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Richard H. Grant and Gordon M. Heisler

Abstract

Sky radiance measurements in the wavelength bands of ultraviolet-B (0.28–0.32 μm), ultraviolet-A (0.32–0.40 μm), and photosynthetically active radiation (0.40–0.70 μm) were made under obscured overcast skies in a rural area. Radiance distributions were modeled for seven measurement scans with solar zenith angles varying from 19° to 49°. For the seven scans, the atmospheric transmittance of photosynthetically active photon flux density varied from 0.16 to 0.25. The corresponding fraction of cloud-free sky photosynthetically active photon flux density ranged from 0.21 to 0.32. The corresponding fraction of ultraviolet-B cloud-free sky irradiance was between 0.20 and 0.34, with typically lower fractions of cloud-free sky irradiance in the ultraviolet-B than in the photosynthetically active photon flux density. The sky radiance was modeled from the ensembled measurements according to the standard overcast sky radiance distribution for each of the wavelength bands.

Although the ultraviolet wave bands had slightly greater normalized radiance at the zenith and smaller radiance at the horizon than has been found for the photosynthetically active radiation wave band, the differences between the photosynthetically active radiation and the ultraviolet-A and ultraviolet-B radiance distributions were not statistically significant. Therefore, the authors concluded that the normalized obscured overcast sky radiance distribution for all three wave bands could be satisfactorily described by the standard overcast sky model of the photosynthetically active radiation distribution.

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conference summary

Conference on metropolitan physical environment: vegetation, space, and structure for human amenities, 25–29 August 1975, Syracuse, N.Y.

Lee P. Herrington and Gordon M. Heisler
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Richard H. Grant and Gordon M. Heisler

Abstract

Methods to estimate the irradiance of ultraviolet-B (UVB; 280–320 nm) radiation are needed to assess biological effects of changes in atmospheric composition. Measurements of the spatial distribution of sky cloud cover, temporal variability of photon flux density of photosynthetically active radiation (PAR; 400–700 nm), and UVB irradiance (I-UVB) were made on 23 days during the summer of 1993 in a rural area (West Lafayette, Indiana). Prediction equations for the measured UVB irradiance under partly cloudy skies were developed based on the photosynthetically active photon flux density (PPFD), cloud cover fraction, probability of cloud obstruction of the sun, and a semiempirical combination of cloud probability and cloud cover. The I-UVB was linearly related to the PPFD, with the variability in PPFD accounting for 77% of the I-UVB variability. Normalized PPFD (PAR F) and I-UVB (UVB F) values, calculated by dividing the observed value by the expected cloud-free sky PPFD and I-UVB, were also linearly related. Linear models based on either the spatial cloud fraction or the probability of clouds blocking the sun accounted for less than 30% of the UVB F variability. A two-component semiempirical model was developed to predict UVB F based on cloud-cover fraction, probability of sun obscuration by clouds, the predicted cloud-free sky diffuse fraction, and solar zenith angle. This model accounted for 60% of the variability in UVB F. Results indicate the best estimation of I-UVB under the partly cloudy sky conditions is made using PPFD measurements and referencing the measured PPFD to cloud-free sky PPFD at the same sun angle. Alternative approaches, such as the developed two-component model, should be used only if on-site PPFD measurements are unavailable.

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C. M. Roberts, C. Gordon, and C. Cooper

Abstract

It is shown that the magnitude of the flux adjustments required in a coupled model simulation to prevent climate drift does not reduce when a higher resolution, and more realistic, ocean component is used. This is because much of the flux adjustment field is dominated by regions of high horizontal SST gradient, where even small shifts in positioning lead to large SST errors and therefore large flux adjustments. A simple scheme is used to remove the effects of these regions in the calculation of flux adjustment and this results in adjustments considerably smaller than those usually obtained. Comparison of the resulting flux adjustment field with an estimate of the error in surface flux from the atmospheric model shows the remaining peak values are generally identifiable with known systematic errors in the atmospheric model.

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Randall M. Dole and Neil D. Gordon

Abstract

We have studied the geographical and regional persistence characteristics of wintertime Northern Hemisphere 500 mb height anomalies, focusing particular attention on the behavior of strong anomalies that persist beyond the durations associated with synoptic-scale variability (“persistent anomalies”). We have also examined the persistence characteristics of certain dominant regional patterns of low-frequency variability.

There are three major regions for the occurrence of persistent anomalies: the North Pacific to the south of the Aleutians, the North Atlantic to the southeast of Greenland, and from the northern Soviet Union northeastward to over the Arctic Ocean. These regions have relatively high numbers of both persistent positive anomaly and persistent negative anomaly cases. For moderate magnitudes and durations, the numbers of positive and negative cases in each region are about the same; however, for larger magnitudes and longer durations, the number of positive cases exceeds the corresponding number of negative cases. Analyses with data that have been low-pass filtered (removing periods of less than 6 days) reveal that part (but not all) of the discrepancy between positive and negative cases results from the relatively greater likelihood that negative anomalies will experience brief interruptions by transient disturbances.

For durations beyond about 5 days, the probability that an anomaly which has lasted n days will last at least one more day is nearly constant. This nearly constant probability of continuation resembles the behavior obtained for a linear first-order autoregressive process (red noise). Nevertheless, there are significant differences in persistence between the positive and negative anomalies and red noise, particularly at large magnitudes, with the positive anomalies typically more persistent than either the negative anomalies or red noise. A simple nonlinear autoregressive model is described that simulates many of the observed deviations from red noise, and possible physical sources for the nonlinearities are discussed.

Relationships between the initial anomaly value and its subsequent 12 h change are then studied. The height changes are decomposed into two parts: a mean change and a deviation from the mean change. Mean change variations are examined for evidence of multiple “quasi-equilibria” (multiple anomaly values having mean changes of zero). Mean change variations are also determined for the temporal coefficients of certain dominant regional patterns of low-frequency variability. Although the temporal fluctuations of the patterns exhibit considerably more persistence than found for the corresponding local height anomalies, neither the patterns nor the local anomalies display convincing evidence of multiple quasi-equilibria.

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Gordon H. Beck, John M. Davis, and S. K. Cox

Abstract

Beam transmittance, emittance, reflectance, and outgoing radiance are inferred from interferometric measurements in the infrared window region for 14 temperate continental and 12 subtropical cirrus cloud cash observed during FIRE II at Parsons, Kansas (37°18′N, 95°07′W), and the ASTEX at Porto Santo, Madeira (33°5′N, 16°21′W). Cirrus emittances were found to span nearly the entire range from 0 to 1 for cloud systems in each location. Spectrally averaged volume extinction coefficients of 0.19 and 0.62 km−1 were found for the respective continental and subtropical samples. A delta-Eddington routine was incorporated into the inference technique to examine the sensitivity of the inferences to the upwelling surface and subcloud-layer emission reflected by the cloud assuming spherical and nonspherical cloud particles. Including reflectance had only a small effect on the spectrally averaged values of the radiative parameters; however, the slope of outgoing longwave radiation across the window region was altered with the introduction of smaller particles. The iterative method is structured in a manner that does not constrain the transmittances of the clear atmosphere to line-by-line model results. Inferred emittances and extinction coefficients are compared to previously published results.

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Marysa M. Laguë, Gordon B. Bonan, and Abigail L. S. Swann

Abstract

Changes in the land surface can drive large responses in the atmosphere on local, regional, and global scales. Surface properties control the partitioning of energy within the surface energy budget to fluxes of shortwave and longwave radiation, sensible and latent heat, and ground heat storage. Changes in surface energy fluxes can impact the atmosphere across scales through changes in temperature, cloud cover, and large-scale atmospheric circulation. We test the sensitivity of the atmosphere to global changes in three land surface properties: albedo, evaporative resistance, and surface roughness. We show the impact of changing these surface properties differs drastically between simulations run with an offline land model, compared to coupled land–atmosphere simulations that allow for atmospheric feedbacks associated with land–atmosphere coupling. Atmospheric feedbacks play a critical role in defining the temperature response to changes in albedo and evaporative resistance, particularly in the extratropics. More than 50% of the surface temperature response to changing albedo comes from atmospheric feedbacks in over 80% of land areas. In some regions, cloud feedbacks in response to increased evaporative resistance result in nearly 1 K of additional surface warming. In contrast, the magnitude of surface temperature responses to changes in vegetation height are comparable between offline and coupled simulations. We improve our fundamental understanding of how and why changes in vegetation cover drive responses in the atmosphere, and develop understanding of the role of individual land surface properties in controlling climate across spatial scales—critical to understanding the effects of land-use change on Earth’s climate.

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Asmi M. Napitu, Arnold L. Gordon, and Kandaga Pujiana

Abstract

Sea surface temperature (SST) variability at intraseasonal time scales across the Indonesian Seas during January 1998–mid-2012 is examined. The intraseasonal variability is most energetic in the Banda and Timor Seas, with a standard deviation of 0.4°–0.5°C, representing 55%–60% of total nonseasonal SST variance. A slab ocean model demonstrates that intraseasonal air–sea heat flux variability, largely attributed to the Madden–Julian oscillation (MJO), accounts for 69%–78% intraseasonal SST variability in the Banda and Timor Seas. While the slab ocean model accurately reproduces the observed intraseasonal SST variations during the northern winter months, it underestimates the summer variability. The authors posit that this is a consequence of a more vigorous cooling effect induced by ocean processes during the summer. Two strong MJO cycles occurred in late 2007–early 2008, and their imprints were clearly evident in the SST of the Banda and Timor Seas. The passive phase of the MJO [enhanced outgoing longwave radiation (OLR) and weak zonal wind stress) projects on SST as a warming period, while the active phase (suppressed OLR and westerly wind bursts) projects on SST as a cooling phase. SST also displays significant intraseasonal variations in the Sulawesi Sea, but these differ in characteristics from those of the Banda and Timor Seas and are attributed to ocean eddies and atmospheric processes independent from the MJO.

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GORDON E. DUNN, R. CECIL GENTRY, and BILLY M. LEWIS

Abstract

The National Hurricane Center and the National Hurricane Research Laboratory joined forces in an effort to improve techniques for forecasting hurricane motion in the spring of 1959 when the latter moved its headquarters from West Palm Beach to Miami into offices adjacent to those occupied by the principal hurricane forecast office in the United States. Results now available from verification of forecasts made during the period 1954 through 1966 show that there has been a significant improvement in the accuracy of hurricane forecasts during the period of increased cooperation between the research and operational forecasting groups. This improvement is indicated by a reduction in the mean error of hurricane forecasts of approximately 10 and 12 percent, respectively, for the two principal hurricane forecast areas near the eastern coasts of the United States.

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G. A. Gordon, J. M. Lough, H. C. Fritts, and P. M. Kelly

Abstract

Reconstructions of winter (December-February) sea level pressure (SLP) from western North American tree-ring chronologies are compared with a proxy record of winter severity in Japan derived from the historically documented freeze dates of Lake Suwa. The SLP reconstructions extend from 1602 to 1961 and freeze dates from 1443 to 1954. The instrumental and reconstructed SLP for the 20th century reveal two distinct circulation regimes (teleconnection patterns) over the North Pacific that appear to be associated with severe and mild winters and, consequently, with early and late freezing of the lake. The reconstructed SLP anomaly map for severe winters prior to 1683 shows a pattern similar to those in the instrumental and reconstructed records of the 20th century. The analysis reveals that the reliability of the reconstruction may vary with the configuration of the actual SLP pattern as the mild winter pattern is not as well reconstructed as the severe winter pattern. That result illustrates the importance of testing the reliability of a reconstruction within the context of the intended interpretation. This analysis demonstrates how different types of proxy climate data can be compared and verified.

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