Evaluation of the contribution of tropical cyclone seeds to changes in tropical cyclone frequency due to global warming in high-resolution multi-model ensemble simulations

Previous projections of the frequency of tropical cyclone genesis due to global warming, even in terms of sign of the change, depends on the chosen model simulation. Here, we systematically examine projected changes in tropical cyclones using six global atmospheric models with medium-to-high horizontal resolutions included in the sixth phase of the Coupled Model Intercomparison Project/High-Resolution Model Intercomparison Project. Changes in the frequency of tropical cyclone genesis could be broken down into the contributions from (i) the tropical cyclone seed, a depression having a closed contour of sea level pressure with a warm core and (ii) the survival rate, the ratio of the frequency of tropical cyclone genesis to that of tropical cyclone seeds. The multi-model ensemble mean indicates that tropical cyclone genesis frequencies are significantly decreased during the period 1990–2049, which is attributable to changes in tropical cyclone seeds. Analysis of the individual models shows that although most models project a more or less decreasing trend in tropical cyclone genesis frequencies and seeds, the survival rate also contributes to the result in some models. The present study indicates the usefulness of decomposition into the frequency of the tropical cyclone seeds and the survival rate to understand the cause of uncertainty in projected frequencies of tropical cyclone genesis

Global cloud-permitting simulations of Typhoon Fengshen (2008)

Large-scale fields and inner-core processes relevant to the formation and intensification of Typhoon Fengshen (2008) were examined by simulations using a global nonhydrostatic model with a cloud-permitting resolution. Five runs were performed by varying the cloud microphysics or initial condition settings. In three out of five runs, a middle tropospheric trough developed within a few days following a large-scale latent heat release, which enabled the successive occurrence of deep convective events within the 50-km radius of the incipient disturbance and subsequent tropical cyclone (TC) formation. In the run initialized by altering the analysis dataset, collocation between latent heat release and the large-scale gyre was less evident, and neither the trough nor a TC developed. In the run with weaker latent heating in the lower troposphere, the trough was weak and TC formation was significantly delayed. These results indicate that the superposition of large-scale disturbances in the lower and middle troposphere and their linkage through convective enhancement played an important role in the genesis of Fengshen by preconditioning the establishment of a deep upright inner core.CC-BY 4.

How Does the Air‐Sea Coupling Frequency Affect Convection During the MJO Passage?

Abstract The importance of air‐sea coupling in the simulation and prediction of the Madden‐Julian Oscillation (MJO) has been well established. However, it remains unclear how air‐sea coupling modulates the convection and related oceanic features on the subdaily scale. Based on a regional cloud‐permitting coupled model, we evaluated the impact of the air‐sea coupling on the convection during the convectively active phase of the MJO by varying the coupling frequency. The model successfully reproduced the atmospheric and oceanic variations observed by satellite and in situ measurements but with some quantitative biases. According to the sensitivity experiments, we found that stronger convection was mainly caused by the higher sea surface temperatures (SSTs) generated in high‐frequency coupled experiments, especially when the coupling frequency was 1 hr or shorter. A lower coupling frequency would generate the phase lags in the diurnal cycle of SST and related turbulent heat fluxes. Our analyses further demonstrated that the phase‐lagged diurnal cycle of SST suppressed deep convection through a decrease in daytime moistening in the lower troposphere. Meanwhile, in the upper ocean, the high‐frequency air‐sea coupling helped maintain the shallower mixed and isothermal layers by diurnal heating and cooling at the sea surface, which led to a higher mean SST. In contrast, the low‐frequency coupled experiments underestimated the SST and therefore convective activities. Overall, our results demonstrated that high‐frequency air‐sea coupling (1 hr or shorter) could improve the reproducibility of the intensity and temporal variation in both diurnal convection and upper ocean processes

Impact of abrupt stratospheric dynamical change on Tropical Tropopause Layer

We have studied the impact of stratospheric circulation change on water vapor in the Tropical Tropopause Layer (TTL) during the stratospheric sudden warming (SSW) events (e.g., Eguchi and Kodera, 2010). Increased Brewer-Dobson circulation associated with SSW produces cooling in the tropical lower stratosphere (LS). The cooling generally produces more cirrus clouds and decreases of the water vapor mixing ratio (WV) in the TTL, except for some regions over Africa and South American continents where penetrating clouds are expected. This time, we found a new stratospheric phenomenon which produces abrupt warming in the tropical stratosphere converse to the SSW event. The dynamical aspect of this phenomenon and the impact on the convective activity will be discussed. Due to the interaction between the subtropical jet and polar night jet in the upper stratosphere, tropical stratosphere warmed about two weeks in early December 2011. Accordingly, temperature in the TTL suddenly increased (approximately 0.5 K at 100 hPa) and the tropical convection ceased. Further, the downward velocity anomaly appeared from stratosphere to lower troposphere through the TTL. The present study mainly focuses on the variation of WV, temperature and cirrus clouds in the upper troposphere (UT) and LS in the period of the stratospheric dynamical change. The data from EOS MLS (Earth Observing System, Microwave Limb Sounder) is used. Before the start of abrupt warming event, the tropical convection temporarily enhanced at the south of the Equator. Then the temperature in the TTL decreased with the Kelvin wave like vertical structure: the WV at 146 hPa increased, while the WV at 100 hPa decreased, and the dryer air extended to 83 hPa with a few days lag. Following the start of the warming event, the tropical convection was suppressed. In the UT and LS, the warm and wet tendencies were found in the temperature and WV anomalies from seasonal march, respectively, and the ice cloud suddenly disappeared with increasing temperature, suggesting adiabatic heating. The result found of the present study clearly shows that the stratospheric dynamical change controls the WV variation in the UT and LS, as well as the tropical convection