Measurement report: In situ observations of deep convection without lightning during the tropical cyclone Florence 2018

Hurricane Florence was the sixth named storm in the Atlantic hurricane season 2018. It caused dozens of deaths and major economic damage. In this study, we present in situ observations of trace gases within tropical storm Florence on 2 September 2018, after it had developed a rotating nature, and of a tropical wave observed close to the African continent on 29 August 2018 as part of the research campaign CAFE Africa (Chemistry of the Atmosphere: Field Experiment in Africa) with HALO (High Altitude and LOng Range Research Aircraft). We show the impact of deep convection on atmospheric composition by measurements of the trace gases nitric oxide (NO), ozone (O3), carbon monoxide (CO), hydrogen peroxide (H2O2), dimethyl sulfide (DMS) and methyl iodide (CH3I) and by the help of color-enhanced infrared satellite imagery taken by GOES-16. While both systems, i.e., the tropical wave and the tropical storm, are deeply convective, we only find evidence for lightning in the tropical wave using both in situ NO measurements and data from the World Wide Lightning Location Network (WWLLN).

Measurement report: In situ observations of deep convection without lightning during the tropical cyclone Florence 2018

Hurricane Florence was the sixth named storm in the Atlantic hurricane season 2018. It caused dozens of deaths and major economic damage. In this study, we present in situ observations of trace gases within tropical storm Florence on September 2, 2018 after it had developed a rotating nature, and of a tropical wave observed close to the African continent on August 29, 2018 as part of the research campaign CAFE Africa (Chemistry of the Atmosphere – Field Experiment in Africa) with the HALO (High Altitude Long Range) research aircraft. We show the impact of deep convection on atmospheric composition by measurements of the trace gases nitric oxide (NO), ozone (O3), carbon monoxide (CO), hydrogen peroxide (H2O2), dimethyl sulfide (DMS) and methyl iodide (CH3I), and by the help of color enhanced infrared satellite imagery taken by GOES-16. While both systems, the tropical wave and the tropical storm, are deeply convective, we only find evidence for lightning in the tropical wave using both in situ NO measurements and data from the World Wide Lightning Location Network (WWLLN).

Tropical precipitation–buoyancy relationship in Convectively Coupled Waves over South America region

<p>In this work, the tropical wave precipitation-buoyancy relationship is revisited by analyzing 4-times daily wave-filtered brightness temperature, reanalysis, and radiosonde datasets over tropical South America during the wet season. Prior studies demonstrated that an integrated measure of buoyancy well-diagnoses precipitation over land and ocean. However, it is an open question whether the buoyancy-based approach can yield a robust relation to precipitation for equatorial wave disturbances. To advance our understanding of this relationship, a comprehensive analysis of their vertical thermodynamic structure and potential interactions with the basic state is also presented. An emphasis is placed on understanding the convection coupling mechanism in convectively coupled Kelvin and inertia-gravity waves. It will be shown that buoyancy is a better predictor of convection for these disturbances than the often-used moist static energy (MSE). Examination of this discrepancy reveals that a cooling of the lower troposphere by gravity wave motions, which reduces MSE, is key to the production of precipitation in these disturbances.</p>

Convectively Coupled Equatorial Wave Simulations Using the ECMWF IFS and the NOAA GFS Cumulus Convection Schemes in the NOAA GFS Model

There is a longstanding challenge in numerical weather and climate prediction to accurately model tropical wave variability, including convectively coupled equatorial waves (CCEWs) and the Madden–Julian oscillation. For subseasonal prediction, the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) has been shown to be superior to the NOAA Global Forecast System (GFS) in simulating tropical variability, suggesting that the ECMWF model is better at simulating the interaction between cumulus convection and the large-scale tropical circulation. In this study, we experiment with the cumulus convection scheme of the ECMWF IFS in a research version of the GFS to understand which aspects of the IFS cumulus convection scheme outperform those of the GFS convection scheme in the tropics. We show that the IFS cumulus convection scheme produces significantly different tropical moisture and temperature tendency profiles from those simulated by the GFS convection scheme when it is coupled with other physics schemes in the GFS physics package. We show that a consistent treatment of the interaction between parameterized convective plumes in the GFS planetary boundary layer (PBL) and the IFS convection scheme is required for the GFS to replicate the tropical temperature and moisture profiles simulated by the IFS model. The GFS model with the IFS convection scheme, and the consistent treatment between the convection and PBL schemes, produces much more organized convection in the tropics, and generates tropical waves that propagate more coherently than the GFS in its default configuration due to better simulated interaction between low-level convergence and precipitation.

Trends in Tropical Wave Activity from the 1980s to 2016

A frequency–wavenumber power () spectrum was constructed using satellite-derived outgoing longwave radiation (OLR) and brightness temperature for the tropical latitudes. Since the two datasets overlap for over 34 years with nonintersecting sources in error and compare relatively well with each other, it is possible to diagnose trends in the tropical wave activity from the two datasets with confidence. The results suggest a weakening trend in characterized by high interannual variability for wave activity occurring in the low-frequency part of the spectrum and a steady increase in with relatively low interannual variability for wave activity occurring in the high-frequency part of the spectrum. The results show the parts of the spectrum representing the Madden–Julian oscillation and equatorial Rossby wave losing and other parts of the spectrum representing Kelvin waves, mixed Rossby–gravity waves, and tropical disturbance–like wave activity gaining . Similar results were obtained when trends in variance corresponding to the first principal component were produced using spectrally filtered OLR data representative of atmospheric equatorial waves. Spatial trends in the active phase of wave events and the mean duration of events are also shown for the different wave types. Linear trends in for the entire spectrum and regional means in the spectrum corresponding to the abovementioned five wave types with confidence intervals are also presented in the paper. Finally, we demonstrate that El Niño–Southern Oscillation variability does not appear to control the overall spatial patterns and trends observed in the spectrum.

LESSONS LEARNED FROM RAPID DEPLOYMENT OF WAVE GAGES AND CAMERAS DURING HURRICANE IRMA

Hurricane Irma was a category 5 hurricane on the Saffir-Simpson hurricane wind scale. Irma developed from a tropical wave around the Cape Verde Islands. The National Hurricane Center started monitoring it on August 26, and it was classified as a tropical storm named Irma on August 30. Moving across the Atlantic Ocean, Irma increased in strength. On September 5, Irma was classified as a category 5 hurricane with wind speeds up to 175 mph (280 km/h). Irma made landfall in the U.S. on Cudjoe Key (near Big Pine and Summerland Keys) in the morning of September 10, still being a category 4 hurricane, and made a second landfall on Marco Island, south of Naples, on the same day as a category 3 hurricane. This paper describes the lessons learned by the authors when deploying wave gages and cameras to observe the wave run-up.

Michio Yanai and Tropical Waves

Insights by Professor Michio Yanai on tropical waves, which have been vital ingredients for progress in tropical meteorology over the last half-century, are recollected. This study revisits various aspects of research on tropical waves over the last five decades to examine, in Yanai’s words, “the nature of ‘A-scale’ tropical wave disturbances and the interaction of the waves and the ‘B-scale’ phenomena (cloud clusters),” the fundamental problem posed by Yanai at the design phase of the GARP Atlantic Tropical Experiment (GATE) in 1971. The various contributions of Michio Yanai to the current understanding of the dynamics of the tropical atmosphere are briefly reviewed to show how his work has led to several current theories in this field.

A Reanalysis of Hurricane Camille

A reanalysis of 1969’s Hurricane Camille has been completed as part of the Atlantic Hurricane Database Reanalysis Project. The reanalysis of Hurricane Camille has been expedited to allow for a homogeneous comparison of all four of the U.S.-landfalling Saffir–Simpson hurricane wind scale category 5 hurricanes since 1900. A review of the available ship, station, radar, aircraft, and satellite observations is presented, along with the reanalysis methodology. Highlights of the Best-Track Change Committee approved changes to Camille’s genesis, track, intensity, and dissipation are discussed. As part of the preparation for the reanalysis, research on Hurricane Camille uncovered new data useful to the reanalysis. Focus was placed on understanding the internal structure in a modern context, especially whether eyewall replacement cycles occurred, including comparisons with a similar hurricane used as a proxy. A more detailed understanding was gained of the tropical wave and genesis phases. In addition, a 901-mb dropsonde that was later rejected was reanalyzed to find out why and to see if an accurate central pressure could be determined. New landfall surface pressures along the Mississippi coast were discovered and a significant revision is made to the U.S.-landfall central pressure and intensity (maximum sustained surface winds). Additionally, a radar “loop” was constructed from archived Weather Surveillance Radar-1957 (WSR-57) film, including landfall, marking the very first time that this historic hurricane can be viewed in a time-lapse movie format.

The Influence of the Spectral Truncation on the Simulation of Waves in the Tropical Stratosphere

Convectively triggered waves are the main driver of the tropical stratospheric circulation. In atmospheric models, the model’s resolution limits the length of the simulated wave spectrum. In this study, the authors compare the tropical tropospheric wave sources, their projection on the wave field in the lower stratosphere, and the circumstances of their upward propagation in the atmospheric model ECHAM6 with three spectral truncations of T63, T127, and T255. The model internally generates the quasi-biennial oscillation (QBO), which dominates the variability in the tropical stratosphere. This analysis focuses on two opposite phases of the QBO to account for the influence of the background wind field on the wave filtering. It is shown that, compared to the high-resolution model versions, the T63 version has less convective variability and less wave momentum in the lower stratosphere at wavenumbers larger than 20, well below the version’s truncation limit. In the low-resolution version, the upward propagation of the waves is further hindered by the highly active (relative to the high-resolution versions) horizontal diffusion scheme. However, even in the T255 version of ECHAM6, the convective variability is too small compared to TRMM observations at periods shorter than 2 days and wavelengths shorter than 1000 km. Hence, to model a realistic tropical wave activity, the convective parameterization of the model has to improve to increase the day-to-day precipitation variability.