James Bishop

Spatial and temporal variability of global surface solar irradiance

A fast scheme for computing surface solar irradiance using data from the International Satellite Cloud Climatology Project (ISCCP) is described. Daily mean solar irradiances from the fast scheme reproduce the detailed global results from full radiative transfer model calculations to within 6 and 10 W m−2 over the ocean and land, respectively. In particular, the fast scheme reproduces the same dependence of surface irradiance on solar zenith angle which is critical for proper calculation of daily, seasonal, and latitudinal variability. Validation of both model results is limited because globally distributed data sets of high quality are lacking, particularly over the oceans. However, comparison of calculated monthly mean results using 5 months of ISCCP data (July 1983 to July 1984) with climatology from the 1970s at six temperate latitude ocean weather stations shows agreement within published estimates of interannual variability of monthly means at the individual stations. A further test against a 17-day time series at a continental site (43°N, 90°W, October–November 1986; 13–170 W m−2 range of irradiance), where ground and satellite data were spatially and temporally coincident, showed an accuracy of better than 9 W m−2 on a daily basis and less than 4% bias in the 17-day mean. Frequently used bulk formulae for solar irradiance were also evaluated in each of these tests. All suffered in comparison because they did not include a parameterization of the effects of the global variability in mean cloud optical thickness. Data from July (1983 and 1984) and January (1984 and 1985) were used to examine the spatial and temporal variability of surface irradiance and its potential impact on biospheric processes. Results show that the oceans and land experience fundamentally different light regimes, with continents receiving significantly greater irradiance. In summer, major interocean differences in zonally averaged irradiance are found in the northern hemisphere with the Atlantic greater than Pacific by up to 80 W m−2; in the southern hemisphere, interocean differences are small. Regional interannual variability (July 1983 versus 1984) ranged between +100 and −100 W m−2. The variability, perhaps due to the 1982–1983 El Nino event, occurred mostly in the Pacific but extended beyond the tropics over the entire north Pacific basin. The nutrient-rich northern and southern ocean waters are almost perpetually cloud covered; however, there is a correspondence between higher than average surface irradiance and productivity in nutrient-rich areas of the southwest Atlantic and Weddell Sea sector of the circumpolar current. This suggests that solar irradiance must be considered as an important factor governing the productivity of these waters.

Physical and geochemical properties across the Atlantic/Pacific water mass front in the southern Canadian Basin

Temperature, salinity, nutrients, oxygen, and halocarbon data collected in the Arctic Ocean reveal a frontal structure previously unrecognized in the hydrography of the Canadian Basin. Samples were collected on a 1300-km section extending from the Beaufort Sea in the Canada Basin to the East Siberian Sea in the Makarov Basin. These data, collected in 1993 aboard the CCGS Henry Larsen, reveal a lateral boundary between water masses of Atlantic and Pacific origin. The term “water mass assembly” is introduced to describe the basic arrangement or vertical stacking of water masses found in the Arctic Ocean, recognizing that water mass components within each assembly may differ from basin to basin. Using historical data, two primary water mass assemblies are defined, each consisting of three layers: an upper layer, an Atlantic layer, and a deep layer. These two assemblies are marked by important differences. One assembly, here defined as the Western Arctic (WA) assembly, is characterized by an upper layer of relatively fresh, high-nutrient water of Pacific origin; below this, by an Atlantic layer with a core temperature generally below 0.5°C; and, finally, by a deep layer of higher salinities and colder temperatures (about −0.5°C) than found in the overlying Atlantic layer. The second assembly, here defined as Eastern Arctic (EA) assembly, is characterized by the absence of Pacific water in the upper layer; below this, by an Atlantic layer core as warm as 2° to 3°C; and by a colder (about −0.9°C) deep layer. Because the presence or absence of Pacific origin water is a key characteristic distinguishing the two assemblies, we will refer to the water mass boundary between the two assemblies as the Atlantic/Pacific front. Earlier research indicated that water masses in the Arctic Ocean were separated by a front above the Lomonosov Ridge into the Canadian and Eurasian basins. Although all Larsen-93 stations from the Canada Basin (A1–D1) display classic WA assembly characteristics, the Makarov Basin station (E1) shows EA assembly characteristics in the upper and Atlantic layers and a WA assembly deep layer. This suggests a relocation in the position of the Atlantic/Pacific boundary away from the Lomonosov Ridge. Further, Larsen-93 data show the transition region between the Atlantic and deep layers is fresher in the Makarov Basin than corresponding water in either the Canada or Eurasian basins, implying a source of cold, low-salinity water, perhaps from the Laptev and East Siberian shelves. The front separating these two assemblies lies above the Mendeleyev Ridge and is marked by large lateral gradients in all measured properties. In particular, the penetration of anthropogenic halocarbons is 2 to 3 times deeper in the Makarov Basin than in the Canada Basin, implying enhanced rates of ventilation. This suggests that direct exchange between the Canadian and Eurasian basins has occurred recently near the perimeter and that physical and chemical properties, including contaminants, may have been transported by boundary currents more quickly from one basin to the other.

Evaporation and solar irradiance as regulators of sea surface temperature in annual and interannual changes

Seven years of net surface solar irradiance (S) derived from cloud information provided by the International Satellite Cloud Climatology Project and 4 years of surface latent heat flux (E) derived from observations of the special sensor microwave imager were used to examine the relation between surface heat fluxes and sea surface temperature (Ts) in their global geographical distribution, seasonal cycle, and interannual variation. The relations of seasonal changes imply that evaporation cooling is significant over most of the ocean and that solar heating is the main drive for the change of Ts away from the equatorial wave guide where ocean dynamics may be more important. However, Ts is not the most direct and significant factor in the seasonal changes of S and E over most of the ocean; the solar incident angle may be more important to S, and wind speed and air humidity are found to correlate better with E. Significant local correlations between anomalies of Ts and S and between anomalies of Ts and E are found in the central equatorial Pacific; both types of correlation are negative. In this area, organized deep convection overlies the warm ocean, forms high clouds, and reduces S, while the low wind speed and high humidity that result from surface convergence reduce E. The negative correlation is not present in the surrounding areas where equally warm water and strong Ts anomalies are found under a subsiding atmosphere without similarly strong S and E anomalies. Correlation between anomalies of temperature tendency and the fluxes is weak, indicating that other factors are more influential in changing upper ocean heat balance during El Nino. The result shows that the relations between Ts and the flux components, in annual and interannual timescales, are not universal and not consistent with the local negative feedback postulations which require that an increase in Ts would result in an increase in local evaporative cooling and a decrease in local solar heating of the ocean. Large-scale atmospheric circulation changes clouds, winds, and humidity; they, in turn, influence the fluxes significantly. The influence of ocean dynamics in changing Ts in the tropical ocean can not be ignored.

Surface solar irradiance from the International Satellite Cloud Climatology Project 1983–1991

An 8 year (July 1983 through June 1991) time series of daily and monthly mean surface solar irradiance has been produced for the globe using data from the International Satellite Cloud Climatology Project (ISCCP) and a revised Bishop and Rossow [1991] algorithm. We present a detailed validation analysis of the ISCCP solar irradiance fields with contemporaneous surface observations at buoys, at remote islands, and from the Global Energy Balance Archives (GEBA) [Ohmura et al., 1991]. The validation is hampered to some degree by the scale difference between the 280 km ISCCP product and the single-point measurements, some of which are affected by orographic clouds and other local meteorological effects. Our analysis suggests criteria for siting of island or coastal monitoring locations to minimize such biases. Particularly, eastward or poleward facing oceanic exposures are to be avoided. In addition, we suggest that deep sea buoys should be investigated for validation of oceanic surface fluxes. At open-ocean, clean-air sites, the ISCCP product is shown to be good to within 10 W m−2 in the monthly mean. The high-frequency (daily) systematics of solar irradiance variability at the open-ocean sites are also well duplicated by the ISCCP product. An identifiable error in the revised solar irradiance product is the neglect of spatially and temporally varying aerosol extinction. This error, when translated into an equivalent aerosol extinction coefficient, can be as large as 0.6 in known polluted and mineral dust-affected regions. We cannot determine additional satellite sensor calibration errors beyond those already corrected in the ISCCP processing. This uniquely long data set has been publicly available since 1994 at the National Center for Atmospheric Research. The data documents significant differences in solar fluxes received by the major oceans as well as significant flux variability on seasonal to interannual timescales.

A new model of solar EUV irradiance variability 2. Comparisons with empirical models and observations and implications for space weather

[1]xa0Motivated by the need for reliable specification of the Suns electromagnetic radiation in the extreme ultraviolet (EUV) spectrum, we have developed a new model of solar EUV irradiance variability at wavelengths from 50 to 1200 A. Solar images are used to quantify changes in the sources of EUV irradiance during the solar cycle. Optically thin EUV emission line fluxes are estimated from differential emission measures (DEMs) that characterize the properties of the solar atmosphere in the source regions, while fluxes for optically thick lines are modeled directly by specifying the source region contrasts. We compare the new model, NRLEUV, with three different empirical models of solar EUV irradiance since 1975. For solar cycles 21 and 22, NRLEUV predicts overall lower EUV irradiances and smaller solar cycle variability than the empirical models. The average total EUV energy at wavelengths from 50 to 1050 A is 2.9 mW m−2, smaller than the HFG, EUVAC, and SOLAR2000 models for which average energies are 3.7, 4.3, and 5.6 mW m−2, respectively. These differences have distinct wavelength dependencies. The solar cycle variation in total EUV energy is 1.9 for NRLEUV compared with 2.7, 2.9, and 2.3 for HFG, EUVAC, and SOLAR2000. Here, too, the differences are wavelength dependent. We compare both the NRLEUV and the empirically modeled EUV irradiances with selected wavelength bands and emission lines measured during 4 years in cycle 21 by Atmospheric Explorer-E (AE-E) and two broad bands at 170–200 and 260–340 A measured in cycle 23 by the Solar X-Ray Photometer (SXP) on the Student Nitric Oxide Experiment (SNOE) and the Solar EUV Monitor (SEM) on the Solar and Heliospheric Observatory (SOHO), respectively. The NRLEUV model reproduces the variations observed during solar rotation better than, or as well as, the empirical models. Comparisons of solar cycle variations are more ambiguous because undetected instrumental drifts can cause spurious trends in the observations over these longer timescales. Drifts in the AE-E instruments may explain why the HFG and EUVAC models, which are based on parameterizations of these data, have larger solar cycle variations than NRLEUV. We assess the implications for space weather of the significant differences among the modeled EUV irradiances by using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC) to quantify corresponding differences in upper atmosphere energy deposition and photoionization rates.

Ionospheric and dayglow responses to the radiative phase of the Bastille Day flare

[1]xa0The Suns Bastille Day flare on July 14, 2000 produced a variety of geoeffective events. This solar eruption consisted of an X-class flare followed by a coronal mass ejection that produced a major geomagnetic storm. We have undertaken a study of this event beginning with an analysis of the effects of the radiative phase of the flare on the dayglow and the ionosphere. The key new enabling work is a novel method of evaluating the X-ray and extreme ultraviolet (EUV) solar spectral irradiance changes associated with the flare. We find that the solar radiative output enhancements modeled during the flare are consistent with measurements of both solar EUV irradiance and far UV Earth thermospheric dayglow. We use the SAMI2 model to predict global ionospheric changes along a magnetic meridian that show significantly different northern and southern effects, suggesting that flares can be used to study ionospheric dynamics.

Analysis of the Astro‐1/Hopkins Ultraviolet Telescope EUV–FUV dayside nadir spectral radiance measurements

[1]xa0The Hopkins Ultraviolet Telescope (HUT) was one of three ultraviolet Astro-1 observatory instruments on space shuttle Columbia in December 1990 (STS-35), covering the 830–1850 A wavelength interval (first order) at ∼3.3 A spectral resolution. Satellite altitude was 360 km. Dayside nadir measurements were performed during a single orbit on 7 December 1990 under solar maximum (F10.7 = 222), geomagnetically quiet (Ap = 4) conditions, covering late morning local times (solar zenith angle < 48°). These data constitute a reference dayglow radiance spectrum comprising a number of thermospheric emission features, including several weak features neighboring bright optically thick emissions, that have not yet received adequate explanations, in part owing to questions regarding spectral intensity calibration, dynamic range, etc., associated with older data sets. In this paper, the HUT extreme ultraviolet (EUV)–far ultraviolet (FUV) nadir dayglow spectrum is presented along with the results of a modeling analysis using a development version of the Atmospheric Ultraviolet Radiance Integrated Code (AURIC). The analysis relies on currently available laboratory data and first-principles excitation and transport modeling codes and utilizes constraints drawn from the FUV spectral region data to study several EUV emissions of interest for which past airglow data are sparse. The focus is on the airglow in the 980–1200 A region, which is particularly rich in emission features affected by thermospheric conditions. Emissions investigated in detail include (1) the optically thick OI 989 A multiplet and associated 1172 A fluorescence, (2) the OI 1026 A sextuplet blended with atomic hydrogen Lyman β, (3) the atomic nitrogen multiplets at 1134 and 1200 A consisting of optically thick components excited by e− + N and optically thin components excited by e− + N2 and hν + N2, (4) a number of other NI and N+ features (e.g., N+ 1085 A) excited by N2 dissociative ionization, (5) the Birge-Hopfield I (N2 BH-1) system, and (6) the resonance lines of argon (Ar 1048 and 1067 A).

The University of Bern Atmospheric Ion Model: Time-dependent modeling of the ions in the mesosphere and lower thermosphere

In this paper the first time-dependent model of ion chemistry in the mesosphere/lower thermosphere (MLT) region acting within a global, time-dependent, two-dimensional neutral atmosphere model is described. Selected diurnal results are presented for undisturbed solar minimum conditions. The University of Bern Atmospheric Ion Model (UBAIM) is a time-dependent, pseudo-two-dimensional model of the ion chemistry in the Earth atmosphere. It covers latitudes from 85°S to 85°N and (log-pressure) altitudes from 20 to 120 km. On this grid a system of differential equations describing the ion chemistry is integrated numerically until a periodical solution, governed by the diurnal changes in the incident radiation, is reached; this solution constitutes a model for quiet or undisturbed conditions. The basic ion chemistry of the UBAIM contains 311 reactions for 71 charged species. Ionization sources are solar X-ray and EUV radiation, resonantly scattered Lyman α and β photons, and galactic cosmic rays. Densities of main and trace neutral atmospheric constituents are taken from a new version of the bidimensional NCAR model SOCRATES, which has been specifically optimized for mesospheric and lower thermospheric processes with upper boundary conditions set using the empirical MSIS thermosphere model. Direct solar flux inputs are computed by the SOLAR2000 model; scattered Lyman α and β fluxes are calculated using geocoronal hydrogen density profiles consistent with the adopted MSIS density distributions.

Data‐model comparison search analysis of coincident PBO Balmer α, EURD Lyman β geocoronal measurements from March 2000

[1]xa0Recent Lyman series and Balmer series airglow measurements provide a fresh opportunity to investigate the density distribution and variability of atomic hydrogen in the upper atmosphere. Dedicated nightside Balmer α Fabry-Perot spectrometer measurements at the Pine Bluff Observatory (PBO), University of Wisconsin-Madison, have been acquired since late 1999 taking advantage of several technological advances. Extreme ultraviolet spectral radiance measurements by the Espectrografo Ultravioleta extremo para la Radiacion Difusa (EURD) instrument on the Spanish MINISAT-1 satellite from October 1997 to December 2001 provide extensive sets of geocoronal Lyman β, Lyman γ and He 584 A emission intensities. In this paper, coincident EURD Lyman β and PBO Balmer α radiance measurements from the early March 2000 new moon period are presented. In addition to serving as examples of the data sets now available, the data volume poses an analysis challenge not faced in prior geocoronal studies. A data-model comparison search procedure employing resonance radiation transport results for extensive sets of parametric density distribution models is being developed for use in analyses of multiple large data sets; this is described, and example results for the PBO and EURD March 2000 data sets are presented. The tightness of the constraints obtained for the solar line-center Lyman β irradiance and the atomic hydrogen column abundance is somewhat surprising, given the crudeness of the parameter binning in the search procedure and the fact that a small number of recognized corrections remain to be made to each data set.

Molecular nitrogen Carroll-Yoshino v′ = 0 emission in the thermospheric dayglow as seen by the Far Ultraviolet Spectroscopic Explorer

[1]xa0Molecular nitrogen (N2) Carroll-Yoshino excitation is of considerable interest in the analysis of EUV-FUV observations obtained from several solar system objects (Earth, Titan, and Triton). The fundamental c4′(0, 0) band at 958 A is a strong transition excited by photoelectron impact and hence is bright under optically thin conditions. On Earth c4′(0, 0) is optically thick near the altitude of peak photoelectron excitation and the emergent (0, 0) emission is weak. Past analyses of EUV airglow spectra focused on the roles of (0, v″ ≤ 2) photon redistribution and predissociation enhancement resulting from multiple scattering in accounting for reduced emission. The role of (0, v″ > 2) fluorescence was deemed minor. Now, high-resolution dayglow measurements by the Far Ultraviolet Spectroscopic Explorer (FUSE) made in September 1999 reveal c4′(0, v″ = 6–9) fluorescence bands much brighter than expected. The results of an analysis of FUSE dayside disk measurements are presented, and the observed (0, v″) emission is discussed in the context of results from a multiple scattering model. The c4′(0, 9) band in particular stands out as a potentially useful remote sensing observable.