Showing papers in "Monthly Weather Review in 1999"

A Monte Carlo Implementation of the Nonlinear Filtering Problem to Produce Ensemble Assimilations and Forecasts

Abstract: Knowledge of the probability distribution of initial conditions is central to almost all practical studies of predictability and to improvements in stochastic prediction of the atmosphere. Traditionally, data assimilation for atmospheric predictability or prediction experiments has attempted to find a single “best” estimate of the initial state. Additional information about the initial condition probability distribution is then obtained primarily through heuristic techniques that attempt to generate representative perturbations around the best estimate. However, a classical theory for generating an estimate of the complete probability distribution of an initial state given a set of observations exists. This nonlinear filtering theory can be applied to unify the data assimilation and ensemble generation problem and to produce superior estimates of the probability distribution of the initial state of the atmosphere (or ocean) on regional or global scales. A Monte Carlo implementation of the fully n...

First- and Second-Order Conservative Remapping Schemes for Grids in Spherical Coordinates

Abstract: Coupling atmosphere, ocean, sea ice, and land surface models requires a means for remapping fields between grids in an accurate and conservative manner. A method is described here for computing interpolation weights for first- and second-order conservative remappings. The method is completely general and can be used for any grid on a sphere.

A Semi-Implicit Semi-Lagrangian Regional Climate Model: The Canadian RCM

Abstract: A new regional climate model (RCM) is presented in this paper and its performance is investigated through a pair of 60-day simulations. This new model is based on the dynamical formulation of the Cooperative Centre for Research in Mesometeorology (CCRM) mesoscale nonhydrostatic community model and on the complete subgrid-scale physical parameterization package of the second-generation Canadian Centre for Climate modeling and analysis General Circulation Model (CCCma GCMII). The main feature of the Canadian RCM (CRCM) comes from the very efficient semi-implicit and semi-Lagrangian (SISL) numerical scheme used for the integration of the fully elastic nonhydrostatic Euler equations. The efficiency of the SISL scheme allows the use of longer time steps (at least by a factor of 5) for the integration of this model (e.g., the 45-km resolution version of the model uses a 15-min time step). A complete description of the numerical formulation of the model is presented with a review of the principal characteristics of the physical package. A pair of two-month-long winter simulations is also analyzed to investigate the behavior of the model and to evaluate the potential of the SISL integration scheme in the context of regional climate simulation. The two integrations, produced with a 45-km resolution version of the model, developed realistic small-scale details from the low-resolution GCMII fields used to initialize and drive the RCM.

Validation and Sensitivities of Frontal Clouds Simulated by the ECMWF Model

Abstract: Clouds simulated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system. Comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ECMWF model quite accurately simulates the general positioning of clouds relative to a low pressure center. However, the optical depths of the model’s high/low clouds are too small/large relative to the satellite observations, and the model lacks the midlevel topped clouds observed to the west of the surface cold front. Sensitivity studies with the ECMWF model reveal that the error in high-cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0.5625° to 1.69° lat. This reflects the fact that in the ECMWF model gravitational settling is the most rapid process controlling the abundance of ic...

Effects of Environmental Flow upon Tropical Cyclone Structure

Abstract: Numerical simulations of tropical-cyclone-like vortices are performed to analyze the effects of unidirectional vertical wind shear and translational flow upon the organization of convection within a hurricane’s core region and upon the intensity of the storm. A series of dry and moist simulations is performed using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 5 (MM5) with idealized initial conditions. The dry simulations are designed to determine the patterns of forced ascent that occur as the vortex responds to imposed vertical wind shear and translational flow, and the mechanisms that modulate the vertical velocity field are explored. The moist simulations are initialized with the same initial conditions as the dry runs but with a cumulus parameterization and explicit moisture scheme activated. The moist simulations are compared to the dry runs in order to test the hypothesis that the forced vertical circulation modes modulate the convection...

Adaptive Constraints for Feature Tracking

Abstract: In this paper extensions to an existing tracking algorithm are described. These extensions implement adaptive tracking constraints in the form of regional upper-bound displacements and an adaptive track smoothness constraint. Together, these constraints make the tracking algorithm more flexible than the original algorithm (which used fixed tracking parameters) and provide greater confidence in the tracking results. The result of applying the new algorithm to high-resolution ECMWF reanalysis data is shown as an example of its effectiveness.

Large-Scale Patterns Associated with Tropical Cyclogenesis in the Western Pacific

Abstract: Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail. A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in all cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of ...

The mean evolution and variability of the Asian summer monsoon: comparison of ECMWF and NCEP/NCAR reanalyses

Abstract: The behavior of the Asian summer monsoon is documented and compared using the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA) and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis. In terms of seasonal mean climatologies the results suggest that, in several respects, the ERA is superior to the NCEP–NCAR Reanalysis. The overall better simulation of the precipitation and hence the diabatic heating field over the monsoon domain in ERA means that the analyzed circulation is probably nearer reality. In terms of interannual variability, inconsistencies in the definition of weak and strong monsoon years based on typical monsoon indices such as All-India Rainfall (AIR) anomalies and the large-scale wind shear based dynamical monsoon index (DMI) still exist. Two dominant modes of interannual variability have been identified that together explain nearly 50% of the variance. Individually, they have many features in c...

Data Assimilation via Error Subspace Statistical Estimation.Part I: Theory and Schemes

Abstract: A rational approach is used to identify efficient schemes for data assimilation in nonlinear ocean‐atmosphere models. The conditional mean, a minimum of several cost functionals, is chosen for an optimal estimate. After stating the present goals and describing some of the existing schemes, the constraints and issues particular to ocean‐atmosphere data assimilation are emphasized. An approximation to the optimal criterion satisfying the goals and addressing the issues is obtained using heuristic characteristics of geophysical measurements and models. This leads to the notion of an evolving error subspace, of variable size, that spans and tracks the scales and processes where the dominant errors occur. The concept of error subspace statistical estimation (ESSE) is defined. In the present minimum error variance approach, the suboptimal criterion is based on a continued and energetically optimal reduction of the dimension of error covariance matrices. The evolving error subspace is characterized by error singular vectors and values, or in other words, the error principal components and coefficients. Schemes for filtering and smoothing via ESSE are derived. The data‐forecast melding minimizes variance in the error subspace. Nonlinear Monte Carlo forecasts integrate the error subspace in time. The smoothing is based on a statistical approximation approach. Comparisons with existing filtering and smoothing procedures are made. The theoretical and practical advantages of ESSE are discussed. The concepts introduced by the subspace approach are as useful as the practical benefits. The formalism forms a theoretical basis for the intercomparison of reduced dimension assimilation methods and for the validation of specific assumptions for tailored applications. The subspace approach is useful for a wide range of purposes, including nonlinear field and error forecasting, predictability and stability studies, objective analyses, data-driven simulations, model improvements, adaptive sampling, and parameter estimation.

A Variational Method for the Analysis of Three-Dimensional Wind Fields from Two Doppler Radars

Abstract: This paper proposes a new method of dual-Doppler radar analysis based on a variational approach. In it, a cost function, defined as the distance between the analysis and the observations at the data points, is minimized through a limited memory, quasi-Newton conjugate gradient algorithm with the mass continuity equation imposed as a weak constraint. The analysis is performed in Cartesian space. Compared with traditional methods, the variational method offers much more flexibility in its use of observational data and various constraints. Using the radar data directly at observation locations avoids an interpolation step, which is often a source of error, especially in the presence of data voids. In addition, using the mass continuity equation as a weak instead of strong constraint avoids the error accumulation and the subsequent somewhat arbitrary adjustment associated with the explicit vertical integration of the continuity equation. The current method is tested on both model-simulated and observed datasets of supercell storms. It is shown that the circulation inside and around the storms, including the strong updraft and associated downdraft, is well analyzed in both cases. Furthermore, the authors found that the analysis is not very sensitive to the specification of boundary conditions and to data contamination. The method also has the potential for retrieving, with reasonable accuracy, the wind in regions of single-Doppler radar coverage.

Multiscale Analysis of the Summertime Precipitation over the Central Andes

Abstract: Precipitation over the central Andes in South America exhibits a marked annual march, with most of the rainfall concentrated during the austral summer season (December‐February), when the atmospheric circulation favors the uplifting of moist air from the lowlands to the east of the mountain range. Within its rainy season, the central Andes experiences week-long rainy and dry episodes. The large-scale and local conditions during these episodes are investigated using satellite imagery, reanalyzed atmospheric fields, and in situ data. Despite the deep layer of conditional instability prevalent during most summertime afternoons, deep convection can occur only on those days in which the mixing ratio within the local boundary layer exceeds some threshold (; 7gk g 21), yielding saturation of near-surface air parcels rising more than 600 m above ground. Convective cloudiness anomalies over the central Andes extend southeastward and tend to be concurrent with anomalies of opposite sign over the eastern part of the continent. Rainy (dry) episodes are also associated with anticyclonic (cyclonic) anomalies centered over subtropical South America that extend through the depth of the troposphere, accompanied by easterly (westerly) wind anomalies over the central Andes. These anomalies are presumably forced by planetary waves originating in the Southern Hemisphere extratropics. To gain insight into the regional processes linking the large-scale and local conditions, The Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model Version 5.2 was used to simulate contrasting rainy and dry episodes. The most marked and relevant differences are the strength and extent of diurnally varying flow over the eastern slope of the Andes. During the rainy simulation, strong easterly winds reach the upper part of the slope by midmorning, initiating an intrusion of warm and moist air (high ue air originating in the eastern lowlands) into the central Andes. In the dry case, the moisture transport from the east is restricted to the eastern slope of the Andes, and the central Andes is flooded by low ue air from the western foothills that cannot support deep convection even in the presence of localized updrafts. The momentum balance based on the model output indicates that turbulent momentum mixing from aloft (determined by the large-scale anomalies of the upper-level flow) into the convective boundary layer is the leading term causing the differences in the daytime upslope flow (and hence moisture transport) over the upper part of the eastern side of the Andes between rainy and dry simulations.

Using Ensembles for Short-Range Forecasting

Abstract: Numerical forecasts from a pilot program on short-range ensemble forecasting at the National Centers for Environmental Prediction are examined. The ensemble consists of 10 forecasts made using the 80-km Eta Model and 5 forecasts from the regional spectral model. Results indicate that the accuracy of the ensemble mean is comparable to that from the 29-km Meso Eta Model for both mandatory level data and the 36-h forecast cyclone position. Calculations of spread indicate that at 36 and 48 h the spread from initial conditions created using the breeding of growing modes technique is larger than the spread from initial conditions created using different analyses. However, the accuracy of the forecast cyclone position from these two initialization techniques is nearly identical. Results further indicate that using two different numerical models assists in increasing the ensemble spread significantly. There is little correlation between the spread in the ensemble members and the accuracy of the ensemble mean for the prediction of cyclone location. Since information on forecast uncertainty is needed in many applications, and is one of the reasons to use an ensemble approach, the lack of a correlation between spread and forecast uncertainty presents a challenge to the production of short-range ensemble forecasts. Even though the ensemble dispersion is not found to be an indication of forecast uncertainty, significant spread can occur within the forecasts over a relatively short time period. Examples are shown to illustrate how small uncertainties in the model initial conditions can lead to large differences in numerical forecasts from an identical numerical model.

Summertime Precipitation Variability over South America: Role of the Large-Scale Circulation

Abstract: The observed large-scale circulation mechanisms associated with summertime precipitation variability over South America are investigated. Particular attention is paid to the Altiplano where a close relationship has been observed between rainfall and the position and intensity of the Bolivian high. Empirical orthogonal function (EOF), correlation, and composite analyses suggest that on intraseasonal timescales (typically 5–20 days), rainy periods on the Altiplano are associated with at least three types of circulation anomalies, involving either extratropical cyclones, cold-core lows, or the westward enhancement of the South Atlantic high. In each instance, the primary support for high rainfall rates is a moist, poleward flow at low levels along the eastern flank of the central Andes in association with the South Atlantic convergence zone (SACZ). The warm, low-level flow along the SACZ also inflates the overlying atmospheric column, resulting in an intensification and southward shift of the Bolivi...

Blocking over the South Pacific and Rossby Wave Propagation

Abstract: Atmospheric blocking events over the South Pacific are investigated using a 39-yr record of 500-hPa height fields from the NCEP–NCAR reanalysis dataset. The analysis extends earlier work using a 16-yr record and confirms that the occurrence of blocking over the southeast Pacific is strongly modulated by the ENSO cycle during austral spring and summer. Comparison of results at 500 hPa with the 300-hPa meridional wind component showed that blocking events are associated with large-scale wave trains lying across the South Pacific from the region of Australia to southern South America. Similar wave trains are evident in both hemispheres in singular value decomposition analyses between 300-hPa meridional wind components and tropical Pacific outgoing longwave radiation (OLR) anomalies. The hypothesis that the divergence associated with tropical OLR anomalies forces an extratropical wave response that results in enhanced blocking over the southeast Pacific was tested using a linearized, barotropic vorti...

The Influence of Anthropogenic Landscape Changes on Weather in South Florida

Abstract: Using identical observed meteorology for lateral boundary conditions, the Regional Atmospheric Modeling System was integrated for July-August 1973 for south Florida. Three experiments were performed-one using the observed 1973 landscape, another the 1993 landscape, and the third the 1900 landscape, when the region was close to its natural state. Over the 2-month period, there was a 9% decrease in rainfall averaged over south Florida with the 1973 landscape and an 11% decrease with the 1993 landscape, as compared with the model results when the 1900 landscape is used. The limited available observations of trends in summer rainfall over this region are consistent with these trends.

Data Assimilation via Error Subspace Statistical Estimation.

Abstract: Identical twin experiments are utilized to assess and exemplify the capabilities of error subspace statistical estimation (ESSE). The experiments consists of nonlinear, primitive equation–based, idealized Middle Atlantic Bight shelfbreak front simulations. Qualitative and quantitative comparisons with an optimal interpolation (OI) scheme are made. Essential components of ESSE are illustrated. The evolution of the error subspace, in agreement with the initial conditions, dynamics, and data properties, is analyzed. The three-dimensional multivariate minimum variance melding in the error subspace is compared to the OI melding. Several advantages and properties of ESSE are discussed and evaluated. The continuous singular value decomposition of the nonlinearly evolving variations of variability and the possibilities of ESSE for dominant process analysis are illustrated and emphasized.

The Use of Finite-Volume Methods for Atmospheric Advection of Trace Species. Part I: Test of Various Formulations in a General Circulation Model

Abstract: In the context of advection of trace species by 3D atmospheric flows, a comparative test of a hierarchy of finite volume transport schemes initially derived by B. Van Leer is presented. Those schemes are conservative by construction and Van Leer proposed a simple way of ensuring monotonicity. One of the schemes, introduced independently in the atmospheric community by M. J. Prather, is now considered as a reference in the GCM community. An important aspect of the present work is to perform test simulations with various spatial resolutions in order to compare the various schemes at a comparable numerical cost. The result is that higher-order schemes are much more accurate than lower order at a given spatial resolution but much more comparable when the lower-order schemes are run on a finer grid to make the numerical costs equivalent. Moreover, the higher moments of the tracer distribution introduced in the more sophisticated schemes become an issue when other processes such as chemistry or turbule...

A Multiscale Numerical Study of Hurricane Andrew (1992). Part II: Kinematics and Inner-Core Structures

Abstract: Despite considerable research, understanding of the temporal evolution of the inner-core structures of hurricanes is very limited owing to the lack of continuous high-resolution observational data of a storm. In this study, the results of a 72-h explicit simulation of Hurricane Andrew (1992) with a grid size of 6 km are examined to explore the inner-core axisymmetric and asymmetric structures of the storm during its rapid deepening stage. Based on the simulation, a conceptual model of the axisymmetric structures of the storm is proposed. Most of the proposed structures confirm previous observations. The main ingredients include a main inflow (outflow) in the boundary layer (upper troposphere) with little radial flow in between, a divergent slantwise ascent in the eyewall, a penetrative dry downdraft at the inner edge of the eyewall, and a general weak subsiding motion in the eye with typical warming/drying above an inversion located near an altitude of about 2‐3 km. The storm deepens as the axes of these features contract. It is found that the inversion divides the eye of the hurricane vertically into two parts, with a deep layer of warm/dry air above and a shallow pool of warm/moist air below. The air aloft descends at an average rate of 5c m s 21 and has a residency time of several days. In contrast, the warm/moist pool consists of air from the main inflow and penetrative downdrafts, offset somewhat by the air streaming in a returning outflow into the eyewall in the lowest 2 km; it is subject to the influence of the upward heat and moisture fluxes over the underlying warm ocean. The warm/moist pool appears to play an important role in supplying high-ue air for deep convective development in the eyewall. The penetrative downdraft is dry and originates from the return inflow in the upper troposphere, and it is driven by sublimative/evaporative cooling under the influence of the (asymmetric) radial inflow of dry/cold air in the midtroposphere. It penetrates to the bottom of the eye (azimuthally downshear with a width often greater than 100 km) in a radially narrow zone along the slantwise inner edge of the eyewall. It is further shown that all the meteorological fields are highly asymmetric. Whereas the storm-scale flow features a source‐sink couplet in the boundary layer and dual gyres aloft, the inner-core structures exhibit alternative radial inflow and outflow and a series of inhomogeneous updrafts and downdrafts. All the fields tilt more or less with height radially outward and azimuthally downshear. Furthermore, pronounced fluctuations of air motion are found in both the eye and the eyewall. Sometimes, a deep layer of upward motion appears at the center of the eye. All these features contribute to the trochoidal oscillation of the storm track and movement. The main steering appears to be located at the midtroposphere (;4.5 km) and the deep-layer mean winds represent well the movement of the hurricane.

The use and misuse of conditional symmetric instability

Abstract: A commonly employed explanation for single- and multiple-banded clouds and precipitation in the extratropics is slantwise convection due to the release of moist symmetric instability (MSI), of which one type is conditional symmetric instability (CSI). This article presents a review of CSI with the intent of synthesizing the results from previous observational, theoretical, and modeling studies. This review contends that CSI as a diagnostic tool to assess slantwise convection has been, and continues to be, misused and overused. Drawing parallels to an ingredients-based methodology for forecasting deep, moist convection that requires the simultaneous presence of instability, moisture, and lift, some of the misapplications of CSI can be clarified. Many of these pitfalls have been noted by earlier authors, but are, nevertheless, often understated, misinterpreted, or neglected by later researchers and forecasters. Topics include the evaluation of the potential for slantwise convection, the relationshi...

Convective Precursors and Predictability in the Tropical Western Pacific

Abstract: Conditions leading to convective outbreak in the Tropics are investigated by multivariate analysis of sounding and satellite data from the tropical western Pacific area. Circumstances that make the prediction problem difficult are discussed and addressed by applying linear “error-in-variables” and nonlinear statistical simulation techniques to a large dataset. Low- to midtropospheric moisture is identified as the dominant factor regulating convective outbreak in this region. Based on the results it is argued that such moisture is particularly important in regulating spontaneous convective outbreak, but instability and near-surface wind speed probably play some role in allowing previous shallow or midtopped cumulus activity to deepen. Mesoscale-mean convective available potential energy sufficient for convection is found to exist almost 90% of the time. Quantitative estimates of noise in the data are obtained and accounted for in reaching these conclusions. The results imply that large-scale mean ...

The Skill of Ensemble Prediction Systems

Abstract: The performance of ensemble prediction systems (EPSs) is investigated by examining the probability distribution of 500-hPa geopotential height over Europe. The probability score (or half Brier score) is used to evaluate the quality of probabilistic forecasts of a single binary event. The skill of an EPS is assessed by comparing its performance, in terms of the probability score, to the performance of a reference probabilistic forecast. The reference forecast is based on the control forecast of the system under consideration, using model error statistics to estimate a probability distribution. A decomposition of the skill score is applied in order to distinguish between the two main aspects of the forecast performance: reliability and resolution. The contribution of the ensemble mean and the ensemble spread to the performance of an EPS is evaluated by comparing the skill score to the skill score of a probabilistic forecast based on the EPS mean, using model error statistics to estimate a probabili...

Convective Structure of Hurricanes as Revealed by Lightning Locations

Abstract: Cloud-to-ground lightning flash locations were examined for nine Atlantic basin hurricanes using data from the National Lightning Detection Network. A common radial distribution in ground flash density was evident: a weak maximum in the eyewall region, a clear minimum 80‐100 km outside the eyewall, and a strong maximum in the vicinity of outer rainbands (210‐290-km radius). These results are consistent with the authors’ previous study of Hurricane Andrew. None of the storms showed this characteristic radial structure during prehurricane stages. The results support the division of precipitation in the hurricane into three distinct regimes. The eyewall is a unique phenomenon but shares some attributes with deep, weakly electrified oceanic monsoonal convection. The region outside the eyewall and under the central dense overcast has characteristics of the trailing stratiform region of mesoscale convective systems, including a relatively high fraction of positive polarity flashes. The outer bands, with mean maximum flash density at the 250-km radius, contain the vast majority of ground flashes in the storms. Eyewall lightning, defined as that within 40 km of the center, was examined for four moderate-to-strong hurricanes. Such lightning occurred episodically during hurricane stage, with 93% of hourly intervals containing no detected flashes. Eyewall lightning outbreaks over water always occurred at the beginning of or during times of intensification, but often were indicative of the imminent end of deepening. It is proposed that the existence of such inner core lightning might reveal the presence of an eyewall cycle. For the one storm with available aircraft reconnaissance data, eyewall cycles were reliably identified by the occurrence of inner core lightning, and inner core lightning appeared only during such cycles. Suggestions are made as to how eyewall flashes in existing hurricanes might be used to help predict hurricane intensity change.

Maximum-Likelihood Estimation of Forecast and Observation Error Covariance Parameters. Part I: Methodology

Abstract: The maximum-likelihood method for estimating observation and forecast error covariance parameters is described. The method is presented in general terms but with particular emphasis on practical aspects of implementation. Issues such as bias estimation and correction, parameter identifiability, estimation accuracy, and robustness of the method, are discussed in detail. The relationship between the maximum-likelihood method and generalized cross-validation is briefly addressed. The method can be regarded as a generalization of the traditional procedure for estimating covariance parameters from station data. It does not involve any restrictions on the covariance models and can be used with data from moving observers, provided the parameters to be estimated are identifiable. Any available a priori information about the observation and forecast error distributions can be incorporated into the estimation procedure. Estimates of parameter accuracy due to sampling error are obtained as a by-product.

The tropical-extratropical interaction between high-frequency transients and the Madden-Julian oscillation

Abstract: The interaction between high-frequency transient disturbances and convection, and the Madden–Julian Oscillation (MJO), is investigated using NCEP–NCAR reanalysis and satellite outgoing longwave radiation data for 15 northern winters. During the phase of the MJO with enhanced convection over the East Indian Ocean and Indonesia, and suppressed convection over the South Pacific convergence zone, both the Asian–Pacific jet and the region of upper-tropospheric tropical easterlies over the warm pool are displaced westward. These changes in the basic state lead to a weaker or “leakier” waveguide in the Asian–Pacific jet, with a westward-displaced “forbidden” region of tropical easterlies, such that high-frequency transient waves propagate equatorward into the deep Tropics over the central Pacific near the date line. As these waves induce convection in the region of ascent and reduced static stability ahead of the upper-level cyclonic disturbances, there is an enhancement of high-frequency convective var...

Comment on ''Data Assimilation Using an Ensemble Kalman Filter Technique''

Abstract: In an interesting paper Houtekamer and Mitchell (1998; hereafter HM98) introduce a variant of the ensemble Kalman filter (EnKF) as proposed by Evensen (1994). HM98 point to the hitherto unacknowledged problem that the EnKF has an ‘‘inbreeding’’ problem: in the analysis step the ensemble is updated with a gain calculated from that same ensemble. In their new approach a double ensemble (DEnKF) is used and the gain of each ensemble is used to update the other ensemble in the analysis step. The new approach is argued to be much less sensitive to this inbreeding. They strengthen their argument by a specific example, in which the EnKF shows a variance that is too low for small ensemble sizes (#100), but they fail to give a rigorous justification. The purposes of this comment are twofold. First, a theoretical justification of the inbreeding effect is given, and it is shown that the DEnKF has similar, but smaller, problems. Second, a serious concern about the use of small ensemble sizes is expressed, thus bringing into question the use of the DEnKF over the EnKF for real applications.

The Low-Level Structure of African Easterly Waves in 1995

Abstract: The existence of African easterly waves (AEWs) north of the African easterly jet (AEJ) core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the U.K. Meteorological Office global model analysis from the hurricane season of 1995. At Bamako (12.5°N, 8.0°W) the AEWs were characterized mainly by maximum amplitudes at the level of the AEJ (around 700 mb), whereas at Dakar (14.7°N, 17.5°W) the waves were characterized by maxima between 850 and 950 mb. The low-level waves to the north of the AEJ arise in association with baroclinic interactions between the negative meridional potential vorticity (PV) gradients in the jet core and the positive low-level gradient of potential temperature, θ, enhanced by the presence of low-static-stability air north of the AEJ. These waves follow the positive meridional θ gradients over northern Africa in contrast to the jet-level AEWs that follow the meridional PV gradients at the level of the AEJ. Cross-correlation analysis...

Tropical Cyclone Kinematic Structure Retrieved from Single-Doppler Radar Observations. Part I: Interpretation of Doppler Velocity Patterns and the GBVTD Technique

Abstract: Deducing the three-dimensional primary circulation of landfalling tropical cyclones (TCs) from single ground-based Doppler radar data remains a difficult task. The evolution and structure of landfalling TCs and their interactions with terrain are left uncharted due to the lack of dual-Doppler radar observations. Existing ground-based single-Doppler radar TC algorithms provide only qualitative information on axisymmetric TC center location and intensity. In order to improve understanding of the wind structures of landfalling TCs using the widely available WSR-88D data along the U.S. coastal region, a single ground-based radar TC wind retrieval technique, the ground-based Velocity Track Display (GBVTD) technique, is developed. Part I of this paper presents 1) single-Doppler velocity patterns of analytic, asymmetric TCs, 2) derivation of the GBVTD technique, and 3) evaluation of the GBVTD-retrieved winds using analytic TCs. The Doppler velocity patterns of asymmetric TCs display more complex structu...

An Observational Study of the Evolution of Horizontal Convective Rolls

Abstract: A comprehensive observational dataset encompassing the entire temporal evolution of horizontal convective rolls was obtained for the first time. Florida, Illinois, and Kansas measurements from preroll conditions through the development of well-defined rolls to their dissipation were utilized to determine the factors influencing roll evolution. When the buoyancy flux reached a critical value of 35–50 W m−2, the first form of boundary layer convection resolved by radar was rolls. It was noted that two-dimensional convective rolls can evolve in a convective boundary layer in the absence of significant wind speed and shear. In fact, the value of wind speed or shear in itself did not seem to determine when or if rolls would form, although it did influence roll evolution. Well-defined, two-dimensional rolls only occurred while −zi/L, where zi is the convective boundary layer depth and L is the Monin–Obukhov length, was less than ∼25, which is consistent with previous studies. As −zi/L increased through...

Principal Modes of Climatological Seasonal and Intraseasonal Variations of the Asian Summer Monsoon

Abstract: Principal modes of climatological variation of the Asian summer monsoon are investigated. Data used in this study include the high cloud fraction produced by the International Satellite Cloud Climatology Project and sea level pressure, and 850- and 200-mb geopotential heights from ECMWF analysis for the five summers of 1985–89. It is shown that the seasonal evolution of the Asian summer monsoon is adequately described by a few leading EOFs. These EOFs capture the variations of regional rainbands over the East Asian and Indian regions. The first mode is characterized by an increase in large-scale cloud over India and the subtropical western Pacific until mid-August. The second mode depicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the development of summer monsoon circulation: a low pressure system over the Asian continent and a subtropical high over the Pacific. The third eigenmode is characterized by zonal cl...

Alleviation of Stationary Biases in a GCM through a Mountain Drag Parameterization Scheme and a Simple Representation of Mountain Lift Forces

Abstract: The problem of the representation of the orientation of mountain forces in a GCM is examined. First a series of winter simulations is presented with the Laboratoire de Meteorologie Dynamique GCM where the model ground is flat and where the mountains are represented by applying forces to the flow that are either strictly opposite to the local wind (i.e., a drag) or strictly perpendicular to it (i.e., a lift). These experiments show that the drag forces improve the zonal wind but do not improve the steady planetary wave everywhere, while the lift forces scarcely affect the zonal wind but force a steady planetary wave that is everywhere realistic. The drag force is formulated following the realistic subgrid-scale orographic (SSO) scheme, developed recently at ECMWF. The lift force essentially enhances vortex stretching over large-scale mountains. To address issues concerning the representation of SSO in GCMs, experiments with mean orography, the ECMWF SSO scheme, and enhanced lift forces over mounta...