Analysis Of Wind Field Research Articles

A Synoptic Evaluation of Normal Mode Initialization Experiments with the NMC Nested Grid Model

Abstract The Regional Analysis and Forecast System at the National Meteorological Center consists of an optimum interpolation objective analysis scheme, an adiabatic nonlinear normal model initialization (NNMI) and a hemispheric Nested Grid Model (NGM) to provide 48 h forecasts. We investigate here the effect NNMI has on the analyses and forecasts produced by this system. An eight vertical mode NNMI procedure led to significant reductions of the divergent component of the analyzed wind field in regions of heavy precipitation. This is shown to contribute to a systematic spinup error in NGM 0–12 h precipitation forecasts, especially from the 0000 UTC runs. Forecasts starting with no initialization had unacceptable noise levels. NNMI using two vertical modes yielded the best combination of noise-free forecasts and unsuppressed initial precipitation rates. A physical interpretation of this result is provided using the vertical structure functions of the normal modes. Tests of the two-mode NNMI in an operation...

Limited Area Kinematic Analysis by a Multivariate Statistical interpolation Method

Abstract An observational map analysis scheme is described, whereby the vector field derived from discrete observations of wind and geopotential may be partitioned into its geostrophic, nondivergent and divergent parts without the need to solve Poisson-type equations relating streamfunction and velocity potential fields to estimated divergence and vorticity data. The method is based on a multivariate statistical interpolation formulation, which assumes a generalized random field model for scalar streamfunction and velocity potential variables, and a linear correlation between streamfunction and geopotential fluctuations. A similar formulation can also be used for wind field analysis based only on vector wind data. Sample analyses using synoptic upper-air observations over North America illustrate some properties of the method. It is shown, for example, that the estimated partitioning of the large-scale wind field into nondivergent and divergent parts is sensitive to the relative magnitudes of streamfuncti...

An Isentropic Mesoα-Scale Analysis System and Its Sensitivity to Aircraft and Surface Observations

An objective analysis scheme for meteorological variables on constant potential temperature surfaces is presented. The analysis uses a form of multivariate statistical interpolation and is designed th retain mesoscale detail in disparate observations including rawinsonde, surface, aircraft, satellite, and wind profiler data while combining them with a forecast background (first guess) field. The wind and mass field analyses are interdependent. The horizontal correlation of forecast error on isentropic surfaces is modeled with an analytical function from statistics collected for this study; the vertical correlation of forecast error is modeled as a function of potential temperature separation. These correlations determine the weights applied to observed-minus-forecast increments in the analysis. The analysis is two-dimensional except with respect to single-level data where it is three-dimensional. Comparisons of isentropic and isobaric analysts are shown, and examples of the effects of single-level (aircraft and surface) observations on isentropic analyses are presented. Although variable in space and time, these datasets are often of higher density than the rawinsonde network, and they support increased resolution of mesoscale features in the analysis. More importantly, the examples reveal that three-dimensional analysis increment structures, especially in the vicinity of fronts, appear to be more physically reasonable in an isentropic analysis than in an isobaric analysis.

Considerations in Estimating Horizontal Wind Gradients from an Individual Doppler Radar or a Network of Wind Profilers

Applications of Doppler weather radar data to the analysis of wind fields are reviewed. Radial velocity measurements from a single radar are used to estimate horizontal wind vectors within small azimuthal sectors using two different models. One assumes a uniform wind, the other a linear wind within the sector. Errors in wind estimates owing to gradients of wind are derived using harmonic analysis. The radar data analysis techniques are tested on complex wind patterns which were reconstructed from dual-Doppler radar measurements.

Verification of FGGE Assimilations of the Tropical Wind Field: The Effect of Model and Data Bias

Abstract We examine the tropical wind field analyses produced by a recent assimilation of the Final FGGE II-b dataset. Our aim is to estimate the effects, on the tropical wind analyses, of biases in the data and biases in the assimilation system. The assimilation system was similar to that used operationally at ECMWF in the first half of 1985. The period studied is the first Special Observing Period (SOP-1). Important differences occur in the intensity of divergence at upper and lower levels in the western Pacific, as measured by cloud-track winds (SATOBs) and by rawinsondes (TEMPs). There appear to be important biases also in the SATOB estimates of the zonal flow at upper and lower levels in the eastern Pacific. There are substantial biases in the wind directions at some west African stations. The 6-hour forecasts which provide the background fields for the analyses show important underestimates of the mean intensity of the tropical divergence field, particularly in the equatorial western Pacific. The er...

Use of Satellite Radiometric Imagery Data for Improvement in the Analysis of Divergent Wind in the Tropics

A scheme is proposed to incorporate satellite radiometric imagery data into the specification of initial conditions for the National Meteorological Center (NMC) operational global prediction model in order to improve the analysis of the divergent wind field in the tropics. The basic assumptions are that outgoing longwave radiation (OLR) data can provide 1) the division between convective (upward motion) and clear sky (downward motion) areas, and 2) the height of convection cells. The intensity of ascending motion in the convective areas is estimated based on OLR data. The intensity of descending motion is evaluated from the thermodynamic energy balance between radiative cooling and adiabatic warming, since the local time change of temperature is small in the tropics. Once the vertical motion field is determined, the horizontal divergence field can be calculated from the mass continuity equation. Then a divergent wind field is determined. The total wind is the sum of the new divergent wind and the rotational part, which is assumed to be unchanged. The proposed scheme is tested using the NMC analysis dataset of 21 January 1985 with satisfactory results.

Field studies of aldehyde chemistry in the Paris area

Measurements of carbonyl compound concentrations at different sites in the Paris area have been carried out. Interpretation of the results made use of the following data: general meteorological conditions, wind field analysis and type of primary pollutant sources. The principal phenomena observed were: a sharp formaldehyde decrease during rainfall; concentration levels of lower aldehydes in rural sites comparable to those found in the literature; an important variation in the ratio of primary aldehydes to secondary aldehydes depending on meteorological conditions; a significant increase in lower aldehyde concentrations downwind from the urban center despite vertical dispersion of the pollutants.

On the influence of map analysis formulation in the estimation of wind field derivatives

Many wind field analysis formulations which have been described in the meteorological literature do not take into account the cross-coherence of the sampled component variables. As a result, such formulations can produce spurious amplification or damping of derived vorticity and divergence. This effect is examined with particular reference to a linear weighting scheme in which an optimal vector component weighting is derived from a homogeneous and isotropic correlation model for streamfunction and velocity potential variables. It is demonstrated theoretically and in sample analyses that unless the contributions to total vector variance from divergent and solenoidal effects are equal, then estimated fields of both vorticity and divergence derived as linear functions of component data are adversely affected by the exclusion of cross-component weighting. The same effect is shown to occur when empirical distance-weighting or surface-fitting formulations are used, and is particularly serious when such methods are used to estimate divergence from scattered observations sampling a predominantly solenoidal field. It is concluded that univariate methods for analysis of atmospheric winds cannot in general produce unbiased estimates of wind field derivatives unless the scale of spatial sampling is much less than the characteristic spatial scale of velocity variation.

On the influence of map analysis formulation in the estimation of wind field derivatives

AbstractMany wind field analysis formulations which have been described in the meteorological literature do not take into account the cross‐coherence of the sampled component variables. As a result, such formulations can produce spurious amplification or damping of derived vorticity and divergence. This effect is examined with particular reference to a linear weighting scheme in which an optimal vector component weighting is derived from a homogeneous and isotropic correlation model for streamfunction and velocity potential variables. It is demonstrated theoretically and in sample analyses that unless the contributions to total vector variance from divergent and solenoidal effects are equal, then estimated fields of both vorticity and divergence derived as linear functions of component data are adversely affected by the exclusion of cross‐component weighting. The same effect is shown to occur when empirical distance‐weighting or surface‐fitting formulations are used, and is particularly serious when such methods are used to estimate divergence from scattered observations sampling a predominantly solenoidal field. It is concluded that univariate methods for analysis of atmospheric winds cannot in general produce unbiased estimates of wind field derivatives unless the scale of spatial sampling is much less than the characteristic spatial scale of velocity variation.

The Environment of Hurricane Debby (1982). Part I: Winds

Abstract A three-dimensional, nested analysis of wind fields in the environment of Hurricane Debby (1982) has been completed. The basic analysis tool uses a two-dimensional least-squares fitting algorithm combined with a derivative constraint that acts as a spatial low-pass filter on the analyzed field. Gridded results of horizontally analyzed fields are combined into vertical cross sections and then analyzed to produce vertical continuity. Consequently, a three-dimensional analysis is obtained. The database for the analysis comes primarily from Omega dropwindsondes (ODWs), rawinsondes, and satellite-derived winds above 400 mb in the environment of Hurricane Debby near 0000 UTC 16 September 1982. Since these data come from many different sources, and thus are not evenly distributed in the horizontal or vertical, techniques have been developed to alleviate difficulties associated with inhomogeneous data. The analyzed wind fields provide an independent evaluation of satellite-derived winds at and below 400 ...

Scale-Controlled Objective Analysis

Abstract The major topic of this paper is the resolvable spatial scales that can be analyzed by statistical interpolation of an undersampled dataset. The inquiry was motivated by the need to design the most appropriate procedures for spatial analysis of the upper-air sounding data from the GARP Atlantic Tropical Experiment. A reliable representation of horizontal scales in the analyzed wind fields was a matter of utmost concern, since the derived fields of vorticity, divergence and vertical motion were also of vital interest. To achieve our goal, it was found that the traditional promise of statistical interpolation had to be reexamined. The main conclusions of this theoretical inquiry are (i) resolvable scales are determined by the geometrical distribution of observing stations; (ii) precise knowledge of the second-moment statistics improves the analysis by de-aliasing the amplitudes of resolvable scales, but has no effect on the definition of resolvable scales; (iii) residual effects of unresolvable sig...

Some Experiments on the Use of Tropical Diabatic Heating Information for Initial State Specification

Abstract The local response to tropical diabatic heating is studied by using diabatic normal mode initialization procedures. It is shown that diabatic initialization is able to produce a well-balanced state in regions of heating provided the appropriate vertical modes are initialized. However, the persistence of balance during model integrations is strongly dependent on the compatibility between the specified heating during initialization and the heating during the model integration. Finally, a procedure is developed in which cloud-top temperature data is used to identify regions of deep convection where heating rates are specified during initialization. Although the procedure provides a means of combining the analysis of the divergent wind field and initialization, a number of problems remain, namely the specification of the intensity and vertical profiles of the heating rates.

30–60 Day Atmospheric Oscillations: Composite Life Cycles of Convection and Circulation Anomalies

Abstract Life cycles of the 30–60 day atmospheric oscillation were examined by compositing 30–60 day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing longwave radiation (OLR) for the years 1979–84. Separate composite life cycles were constructed for the May–October and November–April seasons using empirical orthogonal function analysis of the large-scale divergent wind field (250 mb velocity potential) to define the oscillation's phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields. Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown using sequences of composite maps. Good agreement exists between areas of upper-air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South Am...

Further Applications of Empirical Orthogonal Functions of Wind Fields for Tropical Cyclone Motion Studies

Abstract A physical basis is provided for representing the large-scale operationally analyzed wind fields around western North Pacific tropical cyclones by use of empirical orthogonal functions (EOFs). The synoptic differences in the environmental flow are demonstrated for cyclones having different initial directions and translation speeds. It is also shown that the wind-based EOF coefficients may be used to differentiate between future tracks of cyclones that have a similar initial direction of motion. Thus, the small sets of EOF coefficients that are used in statistical regression techniques for track prediction by Peak et al. have physical meaning and are not statistical artifacts. A new application of the EOF coefficients is to post-process the track predictions from the One-way influence Tropical Cyclone Model (OTCM), which is presently the best objective aid at the Joint Typhoon Warning Center (Guam). A 30% reduction in the forecast error at 72 h is achieved in the dependent sample. Thus, the synopt...

Moisture Bursts over the Tropical Pacific Ocean

“Moisture bursts” are bands of high clouds or middle and high clouds extending poleward and eastward from deep tropical locations into subtropical and middle latitudes. These events, synoptic in both temporal and spatial scales, are extremely common, particularly over the North Pacific Ocean. We define moisture bursts objectively, in primarily geometric terms, to emphasize both their synoptic scale and their tropical-extratropical interaction. We apply this definition to infrared satellite imagery for four 6-month cool seasons (November–April) in the eastern North Pacific (160°E to the west coast of the Americas). The frequency of thew events is about ten bursts per month during normal cool seasons, distributed uniformly across the Pacific to the wen of 110°W; east of this longitude, few moisture bursts occur. Half of the bursts last 2 to 4 days, and no bunt lasted longer than 10 days. Only 36 moisture bursts occurred during the 6-month El Niño cool season of 1982–83, with the location of occurrence shifted eastward. Few bursts occurred in the region of active tropical convection associated with the El Niño event. Because moisture burst frequency decreases at times when the ITCZ strengthens, we hypothesize two modes in Hadley cell behavior: a strong zonally symmetric mode, and a weaker mode comprised of the statistical ensemble of a large number of transient moisture bursts. Through analysis of wind fields, zonal averages across moisture bursts are shown to resemble transient intensification of the mean meridional circulation in regions where the Hadley cell is typically weak.

Tropical Ocean Circulation Experiments

A primitive equation model of the equatorial Pacific Ocean was forced by realistic wind stress distributions over decades. Results were presented for a set of two experiments. In the first experiment the model was forced by an objectively analyzed wind field, while for the second experiment a subjectively analyzed wind field was used. The results indicate a strong sensitivity of the model to the choice of the wind fields. Especially, model results in the eastern Pacific show big differences between the two model runs. Taking the results of the second model run the performance of the model with respect to interannual variability is investigated. Sea level, temperature and zonal currents show pronounced interannual variations within the equatorial belt from 10°N to 10°S. Special attention is given to the simulation of the 1982/83 El Niño event. The model reproduces most of the basic features, which were observed during this El Niño event. In particular the deceleration of the equatorial undercurrent, the evolution of eastward surface currents and the zonal redistribution of heat associated with an eastward propagation of warm water are simulated by the model.

Evaluation of surface wind and flux analysis techniques using conventional data in marine cyclones

Abstract Data from seven storms from the Storm Transfer and Response Experiment in November 1980 have been used to evaluate the relative accuracy of surface wind and flux fields based on two analysis procedures. Two essentially independent techniques were used; objective analysis which is based on composite data taken at several synoptic intervals to enhance the number of observations and processing carefully hand-analyzed (subjective) surface pressure analyses into wind and flux fields using a planetary boundary layer (PBL) similarity model. Scale and accuracy limits imposed by instrument accuracy, sampling error, and gridding and analysis procedures are evaluated for each of these techniques by comparison with independent data and with each other. Wind field differences between the objective composite analysis and the PBL model predictions are found to be comparable to the measurement-related uncertainty in the observations. Unresolved variability in the 10–100 km scale of the dynamic and thermodynamic variables produces the main source of error in both the objective and model wind fields. Additional wind field differences are contributed by PBL and gradient wind assumptions used in the PBL model. Wind differences between either of the two analyses and individual observations are about ±3 m s−1 and ±30° in the mean, and can be greater than ±5 m s−1 and ±50° for small regions. Comparable differences are found between the two wind field analyses. The wind and thermodynamic field differences combine to produce substantial differences in the derived fields. Mean differences of ±19W m−2 and ±41 W m−2 for the fluxes of sensible and latent heat, respectively, represent differences of about 50% of the mean fluxes, with local differences as much as double or triple the magnitude of these means. Fronts are equally well represented by the two analyses, but values of divergence and curl of surface stress may differ by a factor of 2 or more in regions of fronts. These local differences in the derived fields result primarily from the large wind field differences in these inadequately resolved regions.

Comparison of Global Diabatic Heating Rates from FGGE Level IIIb Analyses with Satellite Radiation Imagery Data

We evaluate the global distribution of diabatic heating rates based on the thermodynamic energy budget using the ECMWF Level IIIb FGGE analyses. We select the two 15-day periods of 27 January to 10 February 1979, during the FGGE Special Observing Period (SOP)-I, and 7–21 June 1979, in the SOP-II. An effort is made to examine the daily variation of tropospheric diabatic heating in contrast with previous investigations which dealt with the climatological aspect. The daily variations of tropospheric diabatic heating appear to be related to synoptic features of the global circulation, indicating that the diagnosed heating distributions provide useful information related to numerical weather prediction. Nevertheless, a question may be raised regarding the accuracy of the daily variations of diagnosed heating rates, since the calculation of diabatic heating involves an observationally sensitive quantity, i.e., the vertical motion. One way to investigate the accuracy of diagnosed heating rates is to compare the present results with similar calculating using different FGGE datasets. In the Appendix, we present the diagnosed heating rats based on the GFDL Level IIIb data for the SOP-I study period. While comparisons of heating rates calculated from different analyses are revealing, we still lack an independent measure of comparison. We propose, therefore, the use of infrared and visible radiometric imagery data from the TIROS-N to examine the reliability of diagnosed heating and vertical velocity distributions in the tropics. A stratification of radiance data in terms of cloud types helps to establish useful relationships between infrared radiance and diabatic heating (and vertical velocity), suggesting that radiance data may be used to infer the distribution of diabatic heating in the tropics and to improve the analysis of the divergent wind field.

Numerical simulation of air pollution in urban areas: Model development

A three-dimensional, grid-based numerical air pollution model for the estimation of air pollutant concentrations in an urban area is developed. Based on the continuity equation, the modeling system incorporates the combined influences of advective transport, turbulent diffusion, chemical transformation, source emissions and surface removal of air contaminants. Recent developments in plume rise and plume penetration processes, objective wind field analysis procedures and numerical solution techniques incorporated into the model are described.

The 40–50 Day Oscillation of the Low-Level Monsoon Circulation over the Indian Ocean

Abstract It has been observed that the low-level monsoon circulation, especially the Somali jet, exhibits a 40–50 day oscillation and obtains its maximum intensity in this oscillation when the migrating transient monsoon trough approaches ∼20°N. The data generated by the FGGE III-b analyses of the European Centre for Medium Range Weather Forecasts for the northern summer were used to explore the air mass source of this oscillation and to explain energetically and synoptically the intensification and decay of the low-level monsoon circulation in association with the 40–50 day oscillation. A synoptic analysis of the divergent wind fields suggests that the convergence induced by the intertropical convergence zone and the deepening of the monsoon trough over northern India supplies the air mass to the 40–50 day oscillation. The energetics analysis shows that the 40–50 day oscillation of the low-level monsoon circulation is essentially described by the rotational mode. The 40–50 day oscillation of this flow fi...