Weather Avoidance Guidelines for NASA Global Hawk High-Altitude Unmanned Aircraft Systems (UAS)

The current Global Hawk flight rules would probably not have been effective in the single event of greatest concern (the Emily encounter). The cloud top had not reached 50,000 ft until minutes before the encounter. The TOT and lightning data would not have been available until near the overflight time since this was a rapidly growing cell. This case would have required a lastminute diversion when lightning became frequent. Avoiding such a cell probably requires continual monitoring of the forward camera and storm scope, whether or not cloud tops have been exceeding specific limits. However, the current overflight rules as strictly interpreted would have prohibited significant fractions of the successful Global Hawk overpasses of Karl and Matthew that proved not to be hazardous. Many other high altitude aircraft (ER2 and Global Hawk) flights in NASA tropical cyclone field programs have successfully overflown deep convective clouds without incident.The convective cell that caused serious concern about the safety of the ER2 in Emily was especially strong for a tropical cyclone environment, probably as strong or stronger than any that was overflown by the ER2 in 20 previous flights over tropical cyclones. Specifically, what made that cell a safety concern was the magnitude of the vertical velocity of the updraft, at least 20 m/s (4000 ft/minute) at the time the ER2 overflew it. Such a strong updraft can generate strong gravity waves at and above the tropopause, posing a potential danger to aircraft far above the maximum altitude of the updraft itself or its associated cloud top. Indeed, the ER2 was probably at least 9000 ft above that cloud top. Cloudtop height, by itself, is not an especially good indicator of the intensity of convection and the likelihood of turbulence. Nor is overflying high cloud tops (i.e. > 50,000 ft) of particular concern unless there is other evidence of very strong convective updrafts beneath those tops in the path of the aircraft. center dot Lightning, especially lightning with a high flash rate, is well correlated with convective intensity. Lightning with a minimal flash rate (say 13 flashes per minute) is indicative of updraft speeds of about 10 m/s in the mixed phase region where charge is being separated, generally at altitudes about 2025 kft in a hurricane. That is still stronger than typical updrafts (more like 5 m/s). An unresolved issue is whether there is a high and instantaneous correlation between vertical velocity in the middle troposphere (necessary for lightning generation) and near cloud top (more direct concern for overflights)

Recommendations for improved tropical cyclone formation and position probabilistic Forecast products

Prediction of the potentially devastating impact of landfalling tropical cyclones (TCs) relies substantially on numerical prediction systems. Due to the limited predictability of TCs and the need to express forecast confidence and possible scenarios, it is vital to exploit the benefits of dynamic ensemble forecasts in operational TC forecasts and warnings. RSMCs, TCWCs, and other forecast centers value probabilistic guidance for TCs, but the International Workshop on Tropical Cyclones (IWTC-9) found that the “pull-through” of probabilistic information to operational warnings using those forecasts is slow. IWTC-9 recommendations led to the formation of the WMO/WWRP Tropical Cyclone-Probabilistic Forecast Products (TC-PFP) project, which is also endorsed as a WMO Seamless GDPFS Pilot Project. The main goal of TC-PFP is to coordinate across forecast centers to help identify best practice guidance for probabilistic TC forecasts. TC-PFP is being implemented in 3 phases: Phase 1 (TC formation and position); Phase 2 (TC intensity and structure); and Phase 3 (TC related rainfall and storm surge). This article provides a summary of Phase 1 and reviews the current state of the science of probabilistic forecasting of TC formation and position. There is considerable variability in the nature and interpretation of forecast products based on ensemble information, making it challenging to transfer knowledge of best practices across forecast centers. Communication among forecast centers regarding the effectiveness of different approaches would be helpful for conveying best practices. Close collaboration with experts experienced in communicating complex probabilistic TC information and sharing of best practices between centers would help to ensure effective decisions can be made based on TC forecasts. Finally, forecast centers need timely access to ensemble information that has consistent, user-friendly ensemble information. Greater consistency across forecast centers in data accessibility, probabilistic forecast products, and warnings and their communication to users will produce more reliable information and support improved outcomes

A View of Tropical Cyclones from Above: The Tropical Cyclone Intensity Experiment

Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes

The Saharan Air Layer and the Fate of African Easterly Waves—NASA's AMMA Field Study of Tropical Cyclogenesis

In 2006, NASA led a field campaign to investigate the factors that control the fate of African easterly waves (AEWs) moving westward into the tropical Atlantic Ocean. Aircraft and surface-based equipment were based on Cape Verde's islands, helping to fill some of the data void between Africa and the Caribbean. Taking advantage of the international African Monsoon Multidisciplinary Analysis (AMMA) program over the continent, the NASA–AMMA (NAMMA) program used enhanced upstream data, whereas NOAA aircraft farther west in the Atlantic studied several of the storms downstream. Seven AEWs were studied during AMMA, with at least two becoming tropical cyclones. Some of the waves that did not develop while being sampled near Cape Verde likely intensified in the central Atlantic instead. NAMMA observations were able to distinguish between the large-scale wave structure and the smaller-scale vorticity maxima that often form within the waves. A special complication of the east Atlantic environment is the Saharan air layer (SAL), which frequently accompanies the AEWs and may introduce dry air and heavy aerosol loading into the convective storm systems in the AEWs. One of the main achievements of NAMMA was the acquisition of a database of remote sensing and in situ observations of the properties of the SAL, enabling dynamic models and satellite retrieval algorithms to be evaluated against high-quality real data. Ongoing research with this database will help determine how the SAL influences cloud micro-physics and perhaps also tropical cyclogenesis, as well as the more general question of recognizing the properties of small-scale vorticity maxima within tropical waves that are more likely to become tropical cyclones

Tropical Cyclone Diurnal Cycle Signals in a Hurricane Nature Run

The diurnal cycle of tropical convection and tropical cyclones (TCs) has been previously described in observational-, satellite-, and modeling-based studies. The main objective of this work is to expand on these earlier studies by identifying signals of the TC diurnal cycle (TCDC) in a hurricane nature run, characterize their evolution in time and space, and better understand the processes that cause them. Based on previous studies that identified optimal conditions for the TCDC, a select period of the hurricane nature run is examined when the simulated storm was intense, in a low shear environment, and sufficiently far from land. When analyses are constrained by these conditions, marked radially propagating diurnal signals in radiation, thermodynamics, winds, and precipitation that affect a deep layer of the troposphere become evident in the model. These propagating diurnal signals, or TC diurnal pulses, are a distinguishing characteristic of the TCDC and manifest as a surge in upper-level outflow with underlying radially propagating tropical squall-line-like features. The results of this work support previous studies that examined the TCDC using satellite data and have implications for numerical modeling of TCs and furthering our understanding of how the TCDC forms, evolves, and possibly impacts TC structure and intensity

In Situ Observations of the Diurnal Variation in the Boundary Layer of Mature Hurricanes

Recent studies have suggested that the structure of tropical cyclones (TCs), especially the upper‐level clouds as indicated by satellite infrared brightness temperatures and precipitation, fluctuates with the diurnal cycle. The diurnal cycle of the low‐level structure, including the boundary layer, has not yet been investigated with observations. This study analyzes data from 2242 GPS dropsondes collected in mature hurricanes to investigate the diurnal variation of the mean boundary layer structure. A composite analysis is conducted to compare the kinematic and thermodynamic structure during nighttime (0–6 local time) vs in the afternoon (12–18 local time). The composites show that much stronger inflow occurs during nighttime and the moist entropy is also larger than that in the daytime. Grouping the dropsonde data into 6‐h time windows relative to the local time shows a clear diurnal signal of boundary layer inflow. The amplitude of the diurnal signal is largest at a radius of 250–500 km.Plain Language SummaryThe upper‐level clouds that we see in satellite images of tropical cyclones (also known as hurricanes) are often seen to expand and contract over the course of each day. These expansions are associated with a pulse of thunderstorms and rain that travel hundreds of kilometers away from the storm center. Although this daily cycle at upper levels of the atmosphere is well established, it remains unknown whether there are similar changes in winds and moisture near the surface. This study uses observations from hundreds of dropsondes – instruments on parachutes that are dropped out of airplanes – to determine whether there are similar daily changes in the hurricane winds at low altitudes. These winds are indeed shown to have a daily pattern, with stronger inflow (wind flowing toward the storm center) and increased moisture occurring in the overnight hours as compared to the rest of the day. These periods of increased inflow and moisture precede the outward moving bands of thunderstorms, and then diminish as the bands steadily move outward to larger distances. This study could help us better understand how the TC diurnal cycle affects the low‐level structure of storms.Key PointsDropsonde data from mature hurricanes are composited to study the diurnal variation of the boundary layer structureBoth the inflow speed and moist entropy values are greater in the nighttime boundary layer than in the daytime boundary layerThe diurnal cycle of tropical cyclone low‐level structure is strongest at a radius range of 250–500 k