Abstract

Abstract

Eleven days of energy balance data from three unsaturated land surfaces were collected in the White Mountains of Arizona. These data indicate the variability in α for unsaturated conditions, and seem to verify that a constant value of 1.26 exists for saturated (LE/R → 1) conditions. The “index of aridity” (α/1.26), though not entirely logical for small-scale use, also seems to substantiate the large-scale parameter.

Abstract

A model output statistics (MOS) scheme, using a stepwise-selection multiple linear regression approach, is used to estimate marine fog probability at 24 h intervals from 0 to 48 h, for the North Pacific Ocean (30–60°N) summer season. The predictand is uniquely determined by present and past weather, visibility and low-level cloud information in the primary synoptic report. Available predictors include 158 Fleet Numerical Oceanography Center model output parameters as well as monthly climatological fog frequencies. The regression equations, containing at most seven terms, are derived from 0000 GMT synoptic ship report data in the months of July 1976, 1977 and 1979. Evaporative heat flux and fog frequency are among the first four parameters selected for each equation. Reliability and sharpness diagrams are presented to identify specific biases in estimating fog probability. Categorical (percentage correct, threat and Heidke skill) and probabilistic (Brier) scoring methods are used to establish the credibility level of the MOS scheme in relation to climatology and FNOC's advective-fog probability program. The MOS scheme outperforms its competitors with score ranges for an independent August 1979 data set as follows: Heidke-skill, 0.47 to 0.40; threat; 0.45 to 0.42; Brier, 0.27 to 0.34; percentage correct, 78% to 70%.

Abstract

The large variability of the Gulf of Mexico wind field indicates that high-resolution wind data will be required to represent the weather systems affecting ocean circulation. This report presents methods and results of the calculation of a corrected geostrophic wind data set with high temporal and spatial resolution.

Corrected geostrophic wind was calculated from surface pressure analyses compiled by the Fleet Numerical Oceanography Center. The correction factors for wind magnitude and direction were calculated using linear regressions of observed Gulf buoy winds and geostrophic winds derived at the buoys. The regressions were performed for each month to determine the seasonal variability of the correction factors. The magnitude correction was found to be nearly constant (0.675) throughout the year, but the direction correction varied seasonally from 8.5 to 26.5 degrees.

The corrected geostrophic wind was calculated twice daily store 1967–1982 on a spherical grid over the Gulf, together with the corresponding wind stress and wind stress curl fields. The 12-hourly stress fields show large temporal variations of the wind field for both winter and summer months. Seasonal and monthly climatologies of the stress and corresponding curl show positive curl over the Yucatan and negative curl in the southwest Gulf, which are features not seen in any previous study of Gulf wind stress.

Abstract

A statistical analysis is performed of the tropical cyclone forecast advisories and bulletins issued by the Eastern Pacific Hurricane Center, Redwood City, California, during the 1971–78 seasons. Each forecast is normalized by comparison with the performance of an objective model (EPCLPR) that is based on climatology and persistence. The normalized official forecasts show an improvement in skill during the period. This improvement is attributed to the availability of satellite data for determining the storm positions and to the introduction of objective forecast techniques. Forecast errors are related to a number of storm-related variables, such as initial latitude and longitude and deviations from the climatological track. Stepwise discriminant analysis is used to classify the forecasts into groups of above or below average errors.

Abstract

The differential phase (Φ_DP) measured by polarimetric radars is recognized to be a very good indicator of the path integrated by rain. Moreover, if a linear relationship is assumed between the specific differential phase (K _DP) and the specific attenuation (A_H ) and specific differential attenuation (A _DP), then attenuation can easily be corrected. The coefficients of proportionality, γ_H and γ _DP, are, however, known to be dependent in rain upon drop temperature, drop shapes, drop size distribution, and the presence of large drops causing Mie scattering. In this paper, the authors extensively apply a physically based method, often referred to as the “Smyth and Illingworth constraint,” which uses the constraint that the value of the differential reflectivity Z _DR on the far side of the storm should be low to retrieve the γ _DP coefficient. More than 30 convective episodes observed by the French operational C-band polarimetric Trappes radar during two summers (2005 and 2006) are used to document the variability of γ _DP with respect to the intrinsic three-dimensional characteristics of the attenuating cells. The Smyth and Illingworth constraint could be applied to only 20% of all attenuated rays of the 2-yr dataset so it cannot be considered the unique solution for attenuation correction in an operational setting but is useful for characterizing the properties of the strongly attenuating cells. The range of variation of γ _DP is shown to be extremely large, with minimal, maximal, and mean values being, respectively, equal to 0.01, 0.11, and 0.025 dB °⁻¹. Coefficient γ _DP appears to be almost linearly correlated with the horizontal reflectivity (Z_H ), differential reflectivity (Z _DR), and specific differential phase (K _DP) and correlation coefficient (ρ _HV) of the attenuating cells. The temperature effect is negligible with respect to that of the microphysical properties of the attenuating cells. Unusually large values of γ _DP, above 0.06 dB °⁻¹, often referred to as “hot spots,” are reported for 15%—a nonnegligible figure—of the rays presenting a significant total differential phase shift (Δϕ _DP > 30°). The corresponding strongly attenuating cells are shown to have extremely high Z _DR (above 4 dB) and Z_H (above 55 dBZ), very low ρ _HV (below 0.94), and high K _DP (above 4° km⁻¹). Analysis of 4 yr of observed raindrop spectra does not reproduce such low values of ρ _HV, suggesting that (wet) ice is likely to be present in the precipitation medium and responsible for the attenuation and high phase shifts. Furthermore, if melting ice is responsible for the high phase shifts, this suggests that K _DP may not be uniquely related to rainfall rate but can result from the presence of wet ice. This hypothesis is supported by the analysis of the vertical profiles of horizontal reflectivity and the values of conventional probability of hail indexes.

Abstract

Data from a dense network of ship observations are used to study the structure and properties of westward-moving wave disturbances observed in the eastern Atlantic Intertropical Convergence Zone (ITCZ) during Phase III of the GAPP Atlantic Tropical Experiment (GATE). Comparisons are made with similar disturbances found in the ITCZ of the western Pacific. Wave fields are determined by fitting low-order polynomials to the ship data with use of the method of least squares.

The wave structures in the two regions are found to be similar in many respects, the principal difference being in the divergence field and associated vertical motion. Unlike in the Pacific a multi-layer divergence pattern exists in the eastern Atlantic, leading us to hypothesize the existence of three main cloud populations with outflow levels near 800, 500 and 250 mb. The soundings for the Atlantic exhibit lesser parcel instability then the Pacific soundings in agreement with the reduced vigor of the convective cells and the greater tendency for multiple cloud layers. The strongest upward motion (∼150 mb day⁻¹) occurs in and somewhat ahead of the wave trough, as in the Pacific, but at a much lower level (800–700 mb). A secondary maximum appears near 350 mb, where the primary maximum appears in the Pacific. The maximum precipitation rate of 22 mm day⁻¹ is observed in the region of strongest upward motion. The rate decreases to 4 mm day⁻¹ in the region of suppressed convection near the wave ridge. Vertical eddy flux of total heat is largest at the 800 mb level in the wave trough (225 W m⁻²) and produces cumulus heating and cooling of about 5°C day⁻¹ above and below the maximum, respectively.

A nearly balanced moisture budget for the inner ship array or B-scale area was obtained from the fitted fields when data from both outer and inner ships were employed in the fitting. In particular, two individual waves and the composite or average wave yielded sufficiently accurate budgets to encourage their use in quantitative studies of interactions between synoptic-scale and convective-scale systems. The residual in the heat budget suggests a radiational cooling rate of 0.9°C day⁻¹. The surface energy budget indicates a net radiative flux at the surface of 129 W m⁻² of which 106 W m⁻² was used for evaporation and 12 W m⁻² for sensible heat flux to the atmosphere, leaving 11 W m⁻² for heating of the ocean mixed layer. The heat exchange between ocean and atmosphere underwent a pronounced variation with passage of the synoptic disturbances, causing sea surface temperatures to be 0.3°C warmer ahead of the wave troughs than behind. Precipitation rates employed in the budgets were based on radar measurements; surface sensible and latent heat fluxes were computed by the bulk aerodynamic method with use of temperatures, humidities and winds from the booms of four B-scale ships; and net radiation at the surface was obtained from measurements made aboard the same four ships.

The kinetic energy of the waves was provided by the barotropic conversion process (conversion from zonal kinetic energy), the baroclinic conversion being negative and thus a sink for the eddy kinetic energy. Likewise, the generation of eddy available potential energy was negative, implying that latent heat release opposed, rather than contributed, to the wave growth. The described conditions are quite unlike those in the western Pacific ITCZ where condensation heating provides the source for the wave energy and the barotropic conversion constitutes a weak sink.

Abstract

Ocean heat transport (OHT) plays a key role in climate and its variability. Here, we identify modes of low-frequency North Atlantic OHT variability by applying a low-frequency component analysis (LFCA) to output from three global climate models. The first low-frequency component (LFC), computed using this method, is an index of OHT variability that maximizes the ratio of low-frequency variance (occurring at decadal and longer time scales) to total variance. Lead–lag regressions of atmospheric and ocean variables onto the LFC time series illuminate the dominant mechanisms controlling low-frequency OHT variability. Anomalous northwesterly winds from eastern North America over the North Atlantic act to increase upper ocean density in the Labrador Sea region, enhancing deep convection, which later increases OHT via changes in the strength of the Atlantic meridional overturning circulation (AMOC). The strengthened AMOC carries warm, salty water into the subpolar gyre, reducing deep convection and weakening AMOC and OHT. This mechanism, where changes in AMOC and OHT are driven primarily by changes in Labrador Sea deep convection, holds not only in models where the climatological (i.e., time-mean) deep convection is concentrated in the Labrador Sea, but also in models where the climatological deep convection is concentrated in the Greenland–Iceland–Norwegian (GIN) Seas or the Irminger and Iceland Basins. These results suggest that despite recent observational evidence suggesting that the Labrador Sea plays a minor role in driving the climatological AMOC, the Labrador Sea may still play an important role in driving low-frequency variability in AMOC and OHT.

Abstract

A field project was carried out offshore of central Oregon during August 1999 to evaluate mesoscale model simulations of coastal stratiform cloud layers. Procedures for mapping cloud physical parameters such as cloud optical depth, droplet effective radius, and liquid water path retrieved from Geostationary Operational Environmental Satellite (GOES) Imager multichannel data were developed and implemented. Aircraft measurements by the University of Wyoming provided in situ verification for the satellite retrieval parameters and for the forecast model simulations of the U.S. Navy's nonhydrostatic mesoscale prediction system, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). Case studies show that the satellite retrieval methods are valid within the range of uncertainty associated with aircraft measurements of the microphysical parameters and demonstrate how the gridded cloud parameters retrieved from satellite data can be utilized for mesoscale model verification. Satellite-derived products with applications to forecasting, such as temporal trends and composites of droplet size and liquid water path, are also discussed.

Abstract

Paleoclimatic data and climate model simulations have demonstrated that orbitally forced changes in solar radiation can have a pronounced effect on global climate. Key questions remain, however, about the spatial patterns in the climatic sensitivity to these changes in solar radiation. The authors use GCM simulations of Kutzbach and Guetter and Prell and Kutzbach that were made with the NCAR Community Climate Model (CCM), version CCM0. The results of these simulations are employed to compute linear equilibrium sensitivity coefficients and jackknife uncertainties relating the response of key climate variables to orbitally forced changes in solar radiation. The spatial distributions of the sensitivities and the corresponding uncertainties reveal the synoptic patterns of climate response for these climate variables and identify areas of high and low sensitivity.

The sensitivity of CCM0 to solar radiation changes such as those experienced during the Quaternary is large and predominately linear for many climatic variables. The climatic response is always greatest in the summer hemisphere, because the orbitally induced radiation changes are more pronounced during the summer. The larger landmasses also show a greater climatic response than the smaller ones, due to both the larger heat capacity of the land relative to the oceans, and to the effects of the fixed SSTs. The land surface temperature always increases with increased radiative heating. The surface pressure generally decreases with increasing solar insulation over the landmasses, which were heated, with corresponding increases over the oceans. The net change in moisture (precipitation - evaporation) to increasing solar radiation is greatest over the summer hemisphere Tropics. All three of these variables combine to produce stronger summer monsoons with increasing solar radiation.