The influence of spectral nudging on typhoon formation in regional climate models

Abstract

Regional climate models can successfully simulate tropical cyclones and typhoons. This has been shown and was evaluated for hindcast studies of the past few decades. But often global and regional weather phenomena are not simulated at the observed location, or occur too often or seldom even though the regional model is driven by global reanalysis data which constitute a near-realistic state of the global atmosphere. Therefore, several techniques have been developed in order to make the regional model follow the global state more closely. One is spectral nudging, which is applied for horizontal wind components with increasing strength for higher model levels in this study.The aim of this study is to show the influence that this method has on the formation of tropical cyclones (TC) in regional climate models. Two ensemble simulations (each with five simulations) were computed for Southeast Asia and the Northwestern Pacific for the typhoon season 2004, one with spectral nudging and one without. First of all, spectral nudging reduced the overall TC number by about a factor of 2. But the number of tracks which are similar to observed best track data (BTD) was greatly increased. Also, spatial track density patterns were found to be more similar when using spectral nudging. The tracks merge after a short time for the spectral nudging simulations and then follow the BTD closely; for the no nudge cases the similarity is greatly reduced. A comparison of seasonal precipitation, geopotential height, and temperature fields at several height levels with observations and reanalysis data showed overall a smaller ensemble spread, higher pattern correlations and reduced root mean square errors and biases for the spectral nudged simulations. Vertical temperature profiles for selected TCs indicate that spectral nudging is not inhibiting TC development at higher levels. Both the Madden-Julian Oscillation and monsoonal precipitation are reproduced realistically by the regional model, with results slightly closer to reanalysis data for the spectral nudged simulations. On the basis of this regional climate model hindcast study of a single typhoon season, spectral nudging seems to be favourable since it has mostly positive effects on typhoon formation, location and general circulation patterns in the generation areas of TCs.

Megacity-Induced Mesoclimatic Effects in the Lower Atmosphere: A Modeling Study for Multiple Summers over Moscow, Russia

“…For the first step of downscaling, 6-hourly reanalysis data were used as the initial and boundary conditions for running the model for a "basic" domain D1 covering the central part of European Russia with a size of approximately 1700 km × 1700 km and a 12-km horizontal grid step, 140 × 140 grid cells ( Figure 1a). In addition, to ensure a more reliable binding of the internal model mode to the real atmospheric dynamics, the spectral nudging technique [52] was applied, which is important for such studies [53,54]. Modeling results for D1 were used as the initial and boundary conditions for intermediate domain D2 with a size of 600 km × 600 km (200 × 200 grid cells) and a 3-km grid spacing covering the Moscow region and the neighboring regions of Russia.…”

Changes in intense tropical cyclone activity for the western North Pacific during the last decades derived from a regional climate model simulation

“…The technique was applied here only for horizontal wind components. It was shown by Feser and Barcikowska (2012) that spectral nudging is very efficient at reducing the RCM's internal variability. Consequently, the simulated large-scale climate was in very good agreement with the driving fields, and the representation of TC climatology significantly improved.…”

“…This has been achieved with an application of a spectral nudging technique. Feser and Barcikowska (2012) have shown that the technique substantially reduces both the simulated positive bias of the Asian monsoon circulation (e.g. westerly winds, relative vorticity and precipitation) and the negative bias of the subtropical high.…”

Impact of the internal variability on the cyclone tracks simulated by a regional climate model over the Med-CORDEX domain

“…They showed that the spectrally nudged version of the regional model reproduces the observed polar lows in all the members of the ensemble, reducing significantly the inter-members spread. In a similar study, Feser and Barcikowska (2012) showed that the spectral nudging technique improves the representation of typhoons tracks in the Pacific ocean, reducing some of the biases with respect to the observed best track data.…”