A. da Silva

Quantification of dust-forced heating of the lower troposphere

Aerosols may affect climate through the absorption and scattering of solar radiation and, in the case of large dust particles, by interacting with thermal radiation. But whether atmospheric temperature responds significantly to such forcing has not been determined; feedback mechanisms could increase or decrease the effects of the aerosol forcing. Here we present an indirect measure of the tropospheric temperature response by explaining the ‘errors’ in the NASA/Goddard model/data-assimilation system. These errors, which provide information about physical processes missing from the predictive model, have monthly mean patterns that bear a striking similarity to observed patterns of dust over the eastern tropical North Atlantic Ocean. This similarity, together with the high correlations between latitudinal location of inferred maximum atmospheric heating rates and that of the number of dusty days, suggests that dust aerosols are an important source of inaccuracies in numerical weather-prediction models in this region. For the average dust event, dust is estimated to heat the lower atmosphere (1.5–3.5 km altitude) by ∼0.2 K per day. At about 30 dusty days per year, the presence of the dust leads to a regional heating rate of ∼6 K per year.

Central American biomass burning smoke can increase tornado severity in the U.S.

Tornadoes in the Southeast and central U.S. are episodically accompanied by smoke from biomass burning in central America. Analysis of the 27 April 2011 historical tornado outbreak shows that adding smoke to an environment already conducive to severe thunderstorm development can increase the likelihood of significant tornado occurrence. Numerical experiments indicate that the presence of smoke during this event leads to optical thickening of shallow clouds while soot within the smoke enhances the capping inversion through radiation absorption. The smoke effects are consistent with measurements of clouds and radiation before and during the outbreak. These effects result in lower cloud bases and stronger low-level wind shear in the warm sector of the extratropical cyclone generating the outbreak, two indicators of higher probability of tornadogenesis and tornado intensity and longevity. These mechanisms may contribute to tornado modulation by aerosols, highlighting the need to consider aerosol feedbacks in numerical severe weather forecasting.

The Earth System Prediction Suite: Toward a Coordinated U.S. Modeling Capability

The Earth System Prediction Suite (ESPS) is a collection of flagship U.S. weather and climate models and model components that are being instrumented to conform to interoperability conventions, documented to follow metadata standards, and made available either under open source terms or to credentialed users. The ESPS represents a culmination of efforts to create a common Earth system model architecture, and the advent of increasingly coordinated model development activities in the U.S. ESPS component interfaces are based on the Earth System Modeling Framework (ESMF), community-developed software for building and coupling models, and the National Unified Operational Prediction Capability (NUOPC) Layer, a set of ESMF-based component templates and interoperability conventions. This shared infrastructure simplifies the process of model coupling by guaranteeing that components conform to a set of technical and semantic behaviors. The ESPS encourages distributed, multi-agency development of coupled modeling systems, controlled experimentation and testing, and exploration of novel model configurations, such as those motivated by research involving managed and interactive ensembles. ESPS codes include the Navy Global Environmental Model (NavGEM), HYbrid Coordinate Ocean Model (HYCOM), and Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS®); the NOAA Environmental Modeling System (NEMS) and the Modular Ocean Model (MOM); the Community Earth System Model (CESM); and the NASA ModelE climate model and GEOS-5 atmospheric general circulation model.

Reassessment of the moisture source over the Sahara Desert based on NASA reanalysis

The components of the moisture balance equation are calculated for the Middle East/North Africa regions based on NASA/GEOS-1 multiyear reanalysis data set. These include the Evaporation (E), Precipitation (P), moisture flux divergence (∇ · Q), and errors associated with the incremental analysis updates of the specific humidity, or IAU(q). The Annual mean ∇ · Q corresponds well to the results of Vitart et al. [1996], based on NCEP data. IAU (q) reveals a strong moisture source over the eastern Mediterranean and also confirms the paradoxical net moisture sink over the Arabian-Iraqi desert found by Alpert and Shay-El [1993]. Over the North African Sahara Desert the moisture flux was shown to converge through the northern and southern boundaries mainly at low levels (∼900 hPa) and to diverge through the eastern and western boundaries at higher levels (∼700 hPa). Starr and Peixoto [1958] have classified North Africa as a net moisture source. Area averaging of ∇ · Q over a box with varying dimensions reveals that it can be classified as a net sink if the box is small enough and located over the center of the desert. If the box is big enough to include the boundaries of the continent only then can it be classified as net source or divergence zone. Inspection of the intermonthly and diurnal variability, as well as the model biases, weakens also the net source argument. It is suggested that the earlier finding of a net source might be due to the smoothing of the water/land boundary, or due to various atmospheric diffusion processes such as the sea breeze cycle and cloud intrusion and evaporation.

Development of a grid-independent GEOS-Chem chemical transport model (v9-02) as an atmospheric chemistry module for Earth system models

The GEOS-Chem global chemical transport model (CTM), used by a large atmospheric chemistry research community, has been re-engineered to also serve as an atmospheric chemistry module for Earth system models (ESMs). This was done using an Earth System Modeling Framework (ESMF) interface that operates independently of the GEOSChem scientific code, permitting the exact same GEOSChem code to be used as an ESM module or as a standalone CTM. In this manner, the continual stream of updates contributed by the CTM user community is automatically passed on to the ESM module, which remains state of science and referenced to the latest version of the standard GEOS-Chem CTM. A major step in this re-engineering was to make GEOS-Chem grid independent, i.e., capable of using any geophysical grid specified at run time. GEOS-Chem data sockets were also created for communication between modules and with external ESM code. The grid-independent, ESMF-compatible GEOS-Chem is now the standard version of the GEOS-Chem CTM. It has been implemented as an atmospheric chemistry module into the NASA GEOS5 ESM. The coupled GEOS-5–GEOS-Chem system was tested for scalability and performance with a tropospheric oxidant-aerosol simulation (120 coupled species, 66 transported tracers) using 48–240 cores and message-passing interface (MPI) distributed-memory parallelization. Numerical experiments demonstrate that the GEOS-Chem chemistry module scales efficiently for the number of cores tested, with no degradation as the number of cores increases. Although inclusion of atmospheric chemistry in ESMs is computationally expensive, the excellent scalability of the chemistry module means that the relative cost goes down with increasing number of cores in a massively parallel environment.

Observations of the Interaction and Transport of Fine Mode Aerosols With Cloud and/or Fog in Northeast Asia From Aerosol Robotic Network and Satellite Remote Sensing

Analysis of sun photometer measured and satellite retrieved aerosol optical depth (AOD) data has shown that major aerosol pollution events with very high fine mode AOD (>1.0 in mid-visible) in the China/Korea/Japan region are often observed to be associated with significant cloud cover. This makes remote sensing of these events difficult even for high temporal resolution sun photometer measurements. Possible physical mechanisms for these events that have high AOD include a combination of aerosol humidification, cloud processing, and meteorological co-variation with atmospheric stability and convergence. The new development of Aerosol Robotic network (AERONET) Version 3 Level 2 AOD with improved cloud screening algorithms now allow for unprecedented ability to monitor these extreme fine mode pollution events. Further, the Spectral Deconvolution Algorithm (SDA) applied to Level 1 data (L1; no cloud screening) provides an even more comprehensive assessment of fine mode AOD than L2 in current and previous data versions. Studying the 2012 winter-summer period, comparisons of AERONET L1 SDA daily average fine mode AOD data showed that Moderate Resolution Imaging Spectroradiometer (MODIS) satellite remote sensing of AOD often did not retrieve and/or identify some of the highest fine mode AOD events in this region. Also, compared to models that include data assimilation of satellite retrieved AOD, the L1 SDA fine mode AOD was significantly higher in magnitude, particularly for the highest AOD events that were often associated with significant cloudiness.

On the Parcel Method and the Baroclinic Wedge of Instability

The heuristic explanation, suggested by the parcel method, for the baroclinic instability mechanism is reexamined. The parcel method argues that an air parcel displaced within the wedge of instability, that is, between the horizontal and the isentropes, is vertically accelerated by the buoyancy force and hence becomes unstable. However, in the synoptic scale, the buoyancy is balanced by the vertical pressure gradient force perturbation, which is neglected by the parcel method, and thus the parcel acceleration is essentially horizontal. For the unstable Eady normal modes, the horizontally averaged buoyancy work is found to maximize at the steering level and to vanish at the boundaries, but the horizontally averaged parcel kinetic energy growth is minimized at the steering level and maximized at the boundaries. It is shown that the buoyancy work is vertically redistributed by the pressure gradient force perturbation throughout the secondary circulation. The parcel method also assumes that a parcel displaced adiabatically within the wedge of instability finds itself warmer than its new surroundings and thus contributes toward both vertical and meridional positive heat fluxes. However, since the temperature difference between the parcel and the environment from which it departed cannot be neglected, the slope of the instantaneous displacement is not a sufficient criterion to determine the signs of the heat fluxes. It is shown here that for the Eady normal modes solution, the four combinations of ascending or descending of initially colder or warmer parcels make jointly the vertical heat flux maximize at the steering level and the meridional heat flux remain constant with height.

Dust Impacts on the 2012 Hurricane Nadine Track during the NASA HS3 Field Campaign

During the 2012 deployment of the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign, several flights were dedicated to investigating Hurricane Nadine. Hurricane Nadine developed in close proximity to the dust-laden Saharan Air Layer, and is the fourth longest-lived Atlantic hurricane on record, experiencing two strengthening and weakening periods during its 22-day total lifecycle as a tropical cyclone. In this study, the NASA GEOS-5 atmospheric general circulation model and data assimilation system was used to simulate the impacts of dust during the first intensification and weakening phases of Hurricane Nadine using a series of GEOS-5 forecasts initialized during Nadines intensification phase (12 September 2012). The forecasts explore a hierarchy of aerosol interactions within the model: no aerosol interaction, aerosol-radiation interactions, and aerosol-radiation and aerosol-cloud interactions simultaneously, as well as variations in assumed dust optical properties. When only aerosolradiation interactions are included, Nadines track exhibits sensitivity to dust shortwave absorption, as a more absorbing dust introduces a shortwave temperature perturbation that impacts Nadines structure and steering flow, leading to a northward track divergence after 5 days of simulation time. When aerosol-cloud interactions are added, the track exhibits little sensitivity to dust optical properties. This result is attributed to enhanced longwave atmospheric cooling from clouds that counters shortwave atmospheric warming by dust surrounding Nadine, suggesting that aerosol-cloud interactions are a more significant influence on Nadines track than aerosol-radiation interactions. These findings demonstrate that tropical systems, specifically their track, can be impacted by dust interaction with the atmosphere.

Link Between Arctic Tropospheric BrO Explosion Observed From Space and Sea‐Salt Aerosols From Blowing Snow Investigated Using Ozone Monitoring Instrument BrO Data and GEOS‐5 Data Assimilation System

Bromine radicals (Br + BrO) are important atmospheric species owing to their ability to catalytically destroy ozone as well as their potential impacts on the oxidative pathways of many trace gases, including dimethylsulfide and mercury. Using space‐based observations of BrO, recent studies have reported rapid enhancements of tropospheric BrO over large areas (so called “BrO explosions”) connected to near‐surface ozone depletion occurring in polar spring. However, the source(s) of reactive bromine and mechanism(s) that initiate these BrO explosions are uncertain. In this study, we investigate the relationships between Arctic BrO explosions and two of the proposed sources of reactive bromine: sea‐salt aerosol (SSA) generated from blowing snow and first‐year (seasonal) sea ice. We use tropospheric column BrO derived from the Ozone Monitoring Instrument (OMI) in conjunction with the Goddard Earth Observing System Version 5 (GEOS‐5) data assimilation system provided by National Aeronautics and Space Administration Global Modeling and Assimilation Office. Case studies demonstrate a strong association between the temporal and spatial extent of OMI‐observed BrO explosions and the GEOS‐5 simulated blowing snow‐generated SSA during Arctic spring. Furthermore, the frequency of BrO explosion events observed over the 11‐year record of OMI exhibits significant correlation with a time series of the simulated SSA emission flux in the Arctic and little to no correlation with a time series of satellite‐based first‐year sea ice area. Therefore, we conclude that SSA generated by blowing snow is an important factor in the formation of the BrO explosion observed from space during Arctic spring.

Using CATS near‐real‐time lidar observations to monitor and constrain volcanic sulfur dioxide (SO2) forecasts

An eruption of Italian volcano Mount Etna on 3 December 2015 produced fast-moving sulfur dioxide (SO2) and sulfate aerosol clouds that traveled across Asia and the Pacific Ocean, reaching North America in just 5 days. The Ozone Profiler and Mapping Suites Nadir Mapping UV spectrometer aboard the U.S. National Polar-orbiting Partnership satellite observed the horizontal transport of the SO2 cloud. Vertical profiles of the colocated volcanic sulfate aerosols were observed between 11.5 and 13.5 km by the new Cloud Aerosol Transport System (CATS) space-based lidar aboard the International Space Station. Backward trajectory analysis estimates the SO2 cloud altitude at 7–12 km. Eulerian model simulations of the SO2 cloud constrained by CATS measurements produced more accurate dispersion patterns compared to those initialized with the back trajectory height estimate. The near-real-time data processing capabilities of CATS are unique, and this work demonstrates the use of these observations to monitor and model volcanic clouds.