闪电的光辐射能分布特征

对TRMM卫星上装载的闪电成像仪(LIS)所获取的闪电光辐射能进行了分析,发现在不同地区和不同的空间、时间尺度上闪电光辐射能都很好地遵循对数正态分布。文中利用了一个国际上公认的一维雷暴云起电和放电模式,对闪电放电中和的电荷量进行了数值模拟。蒙特卡罗算法表明,闪电放电所中和的电荷量也遵循对数正态分布,云中的软雹浓度、破碎系数、温度递减率、放电的电场阈值等参量的综合作用可以对其进行较好的解释,而其中任意一种参量的单独作用则不然。

卫星观测到的青藏高原雷电活动特征

利用TRMM卫星上携带的闪电探测系统获取的闪电定位资料首次对青藏高原上的闪电活动进行了研究。研究发现,青藏高原上的闪电活动呈现出大陆性气候特征,但是受其特殊地形的热力和动力特征所调制,高于93%的闪电活动发生于5～9月之间,并在夏季出现单一闪电活动峰值。随着地表加热和湿度的增加,闪电活动在5月份开始明显增加。较高的闪电密度发生在高原的中部,揭示了闪电活动与地形特征的相关性。63%的闪电活动发生于14:00～18:00(地方时,下同)之间,而只有<3%的闪电发生于00:00～10:00之间。闪电光辐射能的时平均值也呈现出明显的日变化,随着太阳的升起,闪电光辐射能在08:00开始增加,并在10:00达到稳定的极大值,直到02:00开始明显减小。闪电的光辐射能在高原北部呈现最大值,高原西部呈现最小值。青藏高原上的闪电放电强度比其它地区弱得多,其差别归因于高原上较低的对流不稳定能量。

Reduced Sensitivity of Tropical Cyclone Intensity and Size to Sea Surface Temperature in a Radiative-Convective Equilibrium Environment

It has been challenging to project the tropical cyclone （TC） intensity, structure and destructive potential changes in a warming climate. Here, we compare the sensitivities of TC intensity, size and destructive potential to sea surface warming with and without a pre-storm atmospheric adjustment to an idealized state of Radiative-Convective Equilibrium （RCE）. Without RCE, we find large responses of TC intensity, size and destructive potential to sea surface temperature （SST） changes, which is in line with some previous studies. However, in an environment under RCE, the TC size is almost insensitive to SST changes, and the sensitivity of intensity is also much reduced to 3% ~C-1-4% ~C-1. Without the pre-storm RCE adjustment, the mean destructive potential measured by the integrated power dissipation increases by about 25% ~C-1 during the mature stage. However, in an environment under RCE, the sensitivity of destructive potential to sea surface warming does not change significantly. Further analyses show that the reduced response of TC intensity and size to sea surface warming under RCE can be explained by the reduced thermodynamic disequilibrium between the air boundary layer and the sea surface due to the RCE adjustment. When conducting regional-scale sea surface warming experiments for TC case studies, without any RCE adjustment the TC response is likely to be unrealistically exaggerated. The TC intensity-temperature sensitivity under RCE is very similar to those found in coupled climate model simulations. This suggests global mean intensity projections under climate change can be understood in terms of a thermodynamic response to temperature with only a minor contribution from any changes in large-scale dynamics.

Importance of Air-Sea Coupling in Simulating Tropical Cyclone Intensity at Landfall

An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(TCs)along the South China coast is upgraded by including ocean and wave models.A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea.Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size.In particular,interaction with the ocean does not necessarily imply a weakening of the TC,with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC.These results suggest that to simulate the evolution of TC structure more accurately,it is essential to use an air-sea coupled model instead of an atmosphere-only model.

Recent advances in operational tropical cyclone genesis forecast

Tropical cyclone(TC)genesis prediction is a major scientific challenge to the TC operation and research community.This report surveys the current status of TC genesis forecasts by a number of major operational centers covering the key ocean basins across both hemispheres.Since IWTC-9,we see an emergence of probabilistic TC genesis forecast products by operational centers,typically supported by the statistical processing of a combination of ensemble prediction and satellite analysis,covering time periods of couple of days to weeks ahead.The prevalence of multi-center grand ensemble approach highlights the uncertainties involved and the forecast challenges in quantitative genesis prediction.While operational practice might differ across agencies,verification efforts generally report a steady or slightly improving skill level in terms of reliability,which likely results from the continual improvement in global numerical weather prediction capability.

Recent advances in seasonal and multi-annual tropical cyclone forecasting

Seasonal tropical cyclone(TC)forecasting has evolved substantially since its commencement in the early 1980s.However,present operational seasonal TC forecasting services still do not meet the requirements of society and stakeholders:current operational products are mainly basin-scale information,while more detailed sub-basin scale information such as potential risks of TC landfall is anticipated for decision making.To fill this gap and make the TC science and services move forward,this paper reviews recent research and development in seasonal tropical cyclone(TC)forecasting.In particular,this paper features new research topics on seasonal TC predictability in neutral conditions of El Ni˜no–Southern Oscillation(ENSO),emerging forecasting techniques of seasonal TC activity including Machine Learning/Artificial Intelligence,and multi-annual TC predictions.We also review the skill of forecast systems at predicting landfalling statistics for certain regions of the North Atlantic,Western North Pacific and South Indian oceans and discuss the gap that remains between current products and potential user's expectations.New knowledge and advanced forecasting techniques are expected to further enhance the capability of seasonal TC forecasting and lead to more actionable and fit-for-purpose products.