Low level jet intensification by mineral dust aerosols

Modification of the intensity of a low level jet (LLJ) and near-surface wind speed by mineral dust is important as it has implications for dust emission and its long-range transport. Using the Weather Research and Forecasting with Chemistry (WRF/Chem) regional model, it is shown that direct radiative forcing by mineral dust reduces temperature in the lower atmosphere, but increases it in the layers aloft. The surface cooling is shown to be associated with a reduction of turbulent kinetic energy (TKE) and hence vertical mixing of horizontal momentum. Changes in the vertical profile of temperature over the regions that are under the influence of a LLJ are shown to result in an intensification of the LLJ and near-surface wind speed, but a decrease of winds aloft. These changes in the wind speed profile differ from results of previous research which suggested a decrease of wind speed in the lower atmosphere and its increase in the upper boundary layer. © Author(s) 2013

Atmospheric forcing of the three-dimensional distribution of dust particles over Australia: A case study

Knowledge of atmospheric processes responsible for horizontal and vertical distribution of dust particles is important for identifying their transport pathways. The Weather Research and Forecasting with Chemistry (WRF/Chem) model, complemented by observations, was used to simulate and observe the three-dimensional distribution of Australian dust for a severe dust event during 22-23 September 2009. The passage of a cold front modified the boundary layer structure during this event, allowing dust to be lifted and subsequently transported for a significant distance. The model simulated the maximum dust concentrations to be located behind the cold front as a result of strong post-frontal wind speeds. The boundary layer depths were also shown to be significantly deeper behind the cold front where dust particles could be lifted to the top of the boundary layer by turbulent fluxes associated with strong mixing. However, dust was predicted to reach its maximum height ahead of the cold front as a result of lifting in the warm conveyor belt. Both Moderate Resolution Imaging Spectroradiometer (MODIS) data and the WRF/Chem model results for this event highlight two transport pathways of dust: southeastward to the Tasman Sea and northward toward tropical regions of Australia. The vertical distribution of dust from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite data and cross-sectional analysis of the model results indicate that significant amounts of dust aerosols were transported over the Tasman Sea toward New Zealand within the lower atmosphere. Copyright 2012 by the American Geophysical Union

Footprint methods to separate N2O emission rates from adjacent paddock areas

Using micrometeorological techniques to measure greenhouse gas emissions from differently treated adjacent plots is a promising avenue to verify the effect of mitigation strategies at the field scale. In pursuing such an approach, it is crucial to accurately characterize the source area of the fluxes measured at each sampling point. Hence, a comprehensive footprint analysis method is required so that emission rates can be obtained for a specific field within a biochemically heterogeneous area. In this study, a footprint analysis method is developed to estimate the emission for an experiment where the flux of N2O is measured from several control and treated plots. The emission rate of an individual plot is estimated using an inverse footprint fraction approach where the footprint fractions are obtained from an analytical footprint model. A numerical solution for obtaining the background flux for such a multiplot measurement system is also provided. Results of the footprint analysis method are assessed, first, by comparing footprint fractions obtained from both an analytical footprint model and a “forward” simulation of a backward Lagrangian stochastic (bLs) model; and second, by comparing the emission rates of a control plot obtained from the footprint analysis method and from the “backward” simulation of the bLs model. It is found that the analytical footprint fractions compare well with the values obtained from the bLs model (correlation coefficient of 0.58 and 0.66 within p value <0.001). An average of 4.3 % of the measured fluxes is found to be contributed by sources outside the measured area and, excluding this outside area contribution to the measured flux, footprint corrected emission rates within the defined domain are found to increase by 2.1 to 5.8 % of the measured flux. Also, the proposed method of emission rate estimation is found to work well under a wide range of atmospheric stability

Measurements of the local energy balance over a coral reef flat, Heron Island, southern Great Barrier Reef, Australia

Coral reefs are thought to face significant threat from global warming due to increased water temperatures and ocean acidity. However, research into the surface energy balance of coral reefs and their associated micrometeorology is rare. Here we present, through a case study approach, the first direct in situ measurements of the surface energy balance of Heron Reef, a small platform coral reef in the southern Great Barrier Reef, Australia. Surface energy exchanges were measured using the eddy covariance method and show that during winter and spring an estimated 80-98% of net radiation goes into heating of the water overlaying the reef and reef substrate. As a result, cloud cover is considered the dominant control on heating of the reef flat environment. Change in cloud cover may therefore significantly affect the thermal environment of coral reefs and their ecology. Sensible and latent heat fluxes reached their highest values during wintertime advection of dry and cool continental air blowing from mainland Australia. This resulted in a net loss of energy from the reef flat and a decreasing trend in water temperature. Turbulent fluxes otherwise remained small, with sensible heat flux often close to zero. Results indicate that coral reefs may act as heat sinks during winter and as heat sources during spring, thereby affecting local water and atmosphere heat budgets and associated thermodynamics. Copyright 2010 by the American Geophysical Union

Analysis of the wind field and heat budget in an alpine lake basin during summertime fair weather conditions

Observational data collected in the Lake Tekapo hydro catchment of the Southern Alps in New Zealand are used to analyse the wind and temperature fields in the alpine lake basin during summertime fair weather conditions. Measurements from surface stations, pilot balloon and tethersonde soundings, Doppler sodar and an instrumented light aircraft provide evidence of multi-scale interacting wind systems, ranging from microscale slope winds to mesoscale coast-to-basin flows. Thermal forcing of the winds occurred due to differential heating as a consequence of orography and heterogeneous surface features, which is quantified by heat budget and pressure field analysis. The daytime vertical temperature structure was characterised by distinct layering. Features of particular interest are the formation of thermal internal boundary layers due to the lake-land discontinuity and the development of elevated mixed layers. The latter were generated by advective heating from the basin and valley sidewalls by slope winds and by a superimposed valley wind blowing from the basin over Lake Tekapo and up the tributary Godley Valley. Daytime heating in the basin and its tributary valleys caused the development of a strong horizontal temperature gradient between the basin atmosphere and that over the surrounding landscape, and hence the development of a mesoscale heat low over the basin. After noon, air from outside the basin started flowing over mountain saddles into the basin causing cooling in the lowest layers, whereas at ridge top height the horizontal air temperature gradient between inside and outside the basin continued to increase. In the early evening, a more massive intrusion of cold air caused rapid cooling and a transition to a rather uniform slightly stable stratification up to about 2000 m agl. The onset time of this rapid cooling varied about 1-2 h between observation sites and was probably triggered by the decay of up-slope winds inside the basin, which previously countered the intrusion of air over the surrounding ridges. The intrusion of air from outside the basin continued until about mid-night, when a northerly mountain wind from the Godley Valley became dominant. The results illustrate the extreme complexity that can be caused by the operation of thermal forcing processes at a wide range of spatial scales

A fine scale approach to map bioclimatic indices using and comparing dynamical and geostatistical methods

Climate, especially temperature, plays a major role in grapevine development. Several bioclimaticindices have been created to relate temperature to grapevine phenology (e.g. Winkler Index, Huglin Index, Grapevine Flowering Véraison model [GFV]). However, temperature variability can be significant at vineyard scale, so knowledge of the various climatic mechanisms leading to this variability is essential in order to improve local management of vineyards in response to climate change. Indeed, current climate change models are not accurate enough to take into account temperature variability at the vineyard scale (Dunn et al, 2015). This study therefore proposes a method for compare regional modelling and fine-scale observations to map temperatures and bioclimatic indices at fine spatial resolution for some recent growing seasons. This study focuses on two vineyard areas, the Saint-Emilion and Pomerol region in France and the Marlborough vineyard region in New Zealand. A regression model using temperature from networks of measurements has been created in order to map temperature and bioclimatic indices at vineyard scale (100 metres for Marlborough and 25 metres for SaintEmilion and Pomerol). To complement the field measurements, the advanced physics-based three-dimensional numerical weather model Weather Research and Forecasting – WRF (http://wrf-model.org/index.php) has been used, providing hourly meteorological parameters over a complete growing season for each site at 1, 3 and 9 and 27 kilometre resolution. The output of the WRF model provides temperature, wind speed and direction, pressure, and solar radiation data at these different resolutions. The application of different scales of modelling allows improvement in understanding the climate component of the specific terroirs of the study areas

Late-Holocene climatic variability indicated by three natural archives in arid southern Australia

Three terrestrial climate proxies are used to investigate the evolution of Holocene palaeoenvironments in southern central Australia, all of which present a coherent record of palaeohydrology. Single-grain optically stimulated luminescence from sediments supplemented by 14C from charcoal and lacustrine shells was obtained to date shoreline deposits (Lake Callabonna) and the adjacent Mt Chambers Creek alluvial fan. Our findings are complemented by a U/Th-based record of speleothem growth in the Mt Chambers Creek catchment, which we interpret to reflect increased precipitation. Together, these archives shed light on the timing of, and possible sources of water for, Holocene pluvial intervals. We identified several phases of elevated lake levels dated at ~5.8-5.2, 4.5, 3.5-2.7 and 1 kyr, most of which correspond to fluvial activity resulting from increased precipitation in the adjacent ranges. The enhanced hydrology during phases of the late Holocene likely increased the reliability of resources for regional human populations during a time of reduced winter rainfall. When considered within the framework of the current understanding of Holocene palaeoclimate in central Australia, our data suggest that the pattern of landscape response was broadly synchronous with larger scale climatic variability and punctuated by pluvial periods greater than today