Showing papers in "Monthly Weather Review in 1998"
TL;DR: In this article, the authors proposed an ensemble Kalman filter for data assimilation using the flow-dependent statistics calculated from an ensemble of short-range forecasts (a technique referred to as Ensemble Kalman filtering) in an idealized environment.
Abstract: The possibility of performing data assimilation using the flow-dependent statistics calculated from an ensemble of short-range forecasts (a technique referred to as ensemble Kalman filtering) is examined in an idealized environment. Using a three-level, quasigeostrophic, T21 model and simulated observations, experiments are performed in a perfect-model context. By using forward interpolation operators from the model state to the observations, the ensemble Kalman filter is able to utilize nonconventional observations. In order to maintain a representative spread between the ensemble members and avoid a problem of inbreeding, a pair of ensemble Kalman filters is configured so that the assimilation of data using one ensemble of shortrange forecasts as background fields employs the weights calculated from the other ensemble of short-range forecasts. This configuration is found to work well: the spread between the ensemble members resembles the difference between the ensemble mean and the true state, except in the case of the smallest ensembles. A series of 30-day data assimilation cycles is performed using ensembles of different sizes. The results indicate that (i) as the size of the ensembles increases, correlations are estimated more accurately and the root-meansquare analysis error decreases, as expected, and (ii) ensembles having on the order of 100 members are sufficient to accurately describe local anisotropic, baroclinic correlation structures. Due to the difficulty of accurately estimating the small correlations associated with remote observations, a cutoff radius beyond which observations are not used, is implemented. It is found that (a) for a given ensemble size there is an optimal value of this cutoff radius, and (b) the optimal cutoff radius increases as the ensemble size increases.
1,827 citations
TL;DR: In this article, it is shown that the observations must be treated as random variables at the analysis steps, which results in a completely consistent approach if the covariance of the ensemble of model states is interpreted as the prediction error covariance, and there are no further requirements on the ensemble Kalman filter method.
Abstract: This paper discusses an important issue related to the implementation and interpretation of the analysis scheme in the ensemble Kalman filter. It is shown that the observations must be treated as random variables at the analysis steps. That is, one should add random perturbations with the correct statistics to the observations and generate an ensemble of observations that then is used in updating the ensemble of model states. Traditionally, this has not been done in previous applications of the ensemble Kalman filter and, as will be shown, this has resulted in an updated ensemble with a variance that is too low. This simple modification of the analysis scheme results in a completely consistent approach if the covariance of the ensemble of model states is interpreted as the prediction error covariance, and there are no further requirements on the ensemble Kalman filter method, except for the use of an ensemble of sufficient size. Thus, there is a unique correspondence between the error statistics from the ensemble Kalman filter and the standard Kalman filter approach.
1,801 citations
TL;DR: An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada.
Abstract: An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada. Part I of this two-part paper motivates the development of the new system, summarizes various considerations taken into its design, and describes its main characteristics.
1,011 citations
TL;DR: In this paper, a new algorithm is applied to study water vapor fluxes in the troposphere using wind and moisture data from the European Centre for Medium-Range Weather Forecasts.
Abstract: A new algorithm is applied to study water vapor fluxes in the troposphere using wind and moisture data from the European Centre for Medium-Range Weather Forecasts. The fluxes are divided into filamentary structures known as tropospheric rivers and what are termed here broad fields. The results show that the tropospheric rivers may carry essentially the total meridional transport observed in the extratropical atmosphere but may occupy only about 10% of the total longitudinal length at a given latitude. The transient fluxes in traditional studies do not catch the filamentary structures completely and may therefore underestimate the fraction of transport assigned to moving systems, as well as omitting the geographical concentration. The mean flow and eddy fluxes evaluated by the new algorithm are considered to be more physically realistic.
823 citations
TL;DR: In this article, the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989 were studied, and the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes was paid to the diagnosis.
Abstract: Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May, then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs. Fina...
563 citations
TL;DR: In this paper, an integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada.
Abstract: An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada. Part II of this two-part paper presents the objective and subjective evaluations of the intercomparison process that led to the operational implementation of the new Global Environmental Multiscale model. The results of a “proof of concept” experiment and those of a meso-γ-scale simulation further demonstrate the validity and versatility of this model.
535 citations
TL;DR: The use of cloud-cleared radiances became the operational technique for using satellite sounding data at the National Centers for Environmental Prediction (NCEP) in October 1995 as discussed by the authors.
Abstract: With improved assimilation techniques, it is now possible to directly assimilate cloud-cleared radiances, rather than temperature and moisture retrievals, in objective analyses. The direct use of the cloud-cleared radiances became the operational technique for using satellite sounding data at the National Centers for Environmental Prediction (NCEP) in October 1995. The methodology for using the data (including bias correction, ozone analysis, skin temperature analysis, and quality control) are described in this paper. The impact of the direct use of the radiances compared to the previously operational use of satellite sounding data shows considerable improvement in NCEP’s forecast skill, especially in the Southern Hemisphere. It is anticipated that additional positive impacts will occur with application of the technique to other remotely sensed data.
498 citations
TL;DR: In this article, atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multi-year global reanalyses produced by NCEP-NCAR and NASA-DAO.
Abstract: Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation. A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase...
371 citations
TL;DR: An improved forecast system has been developed for El Nino-Southern Oscillation (ENSO) prediction at the National Centers for Environmental Prediction as mentioned in this paper, incorporating vertical variation in the first-guess error variance.
Abstract: An improved forecast system has been developed for El Nino–Southern Oscillation (ENSO) prediction at the National Centers for Environmental Prediction. Improvements have been made both to the ocean data assimilation system and to the coupled ocean–atmosphere forecast model. In Part I of a two-part paper the authors describe the new assimilation system. The important changes are 1) the incorporation of vertical variation in the first-guess error variance that concentrates temperature corrections in the thermocline and 2) the overall reduction in the magnitude of the estimated first-guess error. The new system was used to produce a set of retrospective ocean analyses for 1980–95. The new analyses are less noisy than their earlier counterparts and compare more favorably with independent measurements of temperature, currents, and sea surface height variability. Part II of this work presents the results of using these analyses to initialize the coupled forecast model for ENSO prediction.
362 citations
TL;DR: In this paper, the authors develop and test computational methods for advection of a scalar field that also include a minimal dissipation of its variance in order to preclude the formation of false extrema.
Abstract: The authors develop and test computational methods for advection of a scalar field that also include a minimal dissipation of its variance in order to preclude the formation of false extrema. Both of these properties are desirable for advectively dominated geophysical flows, where the relevant scalars are both potential vorticity and material concentrations. These methods are based upon the sequential application of two types of operators: 1) a conservative and nondissipative (i.e., preserving first and second spatial moments of the scalar field), directionally symmetric advection operator with a relatively high order of spatial accuracy; and 2) a locally adaptive correction operator of lower spatial accuracy that eliminates false extrema and causes dissipation. During this correction phase the provisional distribution of the advected quantity is checked against the previous distribution, in order to detect places where the previous values were overshot, and thus to compute the excess. Then an iterative diffusion procedure is applied to the excess field in order to achieve approximate monotone behavior of the solution. In addition to the traditional simple flow tests, we have made long-term simulations of freely evolving twodimensional turbulent flow in order to compare the performance of the proposed technique with that of previously known algorithms, such as UTOPIA and FCT. This is done for both advection of vorticity and passive scalar. Unlike the simple test flows, the turbulent flow provides nonlinear cascades of quadratic moments of the advected quantities toward small scales, which eventually cannot be resolved on the fixed grid and therefore must be dissipated. Thus, not only the ability of the schemes to produce accurate shape-preserving advection, but also their ability to simulate subgrid-scale dissipation are being compared. It is demonstrated that locally adaptive algorithms designed to avoid oscillatory behavior in the vicinity of steep gradients of the advected scalars may result in overall less dissipation, yet give a locally accurate and physically meaningful solution, whereas algorithms with built-in hyperdiffusion (i.e., those traditionally used for direct simulation of turbulent flows) tend to produce a locally unsufficient and, at the same time, globally excessive amount of dissipation. Finally, the authors assess the practial trade-offs required for large models among the competing attributes of accuracy, extrema preservation, minimal dissipation (e.g., appropriate to large Reynolds numbers), and computational cost.
343 citations
TL;DR: In this paper, the evolution of the morphological and radiative properties of clouds over the life cycles of deep convective systems (CS) over the Americas at both tropical and middle latitudes was studied using GOES-7 ISCCP-B3 satellite data.
Abstract: Using GOES-7 ISCCP-B3 satellite data for 1987–88, the authors studied the evolution of the morphological and radiative properties of clouds over the life cycles of deep convective systems (CS) over the Americas at both tropical and middle latitudes. A deep convective cloud system is identified by adjacent satellite image pixels with infrared brightness temperatures, TIR < 245 K (−28°C), that at some time contain embedded convective clusters that are defined by pixel values of TIR < 218 K (−55°C). The first part of the analysis computes parameters for each convective system that describe the system areal size, number of convective clusters, fractional convective area, average and maximum size of the convective clusters, shape eccentricity and orientation, mean TIR, variance of TIR, TIR gradient, and the mean, variance, and gradient of collocated visible reflectances (when available). The second part of the analysis searches a 5° × 5° region centered on each convective system, but in the subsequent...
TL;DR: The Physical Space Statistical Analysis System (PSAS) as mentioned in this paper is a global analysis system that operates directly in physical space and employs error covariance models identical to those of the predecessor OI system, as well as more advanced models.
Abstract: Conventional optimal interpolation (OI) analysis systems solve the standard statistical analysis equations approximately, by invoking a local approximation and a data selection procedure. Although solution of the analysis equations is essentially exact in the recent generation of global spectral variational analysis systems, these new systems also include substantial changes in error covariance modeling, making it difficult to discern whether improvements in analysis and forecast quality are due to exact, global solution of the analysis equations, or to changes in error covariance modeling. The formulation and implementation of a new type of global analysis system at the Data Assimilation Office, termed the Physical-space Statistical Analysis System (PSAS), is described in this article. Since this system operates directly in physical space, it is capable of employing error covariance models identical to those of the predecessor OI system, as well as more advanced models. To focus strictly on the effect of global versus local solution of the analysis equations, a comparison between PSAS and OI analyses is carried out with both systems using identical error covariance models and identical data. Spectral decomposition of the analysis increments reveals that, relative to the PSAS increments, the OI increments have too little power at large horizontal scales and excessive power at small horizontal scales. The OI increments also display an unrealistically large ratio of divergence to vorticity. Dynamical imbalances in the OI-analyzed state can therefore be attributed in part to the approximate local method of solution, and are not entirely due to the simple geostrophic constraint built into the forecast error covariance model. Root-mean-square observation minus 6-h forecast errors in the zonal wind component are substantially smaller for the PSAS system than for the OI system.
TL;DR: The accuracy of short-range probabilistic forecasts of quantitative precipitation from the experimental Eta-Regional Spectral Model ensemble is compared with the accuracy of forecasts from the Nested Grid Model's model output statistics (MOS) over a set of 13 case days from September 1995 through January 1996.
Abstract: The accuracy of short-range probabilistic forecasts of quantitative precipitation (PQPF) from the experimental Eta‐Regional Spectral Model ensemble is compared with the accuracy of forecasts from the Nested Grid Model’s model output statistics (MOS) over a set of 13 case days from September 1995 through January 1996. Ensembles adjusted to compensate for deficiencies noted in prior forecasts were found to be more skillful than MOS for all precipitation categories except the basic probability of measurable precipitation. Gamma distributions fit to the corrected ensemble probability distributions provided an additional small improvement. Interestingly, despite the favorable comparison with MOS forecasts, this ensemble configuration showed no ability to ‘‘forecast the forecast skill’’ of precipitation—that is, the ensemble was not able to forecast the variable specificity of the ensemble probability distribution from day-to-day and location-to-location. Probability forecasts from gamma distributions developed as a function of the ensemble mean alone were as skillful at PQPF as forecasts from distributions whose specificity varied with the spread of the ensemble. Since forecasters desire information on forecast uncertainty from the ensemble, these results suggest that future ensemble configurations should be checked carefully for their presumed ability to forecast uncertainty.
TL;DR: In this paper, the authors examined the interannual variation in tropical cyclone genesis frequency over the western North Pacific during 1979-94 and found that a distinct increase (decrease) occurs north of the climatological location of the monsoon trough in the Philippine Sea during summers with anomalous cold sea surface temperature (SST) over the NINO3 region.
Abstract: The interannual variation in tropical cyclone genesis frequency over the western North Pacific was examined for the active tropical cyclone (including summer and fall) during 1979–94. An emphasis was put on the possible effect of the interannual variation of atmospheric circulation and monsoon trough on tropical cyclone occurrence. The major findings of this study are the following. A distinct increase (decrease) of tropical cyclone genesis frequency occurs north of the climatological location of the monsoon trough in the Philippine Sea during summers (June–August) with anomalous cold (warm) sea surface temperature (SST) over the NINO3 region. The interannual variation of tropical cyclone genesis in this region results from the appearance of an anomalous cyclonic (anticyclonic) cell situated in a summer teleconnection wave train emanating from the western tropical Pacific and progressing along the rim of the North Pacific. In addition to the north–south interannual variation, there is also a long...
TL;DR: In this article, a meteorological mesoscale model (BOLAM) was used to simulate and test the intensity and timing of the most intense rain over Piedmont in northwestern Italy.
Abstract: The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM). The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband...
TL;DR: In this article, the authors evaluated the global tropopause pressure for the period between 1979 and 1993, based on gridded data as provided by the ECMWF reanalysis project.
Abstract: Statistics of the global tropopause pressure are evaluated for the period between 1979 and 1993. The analysis is based on gridded data as provided by the ECMWF reanalysis project. The thermal and dynamical definitions of the tropopause are applied in order to derive the surface separating the troposphere from the stratosphere. The dynamical tropopause is evaluated for threshhold values of potential vorticity of ±1.6, ±2.5, and ±3.5 potential vorticity units. Positive (negative) values are applied for the Northern (Southern) Hemisphere. The thermal tropopause is determined using the lapse-rate criteria. Frequency distributions of the tropopause pressure are shown above continental, ocean, polar, midlatitude, and tropical areas. Trends in the tropopause pressure are evaluated and compared with observed trends in total ozone.
TL;DR: In this paper, the authors investigate the relationship between spread and skill in an operational setting by analyzing ensemble predictions produced by the National Centers for Environmental Prediction and find that the geographical dependence of the spread-skill relationship is related to the geographical dependences of day-to-day variability of spread.
Abstract: Statistical considerations suggest that 1) even for a perfect ensemble (one in which all sources of forecast error are sampled correctly) there need not be a high correlation between spread and skill, 2) the correlation between spread and skill should be larger where the day-to-day variability of spread is large, and 3) the spread is likely to be most useful as a predictor of skill when it is “extreme,” that is, when it is either very large or very small compared to its climatological mean value. The authors investigate the relationship between spread and skill in an operational setting by analyzing ensemble predictions produced by the National Centers for Environmental Prediction. The geographical dependence of the spread–skill relationship is found to be related to the geographical dependence of day-to-day variability of spread. Dynamical mechanisms for spread variability are investigated using a linear quasigeostrophic model. Problems associated with the sample size needed to define what const...
TL;DR: In this article, the impact of ensemble size on the performance of the European Centre for Medium-Range Weather Forecasts ensemble prediction system (EPS) is analyzed for a period of 45 days from 1 October to 15 November 1996.
Abstract: The impact of ensemble size on the performance of the European Centre for Medium-Range Weather Forecasts ensemble prediction system (EPS) is analyzed. The skill of ensembles generated using 2, 4, 8, 16, and 32 perturbed ensemble members are compared for a period of 45 days—from 1 October to 15 November 1996. For each ensemble configuration, the skill is compared with the potential skill, measured by randomly choosing one of the 32 ensemble members as verification (idealized ensemble). Results are based on the analyses of the prediction of the 500-hPa geopotential height field. Various measures of performance are applied: skill of the ensemble mean, spread–skill relationship, skill of most accurate ensemble member, Brier score, ranked probability score, relative operating characteristic, and the outlier statistic. The relation between ensemble spread and control error is studied using L2, L8, and L∞ norms to measure distances between ensemble members and the control forecast or the verification. I...
TL;DR: In this paper, the authors examined observational evidence of a positive feedback between the land surface and rainfall in semiarid conditions, investigating interactions between soil moisture patterns and deep convection at scales of less than 20 km.
Abstract: This paper examines observational evidence of a positive feedback between the land surface and rainfall in semiarid conditions. The novelty of the work lies in the length scale of study, investigating interactions between soil moisture patterns and deep convection at scales of less than 20 km. The feedback mechanism was proposed in a previous study to explain the development of an anomalous rainfall gradient in the West African Sahel. The aim here is to assess whether such rainfall persistence occurs elsewhere in the region. Convective-scale rainfall patterns are examined using two years of observations from a dense rain gauge network in southwest Niger. Rainfall differences are analyzed between neighboring gauges separated by 7.5–15 km. Under certain surface conditions, a positive correlation between daily and antecedent rainfall differences is established. These circumstances arise when previous storm patterns have modified local evaporation rates. Rainfall gradients in subsequent events tend t...
TL;DR: In this paper, a precipitation physics package for the National Centers for Environmental Prediction Regional Spectral Model designed to improve the skill of precipitation forecasts is proposed, which incorporates a prognostic grid-resolvable precipitation scheme and a parameterized convection scheme with a convective trigger function that explicitly couples boundary layer and convective precipitation processes.
Abstract: A precipitation physics package for the National Centers for Environmental Prediction Regional Spectral Model designed to improve the skill of precipitation forecasts is proposed. The package incorporates a prognostic grid-resolvable precipitation scheme and a parameterized convection scheme with a convective trigger function that explicitly couples boundary layer and convective precipitation processes. Comprehensive sensitivity experiments were conducted with a grid spacing of approximately 25 km for a heavy rain case over the United States during 15–17 May 1995. In this paper, the trigger function setup in the convective parameterization scheme and its impact on the predicted precipitation are discussed. Special attention is given to the interaction of cloud properties in the parameterized convection with the evolution of grid-resolvable precipitation physics. The impact of convective forcing due to different convective triggers on the large-scale pattern downstream is also discussed. The imple...
TL;DR: The Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Prediction System was adopted by the U.S. National Weather Service as an operational hurricane prediction model in the 1995 hurricane season as mentioned in this paper.
Abstract: The Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Prediction System was adopted by the U.S. National Weather Service as an operational hurricane prediction model in the 1995 hurricane season. The framework of the prediction model is described with emphasis on its unique features. The model uses a multiply nested movable mesh system to depict the interior structure of tropical cyclones. For cumulus parameterization, a soft moist convective adjustment scheme is used. The model initial condition is defined through a method of vortex replacement. It involves generation of a realistic hurricane vortex by a scheme of controlled spinup. Time integration of the model is carried out by a two-step iterative method that has a characteristic of frequency-selective damping. The outline of the prediction system is presented and the system performance in the 1995 hurricane season is briefly summarized. Both in the Atlantic and the eastern Pacific, the average track forecast errors are substantially red...
TL;DR: In this paper, the dispersion relation for topographic waves is accurately represented in discrete z-level models by allowing bottom-most grid cells to be partially filled with land, and a gently sloping bottom topography is resolved on the scale of horizontal grid resolution.
Abstract: Ocean simulations are in part determined by topographic waves with speeds and spatial scales dependent on bottom slope. By their very nature, discrete z-level ocean models have problems accurately representing bottom topography when slopes are less than the grid cell aspect ratio Dz/Dx. In such regions, the dispersion relation for topographic waves is inaccurate. However, bottom topography can be accurately represented in discrete zlevel models by allowing bottom-most grid cells to be partially filled with land. Consequently, gently sloping bottom topography is resolved on the scale of horizontal grid resolution and the dispersion relation for topographic waves is accurately approximated. In contrast to the standard approach using full cells, partial cells imply that all grid points within a vertical level are not necessarily at the same depth and problems arise with pressure gradient errors and the spurious diapycnal diffusion. However, both problems have been effectively dealt with. Differences in flow fields between simulations with full cells and partial cells can be significant, and simulations with partial cells are more robust than with full cells. Partial cells provide a superior representation of topographic waves when compared to the standard method employing full cells.
TL;DR: In this article, a compositing analysis of NCEP-NCAR reanalyzed meteorological fields and satellite-based outgoing long-wave radiation measurements was performed to study the mean synoptic-scale structure of midlatitude air incursions over South America.
Abstract: Transient incursions of midlatitude air to the east of the Andes Mountains into subtropical and tropical latitudes are a distinctive feature of the synoptic climatology over South America. The mean synoptic-scale structure of these incursions is documented in this paper on the basis of a compositing analysis of NCEP–NCAR reanalyzed meteorological fields and satellite-based outgoing longwave radiation measurements. Although these incursions are a year-round phenomenon, with relatively modest seasonal changes in their structure, this analysis is focused on the austral summer, when they have their largest impact in the precipitation field. The summertime incursions move equatorward at a mean speed of 10 m s−1 and retain their identities over intervals of about 5 days. The upper-level circulation is characterized by a midlatitude trough–ridge couplet that provides the quasigeostrophic forcing of the system. Before the onset of the incursions of midlatitude air, the approaching upper-level trough tend...
TL;DR: In the case of tropical cyclones, sea level pressures at the center are 50−100 hPa lower than outside the vortex, but only 10−30 hPa of the total pressure fall occurs inside the eye between the eyewall and the center.
Abstract: In intense tropical cyclones, sea level pressures at the center are 50‐100 hPa lower than outside the vortex, but only 10‐30 hPa of the total pressure fall occurs inside the eye between the eyewall and the center. Warming by dry subsidence accounts for this fraction of the total hydrostatic pressure fall. Convection in the eyewall causes the warming by doing work on the eye to force the thermally indirect subsidence. Soundings inside hurricane eyes show warm and dry air aloft, separated by an inversion from cloudy air below. Dewpoint depressions at the inversion level, typically 850‐500 hPa, are 10‐30 K rather than the ;100 K that would occur if the air descended from tropopause level without dilution by the surrounding cloud. The observed temperature and dewpoint distribution above the inversion can, however, be derived by ;100 hPa of undilute dry subsidence from an initial sounding that is somewhat more stable than a moist adiabat. It is hypothesized that the air above the inversion has remained in the eye since it was enclosed when the eyewall formed and that it has subsided at most a few kilometers. The cause of the subsidence is the enclosed air’s being drawn downward toward the inversion level as the air below it flows outward into the eyewall. Shrinkage of the eye’s volume is more than adequate to supply the volume lost as dry air is incorporated into the eyewall or converted to moist air by turbulent mixing across the eye boundary. The moist air below the inversion is in thermodynamic contact with the sea surface. Its moisture derives from evaporation of seawater inside the eye, frictional inflow of moist air under the eyewall, and from moist downdrafts induced as condensate mixes into the eye. The moist air’s residence time in the eye is much shorter than that of the dry air above the inversion. The height of the inversion is determined by the balance between evaporation, inflow, and inward mixing on one hand and loss to the eyewall updrafts on the other.
TL;DR: In this paper, the relationship between mid-tropospheric dryness and supercell morphology and evolution using a three-dimensional, non-hydrostatic cloud model was studied using a 3D model.
Abstract: This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so Downdraft convective available potential energy (DCAPE) is
TL;DR: In this article, the authors developed a precipitation physics package for the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) designed to improve the skill of precipitation forecasts by incorporating a prognostic grid-resolvable precipitation scheme and a subgrid-scale precipitation parameterization scheme with a convective trigger that explicitly couples boundary layer and convective precipitation processes.
Abstract: The purpose of this study is to develop a precipitation physics package for the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) designed to improve the skill of precipitation forecasts. The package incorporates a prognostic grid-resolvable precipitation scheme and a subgrid-scale precipitation parameterization scheme with a convective trigger that explicitly couples boundary layer and convective precipitation processes. In this paper, the implementation of a prognostic cloud scheme for the NCEP RSM is described. A subgrid-scale precipitation parameterization scheme was described in a companion paper. Dynamical processes such as advection and diffusion processes for liquid species are included. Eleven experiments are conducted with a grid spacing of approximately 25 km for a heavy rain case over the United States during 15‐17 May 1995. Special attention is given to the setup of the prognostic grid-resolvable precipitation scheme on a spectral grid as well as the importance of dynamical processes on a mesoscale grid together with radiation feedback. Different prognostic cloud schemes, classified according to the number of predicted liquid species, are also compared.
TL;DR: The use of potential vorticity (PV) allows the efficient description of the dynamics of nearly balanced atmospheric flow phenomena, but the distribution of PV must be simply represented for ease in interpretation as discussed by the authors.
Abstract: The use of potential vorticity (PV) allows the efficient description of the dynamics of nearly balanced atmospheric flow phenomena, but the distribution of PV must be simply represented for ease in interpretation. Representations of PV on isentropic or isobaric surfaces can be cumbersome, as analyses of several surfaces spanning the troposphere must be constructed to fully apprehend the complete PV distribution. Following a brief review of the relationship between PV and nearly balanced flows, it is demonstrated that the tropospheric PV has a simple distribution, and as a consequence, an analysis of potential temperature along the dynamic tropopause (here defined as a surface of constant PV) allows for a simple representation of the upper-tropospheric and lower-stratospheric PV. The construction and interpretation of these tropopause maps, which may be termed ‘‘isertelic’’ analyses of potential temperature, are described. In addition, techniques to construct dynamical representations of the lower-tropospheric PV and near-surface potential temperature, which complement these isertelic analyses, are also suggested. Case studies are presented to illustrate the utility of these techniques in diagnosing phenomena such as cyclogenesis, tropopause folds, the formation of an upper trough, and the effects of latent heat release on the upper and lower troposphere.
TL;DR: In this paper, a second-order Runge Kutta time integrator (RK2) for the large-time-step integration is combined with the forward-backward scheme in a manner similar to the Klemp and Wilhelmson method.
Abstract: A forward-in-time splitting method for integrating the elastic equations is presented. A second-order Runge‐ Kutta time integrator (RK2) for the large-time-step integration is combined with the forward‐backward scheme in a manner similar to the Klemp and Wilhelmson method. The new scheme produces fully second-order-accurate integrations for advection and gravity wave propagation. The RK2 scheme uses upwind discretizations for the advection terms and is easily combined with standard vertically semi-implicit techniques so as to improve computational efficiency when the grid aspect ratio becomes large. A stability analysis of the RK2 split-explicit scheme shows that it is stable for a wide range of advective and acoustic wave Courant numbers. The RK2 time-split scheme is used in a full-physics nonhydrostatic compressible cloud model. The implicit damping properties associated with the RK2’s third-order horizontal differencing allows for a significant reduction in the value of horizontal filtering applied to the momentum and pressure fields, while qualitatively the solutions appear to be better resolved than solutions from a leapfrog model.
TL;DR: The 1995 Atlantic hurricane season was a year of near-record hurricane activity with a total of 19 named storms (average is 9.3 for the base period 1950-90) and 11 hurricanes as discussed by the authors.
Abstract: The 1995 Atlantic hurricane season was a year of near-record hurricane activity with a total of 19 named storms (average is 9.3 for the base period 1950–90) and 11 hurricanes (average is 5.8), which persisted for a total of 121 named storm days (average is 46.6) and 60 hurricane days (average is 23.9), respectively. There were five intense (or major) Saffir–Simpson category 3, 4, or 5 hurricanes (average is 2.3 intense hurricanes) with 11.75 intense hurricane days (average is 4.7). The net tropical cyclone activity, based upon the combined values of named storms, hurricanes, intense hurricanes, and their days present, was 229% of the average. Additionally, 1995 saw the return of hurricane activity to the deep tropical latitudes: seven hurricanes developed south of 25°N (excluding all of the Gulf of Mexico) compared with just one during all of 1991–94. Interestingly, all seven storms that formed south of 20°N in August and September recurved to the northeast without making landfall in the United S...
TL;DR: In this paper, mesoscale-model simulations are used to examine the structure and dynamics of a gap-outflow event over the Gulf of Tehuantepec, Mexico, that was associated with a surge of cold air along the eastern slopes of the Sierra Madre.
Abstract: Mesoscale-model simulations are used to examine the structure and dynamics of a gap-outflow event over the Gulf of Tehuantepec, Mexico, that was associated with a surge of cold air along the eastern slopes of the Sierra Madre. The simulated gap-outflow winds emerged from Chivela Pass, reached a maximum speed of 25 m s−1, and turned anticyclonically as they fanned out over the gulf. Northerly winds were also able to ascend the mountains east, and to a lesser extent west, of Chivela Pass, indicating that the movement of cold air across the Sierra Madre was not confined to the pass. A mesoscale pressure ridge was aligned along the axis of the gap-outflow jet, which was flanked to the west by an anticyclonic eddy, and to the east by a weaker cyclonic eddy. A model-derived trajectory along the axis of the outflow jet traced an inertial path, with anticyclonic curvature produced primarily by the Coriolis acceleration. The cross-flow pressure-gradient acceleration along this trajectory was negligible be...