forecast skill and for representing th...

Convection Initiation Associated with a Sea-Breeze Front, a Gust Front, and Their Collision

Abstract: The initiation of convection associated with a sea-breeze front, a gust front, and their collision is analyzed using data collected in east-central Florida during the Convection and Precipitation/Electrification project. In conjunction with satellite, surface, and rawinsonde information, dual-Doppler radar-derived winds are used to determine the three-dimensional kinematic factors critical to storm development. The gust front, which emanated from storms on the western half of the peninsula, propagated more rapidly and was deeper than the sea-breeze front, which originated from the east coast and was characterized by a distinctly scalloped appearance. Convection associated with the sea-breeze front appeared to develop preferentially at the vertices of this scalloped pattern where there were enhanced regions of convergence and upward motion. On the gust front, a Helmholtz shearing instability produced an organized configuration of convergence and updraft maxima along its length. However, these were...

Large-Scale Circulation Variability over the Tropical Western North Pacific. Part I: Spatial Patterns and Tropical Cyclone Characteristics

Abstract: The basic structure of the variability of the large-scale circulations over the tropical western Pacific is investigated with respect to its influence on tropical cyclone characteristics. A vector empirical orthogonal function analysis and fuzzy cluster algorithm are applied to a 9-yr dataset to define six recurrent 700-mb circulation patterns that represent large-scale variabilities associated with the monsoon trough and subtropical ridge. Five of the cluster patterns, which contain 48% of the sample, define combinations of active (inactive) monsoon trough and strong (weak) subtropical ridge circulations. The sixth cluster, which contains 26% of the data sample, depicts small deviations from the long-term climatology. After the cluster centers are defined, the fuzzy cluster coefficients are used to identify a seventh cluster, which contains the remaining 26% of the circulation patterns that could not be classified within any of the original six clusters. The 700-mb circulation patterns are physi...

An Observational Study of the South China Sea Monsoon during the 1979 Summer: Onset and Life Cycle

Abstract: The onset and life cycle of the 1979 South China Sea summer monsoon were examined in the context of the 30–60- and 12–24-day monsoon modes. The former intraseasonal mode formed the northward-migrating monsoon trough/ridge, while the latter intraseasonal mode propagated westward in the South China Sea. The monsoon in this region exhibited three cycles over the summer (May–August), with the onset taking place about one cycle ahead of the onset of the Indian and Japanese monsoons. Climatologically, a summer trough line radiated out from the Indian monsoon trough region, across Indochina, to the northern South China Sea. The monsoon onset occurred when the 30–60-day monsoon trough and the 12–24-day low center arrived simultaneously at the northern South China Sea, close to the climatological summer trough line, in the middle of May. The breaks occurred when the 30–60-day monsoon ridge lines and the 12–24-day high center met near 15°–20°N in the northern South China Sea. The South China Sea monsoon wa...

Numerical Integration of the Shallow-Water Equations on a Twisted Icosahedral Grid. Part II. A Detailed Description of the Grid and an Analysis of Numerical Accuracy

Abstract: The finite-difference scheme for the Laplace and flux-divergence operators described in the companion paper (Part I) is consistent when applied on a grid consisting of perfect hexagons. The authors describe a necessary and sufficient condition for this finite-difference scheme to be consistent when applied on a grid consisting of imperfect hexagons and pentagons, and present an algorithm for generating a spherical geodesic grid on a sphere that guarantees that this condition is satisfied. Also, the authors qualitatively describe the error associated with the operators and estimate their order of accuracy when applied on the new grid.

Methods for Ensemble Prediction

Abstract: It is desirable to filter the unpredictable components from a medium-range forecast. Such a filtered forecast can be obtained by averaging an ensemble of predictions that started from slightly different initial atmospheric states. Different strategies have been proposed to generate the initial perturbations for such an ensemble. “Optimal” perturbation give the largest error at a prespecified forecast time. “Bred” perturbations have grown during a period prior to the analysis. “OSSE-MC” perturbations are obtained using a Monte Carlo-like observation system simulation experiment (OSSE). In the current pilot study, the properties of the different strategies are compared. A three-level quasigeostrophic model is used to describe the evolution of the errors. The tangent linear version of this model and its adjoint version are used to generate the optimal perturbations, while bred perturbations are generated using the full nonlinear model. In the OSSE-MC method, random perturbations of model states are ...

Observations of the Sea-Breeze Front during CaPE. Part II: Dual-Doppler and Aircraft Analysis

Abstract: The three-dimensional kinematic structures of offshore and onshore flow sea-breeze fronts observed during the CaPE experiment are shown using high resolution dual-Doppler and aircraft data. The fronts interact with horizontal convective rolls (HCRs) that develop within the convective boundary layer. Nearly perpendicular intersections between the HCRs and sea-breeze front were observed during the offshore flow case. Close to the front, the HCR axes were tilted upward and lifted by the frontal updrafts. Consequently, a deeper updraft was created at the intersection points, providing additional impetus for cloud development. Furthermore, clouds forming at periodic intervals along the NCRs intensified as they propagated over the front. During the onshore flow case, the HCR orientation was nearly parallel to the front. Extended sections of the front “merged” with the HCRs. This process strengthened the front and is explained as the merger of like-sign vortices associated with both the front and HCRs. ...

A Satellite View of the Synoptic-Scale Organization of Cloud Properties in Midlatitude and Tropical Circulation Systems

Abstract: The spatial and temporal variability of various cloud types and cloud optical thickness are investigated using daily global analyses produced by the International Satellite Cloud Climatology Project (ISCCP) for the 1983–90 period. The climatological patterns of the relative abundance of individual cloud types are closely related to prevalent circulation regimes. Composite charts at various time lags relative to selected cloudy episodes are used to describe the representative shape and propagation of the local cloud patterns. These satellite-based findings are consistent with published results obtained from analyses of geopotential height and wind data. The midlatitude baroclinic cyclones along the wintertime storm tracks over the North and South Atlantic, and the summertime synoptic-scale disturbances occurring over the tropical western Pacific, are selected for more in-depth investigation. The variations of different cloud types in the selected sites are examined in conjunction with the concurre...

Postlandfall Tropical Cyclone Reintensification in a Weakly Baroclinic Environment: A Case Study of Hurricane David (September 1979)

Abstract: An analysis is conducted from a potential vorticity (PV) perspective of the reintensification of Tropical Storm David over the northeastern United States in September 1979. David, a major long-lived hurricane, originated near the Cape Verde Islands in late August 1979. It made final landfall in Georgia on 4 September 1979 and weakened rapidly thereafter. The noteworthy aspect of David was its subsequent reintensification approximately 27 h after landfall as a warm-core disturbance in a weakly baroclinic environment. In this regard the redevelopment of David is unlike the classical extratropical transformation of a tropical storm in a strongly baroclinic environment that has been documented in the literature. The authors' analysis of the evolution of the dynamical tropopause subsequent to storm landfall revealed that David reintensified in response to “tropopause lifting” (upward displacement of the dynamic tropopause) ahead of a nondeepening and otherwise very weak upper-tropospheric disturbance....

Three-Dimensional Kinematic and Microphysical Evolution of Florida Cumulonimbus. Part I: Spatial Distribution of Updrafts, Downdrafts, and Precipitation

Abstract: This paper is the first in a three-part study that examines the kinematic and microphysical evolution of Florida cumulonimbus and focuses on the convective-to-stratiform transition of the storm. This first paper lays the groundwork for the subsequent papers by defining the problem under study, delineating the setting for the storm, and describing the spatial distribution of updrafts, downdrafts, and precipitation. High-resolution radar data of a typical line of storms associated with the Florida sea breeze is the centerpiece of this study. The high-resolution data reveal details of the internal structure of the squall line that were beyond the resolution of previous squall-line studies. Radar reflectivity filled in between cells at upper levels as the storm evolved. Reflectivity values were only weakly associated with updraft and downdraft magnitude. The updrafts and downdrafts in the storm tended to be irregular in their three-dimensional shape and less than 5 km in horizontal extent. At any giv...

Four-dimensional variational assimilation of precipitation data

Abstract: The benefits of assimilation of precipitation data had been demonstrated in diabetic initialization and nudging-type experiments some years ago. In four-dimensional variational (4DVAR) data assimilation, however, the precipitation data have not yet been used. To correctly assimilate the precipitation data by the 4DVAR technique, the problems related to the first-order discontinuities in the “full-physics” forecast model should be solved first. To address this problem in the full-physics regional NMC eta forecast model, a modified, more continuous version of the Beta-Miller cumulus convection scheme is defined and examined as a possible solution. The 4DVAR data assimilation experiments ate performed using the conventional data (in this case, analyses of T, ps, u, v, and q) and the precipitation data (the analysis of 24-h accumulated precipitation). The full-physics NMC eta model and the adjoint model with convective processes are used in the experiments. The control variable of the minimization pr...

Assimilation of Atmospheric Radio Refractivity Using a Nonhydrostatic Adjoint Model

Abstract: Recently, a new approach to remote sensing of water vapor based on the Global Positioning System (GPS) has been proposed. Specifically, the bending of radio signals propagating from GPS satellites to a receiver on a low earth-orbiting satellite can be used to derive vertical profiles of atmospheric refractivity. Vertical profiles of temperature and water vapor can then be retrieved from the refractivity measurements. This is potentially a valuable data source for the meteorological community. However, before such measurements are used for operational numerical weather prediction, we need to assess the accuracy of the retrieved temperature and moisture fields and properly assimilate these observations into a numerical model. A 4D data assimilation system based on the adiabatic version of the Penn State-NCAR Mesoscale Model and its adjoint was developed. A series of observing system simulation experiments was then conducted to assess the impact of GPS-derived atmospheric refractivity data. Specific...

The Structure and Evolution of Cold Surges East of the Rocky Mountains

Abstract: Northerly surges of cold air often move southward along the eastern side of the Rockies from southern Canada into Mexico. The strongest surges, which generally develop in midwinter, are associated with temperature decreases and pressure rises of 20°–30°C and 15–30 mb, respectively, within 24 h. Surges are usually accompanied by a meridionally elongated pressure ridge and strong low-level ageostrophic winds that parallel the terrain. The width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters Mexico. This paper provides a detailed description of a northerly surge to the east of the Rocky Mountains that occurred on 12–14 November 1986. Using both observational and model data, the structural evolution of the surge is analyzed; in addition, the dynamics of the event is explored by diagnosing the momentum, thermodynamic energy, and vorticity equations. To determine the typical synoptic-scale evolution of these...

Simulation of the Mesocyclonic Activity in the Ross Sea, Antarctica

Abstract: The mesocyclonic activity in the southwestern Ross Sea is examined, with emphasis on its forcing by katabatic winds. The three-dimensional version of the meso-gamma-scale primitive equation model Modele Atmospherique Regional is used in which a representation of cloud microphysical processes has been introduced. Idealized boundary conditions are prescribed. In particular, the ocean is assumed to be completely ice-free or partially ice-covered. The former case corresponds to a fall situation that coincides with the climatological maximum of estimated precipitation at McMurdo Station on Ross Island. Due to the propagation of katabatic winds over the ocean, boundary layer fronts form. Clouds are generated in the fronts. A surface pressure trough also forms and extends northeastward from Terra Nova Bay. Mesocyclones are simulated in the trough. When the ocean is ice-free, strong sensible heat fluxes toward the atmosphere amplify the strength of the trough up to typically observed values. Tn this case, snow precipitation is associated with the boundary layer fronts and occurs in particular over McMurdo Sound. It is also found that latent heat release due to precipitation formation in the fronts does not significantly affect the deepening of the trough.

Monsoonal Interactions Leading to Sudden Tropical Cyclone Track Changes

Abstract: Sudden poleward track changes of tropical cyclones embedded in monsoon gyres in the western North Pacific are documented. During these track changes, which are generally not well forecast, the cyclones are often accompanied by a separate comma-shaped area of gale-force winds and deep convection along the eastern periphery. This monsoon surge is distinct from the tropical cyclone. Synoptic analyses often reveal a building anticyclone to the east or southeast of the monsoon gyre. The hypothesis that the sudden track change is initiated by a binary interaction of the tropical cyclone and monsoon gyre is tested with a nondivergent barotropic model. Tropical cyclone-scale vortices with initial positions within the eastern semicircle of a larger monsoon gyre-scale vortex initially coalesce with the monsoon gyre and then exhibit sudden poleward track changes that are similar to the observations. During the coalescence phase, the large and relatively weak monsoon gyre undergoes a β-induced dispersion in ...

Secondary Wind Maxima in Hurricanes: Airflow and Relationship to Rainbands

Abstract: Aircraft flight-level data from 787 radial legs in 20 hurricanes are analyzed to identify the composite kinematic structure in the hurricane eyewall, and especially with secondary horizontal wind maxima (SHWM) that occur outside the eyewall. Similar to previous studies, analysis of the flight-level wind data in the eyewall reveals radial convergence near the radius of maximum wind (RMW), and the highest frequency of updraft and the largest upward mass transport radially inward of the RMW. More than 20% of the flight legs contain substantial secondary horizontal wind maxima of specified strength and length. The kinematic structure associated with SHWM is similar to that of the hurricane eyewall with radial convergence near the radius of maximum wind and a preferred location for maximum upward motions and upward mass transport just inside the RMW. Statistical analysis confirms the similarity in characteristics between radial and vertical velocities of the eyewall and near the SHWM. In addition, for...

The Summertime Low-Level Jet over the Gulf of California

Abstract: Special pilot balloon and aircraft observations made during the 1990 Southwest Area Monsoon Project (SWAMP-90) are used to describe the structure of a low-level jet (LLJ) observed in the southerly flow over the Gulf of California and surroundings. Mean wind fields based on the 35 days of pilot balloon observations reveal an LLJ strongest over the northern Gulf of California, with southerly flow approximately parallel to the axis of the gulf. The diurnal variation of the mean wind fields is also shown; there is upslope flow, away from the gulf, in the afternoon and downslope flow, toward the gulf, in the morning. Over land, the LLJ is strongest at Yuma, Arizona, where it shows a strong diurnal variation. Here the morning maximum wind speeds in the jet reach 20 m s−1 and are strongest approximately 300–600 m above the surface. The low-level flow undergoes synoptic timescale fluctuations in intensity, but the jet is present at Yuma on about 75% of the mornings during SWAMP-90. SWAMP aircraft observa...

A Reduced-Gravity Isopycnal Ocean Model: Hindcasts of El Niño

Abstract: A global isopycnal ocean model is presented for the study of interannual to interdecadal variability in the global ocean. The model treats the primitive equations on a sphere with a generalized vertical coordinate. This coordinate is designed to represent a turbulent well-mixed surface layer and nearly isopycnal deeper layers. Disappearing isopycnics are treated through the quasi-isopycnal technique, in which the coordinate separates from the isopycnic in order to maintain a minimum layer thickness. A reduced gravity treatment is made, with the deepest interface at a mean depth of 2300 m. Coastal topography is represented, but the reduced gravity treatment precludes the use of variable bottom depth. The model is used for hindcast studies of El Nino during the decade from 1982 through 1991 using a combination of climatological wind forcing and wind anomalies derived from various sources. In order to carry out the hindcast experiments, a technique is developed for constructing a mean climatological...