Reginald E. Newell

A Proposed Algorithm for Moisture Fluxes from Atmospheric Rivers

Abstract A new algorithm is applied to study water vapor fluxes in the troposphere using wind and moisture data from the European Centre for Medium-Range Weather Forecasts. The fluxes are divided into filamentary structures known as tropospheric rivers and what are termed here broad fields. The results show that the tropospheric rivers may carry essentially the total meridional transport observed in the extratropical atmosphere but may occupy only about 10% of the total longitudinal length at a given latitude. The transient fluxes in traditional studies do not catch the filamentary structures completely and may therefore underestimate the fraction of transport assigned to moving systems, as well as omitting the geographical concentration. The mean flow and eddy fluxes evaluated by the new algorithm are considered to be more physically realistic.

A Stratospheric Fountain

Abstract A “stratospheric fountain”, or area where air enters the stratosphere from the troposphere, is postulated based on an analysis of global 100 mb monthly mean temperatures. The temperature threshold is calculated from the observed global stratospheric water mixing ratio and fulfills the requirement of preserving low stratospheric humidity. The fountain occurs over the western tropical Pacific, northern Australia, Indonesia, and Malaysia in the November–March period and over the Bay of Bengal and India during the Monsoon. It is suggested that the major portion of the stratospheric air supply enters through these areas with most of the exchange occurring in the November–March period. Methods are proposed for substantiating the existence of the stratospheric fountain.

The Pacific Exploratory Mission-West Phase B: February-March, 1994

The NASA Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West) is a major component of the East Asia/North Pacific Regional Study (APARE), a project within the International Global Atmospheric Chemistry (IGAC) Program. The broad objectives of the PEM-West/APARE initiative are to study chemical processes and long-range transport of atmospheric trace species over the north-west Pacific Ocean and to estimate the magnitude of the human impact on these species over this region. The first phase of PEM-West (PEM-West A) was conducted in September-October 1991, a period characterized by minimum outflow from the Asian continent. The second phase of this mission, PEM-West B, was conducted during February-March 1994, a period characterized by enhanced outflow from the Asian continent. Both field campaigns of PEM-West included intensive airborne measurements of trace gases and aerosols from the NASA DC-8 aircraft coordinated with measurements at surface sites. This paper reports the experimental design for PEM-West B and provides a brief summary of the salient results of the PEM-West B campaign with particular emphases on the difference/similarities between phases A and B. Results from the two campaigns clearly quantify, from a trace gas perspective, the seasonal differences in the continental outflow that were qualitatively anticipated based upon meteorological considerations, and show the impact of major meteorological features within the region on the quality of tropospheric air over the North Pacific Ocean regions. The PEM-West database provides a “baseline” tool by which future assessments of a continuing impact of Asian emissions on remote Pacific regions can be judged. [These data are currently available through the Global Troposhperic Experiment Data Archive at NASAs Langley Research Center (http://www-gte.larc.nasa.gov) and the Langley Distributed Archive Center (http://eosdis.larc.nasa.gov)].

Pacific Exploratory Mission-West A (PEM-West A): September–October 1991

The NASA Pacific Exploratory Mission-West (PEM-West) is a major component of the East Asia/North Pacific Regional Study (APARE), a project within the International Global Atmospheric Chemistry (IGAC) program. The broad objective of the PEM-West/APARE initiative is to study chemical processes and long-range transport over the northwestern Pacific Ocean and to estimate the magnitude of the human impact on the oceanic atmosphere over this region particularly for tropospheric ozone and its precursors as well as for sulfur species. The first phase of this mission, PEM-West A, was conducted during September–October 1991. The PEM-West A included intensive airborne measurements of trace gases from the NASA DC-8 aircraft coordinated with measurements at PEM-West A surface sites as well as with measurements obtained from collaborating APARE ground and airborne platforms. This paper reports the experimental design for PEM-West A with a brief summary of the general content and focus of companion papers in this special issue.

Empirical Orthogonal Analysis of Pacific Sea Surface Temperatures

Abstract An empirical orthogonal function analysis has been performed on monthly mean sea surface temperatures for the greater part of the Pacific Ocean between 55°N and 20°S. The analysis identifies the most important modes of seasonal and non-seasonal variability during the period 1949–73. A mode is defined spatially in terms of an empirical orthogonal function which describes the degree of coherence of variation. The functions corresponding coefficient portray the evolution of the mode in time. The seasonal variation is dominated by a mode having a 12-month periodicity and greatest coherence in the higher latitudes. A second important seasonal mode has a period of approximately 6 months and is dominated by deviations in the North Pacific. The most important non-seasonal variation is identified with the, long-recognized El Nino. The spatial pattern of this mode demonstrates the large-scale nature of the El Nino phenomenon. Other important non-seasonal modes are discussed.

Assessment of ozone photochemistry in the western North Pacific as inferred from PEM-West A observations during the fall 1991

This study examines the influence of photochemical processes on ozone distributions in the western North Pacific. The analysis is based on data generated during NASAs western Pacific Exploratory Mission (PEM-West A) during the fall of 1991. Ozone trends were best described in terms of two geographical domains: the western North Pacific rim (WNPR) and the western tropical North Pacific (WTNP). For both geographical regions, ozone photochemical destruction, D(O3), decreased more rapidly with altitude than did photochemical formation, F(O3). Thus the ozone tendency, P(O3), was typically found to be negative for z 6–8 km. For nearly all altitudes and latitudes, observed nonmethane hydrocarbon (NMHC) levels were shown to be of minor importance as ozone precursor species. Air parcel types producing the largest positive values of P(O3) included fresh continental boundary layer (BL) air and high-altitude (z > 7 km) parcels influenced by deep convection/lightning. Significant negative P(O3) values were found when encountering clean marine BL air or relatively clean lower free-tropospheric air. Photochemical destruction and formation fluxes for the Pacific rim region were found to exceed average values cited for marine dry deposition and stratospheric injection in the northern hemisphere by nearly a factor of 6. This region was also found to be in near balance with respect to column-integrated O3 photochemical production and destruction. By contrast, for the tropical regime column-integrated O3 showed photochemical destruction exceeding production by nearly 80%. Both transport of O3 rich midlatitude air into the tropics as well as very high-altitude (10–17 km) photochemical O3 production were proposed as possible additional sources that might explain this estimated deficit. Results from this study further suggest that during the fall time period, deep convection over Asia and Malaysia/Indonesia provided a significant source of high-altitude NOx to the western Pacific. Given that the high-altitude NOx lifetime is estimated at between 3 and 9 days, one would predict that this source added significantly to high altitude photochemical O3 formation over large areas of the western Pacific. When viewed in terms of strong seasonal westerly flow, its influence would potentially span a large part of the Pacific.

Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3−, NOy), C1 to C6 hydrocarbons, O3, chemical tracers (C2Cl4, CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission-West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO2, NOy, and O3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NOy concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NOx, PAN, and HNO3), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO3 mixing ratios (>100 ppt). A global three-dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NOy and NOx. A large disagreement between measurement and theory exists in the atmospheric distribution of HNO3. It appears that surface-based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equilibrium and the NOy reservoir may be an important in situ source of atmospheric NOx. Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NOx is directly attributable to its surface NOx source, and free tropospheric sources are dominant. In situ release of NOx from the NOy reservoir, lightning, direct transport of surface NOx, aircraft emissions, and small stratospheric input collectively maintain the NOx balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O3 production. Sources of tropospheric NOx cannot yet be accurately defined due to shortcomings in measurements and theory.

Ozone and aerosol distributions and air mass characteristics over the South Pacific during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to establish a baseline chemical signature of the atmosphere above the South Pacific Ocean during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission-Tropics A (PEM-Tropics A) conducted in August-October 1996. This paper discusses general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large-scale variations in ozone (O3) and aerosol distributions across the troposphere, calculated potential vorticity (PV) from the European Centre for Medium-Range Weather Forecasting (ECMWF), and in situ measurements for comprehensive air mass composition. Between 8°S and 52°S, biomass burning plumes containing elevated levels of O3, over 100 ppbv, were frequently encountered by the aircraft at altitudes ranging from 2 to 9 km. Air with elevated O3 was also observed remotely up to the tropopause, and these air masses were observed to have no enhanced aerosol loading. Frequently, these air masses had some enhanced PV associated with them, but not enough to explain the observed O3 levels. A relationship between PV and O3 was developed from cases of clearly defined O3 from stratospheric origin, and this relationship was used to estimate the stratospheric contribution to the air masses containing elevated O3 in the troposphere. The frequency of observation of the different air mass types and their average chemical composition is discussed in this paper.

Pacific Exploratory Mission in the tropical Pacific: PEM‐Tropics A, August‐September 1996

The NASA Pacific Exploratory Mission to the Pacific tropics (PEM-Tropics) is the third major field campaign of NASAs Global Tropospheric Experiment (GTE) to study the impact of human and natural processes on the chemistry of the troposphere over the Pacific basin. The first two campaigns, PEM-West A and B were conducted over the northwestern regions of the Pacific and focused on the impact of emissions from the Asian continent. The broad objectives of PEM-Tropics included improving our understanding of the oxidizing power of the tropical atmosphere as well as investigating oceanic sulfur compounds and their conversion to aerosols. Phase A of the PEM-Tropics program, conducted between August-September 1996, involved the NASA DC-8 and P-3B aircraft. Phase B of this program is scheduled for March/April 1999. During PEM-Tropics A, the flight tracks of the two aircraft extended zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand. Both aircraft were instrumented for airborne measurements of trace gases and aerosols and meteorological parameters. The DC-8, given its long-range and high-altitude capabilities coupled with the lidar instrument in its payload, focused on transport issues and ozone photochemistry, while the P-3B, with its sulfur-oriented instrument payload and more limited range, focused on detailed sulfur process studies. Among its accomplishments, the PEM-Tropics A field campaign has provided a unique set of atmospheric measurements in a heretofore data sparse region; demonstrated the capability of several new or improved instruments for measuring OH, H2SO4, NO, NO2, and actinic fluxes; and conducted experiments which tested our understanding of HOx and NOx photochemistry, as well as sulfur oxidation and aerosol formation processes. In addition, PEM-Tropics A documented for the first time the considerable and widespread influence of biomass burning pollution over the South Pacific, and identified the South Pacific Convergence Zone as a major barrier for atmospheric transport in the southern hemisphere.

Ubiquity of quasi-horizontal layers in the troposphere

Fine laminar structures in the atmosphere have been described previously, but their characterization has been limited. The modern global coverage of aircraft flights offers an opportunity to provide such a characterization, and examine the ubiquity of such structures, in space and time. Research aircraft measuring vertical profiles of atmospheric chemical constituents frequently discern quasi-horizontal atmospheric layers with mean thicknesses of the order of 1 km and mean altitudes between 5 and 7 km (refs 10,11,12). These layers can be characterized and categorized by various combinations of ozone, water vapour, carbon monoxide and methane deviations from background profiles. Five commercial aircraft have been recently equipped to measure water vapour and ozone concentrations, and automatically collect vertical profile information on landing and take-off (refs 13,14,15). Here we synthesize measurements from both research and commercial flights and demonstrate the ubiquity in space and time of four layer types (as categorized by their chemical signatures). Up to one-fifth of the lowest 12 km of the atmosphere is occupied by such layers. We suggest that this universality reflects basic characteristics of the atmosphere hitherto unexplored, with potential implications for present understanding of a wide variety of dynamic and chemical atmospheric processes.