Convectively coupled Kelvin and easterly waves in a regional climate simulation of the tropics

Abstract

This study evaluates the performance of a regional climate model in simulating two types of synoptic tropical weather disturbances: convectively-coupled Kelvin and easterly waves. Interest in these two wave modes stems from their potential predictability out to several weeks in advance, as well as a strong observed linkage between easterly waves and tropical cyclogenesis. The model is a recent version of the weather research and forecast (WRF) system with 36-km horizontal grid spacing and convection parameterized using a scheme that accounts for key convective triggering and inhibition processes. The domain spans the entire tropical belt between 45°S and 45°N with periodic boundary conditions in the east-west direction, and conditions at the meridional/lower boundaries specified based on observations. The simulation covers 6 years from 2000 to 2005, which is long enough to establish a statistical depiction of the waves through space-time spectral filtering of rainfall data, together with simple lagged-linear regression. Results show that both the horizontal phase speeds and three-dimensional structures of the waves are qualitatively well captured by the model in comparison to observations. However, significant biases in wave activity are seen, with generally overactive easterly waves and underactive Kelvin waves. Evidence is presented to suggest that these biases in wave activity (which are also correlated with biases in time-mean rainfall, as well as biases in the model's tropical cyclone climatology) stem in part from convection in the model coupling too strongly to rotational circulation anomalies. Nevertheless, the model is seen to do a reasonable job at capturing the genesis of tropical cyclones from easterly waves, with evidence for both wave accumulation and critical layer processes being importantly involved.

The role of wave energy accumulation in tropical cyclogenesis over the tropical North Atlantic

“…The phase speeds and 3D structure of easterly waves are qualitatively well captured by the NRCM (Tulich et al 2009). However, the easterly wave filtered rainfall variance is shown to be too low over the North Atlantic but this may be due to a dry bias associated with the model waves.…”

Tropical cyclone activity in nested regional and global grid-refined simulations

“…Tulich et al 2011;Bruyère et al 2014;Done et al 2013). MPAS simulations compared favorably with WRF model simulations in the capacity to capture largescale climate features and TC activity.…”

Role of the atmospheric mean state on the initiation of the Madden-Julian oscillation in a tropical channel model

“…The bias towards westward propagation stems from convection coupling too strongly to rotationally dominated phenomena (e.g., easterly waves and tropical cyclones are overly active), a possible cause or a consequence of the time-mean off-equatorial rain bias in the model (see Tulich et al 2010 for details).…”