Numerical simulation of sea and bay breezes in a weak shear environment

Summary A numerical mesoscale model (COAMPS) is used to study some of the features associated with the evolution of the kinematic, thermodynamic, and physical structure of the Alabama sea and bay breeze circulations and convections in weak shear environments based on five cases from Medlin and Croft (1998). The general and expected features and evolution of sea and bay breeze circulations are captured by the model simulations, including horizontal and vertical wind shifts, thermal contrast between land and water surface, vertical stability over water and land, return currents and moisture increase. The relationship of the circulations to specific synoptic flow regimes and local physiographic features was investigated. The sea breeze triggered convective cells are confirmed to have a preferred location according to the flow regime and local conditions. This result can assist the forecasters in understanding the anticipated convective cell initiation and development on a given day as related to sea and bay breeze cells as well as improve the short-term forecast accuracy of the location of thunderstorm initiation based on routine observations and subsequent convective activity. If local NWS office model a selective subset of cases then they can better visualize and forecast those cases operationally.     

Taylor's hypothesis and spectra of velocity and temperature derivatives in a turbulent shear flow

Spectral densities of temporal velocity and temperature derivatives, measured in the atmospheric turbulent boundary layer and on the axis of a turbulent plane jet, are compared with the spectral densities of the ratios of these derivatives and of the local longitudinal instantaneous velocity. The comparison between the spectral densities, when both are normalized such that the areas under the spectra are equal, shows that the corrected spectrum is lower than the measured spectrum at frequencies larger than about one-seventh of the Kolmogorov frequency while the opposite trend is observed at low frequencies. A qualitatively similar result has been obtained when the measured velocity derivative spectrum is compared with that corrected using Lumley's method. While the unnormalized spectral densities and moments of the derivatives, as corrected by Lumley's method are smaller than the corresponding measured quantities, the opposite trend is found when the correction is made by dividing the temporal derivative by the longitudinal instantaneous velocity.     

The effects of mesoscale turbulence on plume rise in a land/lake environment

An extensive data base of LIDAR and photographically measured plumes from the Nanticoke thermal generating station (located on the north shore of Lake Erie) has been accumulated over several years of full-scale atmospheric experiments. Analysis of the data shows that the empirical entrainment constant used in the Briggs simple plume rise formulation has a value of 0.8, which is significantly larger than 0.6 found by Briggs (1975). A hypothesis is proposed that mesoscale eddies present in the land/lake environment are responsible for vertical plume meander leading to the observed enhanced vertical plume spread and lower mean plume rise at this site. This is substantiated through theoretical considerations and experimental data. The results are significant in the computation of ground-level concentrations at such complex terrain sites.     

The interaction of ellipsoidal vortices with background shear flows in a stratified fluid

We show that an arbitrarily oriented ellipsoid of uniform potential vorticity, embedded in a background flow described by a quadratic streamfunction, is an exact solution of the quasigeostrophic equations governing motion in a uniformly stratified, unbounded fluid. This type of flow includes plane horizontal shear and strain as well as uniform vertical shear of a unidirectional horizontal flow. We derive ordinary differential equations describing the motion of such a vortex and discuss some aspects of their solutions. We note the existence of steady states (solutions in which the vortex is in equilibrium with the background flow), of periodic solutions near these steady states, of non-periodic trajectories which nervertheless remain in the vicinity of the steady states, and of solutions which represent the shearing out of the vortex by the background flow. We try to use this information to propose partial answers to the question of when a given horizontal or vertical shear flow is likely to destroy a vortex and when a vortex might survive external shear and strain.     

Determination of the momentum flux at the air-sea interface under variable meteorological and oceanographic conditions: Further application of the wind-wave interaction method

Simultaneous observations of wind, wave, and stability parameters made recently by several authors provide an evaluation of the contribution of these factors to the determination of wind stress on the sea surface. It is shown that under diabatic conditions the wind-wave interaction method of determining wind stress is superior to the method utilizing correction for stability. The implication is that the contribution from waves is more important to the stress than that from stability. Thus, the wind-wave interaction method may be applicable under a variety of conditions. For general meteorological-oceanographic applications, a nomograph is also provided for estimating the wind stress from commonly available wind and wave parameters.     

层结流体中具有纬向基本剪切流的Rossby波振幅的mKdV方程

In this paper,modified Korteweg-de Vries (mKdV) equations for the amplitude of solitary Rossby waves in stratified fluids with a zonal shear flow are derived by using a weakly nonlinear method.It is found that the coefficients of mKdV equations depend not only on the β effect and the Visl-Brunt frequency,but also on the basic shear flow.     

A method for solving the planetary boundary-layer equations

A method for solving the nonlinear three-dimensional steady-state equations which govern planetary boundary layer flow is described. The method is applicable to air motions over terrain with horizontally varying surface roughness, temperature and moisture. It can also be applied to a physical system consisting of the air and the sea (or earth) boundary layers taken together. Examples of calculations for selected cases of terrain variations are presented. Convergence of the method has been assessed by determining the degree to which the solutions satisfy the set of equations.The results of the calculations show that the method produces qualitatively realistic distributions of the different meteorological variables.Contribution No. 1251 from the University of Miami, Rosenstiel School of Marine and Atmospheric Sciences.     

An ocean wind-wave climatology for the Southern Brazilian Shelf. Part II: Variability in space and time

Previously validated model results were used to characterize the wave climate over the Southern Brazilian Shelf (SBS). The low mean significant wave height over the western South Atlantic shelves was shown together with examples of cyclone-induced extreme wave fields and other typical wave conditions. The mean offshore spectra showed a bimodal shape with a predominance of S/SSW and ENE/E waves with distinctive interannual rising periods in wave energy density. Along-shelf wave energy gradients were seen near the coast with higher energy located off capes and coastal projections and energy minima between them. A considerable drop in wave energy suggests the 40 m depth as the mean wave base and consequently the lower limit of the SBS shoreface. The upper shoreface mean wave energy density varied abruptly along the shelf in response to differences in bottom declivities. The large and shallow shoreface was responsible for an intense refraction of the waves and hence very small angles of attack. Additionally, it was shown the sheltering effect caused by capes and coastal projections and a remarkable north/south energy asymmetry between them, caused by a windowing on the wave propagation to the shore. Altogether, it was possible to state that bottom friction plays a major role in wave differentiation along the SBS shoreface, thus suggesting that shelf morphology might indeed be more important to generate wave variability than the offshore wave variation itself.     

A numerical study of the interaction between two simultaneous storms: Goni and Morakot in September 2009

Significant anomalous tracks were observed when the severe tropical storm Goni(0907) and typhoon Morakot(0908) in September 2009 were evaluated in short distances.The relationship between the two is regarded as a case of binary interaction.Based on an MM5 model(fifth-generation mesoscale model of Pennsylvania State University-National Center for Atmospheric Research),in this study a series of sensitivity experiments were designed to determine the binary interaction between them.The sensitivity of the storm characteristics to the binary interaction was demonstrated through modeling experiments with different TC intensities and sizes based on the bogus vortices initialization.Furthermore,the contributions of large-scale environmental flow and the effects of interaction between the motions of the cyclones were distinguished by numerical experiments using only one of the TC vortices.Results from these experiments show that Morakot(0908) had a greater impact on the motion of Goni(0907),whereas Goni(0907) had a relatively limited impact on Morakot(0908).At the upper level,the northeasterly jet flow in the third quadrant of Morakot(0908) enhanced the upper-level divergence of Goni(0907) and had an important role in maintaining and increasing Goni’s(0907) intensity.And at the lower level,Morakot(0908),with strong convergence and ascending airflow,made a stable transport channel of southwesterly warm and wet flow,thus supporting the lower-level water vapor convergence of Goni(0907).Goni(0907),which was located upriver of the southwesterly flow,intercepted part of the water vapor transportation in the southwesterly flow,causing the water vapor convergence to strengthen while the water vapor convergence of Morakot(0908) weakened.     

A simple model for describing the heat balance of a shallow lake with application to Lake Chad

A model is described allowing the computation of a shallow lake's surface temperature as a function of the meteorological conditions of the surroundings. An equilibrium temperature with no heat flux through the surface is defined. The daily cycle of the equilibrium temperature permits computation of the average and the amplitude of the lake's temperature in the form of a simple sine wave. The computed values are compared with satellite measurements of the surface temperature and good agreement is achieved. When the computed surface temperature is applied, the values of the fluxes of latent and sensible heat are in good agreement with observational data.     

A numerical method for solving the evolution equation of solitary Rossby waves on a weak shear

In this paper an evolution equation in integral-differential form for finite amplitude Rossby waves on a weak shear is presented and an efficient method for its numerical solution is set up. It is shown that a propa-gation of solitary wave is possible whenever a proper weak shear in basic flows acts with the nonlinear effects and dispersion of the media, both in the atmosphere and in the ocean. To test the numerical method for solving the evolution equation, a series of experiments are carried out. The results indicate that the solitary solutions do exist and interact with each other in quite a succinct, manner. Therefore the method is successful and efficient for solving initial value problems of the above equation. The time decoupling problem arising in the numerical scheme and the related filtering technique are discussed. A variety of interesting phenomena such as the interaction of solitary Rossby waves, damping, dispersion and the development of nonlinear wave train are numerically studied.     

A radar-based verification of precipitation forecast for local convective storms

Local flash flood storms with a rapid hydrological response are a real challenge for quantitative precipitation forecasting (QPF). It is relevant to assess space domains, to which the QPF approaches are applicable. In this paper an attempt is made to evaluate the forecasting capability of a high-resolution numerical weather prediction (NWP) model by means of area-related QPF verification. The results presented concern two local convective events, which occurred in the Czech Republic (CR) on 13 and 15 July 2002 and caused local flash floods. We used the LM COSMO model (Lokall Model of the COSMO consortium) adapted to the horizontal resolution of 2.8 km over a model domain covering the CR. The 18 h forecast of convective precipitation was verified by using radar rainfall totals adjusted to the measured rain gauge data. The grid point-related root mean square error (RMSE) value was calculated over a square around the grid point under the assumption that rainfall values were randomly distributed within the square. The forecast accuracy was characterized by the mean RMSE over the whole verification domain. We attempt to show a dependence of both the RMSE field and the mean RMSE on the square size. The importance of a suitable merger between the radar and rain gauge datasets is demonstrated by a comparison between the verification results obtained with and without the gauge adjustment. The application of verification procedure demonstrates uncertainties in the precipitation forecasts. The model was integrated with initial conditions shifted by 0.5° distances. The four verifications, corresponding to the shifts in the four directions, show differences in the resulting QPF, which depend on the size of verification area and on the direction of the shift.     

A numerical study of the Chicago lake breeze

A two-dimensional numerical model is presented for the study of the Chicago area lake breeze. The hydrostatic approximation used by previous workers in this field is not employed. Instead, the complete z-momentum equation is solved and the equation of continuity is retained in its original form. The numerical model employs a staggered mesh, and Alternating Direction Implicit methods are used for the integration of the equations. Computational stability is achieved without external filters, upstream space differencing, or artificial horizontal diffusion terms.The results obtained for realistic initial conditions but simplified boundary conditions compare reasonably well with observations obtained by Lyons in his 1967 observational study of the Chicago lake breeze. Limiting forms of the lower boundary condition bracket the observed wind speeds and other kinematic features of the breeze. Among the various eddy diffusivity models, that with constant eddy diffusivity with height produces the most realistic results. The models with a decrease of eddy diffusivity with height show unrealistic flow patterns in the upper layers. The stability of the atmosphere at the onset of the breeze is shown to have a pronounced effect on the intensity of the circulation, but not on the thickness of the inflow layer, which appears quite insensitive to large changes in the parameters involved.     

Sensitivity analysis of the equations for a convective mixed layer

Jump or slab models are frequently used to calculate the depth of the convectively mixed layer and its potential temperature during the course of a clear day. Much attention has been paid theoretically to the parameterization of the budget for turbulent kinetic energy that is required in these models. However, for practical applications the sensitivity of the solutions of the model equations to variations in the entrainment formulation and in the initial and boundary conditions is also very important. We analyzed this sensitivity on the basis of an analytical solution for the model which uses the well-known constant heat flux ratio. The initial conditions for the mixed-layer height (h) and potential temperature ( _m ) quickly lose their influence. Only the initial temperature deficit is important. The mixed-layer temperature at noon on convective days is insensitive to the entrainment coefficient c. It is governed by the integral of the heat input and by the stable lapse rate. A change in c from 0.2 to 0.5 leads to a variation of 20% in h. This is not very much considering the accuracy in the determination of h from actual observations.     

A cold air lake formation in a basin — a simulation with a mesoscale numerical model

Summary A formation of a cold air lake in a basin is studied with a mesometeorological model.A dynamic Boussinesq hydrostatic mesoscale numerical model is developed in a staggered orthogonal grid with a horizontal resolution of 1 km and with a varying vertical grid. The topography is presented in a block shape so that computation levels are horizontal.The mesometeorological model is tested in three idealized topography cases (a valley, a single mountain, a basin) and test results are discussed.In an alpine basin surrounded by mountains and plateaus the air is supposed to be stagnant at the beginning of the night. Due to differences in radiation cooling an inversion layer is formed in the basin and local wind circulation is studied by model simulations.With 14 Figures     

Trends in evaporation of a large subtropical lake

In order to further investigate the capability of the Standardized Precipitation Index (SPI) to identify flood/drought events, monthly precipitation data from 26 precipitation stations and monthly discharge data from four hydrological stations from 1960 to 2006 in the Minjiang River basin were used to analyze the correlations between multiple time scales of the SPI and river discharge. The SPI series that had a maximum correlation with discharge was chosen to detect flood/drought events in the basin, and the results were compared to historical flood/drought events. The results indicated the following. (1) High Pearson correlations between the SPI and discharge were identified at shorter time scales (1 to 3 months), and the maximum correlation was found on the time scale of 2 months. (2) Five floods among the six largest historical flood events in the Minjiang River basin were identified with the 2-month SPI, but the SPI does have shortcomings in identifying more general floods. The SPI also identified major drought events that were consistent with historical data. This demonstrates that the 2-month SPI is an effective indicator for the identification of major flood/drought events in the Minjiang River basin.     

A regional model for studies of atmosphere-ice-ocean interaction in the western Arctic

Summary Asa step in the development of a fully coupled regional model of the atmosphere-ice-ocean system, atmospheric and sea ice models have been adapted to a western Arctic domain centered on the Bering Strait. Lateral boundary conditions derived from operational analyses drive the models through simulations on grids having horizontal resolutions of 21 km and 7 km. Sensitivities to the presence of sea ice are large after only 48 hours, by which time the surface temperatures in the Bering and Chukchi Seas are 10–15°C higher without sea ice than with sea ice. The temperatures, in turn, modify the fields of sea level pressure, surface wind and precipitation. By influencing the surface wind stress through the static static stability, the surface state feeds back to the surface momentum exchange, ice/ocean transport, and the rate of formation of new ice. The results also show a resolution-dependence of the surface winds, precipitation rates and new ice formation rates, particularly in areas in which the coastal configuration and topography are spatially complex. The experiments will be augmented by the implementation of an ocean model on the same grids.With 12 Figures     

A verification of an analytical formula for estimating the height of the stable internal boundary layer

A stable thermal internal boundary layer (IBL) develops when warm air is advected from warmer land upstream to a cooler sea downstream. It is shown that the analytical model for estimating the height (h) of this stable IBL as formulated by Garratt (1987) is verified. It is also demonstrated that a simpler equation, % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWGObGaeyisIS% RaaGymaiaaiAdacaWGybWaaWbaaSqabeaadaWcgaqaaiaaigdaaeaa% caaIYaaaaaaaaaa!390B!\[h \approx 16X^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \] (where h is in meters and X, the fetch downwind, is in kilometers), is useful operationally as a first approximation.