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This study evaluates the representation of tropical cyclone precipitation (TCP) in reforecasts from the Subseasonal to Seasonal (S2S) Prediction Project. The global distribution of precipitation in S2S models shows relevant biases in the multimodel mean ensemble that are characterized by wet biases in total precipitation and TCP, except for the Atlantic. The TCP biases can contribute more than 50% of the total precipitation biases in basins such as the southern Indian Ocean and South Pacific. The magnitude and spatial pattern of these biases exhibit little variation with lead time. The origins of TCP biases can be attributed to biases in the frequency of tropical cyclone occurrence. The S2S models simulate too few TCs in the Atlantic and western North Pacific and too many TCs in the Southern Hemisphere and eastern North Pacific. At the storm scale, the average peak precipitation near the storm center is lower in the models than observations due to a too high proportion of weak TCs. However, this bias is offset in some models by higher than observed precipitation rates at larger radii (300–500 km). An analysis of the mean TCP for each TC at each grid point reveals an overestimation of TCP rates, particularly in the near-equatorial Indian and western Pacific Oceans. These findings suggest that the simulation of TC occurrence and the storm-scale precipitation require better representation in order to reduce TCP biases and enhance the subseasonal prediction skill of mean and extreme total precipitation. [ABSTRACT FROM AUTHOR]

A significant fraction of tropical cyclones develop in baroclinic environments, following tropical cyclogenesis "pathways" that are characterized by dynamical processes often associated with mid-latitudes. This study investigates whether such storms are more likely to undergo subsequent extratropical transition than those that develop in more typical, non-baroclinic environments. We consider tropical cyclones globally in the period of 1979 - 2011 using best-track datasets and define the genesis pathway of each storm using McTaggart-Cowan's classification: non-baroclinic, low-level baroclinic, trough-induced, and weak and strong tropical transition. In each basin, we analyze the total number and the fraction of storms that underwent extratropical transition as well as their seasonality and storm tracks according to their genesis pathways. The relationship between the pathways and extratropical transition is statistically significant in the North Atlantic and Western North Pacific, where the strong tropical transition and the trough-induced pathways have a significantly greater extratropical fraction compared with all other pathways, respectively. Latitude, longitude, and environmental factors, such as sea surface temperature and vertical shear, were further analyzed to explore whether storms in these pathways occur in environments conducive to extratropical transition, or whether a "memory" of the genesis pathway persists throughout the storm life cycle. After controlling for genesis latitude, the relationship between the strong tropical transition and troughinduced pathways and the extratropical transition occurrence remains statistically significant, implying a lasting effect from the pathway on the probability of an eventual extratropical transition. [ABSTRACT FROM AUTHOR]

The intensity and spatial extent of tropical cyclone precipitation (TCP) often shapes the risk posed by landfalling storms. Here we provide a comprehensive climatology of landfalling TCP characteristics as a function of tropical cyclone strength, using daily precipitation station data and Atlantic U.S. landfalling tropical cyclone tracks from 1900 to 2017. We analyze the intensity and spatial extent of ≥1 mm/day TCP (Z1) and ≥50 mm/day TCP (Z50) over land. We show that the highest median intensity and largest median spatial extent of Z1 and Z50 occur for major hurricanes that have weakened to tropical storms, indicating greater flood risk despite weaker wind speeds. We also find some signs of TCP change in recent decades. In particular, for major hurricanes that have weakened to tropical storms, Z50 intensity has significantly increased, indicating possible increases in flood risk to coastal communities in more recent years. Plain Language Summary: Heavy and widespread rainfall during landfalling tropical cyclones can cause severe damage and large financial losses. Here we investigate the differences in rainfall along tracks of tropical cyclones of different intensities. To do this, we examine the tracks of Atlantic tropical cyclones that made landfall in the southeastern and eastern United States during the 20th century. Across all major hurricanes, the largest areas and heaviest intensities of rainfall over land occur after they have weakened to tropical storms. These major hurricanes that have weakened to tropical storms also have heavier rainfall over land during the most recent six decades compared to the first six decades of our study period. Our findings indicate that after landfall occurs, the greatest risks of heavy and widespread rainfall are associated with major hurricanes that have weakened to tropical storms and that these risks may have grown in the past century. Key Points: Precipitation extent and intensity vary strongly among categories of tropical cyclonesThe largest extents and heaviest intensities of overall rainfall over land occur for major hurricanes that have weakened to tropical stormsHeavy precipitation has significantly increased between 1900–1957 and 1958–2017 for major hurricanes that have weakened to tropical storms [ABSTRACT FROM AUTHOR]

An autoregressive model is developed to simulate the climatological distribution of global tropical cyclone (TC) intensity. The model consists of two components: a regression-based deterministic component that advances the TC intensity in time and depends on the storm state and surrounding large-scale environment and a stochastic forcing. Potential intensity, deep-layer mean vertical shear, and midlevel relative humidity are the environmental variables included in the deterministic component. Given a storm track and its environment, the model is initialized and then iterated along the track. Model performance is evaluated by its ability to represent the observed global and basin distributions of TC intensity as well as lifetime maximum intensity (LMI). The deterministic model alone captures the spatial features of the climatological TC intensity distribution but with intensities that remain below 100 kt (1 kt ≈ 0.51 m s−1). Addition of white (uncorrelated in time) stochastic forcing reduces this bias by improving the simulated intensification rates and the frequency of major storms. The model simulates a realistic range of intensities, but the frequency of major storms remains too low in some basins. [ABSTRACT FROM AUTHOR]

Recent research has strengthened the understanding of the links between climate and tropical cyclones (TCs) on various timescales. Geological records of past climates have shown century-long variations in TC numbers. While no significant trends have been identified in the Atlantic since the late 19th century, significant observed trends in TC numbers and intensities have occurred in this basin over the past few decades, and trends in other basins are increasingly being identified. However, understanding of the causes of these trends is incomplete, and confidence in these trends continues to be hampered by a lack of consistent observations in some basins. A theoretical basis for maximum TC intensity appears now to be well established, but a climate theory of TC formation remains elusive. Climate models mostly continue to predict future decreases in global TC numbers, projected increases in the intensities of the strongest storms and increased rainfall rates. Sea level rise will likely contribute toward increased storm surge risk. Against the background of global climate change and sea level rise, it is important to carry out quantitative assessments on the potential risk of TC-induced storm surge and flooding to densely populated cities and river deltas. Several climate models are now able to generate a good distribution of both TC numbers and intensities in the current climate. Inconsistent TC projection results emerge from modeling studies due to different downscaling methodologies and warming scenarios, inconsistencies in projected changes of large-scale conditions, and differences in model physics and tracking algorithms. WIREs Clim Change 2016, 7:65-89. doi: 10.1002/wcc.371 For further resources related to this article, please visit the . [ABSTRACT FROM AUTHOR]

The thermodynamic factors related to tropical cyclone genesis are examined in several simulations of the middle part of the Holocene epoch when the precession of Earth's orbit altered the seasonal distribution of solar radiation and in one transient simulation of the millennium preceding the industrial era. The thermodynamic properties most crucial for genesis display a broad stability across both periods, although both orbital variations during the mid-Holocene (MH) 6000 years ago (6ka) and volcanic eruptions in the transient simulation have detectable effects. It is shown that the distribution of top-of-the-atmosphere radiation 6ka altered the Northern Hemisphere seasonal cycle of the potential intensity of tropical cyclones in addition to slightly increasing the difference between middle tropospheric and boundary layer entropy, a parameter that has been related to the incubation period required for genesis. The Southern Hemisphere, which receives more solar radiation during its storm season today than it did 6ka, displays slightly more favorable thermodynamic properties during the MH than in the preindustrial era control. Surface temperatures over the ocean in both hemispheres respond to radiation anomalies more slowly than those in upper levels, altering the thermal stability. Volcanism produces a sharp but transient temperature response in the last-millennium simulation that strongly reduces potential intensity during the seasons immediately following a major eruption. Here, too, the differential vertical temperature response is key: temperatures in the lower and middle troposphere cool, while those near the tropopause rise. Aside from these deviations, there is no substantial variation in thermodynamic properties over the 1000-yr simulation. [ABSTRACT FROM AUTHOR]

The properties of tropical cyclones in three low-resolution atmospheric general circulation models (AGCMs) in seven ocean basins are discussed. The models are forced by prescribed, observed sea surface temperatures over a period of 40 yr, and their simulations of tropical cyclone activity are compared with observations. The model cyclone characteristics considered include genesis position, number of cyclones per year, seasonality, accumulated cyclone energy, track locations, and number of storm days. Correlations between model and observed interannual variations of these characteristics are evaluated. The models are found able to reproduce the basic features of observed tropical cyclone behavior such as seasonality, general location and interannual variability, but with identifiable biases. A bias correction is applied to the tropical cyclone variables of the three models. The three AGCMs have different levels of realism in simulating different aspects of tropical cyclone activity in different ocean basins. Some strengths and weaknesses in simulating certain tropical cyclone activity variables are common to the three models, while others are unique to each model and/or basin. Although the overall skill of the models in reproducing observed interannual variability of tropical cyclone variables has not surpassed or often even equalled that of statistical models, there exists potential for higher future skills using improved versions of dynamical approaches. [ABSTRACT FROM AUTHOR]

The formation of tropical storms in a low-resolution Atmospheric General Circulation model is studied over the western North Pacific region during the June–October season. The model simulates the mean annual cycle of storm number in this basin quite well. Time-dependent composites of the storms are formed and analyzed, with a focus on the temporal evolution of quantities averaged in space around the storm centers. Day zero of each composite corresponds to the time at which the disturbance passes criteria for detection. The composites depict the model storms as convectively coupled, synoptic-scale vortices whose degree of coupling to convection increases at some point, leading to intensification. Variables related to disturbance intensity have significant anomalies at day −7, indicating a finite amplitude disturbance prior to “genesis”. Many of these variables show similar temporal evolution, with a local minimum two or three days before day zero, and a strong increase after that for several days, followed by an eventual decrease. The precipitation reaches its maximum on day 2, the net moist static energy forcing (surface fluxes minus net tropospheric radiative cooling, each of which has an anomaly of 20–30 W m−2 in the sense of warming the atmosphere) a day later, and dynamical variables such as vorticity and temperature still later, with broad plateaus centered around day 4 or 5. The vorticity increases at the surface at the same time as at midlevels, unlike in observed storms. The mean composite environmental vertical wind shear has a maximum amplitude on day −2 and then decreases. This could indicate a causal role of shear in limiting development, but would also be consistent with a coincidental storm motion to regions of lower shear, with development controlled by other factors. A signal in the skewness of the lower-level relative humidity distribution over the ensemble suggests that a dry lower troposphere can prevent development of a model tropical disturbance. [ABSTRACT FROM AUTHOR]