Wind profiles over complex terrain

A large set of routine wind profiles has been analyzed at three towers, located in various types of complex terrain in New England. After allowing for effects of roughness change, uphill flow and stability, roughness lengths have been estimated. In general, the profiles and roughness lengths could be explained by differences in terrain features and stability.     

On equilibrium profiles of suspended particles

A power law is often used to represent the vertical profile of uniform suspended particles above a horizontally homogeneous surface. It serves as an analytical solution representing an equilibrium between vertical turbulent diffusion and gravitational settling, andcan be used to extract settling velocity information from observed particle number density profiles. In this note, we analyse this situation and use a numerical model to investigate the temporal change of particle number density and of the net vertical flux due to turbulent diffusion and gravitational settling. The results show that the net flux approaches zero very slowly for small particles (w_s/ u_* < 1), and show that the power law does not hold for small particles. If the power law is used to extract settling velocities from observed vertical distributions of particle number density in these cases, the estimated settling velocity may be unrealistically large.     

Vertical radar reflectivity profiles in Slovenia

Summary Radar reflectivity from hydrometeors is used for the estimation of the precipitation rate at ground level. If the vertical reflectivity profile is taken into account, the estimate can be improved considerably. In the first part of the article some theoretical explanations are given for the two most pronounced characteristic of the vertical radar reflectivity profile from clouds. In general, the observed values decrease with height in the upper part of the radar echo due to the growth of precipitation particles by collision and coalescence. The effect of the bright band, especially in more stratiform types of precipitation, adds a significant strong peak to the profile, at the approximate height of the 0 °C isotherm. These explanations, although being simplified, also provide a quantitative explanation of the two characteristics mentioned previously. Averaged seasonal characteristics of vertical profiles in Slovenia are used as the climatological basis for the construction of an idealised profile for correcting the precipitation estimate. For individual cases, and also after averaging, a maximum in the profiles can clearly be detected. This maximum is much sharper if the profiles are normalised. When looking at time changes, it is shown that most of the changes in radar reflectivity, on average, occur during a roughly 6-hour time-lag between the two measurements. With greater time-lags, the differences are smaller on average. This is caused by the local natural evolution of the precipitation field and indicates that a 6-hour to 12-hour accumulating and averaging of data could diminish much of the error due to the time variation in radar estimated precipitation.With 5 Figures     

Wind profiles at the Boulder Tower

Analysis of wind profiles at the Boulder Tower (BAO) leads to these conclusions:

1. The variation of roughness with wind direction found earlier is confirmed. Roughness lengths measured on the tower are larger than those measured close to the surface.
2. The profiles and measurements of Reynolds stress are consistent with a von-Karman constant of 0.35.
3. The form φ_m=(1?15z/L)^-1/3 fits best in the range -0.6 < z/L < 0. In the range 0 < z/L < 0.5, θ _m ～ 1 + 4.7z/L provides a good fit to the observations. For z/L < 0.1, φ _m also depends on h, the thickness of the PBL. For z/L < -0.6, Φ _m approaches the constant 0.5, in contrast to all previous suggestions. For larger stabilities, the upper level is usually not in the surface layer, and wind ratios become independent of z/L.
4. With snow cover, the effective roughness diminishes to about 1 cm, even for directions for which the roughness length without snow is large.
5. Estimation of winds at 100 or 150 m from information near the surface is best for similarity theory provided that the ratio of height to Monin-Obukhov L is less than 0.1. For larger z/L, simple power laws seem more appropriate.

     

Wind profiles through boundary-layer capping inversions

Wind profiles have been measured through daytime boundary-layer capping inversions at a site surrounded by fairly flat terrain. The measurements were made by an instrument package mounted on a tethered balloon cable. Analysis of the results has shown that in the region of the inversion, the wind profiles generally show strong shears and also relatively narrow jet or minima features that vary with time. The profiles have been compared with the results of simple one-dimensional models which reproduce several of the observed features. The comparison has shown that some of these features are a result of inertial oscillations taking place in the stable layer immediately above the inversion, and suggests that the oscillations are coherent over a distance of hundreds of kilometres. Although their origin is related to that of the noctural jet, the daytime jets observed have a different structure due to the elevation of the stable layer. In particular, a narrow jet is formed when the inertial oscillation is at its minimum. This has no counterpart in the nocturnal case.     

Projected changes in vertical temperature profiles for Australasia

The vertical temperature profile in the atmosphere reflects a balance between radiative and convective processes and interactions with the oceanic and land surfaces. Changes in vertical temperature profiles can affect atmospheric stability, which in turn can impact various aspects of weather systems. In this study, we analyzed recent-past trends of temperature over the Australian region using a homogenized monthly upper-air temperature dataset and four reanalysis datasets (NCEP, ERA-Interim, JRA-55 and MERRA). We also used outputs of 12 historical and future regional climate model (RCM) simulations from the NSW/ACT (New South Wales/Australian Capital Territory) Regional Climate Modelling (NARCliM) project and 6 RCM simulations from the CORDEX (Coordinated Regional Downscaling Experiment) Australasian project to investigate projected changes in vertical temperature profiles. The results show that the currently observed positive trend in the troposphere and negative trend in the lower stratosphere will continue in the future with significant warming over the whole troposphere and largest over the middle to upper troposphere. The increasing temperatures are found to be latitude-dependent with clear seasonal variations, and a strong diurnal variation for the near surface layers and upper levels in tropical regions. Changes in the diurnal variability indicate that near surface layers will be less stable in the afternoon leading to conditions favoring convective systems and more stable in the early morning which is favorable for temperature inversions. The largest differences of future changes in temperature between the simulations are associated with the driving GCMs, suggesting that large-scale circulation plays a dominant role in regional atmospheric temperature change.

     

Sensitivity of MJO simulations to diabatic heating profiles

The difficulty for global atmospheric models to reproduce the Madden–Julian oscillation (MJO) is a long-lasting problem. In an attempt to understand this difficulty, simple numerical experiments are conducted using a global climate model. This model, in its full paramterization package (control run), is capable of producing the gross features of the MJO, namely, its planetary-scale, intraseasonal, eastward slow propagation. When latent heating profiles in the model are artificially modified, the characteristics of the simulated MJO changed drastically. Intraseasonal perturbations are dominated by stationary component over the Indian and western Pacific Oceans when heating profiles are top heavy (maximum in the upper troposphere). In contrast, when diabatic heating is bottom heavy (maximum in the lower troposphere), planetary-scale, intraseasonal, eastward propagating perturbations are reproduced with a phase speed similar to that of the MJO. The difference appears to come from surface and low-level moisture convergence, which is much stronger and more coherent in space when the heating profile is bottom heavy than when it is top heavy. These sensitivity experiments, along with other theoretical, numerical, and observational results, have led to a hypothesis that the difficulty for global models to produce the MJO partially is rooted in a lack of sufficient diabatic heating in the lower troposphere, presumably from shallow convection.     

A systematic algorithm for obtaining SODAR wind profiles

Summary Current methods of obtaining wind profiles from SODARs generally employ ad hoc strategies for Doppler spectrum peak selection, profile smoothing, and consistency checks. This paper addresses the problem of systematically obtaining vertical profiles of wind vectors from a SODAR. A new approach is to regard every spectrum frequency as a possible estimator of the radial wind component, but to weight these Doppler estimates according to the spectral power at each frequency. This allows the peak detection, smoothing, and assimilation of a priori information to be combined into a constrained linear inversion methodology. The inversion process also allows an estimation of signal-to-noise ratio, degrees of freedom, and information content versus height. Examples are given of profiles obtained using this new inverse method. The relationship to the physical limitations of the SODAR instrument and the atmosphere is discussed.     

Some characteristics of ozone profiles above Hong Kong

Summary Analysis of ozonesonde data shows that in the lower troposphere above Hong Kong, there is a relative maximum with respect to height in all seasons except winter. In the upper troposphere, there is with respect to height a relative minimum in the seasonally averaged ozone mixing ratio in winter. Ozone mixing ratios in the upper troposphere in winter and spring can be significantly enhanced by stratospheric intrusions associated with the passage of cold fronts and upper cut-off lows.For Hong Kong, the seasonally averaged total ozone has the highest value in spring, and the lowest in winter. The seasonally averaged total tropospheric ozone also has the highest value in spring, but the lowest in summer. In a relative sense, total tropospheric ozone contributes most to the total ozone in spring and the least in summer.The phase of the total ozone anomaly above Hong Kong is influenced by the Quasi-Biennial Oscillation (QBO), with the positive anomaly associated with the easterly phase of QBO, and the negative anomaly the westerly phase.