Showing papers in "Monthly Weather Review in 2003"
TL;DR: In this article, a deterministic analysis ensemble updates are implemented in Kalman square root filters, and the nonuniqueness of the deterministic transformation used in square root Kalman filters provides a framework to compare ensemble data assimilation methods.
Abstract: Ensemble data assimilation methods assimilate observations using state-space estimation methods and low-rank representations of forecast and analysis error covariances. A key element of such methods is the transformation of the forecast ensemble into an analysis ensemble with appropriate statistics. This transformation may be performed stochastically by treating observations as random variables, or deterministically by requiring that the updated analysis perturbations satisfy the Kalman filter analysis error covariance equation. Deterministic analysis ensemble updates are implementations of Kalman square root filters. The nonuniqueness of the deterministic transformation used in square root Kalman filters provides a framework to compare three recently proposed ensemble data assimilation methods.
848 citations
TL;DR: In this paper, the authors provide a clear theoretical framework for the simulation of turbulent flows, and the source of the subgrid terms in the Navier-Stokes equation is clarified.
Abstract: The spatial resolution appropriate for the simulation of deep moist convection is addressed from a turbulence perspective. To provide a clear theoretical framework for the problem, techniques for simulating turbulent flows are reviewed, and the source of the subgrid terms in the Navier‐Stokes equation is clarified. For decades, cloud-resolving models have used large-eddy simulation (LES) techniques to parameterize the subgrid terms. A literature review suggests that the appropriateness of using traditional LES closures for this purpose has never been established. Furthermore, examination of the assumptions inherent in these closures suggests that grid spacing on the order of 100 m may be required for the performance of cloud models to be consistent with their design. Based on these arguments, numerical simulations of squall lines were conducted with grid spacings between 1 km and 125 m. The results reveal that simulations with 1-km grid spacing do not produce equivalent squallline structure and evolution as compared to the higher-resolution simulations. Details of the simulated squall lines that change as resolution is increased include precipitation amount, system phase speed, cloud depth, static stability values, the size of thunderstorm cells, and the organizational mode of convective overturning (e.g., upright towers versus sloped plumes). It is argued that the ability of the higher-resolution runs to become turbulent leads directly to the differences in evolution. There appear to be no systematic trends in specific fields as resolution is increased. For example, mean vertical velocity and rainwater values increase in magnitude with increasing resolution in some environments, but decrease with increasing resolution in other environments. The statistical properties of the simulated squall lines are still not converged between the 250- and 125-m runs. Several possible explanations for the lack of convergence are offered. Nevertheless, it is clear that simulations with O(1 km) grid spacing should not be used as benchmark or control solutions for resolution sensitivity studies. The simulations also support the contention that a minimum grid spacing of O(100 m) is required for traditional LES closures to perform appropriately for their design. Specifically, only simulations with 250- and 125-m grid spacing resolve an inertial subrange. In contrast, the 1-km simulations do not even reproduce the correct magnitude or scale of the spectral kinetic energy maximum. Furthermore, the 1-km simulations contain an unacceptably large amount of subgrid turbulence kinetic energy, and do not adequately resolve turbulent fluxes of total water. A guide to resolution requirements for the operational and research communities is proposed. The proposal is based primarily on the intended use of the model output. Even though simulations with O(1 km) grid spacing display behavior that is unacceptable for the model design, it is argued that these simulations can still provide valuable information to operational forecasters. For the research community, O(100 m) grid spacing is recommended for most applications, because a modeling system that is well founded should be desired for most purposes.
762 citations
TL;DR: A parallel ocean and ice model (POIM) in generalized orthogonal curvilinear coordinates has been developed for global climate studies as mentioned in this paper, which couples the Parallel Ocean Program (POP) with a 12-category thickness and enthalpy distribution (TED) sea ice model.
Abstract: A parallel ocean and ice model (POIM) in generalized orthogonal curvilinear coordinates has been developed for global climate studies. The POIM couples the Parallel Ocean Program (POP) with a 12-category thickness and enthalpy distribution (TED) sea ice model. Although the POIM aims at modeling the global ocean and sea ice system, the focus of this study is on the presentation, implementation, and evaluation of the TED sea ice model in a generalized coordinate system. The TED sea ice model is a dynamic thermodynamic model that also explicitly simulates sea ice ridging. Using a viscous plastic rheology, the TED model is formulated such that all the metric terms in generalized curvilinear coordinates are retained. Following the POP's structure for parallel computation, the TED model is designed to be run on a variety of computer architectures: parallel, serial, or vector. When run on a computer cluster with 10 parallel processors, the parallel performance of the POIM is close to that of a correspon...
551 citations
TL;DR: In this article, the EnKF is used to estimate the covariances between observed variables and the state through an ensemble of forecasts to assimilate radar observations at convective scales.
Abstract: Assimilation of Doppler radar data into cloud models is an important obstacle to routine numerical weather prediction for convective-scale motions; the difficulty lies in initializing fields of wind, temperature, moisture, and condensate given only observations of radial velocity and reflectivity from the radar. This paper investigates the potential of the ensemble Kalman filter (EnKF), which estimates the covariances between observed variables and the state through an ensemble of forecasts, to assimilate radar observations at convective scales. In the basic experiment, simulated observations are extracted from a reference simulation of a splitting supercell and assimilated using the EnKF and the same numerical model that produced the reference simulation. The EnKF produces accurate analyses, including the unobserved variables, after roughly 30 min (or six scans) of radial velocity observations. Additional experiments, in which forecasts are made from the ensemble-mean analysis, reveal that forecast errors grow significantly in this simple system, so that the ability of the EnKF to track the reference solution is not simply because of stable system dynamics. It is also found that the covariances between radial velocity and temperature, moisture, and condensate are important to the quality of the analyses, as is the initialization chosen for the ensemble members prior to assimilating the first observations. These results are promising, especially given the ease of implementing the EnKF. A number of important issues remain, however, including the initialization of the ensemble prior to the first observation, the treatment of uncertainty in the environmental sounding, the role of error in the forecast model (particularly the microphysical parameterizations), and the treatment of lateral boundary conditions.
461 citations
TL;DR: In this article, the authors constructed a composite of SST, wind, rainfall, and humidity for years of high rainfall during September, October, and November (SON) in equatorial and southern-central East Africa.
Abstract: Composites of SST, wind, rainfall, and humidity have been constructed for years of high rainfall during September, October, and November (SON) in equatorial and southern-central East Africa. These show that extreme East African short rains are associated with large-scale SST anomalies in the Indian Ocean that closely resemble those that develop during Indian Ocean dipole or zonal mode (IOZM) events. This is corroborated by the observation that strong IOZM events produce enhanced East African rainfall. However, it is also shown that the relationship between the IOZM and East African rainfall is nonlinear, with only IOZM events that reverse the zonal SST gradient for several months (extreme events) triggering high rainfall. Comparison of the wind anomalies that develop during extreme IOZM events with those that develop during weaker (moderate) events shows that strong easterly anomalies in the northern-central Indian Ocean are a persistent feature of extreme, but not of moderate, IOZM years. It is ...
448 citations
TL;DR: In this paper, the ensemble filter methods derived here make a (local) least squares assumption about the relation between prior distributions of an observation variable and model state variables, and the update procedure applied when a new observation becomes available can be described in two parts.
Abstract: Many methods using ensemble integrations of prediction models as integral parts of data assimilation have appeared in the atmospheric and oceanic literature. In general, these methods have been derived from the Kalman filter and have been known as ensemble Kalman filters. A more general class of methods including these ensemble Kalman filter methods is derived starting from the nonlinear filtering problem. When working in a joint state‐ observation space, many features of ensemble filtering algorithms are easier to derive and compare. The ensemble filter methods derived here make a (local) least squares assumption about the relation between prior distributions of an observation variable and model state variables. In this context, the update procedure applied when a new observation becomes available can be described in two parts. First, an update increment is computed for each prior ensemble estimate of the observation variable by applying a scalar ensemble filter. Second, a linear regression of the prior ensemble sample of each state variable on the observation variable is performed to compute update increments for each state variable ensemble member from corresponding observation variable increments. The regression can be applied globally or locally using Gaussian kernel methods. Several previously documented ensemble Kalman filter methods, the perturbed observation ensemble Kalman filter and ensemble adjustment Kalman filter, are developed in this context. Some new ensemble filters that extend beyond the Kalman filter context are also discussed. The two-part method can provide a computationally efficient implementation of ensemble filters and allows more straightforward comparison of methods since they differ only in the solution of a scalar filtering problem.
416 citations
TL;DR: In this paper, an objectively defined three-dimensional cyclone phase space is proposed and explored, where 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) are analyzed.
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...
402 citations
TL;DR: The inner-core upper-ocean environment for 23 Atlantic, Gulf of Mexico, and Caribbean hurricanes between 1975 and 2002 was analyzed using airborne expendable bathythermograph (AXBT) observations and buoy-derived archived SST data as discussed by the authors.
Abstract: Scientists at NOAA’s Hurricane Research Division recently analyzed the inner-core upper-ocean environment for 23 Atlantic, Gulf of Mexico, and Caribbean hurricanes between 1975 and 2002. The interstorm variability of sea surface temperature (SST) change between the hurricane inner-core environment and the ambient ocean environment ahead of the storm is documented using airborne expendable bathythermograph (AXBT) observations and buoy-derived archived SST data. The authors demonstrate that differences between inner-core and ambient SST are much less than poststorm, ‘‘cold wake’’ SST reductions typically observed (i.e., ;08‐28C versus 48‐58C). These findings help define a realistic parameter space for storm-induced SST change within the important high-wind inner-core hurricane environment. Results from a recent observational study yielded estimates of upper-ocean heat content, upper-ocean energy extracted by the storm, and upper-ocean energy utilization for a wide range of tropical systems. Results from this analysis show that, under most circumstances, the energy available to the tropical cyclone is at least an order of magnitude greater than the energy extracted by the storm. This study also highlights the significant impact that changes in inner-core SST have on the magnitude of air‐ sea fluxes under high-wind conditions. Results from this study illustrate that relatively modest changes in innercore SST (order 18C) can effectively alter maximum total enthalpy (sensible plus latent heat) flux by 40% or more. The magnitude of SST change (ambient minus inner core) was statistically linked to subsequent changes in storm intensity for the 23 hurricanes included in this research. These findings suggest a relationship between reduced inner-core SST cooling (i.e., increased inner-core surface enthalpy flux) and tropical cyclone intensification. Similar results were not found when changes in storm intensity were compared with ambient SST or upper-ocean heat content conditions ahead of the storm. Under certain circumstances, the variability associated with inner-core SST change appears to be an important factor directly linked to the intensity change process.
355 citations
TL;DR: In this article, a variational analysis of covariances is presented, where recursive filter techniques are applied to the synthesis of covariance structures with adaptive variations of amplitude, scale, profile shape, and degrees of local anisotropy.
Abstract: In this second part of a two-part study of recursive filter techniques applied to the synthesis of covariances in a variational analysis, methods by which non-Gaussian shapes and spatial inhomogeneities and anisotropies for the covariances may be introduced in a well-controlled way are examined. These methods permit an analysis scheme to possess covariance structures with adaptive variations of amplitude, scale, profile shape, and degrees of local anisotropy, all as functions of geographical location and altitude. First, it is shown how a wider and more useful variety of covariance shapes than just the Gaussian may be obtained by the positive superposition of Gaussian components of different scales, or by further combinations of these operators with the application of Laplacian operators in order for the products to possess negative sidelobes in their radial profiles. Then it is shown how the techniques of recursive filters may be generalized to admit the construction of covariances whose charact...
334 citations
TL;DR: In this article, the authors investigated the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall.
Abstract: Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate the storm between 0000
265 citations
TL;DR: In this paper, the authors used the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) and further diagnoses of the control run described in Part II, to explore the late night and morning convection over coastal waters throughout the Tropics.
Abstract: Afternoon/evening near-coastal convection over land is easily understood as a response to solar heating of the land, turbulent transfer of heat and moisture to the boundary layer, and lifting of air by vigorous sea-breeze fronts. Subtler processes apparently underlie the late night and morning convection that is prevalent over coastal waters throughout the Tropics. Sensitivity tests using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), and further diagnoses of the control run described in Part II, are used to explore these processes. Prior studies have speculated that “land-breeze” circulations, analogous but opposite to the sea breeze, drive offshore convection at night. However, nighttime radiative cooling of land and the associated thermal breezes are much weaker than the corresponding daytime processes, especially under humid tropical skies. Analysis of model mean soundings reveals that modest (fractions of a degree Celsius) temperature changes near the 800-hPa ...
TL;DR: The impact of irrigation on the surface energy budget in the U.S. high plains is investigated in this paper, where four 15-day simulations were conducted: one using a 1997 satellite-derived estimate of farmland acreage under irrigation in Nebraska (control run), two using the Olson Global Ecosystem (OGE) vegetation dataset (ogE wet run and OGE dry run), and the fourth with the Kuchler vegetation run (natural vegetation run) as lower boundary conditions in the Colorado State University Regional Atmospheric Modeling System (RAMS).
Abstract: The impact of irrigation on the surface energy budget in the U.S. high plains is investigated. Four 15-day simulations were conducted: one using a 1997 satellite-derived estimate of farmland acreage under irrigation in Nebraska (control run), two using the Olson Global Ecosystem (OGE) vegetation dataset (OGE wet run and OGE dry run), and the fourth with the Kuchler vegetation dataset (natural vegetation run) as lower boundary conditions in the Colorado State University Regional Atmospheric Modeling System (RAMS). In the control and OGE wet simulations, the topsoil in the irrigated locations, up to a depth of 0.2 m, was saturated at 0000 UTC each day for the duration of the experiment (1‐15 July 1997). In the other two runs, the soil was allowed to dry out, except when replenished naturally by rainfall. Identical observed atmospheric conditions were used along the lateral boundary in all four cases. The area-averaged model-derived quantities for the grid centered over Nebraska indicate significant differences in the surface energy fluxes between the control (irrigated) and the ‘‘dry’’ simulations. For example, a 36% increase in the surface latent heat flux and a 2.6 8C elevation in dewpoint temperature between the control run and the OGE dry run is shown. Surface sensible heat flux of the control run was 15% less and the near-ground temperature was 1.28C less compared to the OGE dry run. The differences between the control run and the natural vegetation run were similar but amplified compared to the control run‐OGE dry run comparisons. Results of statistical analyses of long-term (1921‐2000) surface temperature data from two sites representing locations of extensive irrigated and nonirrigated land uses appear to support model results presented herein of an irrigationrelated cooling in surface temperature. Growing season monthly mean and monthly mean maximum temperature data for the irrigated site indicate a steady decreasing trend in contrast to an increasing trend at the nonirrigated site.
TL;DR: The recent NCEP-NCAR Reanalysis-2 update as mentioned in this paper provides the highest quality analyses spanning two decades available for the high southern latitudes, and therefore offers an excellent starting point from which to assemble a modern, comprehensive, and reliable picture of synoptic activity in the subantarctic region.
Abstract: The recent NCEP–Department of Energy (DOE) Reanalysis-2 update of the original NCEP–NCAR dataset provides what is arguably the highest quality analyses spanning two decades available for the high southern latitudes. It therefore offers an excellent starting point from which to assemble a modern, comprehensive, and reliable picture of synoptic activity in the subantarctic region. This set, covering the “modern satellite” era from January 1979 to February 2000, is used herein. In addition, the exploration in this study has been conducted with sophisticated feature-tracking and trajectory analysis software. It is shown that the high southern latitude cyclone system density is greatest in the Indian Ocean and to the south of Australia near, or to the south of, 60°S. The numbers in winter exceed those in summer, except over a few, but important, regions such as the Bellingshausen Sea. The Antarctic coastal region is confirmed as one of high cyclonicity, as is that in the northern part of the Antarctic...
TL;DR: In this article, potential vorticity (PVV) and quasigeostrophic perspectives are utilized to better understand and quantify the relevant dynamics during the interaction of a tropical cyclone with a midlatitude trough.
Abstract: Several recent landfalling tropical cyclones (e.g., Dennis, Floyd, and Irene 1999) have highlighted a need for a refinement in the forecasting paradigms and techniques in the area of quantitative precipitation forecasting. Floyd proved to be a particularly challenging forecast problem as it was accompanied by catastrophic flooding over large regions of the East Coast, in spite of its relatively quick northward movement. The extent and intensity of the precipitation distribution was strongly modulated by the storm's interaction with a midlatitude trough. In an attempt to better understand and quantify the relevant dynamics during this interaction, potential vorticity (PV) and quasigeostrophic perspectives are utilized. As Floyd approached the East Coast, precipitation shifted to the left of the storm track due to the presence of a deep midlatitude trough in the Ohio valley. The juxtaposition of a cold-core PV anomaly associated with the midlatitude trough and a warm-core PV anomaly associated with...
TL;DR: In this article, the authors investigated the rainfall regimes of Taiwan using the 18-yr European Centre for Medium-Range Weather Forecasts (ECMWF) data (1980-97), the available 38-yr daily rainfall data from 25 conventional surface stations around Taiwan (1961-98), and the 5-yr hourly rainfall data (1994-98) from 249 high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System (ARMTS) stations.
Abstract: The rainfall regimes of Taiwan are investigated using the 18-yr European Centre for Medium-Range Weather Forecasts (ECMWF) data (1980–97), the available 38-yr daily rainfall data from 25 conventional surface stations around Taiwan (1961–98), and the 5-yr hourly rainfall data (1994–98) from 249 high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System (ARMTS) stations. Rainfall over the island is usually generated either by transient disturbances embedded in the prevailing monsoon flow or local rainshowers related to terrain or local winds. With the change in the direction of the prevailing winds between the warm and cold seasons as well as a variety of transient subsynoptic disturbances occurring in different seasons (e.g., winter monsoon cold surges, springtime cold fronts, mei-yu fronts in the early summer, typhoons in summer months, and cold fronts in fall) and the presence of the Central Mountain Range, the regional rainfall climate over the island shows large spatial and...
TL;DR: In this article, an iterative incremental approach is used to minimize a cost function that measures the statistically weighted squared differences between the observational information and their model equivalent, where the control variable of the minimization problem is an increment to the background estimate of the model initial conditions at the beginning of each assimilation window.
Abstract: Three- and four-dimensional variational assimilation (3DVAR and 4DVAR) systems have been developed for the Ocean Parallelise (OPA) ocean general circulation model (OGCM) of the Laboratoire d'Oceanographie Dynamique et de Climatologie. An iterative incremental approach is used to minimize a cost function that measures the statistically weighted squared differences between the observational information and their model equivalent. The control variable of the minimization problem is an increment to the background estimate of the model initial conditions at the beginning of each assimilation window. In 3DVAR, the increment is transported between observation times within the window using a persistence model, while in 4DVAR a dynamical model derived from the tangent linear (TL) of the OGCM is used. Both the persistence and TL models are shown to provide reasonably good descriptions of the evolution of typical errors over the 10- and 30-day widths of the assimilation windows used in the authors' 3DVAR an...
TL;DR: In this article, four recent reanalysis projects have been diagnosed at the scale of synoptic weather systems using an objective feature tracking method, and the tracking statistics indicate that, overall, the reanalyses correspond very well in the Northern Hemisphere (NH) lower troposphere, although differences for the spatial distribution of mean intensities show that the ECMWF reanalysis is systematically stronger in the main storm track regions but weaker around major orographic features.
Abstract: Data from four recent reanalysis projects [ECMWF, NCEP‐NCAR, NCEP‐Department of Energy (DOE), NASA] have been diagnosed at the scale of synoptic weather systems using an objective feature tracking method. The tracking statistics indicate that, overall, the reanalyses correspond very well in the Northern Hemisphere (NH) lower troposphere, although differences for the spatial distribution of mean intensities show that the ECMWF reanalysis is systematically stronger in the main storm track regions but weaker around major orographic features. A direct comparison of the track ensembles indicates a number of systems with a broad range of intensities that compare well among the reanalyses. In addition, a number of small-scale weak systems are found that have no correspondence among the reanalyses or that only correspond upon relaxing the matching criteria, indicating possible differences in location and/or temporal coherence. These are distributed throughout the storm tracks, particularly in the regions known for small-scale activity, such as secondary development regions and the Mediterranean. For the Southern Hemisphere (SH), agreement is found to be generally less consistent in the lower troposphere with significant differences in both track density and mean intensity. The systems that correspond between the various reanalyses are considerably reduced and those that do not match span a broad range of storm intensities. Relaxing the matching criteria indicates that there is a larger degree of uncertainty in both the location of systems and their intensities compared with the NH. At upper-tropospheric levels, significant differences in the level of activity occur between the ECMWF reanalysis and the other reanalyses in both the NH and SH winters. This occurs due to a lack of coherence in the apparent propagation of the systems in ERA15 and appears most acute above 500 hPa. This is probably due to the use of optimal interpolation data assimilation in ERA15. Also shown are results based on using the same techniques to diagnose the tropical easterly wave activity. Results indicate that the wave activity is sensitive not only to the resolution and assimilation methods used but also to the model formulation.
TL;DR: The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the south slopes of the Himalayas in central Nepal, where Radiosondes were launched around the clock from two sites, one in the Marsyandi River basin on the eastern footslopes of the Annapurna range, and one farther to the southwest near the border with India as mentioned in this paper.
Abstract: The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the south slopes of the Himalayas in central Nepal. Radiosondes were launched around the clock from two sites, one in the Marsyandi River basin on the eastern footslopes of the Annapurna range, and one farther to the southwest near the border with India. The flights supported rainfall and other hydrometeorological observations (including surface winds) from the Marsyandi network that has been operated in this region since the spring of 1999. The thermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall during the monsoon, with total column moisture and instability maximized just before rainfall peaks. Coinciding with the appearance of a monsoon depression over central India, the onset of the monsoon in this region was characterized by a weeklong weakening of the upper-level westerlies, and an increase in moisture and convective instability. The vertical structure of...
TL;DR: In this paper, the Madden-Julian oscillation (MJO) dominates tropical variability on time scales of 30-70 days and is manifested as an eastward propagating disturbance, with a strong convective signature over the Eastern Hemisphere.
Abstract: The Madden–Julian oscillation (MJO) dominates tropical variability on time scales of 30–70 days. During the boreal winter–spring it is manifested as an eastward propagating disturbance, with a strong convective signature over the Eastern Hemisphere. The space–time structure of the MJO is described using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis, Advanced Very High Resolution Radiometer outgoing longwave radiation, observed sea surface temperature, and the Climate Prediction Center Merged Analysis of Precipitation. Empirical orthogonal function analysis is used to identify the convective signature of the MJO, and regression is used to identify key relationships with the convection. Compared to analyzing successive years of data, the selection of years of strong MJO activity results in a more robust lead–lag structure and an increase in explained variance. The MJO exhibits a rich vertical structure, with low-level moisture convergence bein...
TL;DR: One of the rainiest areas on earth, the Panama Bight and Pacific (western) littoral of Colombia, is the focal point for a regional modeling study utilizing the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) with nested grids.
Abstract: One of the rainiest areas on earth, the Panama Bight and Pacific (western) littoral of Colombia, is the focal point for a regional modeling study utilizing the fifth-generation Pennsylvania State University‐NCAR Mesoscale Model (MM5) with nested grids. In this first of three parts, the observed climatology of the region is presented. The seasonal march of rainfall has a northwest‐southeast axis, with western Colombia near the center, receiving rain throughout the year. This study focuses on the August‐September season. The diurnal cycle of rainfall over land exhibits an afternoon maximum over most of South and Central America, typically composed of relatively small convective cloud systems. Over some large valleys in the Andes, and over Lake Maracaibo, a nocturnal maximum of rainfall is observed. A strong night/morning maximum of rainfall prevails over the coastal ocean, propagating offshore and westward with time. This offshore convection often takes the form of mesoscale convective systems with sizes comparable to the region’s coastal concavities and other geographical features. The 10-day period of these model studies (28 August‐7 September 1998) is shown to be a period of unusually active weather, but with a time-mean rainfall pattern similar to longer-term climatology. It is concluded that the rain-producing processes during this time period are likely to be typical of those that shape the seasonal climatology.
TL;DR: In this paper, the authors proposed a fundamental explanation of the genesis, structure, and implications of low-level, meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes.
Abstract: This two-part study proposes a fundamental explanation of the genesis, structure, and implications of lowlevel, meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs, especially when the environmental vertical wind shear is above a minimum threshold and when the Coriolis forcing is nonzero. As illustrated by a QLCS simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and, more notably, planetary vorticity is stretched vertically; hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure-gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line. In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with ‘‘straight line’’ surface winds found at the bow-echo apex and additionally in association with, and in fact induced by, the lowlevel mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or ‘‘upscale’’ vortex growth. In weaker environmental shear—in which significant low-level mesovortices tend not to form—damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.
TL;DR: In this paper, the authors proposed fundamental explanations of the genesis, structure, and implications of low-level meso-g-scale surface vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes.
Abstract: This two-part study proposes fundamental explanations of the genesis, structure, and implications of lowlevel meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized simulations are used herein to study the structure and evolution of meso- g-scale surface vortices within QLCSs and their dependence on the environmental vertical wind shear. Within such simulations, significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s 21 or greater over a 0‐2.5- or 0‐5-km-AGL layer. As similarly found in observations of QLCSs, these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger-scale bow-shaped system. They generally develop first near the surface but can build upward to 6‐8 km AGL. Vortex longevity can be several hours, far longer than individual convective cells within the QLCS; during this time, vortex merger and upscale growth is common. It is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme ‘‘straight line’’ wind damage, as has also been observed with some QLCSs. Surface vortices are also produced for weaker shears but remain shallow, weak, and short-lived. Although similar in size and strength to mesocyclones associated with supercell storms, and also sometimes producing similar hooklike structures in the rain field, it is also shown that the present vortices are quite distinct, structurally and dynamically. Most critically, such vortices are not associated with long-lived, rotating updrafts at midlevels and the associated strong, dynamically forced vertical accelerations, as occur within supercell mesocyclones.
TL;DR: In this article, the structural evolution of a base set of 61 Atlantic tropical cyclones that underwent extratropical transition between 1979 and 1993 is examined, and the onset and completion of the ET process are empirically determined using National Hurricane Center (NHC) best-track data, ECMWF 1.125° × 1125° reanalyses, and operational NCEP Aviation Model (AVN) and U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) numerical analyses.
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...
TL;DR: This paper examined whether these propagating signals are found in two numerical weather prediction (NWP) models commonly used today, namely, the Eta Model from the National Centers for Environmental Prediction and the newly developed Weather Research and Forecast (WRF) model.
Abstract: A recent study by Carbone et al. revealed “episodes” of warm-season rainfall over North America characterized as coherently propagating signals often linking multiple mesoscale convective systems over spatial scales of 1000–3000 km and timescales of 1–3 days. The present study examines whether these propagating signals are found in two numerical weather prediction (NWP) models commonly used today, namely, the Eta Model from the National Centers for Environmental Prediction and the newly developed Weather Research and Forecast (WRF) model. The authors find that the diurnal cycle of rainfall over much of the United States east of the Rockies is poorly represented, particularly over the central United States, where a nocturnal rainfall maximum is observed. Associated with this nocturnal maximum is an axis of propagating rainfall emanating from the western High Plains in the late afternoon, extending across the Midwest overnight, and occasionally continuing to the Appalachians on the second day. This...
TL;DR: In this article, the behavior of convective activity over the Bay of Bengal during the 1988 and 1999 monsoon seasons was examined using 3-hourly satellite infrared data, and the diurnal cycle of all the systems over the bay, regardless of size, shows a 0600 local time (LT) maximum in very cold cloud top (infrared brightness temperature <210 K), with genesis occurring between 2100 and 0300 LT (2100 LT for the larger, longer-lived systems).
Abstract: The behavior of convective activity over the Bay of Bengal during the 1988 and 1999 monsoon seasons is examined using 3-hourly satellite infrared data. More organized convective activity, spreading farther south into the bay, occurred in 1988 than in 1999. A distinct spatial grouping of convective systems by size is found. The east side of the bay experiences most of the rainfall over water, and here the convective systems are relatively small, short lived, and frequent. At the northwest side of the bay near most of the land-based rainfall, convective activity is organized into much larger and longer-lived systems. The diurnal cycle of all the systems over the bay, regardless of size, shows a 0600 local time (LT) maximum in very cold cloud tops (infrared brightness temperature <210 K), with genesis occurring between 2100 and 0300 LT (2100 LT for the larger, longer-lived systems). The cloud systems dissipate after sunrise, with the larger systems lasting until the afternoon. The land–water interfa...
TL;DR: In this article, the authors examined the relationship between the convection and circulation fields of the boreal summer intraseasonal oscillation (ISO) and two types of higher frequency tropical wave activity through a statistical analysis of 22 yr of data.
Abstract: Interactions between the convection and circulation fields of the boreal summer intraseasonal oscillation (ISO) and two types of higher-frequency tropical wave activity are examined through a statistical analysis of 22 yr of data. During the convectively active phase of the ISO, westward-propagating mixed Rossby–gravity (MRG)–tropical depression (TD)-type wave activity is enhanced within the low-frequency ISO convective envelope, and is strongly correlated with low-frequency 850-hPa westerly anomalies. At the same time, eastward-propagating convectively coupled Kelvin wave activity is enhanced well to the east of the active ISO convection, in the central Pacific. A case study of an ISO event during July–September 1987 illustrates these statistically derived relationships. The enhanced phase of the ISO is shown to consist primarily of westward-propagating higher-frequency variability, including seven named tropical cyclones in the western Pacific, two of which project onto MRG–TD-type modes as the...
TL;DR: In this paper, a variance-minimizing filter is introduced and its performance is demonstrated with the Korteweg-DeVries (KdV) equation and with a multilayer quasigeostrophic model of the ocean area around South Africa.
Abstract: A truly variance-minimizing filter is introduced and its performance is demonstrated with the Korteweg–DeVries (KdV) equation and with a multilayer quasigeostrophic model of the ocean area around South Africa. It is recalled that Kalman-like filters are not variance minimizing for nonlinear model dynamics and that four-dimensional variational data assimilation (4DVAR)-like methods relying on perfect model dynamics have difficulty with providing error estimates. The new method does not have these drawbacks. In fact, it combines advantages from both methods in that it does provide error estimates while automatically having balanced states after analysis, without extra computations. It is based on ensemble or Monte Carlo integrations to simulate the probability density of the model evolution. When observations are available, the so-called importance resampling algorithm is applied. From Bayes's theorem it follows that each ensemble member receives a new weight dependent on its “distance” to the obse...
TL;DR: In this article, a method for assimilating Lagrangian tracer positions, observed at discrete times, directly into the model is presented for point vortex flows: an NF point vortex system with a Gaussian noise term is modeled by its deterministic counterpart.
Abstract: Difficulties in the assimilation of Lagrangian data arise because the state of the prognostic model is generally described in terms of Eulerian variables computed on a fixed grid in space, as a result there is no direct connection between the model variables and Lagrangian observations that carry time-integrated information. A method is presented for assimilating Lagrangian tracer positions, observed at discrete times, directly into the model. The idea is to augment the model with tracer advection equations and to track the correlations between the flow and the tracers via the extended Kalman filter. The augmented model state vector includes tracer coordinates and is updated through the correlations to the observed tracers. The technique is tested for point vortex flows: an NF point vortex system with a Gaussian noise term is modeled by its deterministic counterpart. Positions of ND tracer particles are observed at regular time intervals and assimilated into the model. Numerical experiments demon...
TL;DR: In this article, an investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April-August) of 1996-1998 is presented.
Abstract: An investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April–August) of 1996–98 is presented. Circular and elongated MCSs on both the large and small scales were classified and analyzed in this study using satellite and radar data. The satellite classification scheme used for this study includes two previously defined categories and two new categories: mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso-β circular convective systems (MβCCSs), and meso-β elongated convective systems (MβECSs). Around two-thirds of the MCSs in the study fell into the larger satellite-defined categories (MCCs and PECSs). These larger systems produced more severe weather, generated much more precipitation, and reached a peak frequency earlier in the convective season than the smaller, meso-β systems. Overall, PECSs were found to be the dominant satellite-defined MCS, as they were the largest, most common, most severe, and most prolific ...
TL;DR: The first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MMS), the Regional Atmospheric Modeling System (RAMS), and the NCEP Meso-Eta, at horizontal resolution finer than 1 km was presented in this article.
Abstract: This study presents what is, to the authors' knowledge, the first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MMS), the Regional Atmospheric Modeling System (RAMS), and the NCEP Meso-Eta, at horizontal resolution finer than 1 km. Simulations were carried out for both weak and strong synoptic forcing cases during the Vertical Transport and Mixing (VTMX) field campaign conducted in the Salt Lake valley in October of 2000. Both upper-air and surface observations at high spatial and temporal resolution were used to evaluate the simulations with a focus on boundary layer structures and thermally driven circulations that developed in the valley. Despite differences in the coordinate systems, numerical algorithms, and physical parameterizations used by the three models, the types of forecast errors were surprisingly similar. The common errors in predicted valley temperature structure include...