An idealized numerical study of tropical cyclogenesis and evolution at the Equator

Tropical cyclone formation and evolution at, or near, the Equator is explored using idealized three‐dimensional model simulations, starting from a prescribed, initial, weak counterclockwise rotating vortex in an otherwise quiescent, nonrotating environment. Three simulations are carried out in which the maximum tangential wind speed (5 m surn:x-wiley:qj:media:qj3701:qj3701-math-0001) is specified at an initial radius of 50, 100, or 150 km. After a period of gestation lasting between 30 and 60 hr, the vortices intensify rapidly, the evolution being similar to that for vortices away from the Equator. In particular, the larger the initial vortex size, the longer the gestation period, the larger the maximum intensity attained, and the longer the vortex lifetime. Beyond a few days, the vortices decay as the cyclonic vorticity source provided by the initial vortex is depleted and negative vorticity surrounding the vortex core is drawn inwards by the convectively driven overturning circulation. In these negative vorticity regions, the flow is inertially/centrifugally unstable. The vortex evolution during the mature and decay phases differs from that in simulations away from the Equator, where inertially unstable regions are much more limited in area. Vortex decay in the simulations appears to be related intimately to the development of inertial instability, which is accompanied by an outward‐propagating band of deep convection. The degree to which this band of deep convection is realistic is unknown

Trends in reported flooding in the UK: 1884–2013

A long term dataset of reported flooding based on reports from the UK Meteorological Office and the UK Centre for Ecology and Hydrology is described. This is possibly a unique dataset as the authors are unaware of any other 100+ year records of flood events and their consequences on a national scale. Flood events are classified by severity based upon qualitative descriptions. There is an increase in the number of reported flood events over time associated with an increased exposure to flooding as floodplain areas were developed. The data was de-trended for exposure, using population and dwelling house data. The adjusted record shows no trend in reported flooding over time, but there is significant decade to decade variability. This study opens a new approach considering flood occurrence over a long timescale using reported information (and thus likely effects on society) rather than just considering trends in extreme hydrological conditions.<br/

The role of land use change on the development and evolution of the West Coast trough, convective clouds, and precipitation in Southwest Australia

Land clearing for agricultural purposes in southwest Australia has created a landscape where a 750 km rabbit-proof fence separates 13 million hectares of croplands from the remnant native vegetation to the east. The Bunny Fence Experiment (BuFex) was conducted in the vicinity of the intended vermin-proof boundary in December 2005 and August 2007. The experiment examined the role of land cover change (LCC) on the preferential formation of clouds over the native vegetation that often terminates along the vermin-proof fence as well as the regional rainfall reduction observed in this region. Observations and numerical model analysis show that the formation and development of the west coast trough (WCT), which is a synoptic-scale feature that initiates spring and summertime convection, is impacted by land cover change and that the cloud fields induced by the WCT would extend further west in the absence of the LCC. The surface convergence patterns associated with the wintertime WCT circulation is substantially altered by LCC, due to changes in both WCT dynamics and surface aerodynamic roughness, leading to a rainfall decrease to the west of the rabbit fence. It is found that the LCC in southwest Australia is indeed responsible for preferential formation of clouds over native vegetation and contributes to the observed rainfall reduction in this region.

Observational estimates of radiative forcing due to land use change in southwest Australia

Radiative forcing associated with land use change is largely derived from global circulation models (GCM), and the accuracy of these estimates depends on the robustness of the vegetation characterization used in the GCMs. In this study, we use observations from the Clouds and Earth's Radiant Energy System (CERES) instrument on board the Terra satellite to report top-of-the-atmosphere (TOA) radiative forcing values associated with clearing of native vegetation for agricultural purposes in southwest Australia. Over agricultural areas, observations show consistently higher shortwave fluxes at the TOA compared to native vegetation, especially during the time period between harvest and planting. Estimates using CERES observations show that over a specific area originally covered by native vegetation, replacement of half the area by croplands results in a diurnally averaged shortwave radiative forcing of approximately -7 W m-2. GCM-derived estimates for areas with 30% or more croplands range from - 1 to - 2 W m-2 compared to observational estimate of -4.2 W m-2, thus significantly underestimating radiative forcing due to land use change by a factor of 2 or more. Two potential reasons for this underestimation are incorrect specification of the multiyear land use change scenario and the inaccurate prescription of seasonal cycles of crops in GCMs

The impact of plant stomatal control on mesoscale atmospheric circulations

Only a few numerical studies with mesoscale atmospheric models have been undertaken to explore the influence of vegetation on the generation and modification of mesoscale atmospheric circulations. Nevertheless, these few studies have demonstrated the importance of a correct representation of vegetation in the simulation of atmospheric features. In the present study, a mesoscale atmospheric model with a sophisticated land surface parameterization is used to emphasize the major role of plant stomata on the control of the Bowen ratio and on mesoscale atmospheric circulations. This study demonstrates that refinement of the land surface parameterization and stomatal mechanism, including subgrid-scale processes, is required in order to simulate confidently land-atmosphere interactions. This is particularly important when plants experience relatively strong stress and a small change in the stomatal behavior generates large variations in sensible and latent heat fluxes at the Earth's surface

Synoptic and mesoscale weather conditions during air pollution episodes in Athens, Greece

Based on regular climatological and air quality data from the Greater Athens Area (GAA), the air pollution episodes observed in Athens during the period 1983-1990 were analysed and classified. The main characteristics of atmospheric conditions during days with high air pollution concentrations are summarized too. Model simulations show that the worst air pollution episodes in Athens occur during days with a critical balance between synoptic and mesoscale circulations and/or during days with warm advection in the lower troposphere. © 1993 Kluwer Academic Publishers

Coastal zone wind energy. Part I. Potential wind power density fields based on 3-D model simulations of the dominant wind regimes for three east and Gulf coast areas

The results of applying a numerical model of the atmosphere to the problem of locating areas of maximum wind power are presented. Three US coastal regions, of approximately 10/sup 5/ km/sup 2/ area each, are investigated. For each region the spatial distribution of daily average power density (W m/sup -2/) for the lowest 100 m of the atmosphere is given for the three most prevalent weather regimes. These distributions are then combined to form an estimate of the annual average power density for each region. Comparisons with long-term climatological data at stations within each region show good agreement between model estimated and observed wind power density for two of the three regions studied