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Stephen J. Colucci and Timothy L. Alberta

Abstract

A climatology of the relationship between explosive cyclogenesis and atmospheric blocking has been constructed from an investigation of sea level pressure and 500-mb height analyses over the Northern Hemisphere during seven winter seasons. Planetary-scale 500-mb geostrophic u and v wind components and component anomalies (u′ and v′) were calculated over each explosive cyclone center and compared to the times and locations of block onsets. Blocking episodes were defined by the persistence, for at least 5 days, of negative zonal index (500-mb height at 40°N less than that at 60°N) spanning at least 18.75° of longitude. Thirty-three percent (43%) of the explosive cyclones that were not analyzed during blocking episodes over the Atlantic (Pacific) sector of the Northern Hemisphere and over which there were anomalously strong planetary-scale southerly winds (v′ > 0), anomalously weak westerly winds (u′ < 0), and v > u/2 were followed, in not more than 5 days and within 60° of longitude, by the onset of blocking in that sector. These percentages significantly exceed, at the 90% (95%) confidence level, the 12% unconditional probability of an unblocked day in either sector being followed in not more than 5 days by the onset of blocking in that sector. Conversely, while each of the block onsets was preceded, by not more than 5 days, by at least one sea level cyclone analyzed within 60° of longitude, in only 56% of these cases was one of the antecedent cyclones rapidly intensifying. In only 24% of the cases was block onset preceded (within 5 days and 60° of longitude) by explosive cyclogenesis over which v′ gt; 0, u′ < 0, and v/u > 0.5. An empirical rule developed from these findings may thus fail to predict many block onsets. The most common block-onset precursor, noted in 76% of the cases, was anomalously strong planetary-scale southerly winds over the antecedent cyclone. Statistically, the most significant feature distinguishing explosive cyclones followed by blocking from other explosive cyclones was weaker planetary-scale westerlies relative to normal over the block-onset cyclones. It is hypothesized that the onset of blocking in at least some of these cases was due to the anomalous planetary-scale advection of synoptic-scale potential vorticity associated with the antecedent cyclones or to the interaction between synoptic-scale processes and “preconditioned” (anomalously amplified) planetary waves.

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Thomas P. Charlock and Timothy L. Alberta

Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison.

The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insolation is larger than the measurements for clear-sky and total-sky conditions.

This experiment has been developed to test retrievals of radiative fluxes and the associated forcings by clouds, aerosols, surface properties, and water vapor. Collaboration is sought; the goal is to extend the domain of meteorological conditions for which such retrievals can be done accurately. CAGEX Version 1 covers April 1994. Subsequent versions will (a) at first span the same limited geographical area with data from October 1995, (b) then expand to cover a significant fraction of the GEWEX Continental-Scale International Project region for April 1996 through September 1996, and (c) eventually be used in a more advanced form to validate CERES.

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Timothy L. Alberta and Stephen K. Cox

Abstract

Results of experiments conducted using the Cloud Field Optical Simulator (CFOS) to examine the variability in reflectance properties of cloud fields with fixed cloud amount but different cloud patterns are presented. Angular reflectance data from 20 cloud fields with a common cloud amount of 30 percent were analyzed. The experiment demonstrated the problem of changing spot size as a function of view angle for a fixed field of view detector.

Seven different incident solar zenith angles were analyzed for variations in reflected irradiances arising from different cloud field patterns. Results show irradiance variations as great as 31% at large incident zenith angles. Also indicated are increased irradiances and increased anisotropy at large incident zenith angles.

Radiances and irradiances of interacting cloud elements were compared to those of noninteracting clouds. Interacting cloud fields produced larger radiance and irradiance values than noninteracting cloud fields. The differences between interacting and noninteracting cloud fields were greater at smaller source zenith angles. Maximum radiances were found at photodiode locations measuring backscattered radiation in the interacting cloud fields.

Reflectances were integrated as a function of zenith angle to produce daily reflectances for five different latitude/date combinations. Analysis of this study demonstrated the importance of the sides of clouds, and verified irradiance differences due to cloud patterns when examined on a daily scale.

Irradiances calculated utilizing 195 radiances, each measured at different local zenith and azimuth angles, were compared with irradiances calculated from a single radiance assuming isotropy. The isotropic assumption produced overestimates of the measured irradiances at large local zenith angles when the photodiode detectors measured backscattered radiation, and underestimates when the detectors measured forward scattered radiation. Minimum errors were found at small local zenith angles.

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Timothy L. Alberta, Stephen J. Colucci, and J. Clay Davenport

Abstract

Rapid intensification of cyclones and anticyclones in National Meteorological Center 500-mb height analyses is investigated during the 1983–84 through 1986–87 cool seasons (September through May). Rapid intensification is defined by analyzed height decreases (increases) of at least 10(5) dam in 24 h at the centers of 500-mb cyclones (anticyclones), defined here by closed contours at 6-dam intervals around low (high) 500-mb heights. The threshold 24-h central height changes for rapidly intensifying 500-mb cyclones (anticyclones) are two standard deviations lower (higher) than the mean central height changes for all 500-mb cyclones (anticyclones) during the 1983–84 cool season. Rapid 500-mb cyclogenesis is climatologically preferred over the northwestern Pacific Ocean and northeastern North America, while anticyclogenesis is favored in high latitudes with little longitudinal preference, but with some concentration in frequency near Alaska and Greenland. Both phenomena are observed preferentially north of the 546-dam contour at 500 mb; thus, most rapidly intensifying cyclones (anticyclones) apparently are not (are) cut off from the 500-mb westerlies. On the average, rapidly intensifying 500-mb cyclones (anticyclones) are associated with intensifying surface cyclones (anticyclones). The surface cyclones associated with rapid 500-mb cyclogenesis are more likely to be rapidly developing than is expected climatologically. In a limited sample it is seen that, on the average, rapid 500-mb cyclone and anticyclone developments are underpredicted by operational forecast models. A case of nearly simultaneous 500-mb rapid cyclogenesis and anticyclogenesis over the Atlantic Ocean during, January 1985 is diagnosed with a quasi-geostrophic model. Neither system is cut off from the westerlies, and both are associated with rapidly intensifying surface counterparts. The 500-mb anticyclone in turn is preceded by (nearly rapid) surface cyclognesis upstream and evolves into a blocking pattern. The 500-mb height tendencies at the center of the rapidly intensifying 500-mb cyclone are forced primarily by lower-tropospheric cold air advection decreasing upward in magnitude beneath the system. On the other hand, the quasi-geostrophic tendencies underestimate those analyzed at the anticyclone center, suggesting the importance of non-quasi-geostrophic processes in rapid 500-mb anticyclone development.

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Ellsworth G. Dutton, Joseph J. Michalsky, Thomas Stoffel, Bruce W. Forgan, John Hickey, Donald W. Nelson, Timothy L. Alberta, and Ibrahim Reda

Abstract

Diffuse-sky solar irradiance is an important quantity for radiation budget research, particularly as it relates to climate. Diffuse irradiance is one component of the total downwelling solar irradiance and contains information on the amount of downward-scattered, as opposed to directly transmitted, solar radiation. Additionally, the diffuse component is often required when calibrating total irradiance radiometers. A variety of pyranometers are commonly used to measure solar diffuse irradiance. An examination of some instruments for measuring diffuse irradiance using solar tracking shade disks is presented, along with an evaluation of the achieved accuracy. A data correction procedure that is intended to account for the offset caused by thermal IR exchange between the detector and filter domes in certain common diffuse pyranometers is developed and validated. The correction factor is derived from outputs of a collocated pyrgeometer that measures atmospheric infrared irradiance.

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