Showing papers in "Monthly Weather Review in 1989"

A Comprehensive Mass Flux Scheme for Cumulus Parameterization in Large-Scale Models

Abstract: Observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large-scale models, but this approach, through the introduction of a spectral cloud ensemble, leads normally to rather complex schemes. In this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions. This is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection (GATE, Marshall Islands), tradewind cumuli (ATEX, BOMEX) and extratropical organized convection (SESAME-79) and then in a NWP model. The scheme considers a population of clouds where the cloud ensemble is described by a one-dimensional bulk model as earlier applied by Yanai et al. in a diagnostic study of tropical convection. Cumulus scale downdrafts are included. Various types of convection are represented, i.e., penetrative convection in c...

A Simple Parameterization of Land Surface Processes for Meteorological Models

Abstract: A parameterization of land surface processes to be included in mesoscale and large-scale meteorological models is presented. The number of parameters has been reduced as much as possible, while attempting to preserve the representation of the physics which controls the energy and water budgets. We distinguish two main classes of parameters. The spatial distribution of primary parameters, i.e., the dominant types of soil and vegetation within each grid cell, can be specified from existing global datasets. The secondary parameters, describing the physical properties of each type of soil and vegetation, can be inferred from measurements or derived from numerical experiments. A single surface temperature is used to represent the surface energy balance of the land/cover system. The soil heat flux is linearly interpolated between its value over bare ground and a value of zero for complete shielding by the vegetation. The ground surface moisture equation includes the effect of gravity and the thermo-hyd...

Parameterization of Orography-Induced Turbulence in a Mesobeta--Scale Model

Abstract: The possibility of extending existing techniques for turbulence parameterization in the planetary boundary layer to attitude, orography-induced turbulence events is examined. Starting from a well-tested scheme, we show that it is possible to generalize the specification method of the length scales, with no deterioration of the scheme performance in the boundary layer. The new scheme is implemented in a two-dimensional version of a limited-area, numerical model used for the simulation of mesobeta-scale atmospheric flows. Three well-known cases of orographically induced turbulence are studied. The comparison with observations and former studies shows a satisfactory behavior of the new scheme.

A Parameterization of Heterogeneous Land Surfaces for Atmospheric Numerical Models and Its Impact on Regional Meteorology

Abstract: Natural land surfaces are usually heterogeneous over the resolvable scales considered in atmospheric numerical models. Therefore, model surface parameterizations that assume surface homogeneity may fail to represent the surface forcing accurately. In this paper, a parameterization of the subgrid-scale forcing of heterogeneous land surfaces for atmospheric numerical models is suggested. In each surface grid element of the numerical model similar homogeneous land patches located at different places within the element are regrouped into subgrid classes. Then, for each one of the subgrid classes, a sophisticated micrometeorological model of the soil-plant-atmosphere system is applied to assess the surface temperature, humidity, and fluxes to the atmosphere. The global fluxes of energy between the grid and the atmosphere are obtained by averaging according to the distribution of the subgrid classes. In addition to the surface forcing, detailed micrometeorological conditions of the patches are assessed...

Condensation and Cloud Parameterization Studies with a Mesoscale Numerical Weather Prediction Model

Abstract: This paper presents the implementation of a parameterization scheme for convective and stratiform condensation (with cloud water as a prognostic variable) into a fine mesh numerical weather prediction model. The results from a 36 h integration of the model, with grid distance 50 km, indicate that the new condensation scheme contributes to an improved forecast compared to that obtained by the original model. Furthermore, from a qualitative comparison with satellite pictures, it is found that the prediction of condensation-cloud parameters is quite realistic.

A positive definite advection scheme obtained by nonlinear renormalization of the advective fluxes

Abstract: A new method is developed to obtain a conservative and positive definite advection scheme that produces only small numerical diffusion. Advective fluxes are computed utilizing the integrated flux form of Tremback et al. These fluxes are normalized and then limited by upper and lower values. The resulting advection equation is numerically solved by means of the usual upstream procedure. The proposed treatment is not restricted to the integrated flux form but may also be applied to other known advection algorithms which are formulated in terms of advective fluxes. Different numerical tests are presented illustrating that the proposed scheme strongly reduces numerical and diffusion and simultaneously requires only small computational effort. For Corant numbers with absolute values not exceeding one, the scheme preserves numerical stability except in strong deformational flow fields where slight instabilities may occur.

An Ice-Water Saturation Adjustment

Abstract: A reasonably accurate and noniterative saturation adjustment scheme is proposed to calculate: (1) the amount of condensation and/or deposition necessary to remove any supersaturated vapor, or (2) the amount of evaporation and/or sublimation necessary to remove any subsaturation in the presence of cloud droplets and/or cloud ice. This proposed scheme can be implemented for a nonhydrostatic cloud model. The derivation of the scheme, an evaluation of its performance, and tests for sensitivity to variations in a few key parameters are presented.

The climatological skill of a regional model over complex terrain

Abstract: As part of an ongoing study of the regional climate and hydrology of the southwestern United States, in this paper we investigate the systematic biases of two versions of the PSU/NCAR mesoscale model (MM4). These are a standard version and one that includes a more detailed treatment of radiative transfer, surface physics, and soil hydrology. We simulated the period 1–30 January 1979, in which nine Pacific storms moved across the western United States. Results from both model versions are compared to the large scale analysis used to provide initial and lateral boundary conditions. Both models show a lower tropospheric cold bias of 1–3 K near the surface over land and an upper tropospheric warm bias of less than 1 K, which suggest high model stability and reduced vertical mixing. The model atmospheres are wetter than that of the analysis, particularly in the lower troposphere and over the ocean. The wind magnitude bias is positive near the surface (∼1.5–3 m s−1), negative in the upper troposphere (...

Skill Scores and Correlation Coefficients in Model Verification

Abstract: Attributes of the anomaly correlation coefficient, as a model verification measure, are investigated by exploiting a recently developed method of decomposing skill scores into other measures of performance. A mean square error skill score based on historical climatology is decomposed into terms involving the anomaly correlation coefficient, the conditional bias in the forecast, the unconditional bias in the forecast, and the difference between the mean historical and sample climatologies. This decomposition reveals that the square of the anomaly correlation coefficient should be interpreted as a measure of potential rather than actual skill. The decomposition is applied to a small sample of geopotential height field forecasts, for lead times from one to ten days, produced by the medium range forecast (MRF) model. After about four days, the actual skill of the MRF forecasts (as measured by the “climatological skill score”) is considerably less than their potential skill (as measured by the anomaly...

Two-Dimensional Semi-Lagrangian Transport with Shape-Preserving Interpolation

Abstract: The more attractive one dimensional, shape-preserving interpolation schemes as determined from a companion study are applied to two-dimensional semi-Lagrangian advection in plane and spherical geometry. Hermite cubic and a rational cubic are considered for the interpolation form. Both require estimates of derivatives at data points. A cubic derivative form and the derivative estimates of Hyman and Akima are considered. The derivative estimates are also modified to ensure that the interpolant is monotonic. The modification depends on the interpolation form. Three methods are used to apply the interpolators to two-dimensional semi-Lagrangian advection. The first consists of fractional time steps or time splitting. The method has noticeable displacement errors and larger diffusion than the other methods. The second consists of two-dimensional interpolants with formal definitions of a two-dimensional monotonic surface and application of a two-dimensional monotonicity constraint. This approach is exam...

Is the Tropical Atmosphere Conditionally Unstable

Abstract: We examine the mean and standard deviation of the bouyancy of lifted boundary layer parcels from a large sample of soundings from the western equatorial Pacific. The inclusion of condensate loading in the definition of buoyancy is emphasized as is the precise originating level of parcels in the subcloud layer. We confirm the observation of Betts that those parts of the tropical atmosphere experiencing deep convection are nearly neutral to adiabatic parcel ascent from the subcloud layer when adiabatic condensate loading is included in the definition of buoyancy. Parcels lifted from the top of the subcloud layer are more nearly neutral than those originating near the surface, and atmospheres with higher levels of free convection (LFCs) are closer to neutral than those with low LFCs. We explore possible reasons for these observations and discuss their implications for tropical meteorology.

Random Forcing and Forecasting Using Principal Oscillation Pattern Analysis

Abstract: The effects of random forcing and deterministic feedback are combined in a measured multivariate time series. It is shown here how the characteristics of the driving noise can be found after the deterministic effects have been identified by the principal oscillation pattern (POP) analysis. In addition, the POP analysis is extended to enable the prediction of the most probable meteorological pattern at some future time when the present pattern is known, and the conditional probability of finding the process at any location within a range of values given the value of the process at another location at an earlier time. Estimates of how well these predictions can be trusted are also given. The basic assumption of POP analysis is that the system can be optimally modeled by a linear Markov process.

Non-supercell Tornadoes

Abstract: Analyses of tornadoes that are not associated with supercells are presented. The database for this study was collected during CINDE (Convention INitiation and Downburst Experiment), a field project operated during the summer of 1987 in Colorado. A total of 27 visual vortices were studied. They appeared to form as shear instabilities along radar detected convergence lines. The circulations initiated at low levels generally in the absence of precipitation echo. Subsequently as these vortices propagated along the convergence line they appeared to strengthen to tornadic intensity when they became colocated with the updraft of a rapidly developing storm. It is hypothesized that vortex stretching is responsible for intensifying the initial rotation. Although these tornadoes were weaker than those accompanied by strong midlevel mesocyclones, estimates of their strength suggest damage capability as high as F2. The implications for operational radars to detect these types of phenomena were identified. The...

A Variational Continuous Assimilation Technique

Abstract: A variational assimilation technique is presented which continuously adjusts a model solution by introducing a correction term to the model equations. The technique is essentially a modification of the adjoint technique. The Variational Continuous Assimilation (VCA) technique optimizes the correction to the model equations rather than the initial conditions as is done in the adjoint technique. The VCA-technique characteristics were examined by inserting independent analyses into a simple quasi- geostrophic model using both the VCA technique and the adjoint technique. Because the model equations do not have to be satisfied exactly in the VCA technique, some of the effects of systematic model errors can be removed from the assimilation. Thus, the VCA technique was able to consistently fit the data better than the adjoint technique. Predictions from the results from the assimilation techniques showed that the forecast from the adjoint technique's solution was consistently inferior to those from the ...

Inclusion of a TKE Boundary Layer Parameterization in the Canadian Regional Finite-Element Model

Abstract: The formulation of the regional model recently implemented by the Atmospheric Environment Service of Canada for its operational 48 h NWP forecasts is presented. The emphasis is put on the parameterization of the physical processes, especially those affecting the atmospheric boundary layer. The originality of this model, in addition to the use of 3-D finite elements, of variable meshes in both the horizontal and vertical, and of being non-nested (as previously described by Staniforth and Daley), consists in the treatment of the time-dependent turbulent Kinetic energy (TKE) and the inclusion of the full diurnal cycle. The overall organization of the model calculations it also presented in order to convey a more accurate description of this integrated system. Sample results from the well-known case of the Presidents' Day Cyclone of 1979 and general performance are covered in the last section.

Fractal Statistics of Cloud Fields

Abstract: Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM) data, with 80 and 30 m spatial resolution, respectively, have been employed to study the spatial structure of boundary-layer and intertropical convergence zone (ITCZ) clouds The probability distributions of cloud areas and cloud perimeters are found to approximately follow a power-law, with a different power (ie, fractal dimension) for each cloud type They are better approximated by a double power-law behavior, indicating a change in the fractal dimension at a characteristic size which depends upon cloud type The fractal dimension also changes with threshold The more intense cloud areas are found to have a higher perimeter fractal dimension, perhaps indicative of the increased turbulence at cloud top A detailed picture of the inhomogeneous spatial structure of various cloud types will contribute to a better understanding of basic cloud processes, and also has implications for the remote sensing of clouds, for their effects on remote sensing of other parameters, and for the parameterization of clouds in general circulation models, all of which rely upon plane-parallel radiative transfer algorithms

The Impact of Crop Areas in Northeast Colorado on Midsummer Mesoscale Thermal Circulations

Abstract: The present study provides a preliminary evaluation of mesoscale circulations forced by surface gradients of heating arising from irrigated areas adjacent to dry land, utilizing a combination of satellite, observational, and modeling approaches. The irrigated crop areas of northeast Colorado were chosen for the study. For the cases studied satellite surface infrared temperature data indicated a typical temperature contrast of approximately 10 K at noon, between the irrigated area and the adjacent dry land. Surface observations and aircraft measurements within the lower region of the atmospheric boundary layer indicated, in general, a significant temperature contrast and moisture difference, thereby implying a potential thermally driven circulation. The anticipated thermally induced flows, however, were reflected in the measurements only by modest changes in the wind speed and wind direction across the contrast location. It is suggested that the daytime, elevated, terrain-forced flow in the area, ...

Numerical Study of an Observed Orogenic Mesoscale Convective System. Part 1: Simulated Genesis and Comparison with Observations

Abstract: The interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the Rockies is shown to lead to the growth of a mesoscale convective system (MCS). An organic MCS observed during the 1977 combined South Park Area Cumulus Experiment and High Plains Experiment is numerically simulated with a two-dimensional nonhydrostatic cloud model covering spatial scales of 1000 km. In this numerical investigation,mesoγ-, mesoβ- and mesoα-scales of motion are represented simultaneously. As a result, interesting features of cloud-mesoscale interaction are predicted that cannot be represented in cloud parameterization frameworks. Based on the results of this simulation, a six-stage conceptual model of orogenic development is given.

A Composite Model of Mesoscale Convective Complexes

Abstract: A composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes (MCCs). The early stage of the MCC lifecycle is characterized by convergence, vertical motion and heating being centered in the lower troposphere. As the MCC matures the level of peak upward motion and heating shifts to the upper troposphere. The system achieves and maintains its maximum divergence, upward motion, and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle. This is in contrast to GATE tropical clusters where the maximum divergence, upward motion, and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened. This difference might be explained by an MCC being an inertially stable form of mesoscale convective system whose radius exceeds the Rossby radius of deformation. The MCC is shown to be an efficient rain producer, exhibiting a precipitation efficiency exceeding 100% at the mature stage due to th...

Cloud-to-Ground Lightning: Climatological Characteristics and Relationships to Model Fields, Radar Observations, and Severe Local Storms

Abstract: Data for nearly 2 million lightning flashes recorded during the 1985–86 warm seasons by the National Severe Storm Laboratory's (NSSL's) lightning strike locating network were evaluated to determine some of the climatological characteristics of cloud-to-ground lightning. Among the characteristics studied were the seasonal, diurnal, and spatial variations Of Positive and negative lightning strike activity, including flush rates, signal strength, and flash multiplicity. The lightning data were also compared to manually digitized radar data, reports of tornadoes, large hail, and damaging winds, and to analyzed 0000 UTC fields obtained from operational numerical models. An examination of the diurnal distribution of lightning revealed that peak rates occurred later than in other sections of the country, reflecting the prevalence of nocturnal convection within much of the NSSL network. An analysis of the spatial variations in lightning activity also confirmed the existence of distinct climatological reg...

Consistent vertical and horizontal resolution

Abstract: Simple physical relations (namely, the Rossby ratio between vertical and horizontal scales in quasi-geostrophic flow and the dispersion relation for internal gravity waves) are used to estimate the vertical resolution consistent with a given horizontal resolution. Using these relations it is found that virtually all large scale models and observing systems have inadequate vertical resolution. In models, the excess horizontal resolution can lead to increased model 'noise' rather than improved accuracy. In observing systems, the finer horizontal scales can be severely misrepresented.

A New Look at Weather Forecasting through Analogues

Abstract: In the literature, the use of analogues for short-range weather forecasting has practically been discarded. This is because no good matches for today's extratropical large-scale flow patterns can be found in a 30-year data library. We propose here a limited-area model approach for Analogue-Forecasting (AF). In order to make a 12-hour AF valid at a target point, we require analogy in initial states only over a circle with radius of about 900 km. On a limited area there are usually several good analogues, sometimes to within observational error. Different historical analogues may be used at different target points. The usefulness of the limited area approach is first demonstrated with some examples. We then present verification statistics of 3000 12-hour 500-mb height point forecasts in the Northern Hemisphere winter at 38°N, 80°W (over West Virginia, U.S.A.). In order to beat persistence at 12 hours at this point we need an analogue which differs by about 40 geopotential meters or less from the ba...

The Precipitation Life Cycle of Mesoscale Convective Complexes over the Central United States

Abstract: The mesoscale convective complex (MCC) is a common and particularly well-organized class of meso-&α scale storm systems over the central United States. As observed by infrared (IR) satellite, the typical MCC's 10–12 h evolution displays a fairly consistent sequence of events, including the monotonic areal expansion of its anvil from its formation to its maximum size, followed by the monotonic shrinkage of the colder cloud top areas as the system weakens and dissipates. Primarily within the growth phase of this cycle, a characteristic IR signature reflects the MCC in its most intense, mesoconvective stage, which lasts ∼4 h and during which the coldest cloud top area reaches its largest extent. Hourly precipitation data have been analyzed for 122 MCC cases that were selected from June–August 1977–83 and screened to insure a reasonable conformity with the typical IR life cycle. On average. these systems produced a rainfall volume of 3.46 km 3 during their life cycle, over an area of 3.20×105km2and a...

A Vertically Nested Regional Numerical Weather Prediction Model with Second-Order Closure Physics

Abstract: The model we describe involves a unique strategy in which a high vertical resolution grid is nested within the coarse vertical resolution grid of a regional numerical weather prediction (NWP) model. Physics computations performed on the high vertical resolution grid involve time-dependent solution of second-order turbulence equations, the transfer equations for long- and shortwave radiation, and moist thermodynamic calculations which include liquid water content and fractional cloudiness. The dynamical computations involving advection, pressure gradient, and Coriolis terms are performed on the regional model grid. The two grids interact fully each model time step. This approach represents an extension into NWP of the general practice of supplying coarse large-scale dynamical forcing to high-resolution boundary layer models. Aside from the computational savings of performing dynamical calculations only at the coarser resolution, we also avoid difficulties which can arise with high vertical-resolut...

Convection over Tropical Africa and the Atlantic Ocean during Northern Summer. Part I: Interannual and Diurnal Variations

Abstract: Some aspects of the interannual and the diurnal variations of the convection over the tropical Africa and the Atlantic Ocean are derived using Meteosat data. The study is based on four summer months (June, July, August and September) of three years from 1983 to 1985, for regions of 2.5°×2.5° extending from 5°S to 25°N and from 50°E to 50°W. Using ECMWF (European Centre for Medium Range Weather Forecasts) analyses, mean cloud fields and interannual changes are interpreted in terms of dynamical forcing and feedback. Anomalies in the thermal wind at 700 mb between 1985 (relatively wet year) and the two other years are consistent with previous results based on more contrasted wet and dry years. Defining the high cloudiness by a threshold in the infrared signal, the amplitude of the diurnal variation is maximum over land with larger values over regions of large mean fractional cloudiness corresponding generally to regions of highlands. The diurnal cycle of high clouds is generally not sinusoidal and t...

A 15-Year Climatology of Northern Hemisphere 500 mb Closed Cyclone and Anticyclone Centers

Abstract: A 15-year (1963–77) Northern Hemisphere 2 × 5 degree latitude-longitude gridded dataset of 500 mb geopotential heights has been used to construct a climatology of 500 mb closed circulation centers. These centers, defined by at least one closed 30 m contour around a central minimum or maximum geopotential height value, were identified objectively between 24° and 82°N from the twice-daily analysis grids. Tracks for all closed circulation centers were computed to establish genesis and lysis distributions, and to examine the monthly, seasonal and interannual variability characteristics of the closed circulation center distributions for specified regions. The occurrence of closed cyclone centers is maximized north of and within the main belt of westerlies extending from northeast Asia to the Gulf of Alaska near 50°N, and extending from eastern Canada and the extreme northeast United States to southeast of Greenland and west of the United Kingdom. Their occurrence is also maximized south of the main be...

An Extraordinary Katabatic Wind Regime at Terra Nova Bay, Antarctica

Abstract: Three years of automatic weather station observations for the months of February to April show that intense katabatic winds persistently blow across the western shore of Terra Nova Bay. The data demonstrate that the anomalously strong katabatic winds of Adelie Land are not unique, and thus strongly support the proposition that most of the cold boundary layer air from the ice sheet crosses the coastline in a small number of narrow zones. Furthermore the observations prove that katabatic winds can routinely blow for substantial distances across flat terrain in marked contrast to the abrupt dissipation previously monitored just offshore from East Antarctica. Winter wind conditions onset suddenly in mid-February and are characterized by negligible directional variations and by speeds mostly ranging between 10 and 30 m s−1. Katabatic winds at Terra Nova Bay both affect and are affected by the regional atmospheric circulation. This katabatic airflow is a time-averaged source of cold boundary layer air ...

Numerical Study of an Observed Orogenic Mesoscale Convective System. Part 2: Analysis of Governing Dynamics

Abstract: A detailed analysis of the dynamics and thermodynamics responsible for the structure, growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made. The process of scale interaction is addressed through Fourier analysis and Reynolds averaging analysis of representative predicted variables, diabatic forcing and momentum acceleration terms. Additional dynamical analysis is accomplished through sensitivity experiments in which Coriolis, diabatic heating and ambient airflow are varied. The general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems. The heating scales range over a nearly continuous spectrum ranging from 10–250 km. The heating occurs in response to primary advective gravity modes. The larger-scale gravity-wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns. The la...

Objective Analysis of Pseudostress over the Indian Ocean Using a Direct-Minimization Approach

Abstract: A technique not previously used in objective analysis of meteorological data is used here to produce monthly average surface pseudostress data over the Indian Ocean. An initial guess field is derived and a cost functional is constructed with five terms: approximation to initial guess, approximation to climatology, a smoothness parameter, and two kinematic terms. The functional is minimized using a conjugate-gradient technique, and the weight for the climatology term controls the overall balance of influence between the climatology and the initial guess. Results from various weight combinations are presented for January and July 1984. Quantitative and qualitative comparisons to the subject analysis are made to find which weight combination provides the best results. The weight on the approximation to climatology is found to balance the influence of the original field and climatology.

A comparison of three numerical methods for solving differential equations on the sphere

Abstract: We compare three numerical methods for solving vector differential equations on a sphere. A composite mesh finite-difference method using overlapping stereographic coordinate systems is compared to transform methods based on scalar and vector spherical harmonics. The methods are compared in terms of total computer time, memory requirements, and execution rates for relative accuracy requirements of two and four digits in a five-day forecast. The computational requirements of the three methods were well within an order of magnitude of one another. In most of the cases that are examined, the time step was limited by accuracy rather than stability. This problem can be overcome by the use of a higher order time integration scheme, but at the expense of an increase in the memory requirements.