Tropical cloud clusters (TCCs) are traditionally defined as synoptic-scale areas of deep convection and associated cirrus outflow. They play a critical role in the energy balance of the tropics, releasing large amounts of latent heat high in the troposphere. If conditions are favorable, TCCs can develop into tropical cyclones (TCs), which put coastal populations at risk. Previous work, usually connected with large field campaigns, has investigated TCC characteristics over small areas and time periods. Recently, developments in satellite reanalysis and global best track assimilation have allowed for the creation of a much more extensive database of TCC activity. The authors use the TCC database to produce an extensive global analysis of TCCs, focusing on TCC climatology, variability, and genesis productivity (GP) over a 28-yr period . While global TCC frequency was fairly consistent over the time period, with relatively small interannual variability and no noticeable trend, regional analyses show a high degree of interannual variability with clear trends in some regions. Approximately 1600 TCCs develop around the globe each year; about 6.4% of those develop into TCs. The eastern North Pacific Ocean (EPAC) basin produces the highest number of TCCs (per unit area) in a given year, but the western North Pacific Ocean (WPAC) basin has the highest GP (;12%). Annual TCC frequency in some basins exhibits a strong correlation to sea surface temperatures (SSTs), particularly in the EPAC, North Atlantic Ocean, and WPAC. However, GP is not as sensitive to SST, supporting the hypothesis that the tropical cyclogenesis process is most sensitive to atmospheric dynamical considerations such as vertical wind shear and large-scale vorticity.

Previous studies have suggested that tropical cyclones (TCs) in deformation steering flows can be associated with large position errors and uncertainty. The goal of this study is to evaluate the sensitivity of position forecasts for three TCs within deformation wind fields [Debby (2012), Joaquin (2015), and Lionrock (2016)] using the ensemble-based sensitivity technique applied to European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecasts. In all three cases, the position forecasts are sensitive to uncertainty in the steering wind within 500 km of the 0-h TC position. Subsequently, the TC moves onto either side of the axis of contraction due to the ensemble perturbation steering flow. As a TC moves away from the saddle point, the ensemble members subsequently experience different ensemble-mean steering winds, which act to move the TC away from the ensemble-mean TC position along the axis of dilatation. By contrast, the position forecasts appear to exhibit less sensitivity to the steering wind more than 500 km from the initial TC position, even though the TC may interact with these features later in the forecast. Furthermore, forecasts initialized at later times are characterized by significantly lower position errors and uncertainty once it becomes clear on which side of the axis of contraction the TC will move. These results suggest that TCs in deformation steering flow could be inherently unpredictable and may benefit from densely sampling the near-storm steering flow and TC structure early in their lifetimes.

The 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field experiment obtained a diverse set of in-situ and remotely-sensed measurements before and during a pyrocumulonimbus (pyroCb) event over the Williams Flats fire in Washington State. This unique dataset confirms that pyroCb activity is an efficient vertical smoke transport pathway into the upper troposphere and lower stratosphere (UTLS). The magnitude of smoke plumes observed in the UTLS has increased significantly in recent years, following unprecedented wildfire and pyroCb activity observed worldwide. The FIREX-AQ pyroCb dataset is therefore extremely relevant to a broad community, providing the first measurements of fresh smoke exhaust in the upper-troposphere, including from within active pyroCb cloud tops. High-resolution remote sensing reveals that three plume cores linked to localized fire fronts, burning primarily in dense forest fuels, contributed to four total pyroCb “pulses”. Rapid changes in fire geometry and spatial extent dramatically influenced the magnitude, behavior, and duration of pyroCb activity. Cloud probe measurements and weather radar identify the presence of large ice particles within the pyroCb and hydrometers below cloud base, indicating precipitation development. The resulting feedbacks suggest that vertical smoke transport efficiency was reduced slightly when compared with intense pyroCb events reaching the lower stratosphere. Physical and optical aerosol property measurements in pyroCb exhaust are compared with previous assumptions. A large suite of aerosol and gas-phase chemistry measurements sets a foundation for future studies aimed at understanding the composition of smoke plumes lifted by pyroconvection into the UTLS and their role in the climate system.

This study examines climatological potential vorticity streamer (PVS) activity associated with Rossby wave breaking (RWB), which can impact TC activity in the subtropical North Atlantic (NATL) basin via moisture and wind anomalies. PVSs are identified along the 2-PVU (1 PVU = 10−6 K kg−1 m2 s−1) contour on the 350-K isentropic surface, using a unique identification technique that combines previous methods. In total, 21 149 individual PVS instances are identified from the ERA-Interim (ERAI) climatology during June–November over 1979–2015 with a peak in July–August. The total number of PVSs identified in this study is more than previous PVS climatologies for this region, since the new technique identifies a wider range of cases. Variations in PVS size and intensity prompt the development of a new PVS activity index (PVSI), which provides an integrated measure of PVS activity that can improve comparisons with TC activity. For instance, PVSI has a stronger negative correlation with seasonal TC activity (r = −0.55) relative to PVS frequency, size, or intensity alone. PVSI in June–July is also positively correlated with PVSI in August–November (r = 0.67), suggesting predictive capability. Compared to the ERAI and Japan Meteorological Agency 55-Year Reanalysis (JRA-55) climatology, there are more PVSs in the Climate Forecast System Reanalysis (CFSR) but these have weaker average intensity overall. While no long-term trend in PVSI is observed in the ERAI or JRA-55 climatologies, a negative trend is observed in CFSR, which could be related to differences in near tropopause static stability early in the climatological period (1979–86) between the CFSR and ERAI datasets.

Central American gyres (CAGs) are large, closed, cyclonic circulations that occur during the rainy season (May–November), which can yield exceptional rainfall leading to catastrophic flooding and large societal impacts. A reanalysis-based climatology of CAGs is developed from an algorithm that distinguishes CAG cases from other systems. This algorithm identified CAG cases based on circulation intensity, a broad radius of maximum winds, and the existence of closed, Earth-relative, cyclonic flow. Based on these criteria, 47 CAG cases were identified from 1980 to 2010, featuring a bimodal distribution of cases with maxima in May–June and September–November. CAG cases are composited into two categories based on their upper-tropospheric PV structure: nonbaroclinic CAGs are more common (N = 42) and characterized by an upper-tropospheric anticyclone, while baroclinic CAGs are less common (N = 5) and characterized by an upper-tropospheric trough. Whereas a nonbaroclinic CAG has anomalous moisture and precipitation surrounding the center, a baroclinic CAG has anomalous moisture and precipitation concentrated east of the center, with these structural differences attributed to their upper-tropospheric PV structure. Both nonbaroclinic and baroclinic CAGs are preceded by anomalous westerly lower-tropospheric flow in the eastern Pacific before their development, which is linked to a climatological reduction in easterly trade winds and is coincident with MJO phases 1, 2, and 8. Extreme precipitation is observed over multiple days in all available CAG cases, most commonly along the Central American coastline and on average over a large fractional area (25%) within 10° of their center.

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Philippe Papin Geography and politics in Ancient Vietnam This article is concerned with the arrangement of the broad lines of power across Vietnam and with the manner in which the world of the plains and that of the mountains are articulated. Topography has played a huge role in the history of a country the epic of whose foundation is one of a separation between the mountain peoples, recipients of an ancient power, and those who, thanks to the use of metal, headed East to benefit from the delta of the Red River, rice-growing country then still being sealed up. Upon contact with China, in particular with the governor Gao Pian, the society of the plain defined new ways of governing, based in Ancient Hanoi but contested and endangered several times during the course of the country's history by the forces still active in the surrounding mountains. The incursion of the armies of the Nanzhao in the 9th century, the seizure of power by the Dinh a hundred years later, or even the fall of the Ly dynasty in the 13th century are possible illustrations; but many other events, less well-known because less decisive, befell them in the course of this long-lasting opposition. From the 14th century, however, the Vietnamese monarchy made efforts to integrate this outlying rebel society, holding places for its members in the mandarins' examinations and forming firm matrimonial alliances with the potentates of the foothills. In many respects, the birth of the bureaucratic state was a response to the necessity, for the crown, of controlling its own territory by trying to resolve the original partition of a country divided by geography.

A high-impact Atmospheric River (AR) event that made landfall on the US West Coast on Valentine’s Day, 2019 and produced widespread flooding in California is examined. The US Naval Research Laboratory cloud resolving and high-resolution Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) captures the main features impacting the life cycle and structure of the Valentine’s Day AR. Analysis of the model simulated AR reveals the complex processes leading up to the initial northeastward surge of the water vapor and enhanced near-surface flow associated with this AR. These include the pre-existence of a mesoscale cold-core Kona Low, a mesoscale anticyclone, and a strong low-level convergence in the corridor between the Kona Low and mesoscale anticyclone where the environment becomes supersaturated in a region of weak vertical wind shear. Model sensitivity experiments show that the eastward progression and magnitude of the AR water vapor surge is strongly sensitive to the magnitude of Kona Low circulation. Experiments with the Kona Low circulation amplitude reduced to less than 25% showed that the AR is not able to reach the US West Coast. These results help identify key new aspects of an important player – the Kona Low— and its significant contributions to the overall AR characteristics of this particular observed event.