Characterizing mesoscale variability in low-level jet simulations for CBLAST-LOW 2001 campaign

A low-level jet (LLJ) event observed during a frontal passage in the 2001 Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds campaign was simulated using the Weather Research and Forecasting model (WRF). The sensitivity of the modeled LLJ characteristics, such as formation time, height and the strength of the LLJ core, to the choice of initial and boundary conditions, planetary boundary layer (PBL) schemes and vertical resolution was evaluated with a suite of diagnostic tools. The model simulations were compared against available soundings from the campaign observations as well as with surface observations from the Automated Surface Observing Systems. The simulation initialized with ERA-interim reanalysis and using the Mellor–Yamada–Nakanishi–Niino PBL scheme gave the best mix of diagnostic scores for surface temperature and wind speed predictions. The choice of boundary conditions introduced a stronger variability in the LLJ characteristics than the changes in PBL schemes or vertical resolution. The variability emerged primarily due to the timing of the frontal passage in the boundary condition datasets

Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation

A validated large-eddy simulation model was employed to study the effect of the aspect ratio and ground heating on the flow and pollutant dispersion in urban street canyons. Three ground-heating intensities (neutral, weak and strong) were imposed in street canyons of aspect ratio 1, 2, and 0.5. The detailed patterns of flow, turbulence, temperature and pollutant transport were analyzed and compared. Significant changes of flow and scalar patterns were caused by ground heating in the street canyon of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed a change in flow regime (from wake interference flow to skimming flow). The street canyon of aspect ratio 1 does not show any significant change in the flow field. Ground heating generated strong mixing of heat and pollutant; the normalized temperature inside street canyons was approximately spatially uniform and somewhat insensitive to the aspect ratio and heating intensity. This study helps elucidate the combined effects of urban geometry and thermal stratification on the urban canyon flow and pollutant dispersion.Singapore National Research Foundation (Singapore-MIT Alliance for Research and Technology (SMART)

Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating

Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature, and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature, the ground heating facilitated pollutant removal from the street canyon.Singapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Monitorin

Nadir correction of AIRS radiances

A statistical method to correct for the limb effect in off-nadir Atmospheric Infrared Sounder (AIRS) channel radiances is described, using the channel radiance itself and principal components (PCs) of the other channel radiances to account for the multicollinearity. A method of selecting an optimal set of predictors is proposed and demonstrated for one- and two-PC predictors. Validation results with a subset of AIRS channels in the spectral region 649–2664 cm−1 show that the mean nadir-corrected brightness temperature (BT) is largely independent of scan angle. More than 66% of the channels have a root-mean-square (rms) bias less than 0.10 K after nadir correction. Limb effect on the standard deviation (SD) of BT is discernible at larger scan angles, mainly for the atmospheric windows and the water vapor channels around 6.7 μm. After nadir correction, nearly all atmospheric window channels unaffected by solar glint and more than 76% of water vapor channels examined have BT SDs brought closer to nadir values. For the window channels affected by solar glint (wavenumber > 2490 cm−1), BT SDs at the scan angles with the strongest impact from solar reflection were improved on average by more than 0.6 K after nadir correction.Published versio

Lobe dynamics applied to barotropic Rossby-wave breaking

We applied the methods of lobe dynamics to the problem of transport across the edge of a barotropic vortex patch. The model used captures the essential dynamics of filament shedding in the wintertime stratospheric polar vortex. Two approaches were adopted for the problem: (1) the dominant periodic component of the vortical flow was identified and conventional lobe dynamics methods for periodic dynamical systems were applied to it; (2) the full aperiodic, dynamically consistent flow was retained and a modified brand of lobe dynamics was used to quantify the transport. Our results show that in the periodic case, much reversible transport occurs across the lobe dynamical boundary due to overlapping intruding and extruding lobes. In the aperiodic case, a small amount of intrusion was noted, contrary to the well-established fact that potential vorticity shedding in barotropic vortices is uniquely outwards. In our discussion, we argue that while lobe dynamics provides a rigorous framework for quantifying transport across the lobe dynamical boundary, this boundary may not be appropriate for quantifying transport across internal transport barriers, such as the stratospheric polar vortex edge.Published versio

Teleconnection between Australian winter temperature and Indian summer monsoon rainfall

The large-scale circulation over the Indian Ocean during the boreal summer raises the question of whether atmospheric conditions in Australia could influence conditions over the Indian subcontinent, despite the long passage of air over the Indian Ocean. Using a combination of reanalysis, satellite and in situ data, we argue that unusually low temperature over inland Australia during austral winter can enhance evaporation rate over the eastern tropical Indian Ocean, and hence enhance rainfall over western India after 10–18 days. Since extreme winter temperature in Australia is often associated with cold-air outbreaks, the above mechanism can be an example of how southern hemispheric mid-latitude weather can influence northern hemispheric monsoon rainfall.Published versio

Isentropic zonal average formalism and the near-surface circulation

The isentropic zonal average formalism is extended to include a rigorous treatment of the bottom boundary of the atmosphere. We define a ‘surface zone’, where isentropes in the latitudinal plane are interrupted by the earth's surface. The zonal average equations of motion and their time average in isentropic coordinates are rederived in the presence of the surface zone. Applying the extended formalism to a baroclinic wave model, we show that near-surface equatorward mean flow is driven by eastward surface form drag in isentropic coordinates, which in turn is related to poleward geostrophic potential temperature flux at the surface. A potential vorticity–potential temperature picture of extratropical general circulation dynamics above and within the surface zone is presented. We highlight the importance of poleward mean flow in the upper region of the surface zone and investigate the antisymmetric distribution of mean meridional mass flow about the median potential temperature of surface air

Toward a mesoscale observation network in Southeast Asia

The current weather observation network in Southeast Asia is unable to support the accurate monitoring and prediction of the region's predominantly convective weather. Establishing a multisensor mesoscale observation network comprising automated in situ instruments and atmospheric remote sensors (including weather radar) over land and exploiting weather satellite data especially over the sea would significantly improve the quantity and quality of data and benefit numerical weather prediction and tropical atmospheric science research. Several technical and organizational challenges need to be overcome in order to attain this goal. It is hoped that this article would motivate closer regional coordination in plans for developing infrastructure for atmospheric observations for weather research and forecasts in Southeast Asia.Published versio

Improved diagnostics for NWP verification in the tropics

The root-mean-square error (RMSE) is often used to verify forecasts. However, its strong dependence on the observation variability makes it unsuitable for comparing model performance between regions where observation variability is much different, e.g., across vertical levels or between the midlatitudes and the tropics. The alpha index based on the tensor variance of forecast-observation discrepancy was formulated to improve on RMSE (and the closely related bias-corrected RMSE). An “error ellipse” was used to represent the random error in vector wind, yielding two other diagnostics: eccentricity and orientation. These diagnostics were applied to verify Naval Research Laboratory's limited-area model, Coupled Ocean/Atmospheric Mesoscale Prediction System (COAMPS), for the first time in Southeast Asia. COAMPS forecasts were verified against radiosonde data from South China Sea Monsoon Experiment (SCSMEX), May–June 1998. Results revealed falling model performance as forecast time increases but little difference between forecasts at 18-km and 54-km resolution. Systematic errors in the model dynamics were suggested from the biases. The alpha indices show that (after bias correction) COAMPS performs best for wind, followed by temperature and then by dew point depression. In this tropical region, 1-day persistence forecasts were only outperformed by the model for wind predictions between 400 mb and 850 mb at forecast times less than 24 hr. The RMSE diagnostic was shown to sometimes yield misleading evaluation of the model's performance. The wind error ellipses revealed that the random error tended to align more with the background flow than with the model bias, possibly indicating a dynamical reason for its existence.Published versio

Isentropic zonal average formalism and the near-surface circulation

Hoskins (1991) proposed dividing the atmosphere into 3 regions: Overworld, Middleworld and Underworld, using potential temperature θ and potential vorticity (PV) as reference (Fig. 1). In the Underworld (θ < 300 K), isentropes intercept the Earth’s surface and a direct isentropic zonal average circulation exists in the mid-latitudes. Held and Schneider (1999) suggested that this circulation may be understood as follows: the equator-pole temperature gradient determines the near-surface eddy heat flux via an eddydiffusion mechanism. The poleward eddy heat flux in turn drives an equatorward mean flow next to the surface whose horizontal convergence in the subtropics forces the mean quasi-isentropic ascent of air into the troposphere. Radiative cooling causes air to sink back to the surface, thus closing the circulation.Accepted versio