Resolution Dependence of Future Tropical Cyclone Projections of CAM5.1 in the U.S. CLIVAR Hurricane Working Group Idealized Configurations

Abstract

The four idealized configurations of the U.S. CLIVAR Hurricane Working Group are integrated using the global Community Atmospheric Model version 5.1 at two different horizontal resolutions, approximately 100 and 25 km. The publicly released 0.9° × 1.3° configuration is a poor predictor of the sign of the 0.23° × 0.31° model configuration’s change in the total number of tropical storms in a warmer climate. However, it does predict the sign of the higher-resolution configuration’s change in the number of intense tropical cyclones in a warmer climate. In the 0.23° × 0.31° model configuration, both increased CO2 concentrations and elevated sea surface temperature (SST) independently lower the number of weak tropical storms and shorten their average duration. Conversely, increased SST causes more intense tropical cyclones and lengthens their average duration, resulting in a greater number of intense tropical cyclone days globally. Increased SST also increased maximum tropical storm instantaneous precipitation rates across all storm intensities. It was found that while a measure of maximum potential intensity based on climatological mean quantities adequately predicts the 0.23° × 0.31° model’s forced response in its most intense simulated tropical cyclones, a related measure of cyclogenesis potential fails to predict the model’s actual cyclogenesis response to warmer SSTs. These analyses lead to two broader conclusions: 1) Projections of future tropical storm activity obtained by a direct tracking of tropical storms simulated by coarse-resolution climate models must be interpreted with caution. 2) Projections of future tropical cyclogenesis obtained from metrics of model behavior that are based solely on changes in long-term climatological fields and tuned to historical records must also be interpreted with caution.

Hurricanes and Climate: The U.S. CLIVAR Working Group on Hurricanes

“…1 shows the simulated global TC numbers range from small values to numbers similar to those observed (Zhao et al 2013a,b;Shaevitz et al 2014). Better results can also be obtained from higher-resolution versions of the HWG models (finer than 50-km horizontal resolution), including an ability to generate storms of intense tropical cyclone strength, as shown by Wehner et al (2014) for a higher-resolution version of the NCAR CAM5 than that shown in Fig. 1.…”

“…While it is clear that simply increasing the resolution does not necessarily improve intensity distribution (Shaevitz et al 2014), results from the HWG simulations indicate that a very significant improvement in a GCM's ability to simulate both TC formation and intensity occurs at resolutions finer than 50 km, with good results shown at 25 km (Strachan et al 2013;Roberts et al 2015;Lim et al 2015;Wehner et al 2015;Mei et al 2014). In addition, if such high resolution is employed, it is possible to simulate reasonably well the observed intensity distribution of tropical cyclones (Bender et al 2010;Lavender and Walsh 2011; Murakami et al 2012b;Chen et al 2013;Zarzycki and Jablonowski 2014).…”

The influence of model resolution on the simulated sensitivity of North Atlantic tropical cyclone maximum intensity to sea surface temperature

“…For example, results from LaRow et al [2008] demonstrate that a GCM with a T126 horizontal resolution (0.948) reasonably simulates the effect of the El Niño/Southern Oscillation on North Atlantic TC frequency beginning in 1995. Furthermore, recent higher-resolution modeling studies by Wehner et al [2015] and Zarzycki and Jablonowski [2014] successfully generate TCs with wind speed distributions that better match observations. These results suggest that as resolution continues to improve, climate models may be able to better predict not only future changes in TC frequency, but also intensity.…”

“…Much of this previous research examines GCM simulations spanning several resolutions to compare TC frequency and intensity statistics among model runs. For example, Wehner et al [2015] present the effects of model resolution on the ability of the Community Atmosphere Model (CAM5) to generate intense TCs and simulate inter/intraannual variability. As expected, the 0.258 version of the CAM5 generates much stronger TCs (including some Category 5 hurricanes on the Saffir-Simpson scale) when compared to the 18 and 28 model simulations.…”

“…One simulation uses a 0.238 by 0.318 mesh grid (referred to here as 0.258 grid spacing), while the other simulation uses a 0.918 by 1.38, or 18 grid spacing [Wehner et al, 2014[Wehner et al, , 2015. Both the 18 and 0.258 simulations are run with 30 vertical levels extending up to 2 hPa.…”