斜交型不稳定谱点分布侧半圆定理及其增长率上界的估计

本对斜交型扰动不稳定谱点的分布做了理论分析,得到了该谱点分布的半圆定理一该谱点分布在复一面上以原点为圆心以R0为半径的上半平面上,同时还对该不稳定增长率的上界作了估计。发现水平永度越小,模式顶越高则该估计值越大;垂直风切变的增大和纬度的增高对该增长率的增大有正贡献;当层结稳定度减小时,最大增长率随相对最大增长率得增大而减小。     

Stratiform Cloud—Inversion Characterization During the Arctic Melt Season

Data collected during July and August from the Arctic Ocean Experiment 2001 illustrated a common occurrence of specific-humidity (q) inversions, where moisture increases with height, coinciding with temperature inversions in the central Arctic boundary layer and lower troposphere. Low-level stratiform clouds and their relationship to temperature inversions are examined using radiosonde data and data from a suite of remote sensing instrumentation. Two low-level cloud regimes are identified: the canonical case of stratiform clouds, where the cloud tops are capped by the temperature inversion base (CCI—Clouds Capped by Inversion) and clouds where the cloud tops were found well inside the inversion (CII—Clouds Inside Inversion). The latter case was found to occur more than twice as frequently than the former. The characteristic of the temperature inversion is shown to have an influence on the cloud regime that was supported. Statistical analyses of the cloud regimes using remote sensing instruments suggest that CCI cases tend to be dominated by single-phase liquid cloud droplets; radiative cooling at the cloud top limits the vertical extent of such clouds to the inversion base height. The CII cases, on the other hand, display characteristics that can be divided into two situations—(1) clouds that only slightly penetrate the temperature inversion and exhibit a microphysical signal similar to CCI cases, or (2) clouds that extend higher into the inversion and show evidence of a mixed-phase cloud structure. An important interplay between the mixed-phase structure and an increased potential for turbulent mixing across the inversion base appears to support the lifetime of CII cases existing within the inversion layer.     

Indigenous Australians’ knowledge of weather and climate

Although the last 200 years of colonisation has brought radical changes in economic and governance structures for thousands of Aboriginal and Torres Strait Islanders living in remote areas of northern Australia, many of these Indigenous people still rely upon, and live closely connected to, their natural environment. Over millennia, living ‘on country’, many of these communities have developed a sophisticated appreciation of their local ecosystems and the climatic patterns associated with the changes in them. Some of this knowledge is recorded in their oral history passed down through generations, documented in seasonal weather calendars in local languages and, to a limited degree, transcribed and translated into English. This knowledge is still highly valued by these communities today, as it is used to direct hunting, fishing and planting as well as to inform many seasonally dependant cultural events. In recent years, local observations have been recognised by non-Indigenous scientists as a vital source of environmental data where few historic records exist. Similar to the way that phenological observations in the UK and US provide baseline information on past climates, this paper suggests that Indigenous observations of seasonal change have the potential to fill gaps in climate data for tropical northern Australia, and could also serve to inform culturally appropriate adaptation strategies. One method of recording recent direct and indirect climate and weather observations for the Torres Strait Islands is documented in this paper to demonstrate the currency of local observations of climate and its variability. The paper concludes that a comprehensive, participatory programme to record Aboriginal and Torres Strait Islander knowledge of past climate patterns, and recent observations of change, would be timely and valuable for the communities themselves, as well as contributing to a greater understanding of regional climate change that would be useful for the wider Australian population.     

Coupling Mesoscale Modelling with a Simple Urban Model: The Lisbon Case Study

The ongoing trend of urbanisation worldwide is leading to a growing requirement for detailed flow and transport parameterisations to be included within numerical weather prediction (NWP) models. Such models often employ a simple roughness parameterisation for urban areas, which is not particularly accurate in predicting or assessing the flow and dispersion at street scale. Moreover, this kind of parameterisation offers too poor a representation of the mechanical and thermal forcing exerted by urban areas on the larger scale flow. At present, high computational costs and long simulation running times are among the constraints for the implementation of more detailed urban sub-models within NWP models. To overcome such limitations, a downscaling procedure from the atmospheric flow at the synoptic scale to the neighbourhood scale and below, is presented in this study. This is achieved by means of a simple urban model based on a parameterised formulation of the drag exerted by the building on the airflow. Application of the urban model for estimating spatially-averaged mean wind speed and the urban heat island over a selected neighbourhood area in Lisbon, Portugal, is presented. The results show the capability of the urban model to provide more accurate mean wind and temperature profiles. Moreover, the urban model has the advantage of being cost effective, as it requires small computational resources, and thus is suitable to be adopted in an operational context. The model is simple enough to be also used to assess how the resolving of urban surface processes may affect those at the larger scales.     

Coherent Structures and the Dissimilarity of Turbulent Transport of Momentum and Scalars in the Unstable Atmospheric Surface Layer

Atmospheric stability effects on the dissimilarity between the turbulent transport of momentum and scalars (water vapour and temperature) are investigated in the neutral and unstable atmospheric surface layers over a lake and a vineyard. A decorrelation of the momentum and scalar fluxes is observed with increasing instability. Moreover, different measures of transport efficiency (correlation coefficients, efficiencies based on quadrant analysis and bulk transfer coefficients) indicate that, under close to neutral conditions, momentum and scalars are transported similarly whereas, as the instability of the atmosphere increases, scalars are transported increasingly more efficiently than momentum. This dissimilarity between the turbulent transport of momentum and scalars under unstable conditions concurs with, and is likely caused by, a change in the topology of turbulent coherent structures. Previous laboratory and field studies report that under neutral conditions hairpin vortices and hairpin packets are present and dominate the vertical fluxes, while under free-convection conditions thermal plumes are expected. Our results (cross-stream vorticity variation, quadrant analysis and time series analysis) are in very good agreement with this picture and confirm a change in the structure of the coherent turbulent motions under increasing instability, although the exact structure of these motions and how they are modified by stability requires further investigation based on three-dimensional flow data.     

Effect of Ocean Spray on Vertical Momentum Transport Under High-Wind Conditions

Two mathematical models are proposed detailing the influence of ocean spray on vertical momentum transport under high-wind conditions associated with a hurricane or severe storm. The first model is based on a turbulent kinetic energy (TKE) equation and accounts for the so-called lubrication effect due to the reduction of turbulence intensity. The second model is based on Monin–Obukhov similarity (MOS) and uses available experimental data. It is demonstrated that the flow acceleration is negligible for wind speeds below a certain critical value due to the fact that the spray volume concentration is low for such speeds. For wind speeds higher than the critical value, the spray concentration rapidly increases, which results in significant flow acceleration. Both models produce qualitatively similar results for all turbulent flow parameters considered. It was found that the MOS-based model tends to predict a noticeably stronger lubrication effect than the TKE-based model, especially for lower wind speeds. The results of model calculations are in very good agreement with available experimental data for the spray production values near the upper bound. It is also shown that neither the value of the turbulent Schmidt number in the TKE-based model nor the choice of a stability profile function affects the spray-laden flow dynamics significantly.