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- Author or Editor: J. H. Joseph* x
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Abstract
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Abstract
Two extensive sets of spectral pyrheliometric data taken at Jerusalem in the years 1930–34 and 1961–68 have been analyzed.
Seasonal and synoptic variations in the optical depth of the atmospheric aerosol and in its wavelength dependence in the visible part of the spectrum are given and compared with those at other locations.
A secular increase in the turbidity of about 10% per decade, since 1930, could be isolated in this study. Similar increases at various other stations throughout the world seem to indicate a global buildup of turbidity, at least in the Northern Hemisphere.
Abstract
Two extensive sets of spectral pyrheliometric data taken at Jerusalem in the years 1930–34 and 1961–68 have been analyzed.
Seasonal and synoptic variations in the optical depth of the atmospheric aerosol and in its wavelength dependence in the visible part of the spectrum are given and compared with those at other locations.
A secular increase in the turbidity of about 10% per decade, since 1930, could be isolated in this study. Similar increases at various other stations throughout the world seem to indicate a global buildup of turbidity, at least in the Northern Hemisphere.
Abstract
On 14 August 1977, there was a mini-outbreak of three tornadoes about 40 km cut of Denver, Colorado. There were no significant synoptic-scale disturbances affecting Colorado on that day. Mesoscale analysis is used to establish several smaller scale systems that influenced storm development. The most notable feature of the mesoscale band of parent thunderstorms was the active growth along their northwest flank, in spite of cell movement toward the east. On the convective scale, the situation can be described as discrete propagation of multi-cell storms by new cell development on the left rear flank. Two of the three tornadoes were documented photographically, and post-analysis shows that they were of large size and long duration, but slow moving. Structural features of the largest tornado are analyzed in different portions of the life cycle, and compared with other cases in the literature. This tornado moved on a track curving toward the north-northwest, remaining at least 5–10 km distant from any significant precipitation. A dust band believed to represent an inflow jet was observed, which was in a different quadrant from similar features in other cases. Aspects of the tornadoes which could cause public confusion are noted, such as the disproportionately short condensation funnel from high-based cumulus clouds.
Abstract
On 14 August 1977, there was a mini-outbreak of three tornadoes about 40 km cut of Denver, Colorado. There were no significant synoptic-scale disturbances affecting Colorado on that day. Mesoscale analysis is used to establish several smaller scale systems that influenced storm development. The most notable feature of the mesoscale band of parent thunderstorms was the active growth along their northwest flank, in spite of cell movement toward the east. On the convective scale, the situation can be described as discrete propagation of multi-cell storms by new cell development on the left rear flank. Two of the three tornadoes were documented photographically, and post-analysis shows that they were of large size and long duration, but slow moving. Structural features of the largest tornado are analyzed in different portions of the life cycle, and compared with other cases in the literature. This tornado moved on a track curving toward the north-northwest, remaining at least 5–10 km distant from any significant precipitation. A dust band believed to represent an inflow jet was observed, which was in a different quadrant from similar features in other cases. Aspects of the tornadoes which could cause public confusion are noted, such as the disproportionately short condensation funnel from high-based cumulus clouds.
Abstract
The properties of baroclinic, quasigeostrophic Rossby basin waves are examined. Full analytical solutions are derived to elucidate the response in irregular basins, specifically in a (horizontally) tilted rectangular basin and in a circular one. When the basin is much larger than the (internal) deformation radius, the basin mode properties depend profoundly on whether one allows the streamfunction to oscillate at the boundary or not, as has been shown previously. With boundary oscillations, modes occur that have low frequencies and, with scale-selective dissipation, decay at a rate less than or equal to that of the imposed dissipation. These modes approximately satisfy the long-wave equation in the interior. Using both unforced and forced solutions, the variation of the response with basin geometry and dissipation is documented. The long-wave modes obtain with scale-selective dissipation, but also with damping that acts equally at all scales. One finds evidence of them as well in the forced response, even when the dissipation is weak and the corresponding free modes are apparently absent.
Abstract
The properties of baroclinic, quasigeostrophic Rossby basin waves are examined. Full analytical solutions are derived to elucidate the response in irregular basins, specifically in a (horizontally) tilted rectangular basin and in a circular one. When the basin is much larger than the (internal) deformation radius, the basin mode properties depend profoundly on whether one allows the streamfunction to oscillate at the boundary or not, as has been shown previously. With boundary oscillations, modes occur that have low frequencies and, with scale-selective dissipation, decay at a rate less than or equal to that of the imposed dissipation. These modes approximately satisfy the long-wave equation in the interior. Using both unforced and forced solutions, the variation of the response with basin geometry and dissipation is documented. The long-wave modes obtain with scale-selective dissipation, but also with damping that acts equally at all scales. One finds evidence of them as well in the forced response, even when the dissipation is weak and the corresponding free modes are apparently absent.
Abstract
This paper presents a rapid yet accurate method, the “delta-Eddington” approximation, for calculating monochromatic radiative fluxes in an absorbing-scattering atmosphere. By combining a Dirac delta function and a two-term approximation, it overcomes the poor accuracy of the Eddington approximation for highly asymmetric phase functions. The fraction of scattering into the truncated forward peak is taken proportional to the square of the phase function asymmetry factor, which distinguishes the delta-Eddington approximation from others of similar nature. Comparisons of delta-Eddington albedos, transnmissivities and absorptivities with more exact calculations reveal typical differences of 0–0.022 and maximum differences of 0.15 over wide ranges of optical depth, sun angle, surface albedo, single-scattering albedo and phase function asymmetry. Delta-Eddington fluxes are in error, on the average, by no more than 0.5%0, and at the maximum by no more than 2% of the incident flux. This computationally fast and accurate approximation is potentially of utility in applications such as general circulation and climate modelling.
Abstract
This paper presents a rapid yet accurate method, the “delta-Eddington” approximation, for calculating monochromatic radiative fluxes in an absorbing-scattering atmosphere. By combining a Dirac delta function and a two-term approximation, it overcomes the poor accuracy of the Eddington approximation for highly asymmetric phase functions. The fraction of scattering into the truncated forward peak is taken proportional to the square of the phase function asymmetry factor, which distinguishes the delta-Eddington approximation from others of similar nature. Comparisons of delta-Eddington albedos, transnmissivities and absorptivities with more exact calculations reveal typical differences of 0–0.022 and maximum differences of 0.15 over wide ranges of optical depth, sun angle, surface albedo, single-scattering albedo and phase function asymmetry. Delta-Eddington fluxes are in error, on the average, by no more than 0.5%0, and at the maximum by no more than 2% of the incident flux. This computationally fast and accurate approximation is potentially of utility in applications such as general circulation and climate modelling.
Abstract
Four commonly used families of inversion techniques have been investigated. The accuracy and stability of the Chahine, Twomey–Phillips, Backus–Gilbert and non-linear regression techniques have been evaluated and compared. Practical and empirical rules for efficient use have been obtained.
Abstract
Four commonly used families of inversion techniques have been investigated. The accuracy and stability of the Chahine, Twomey–Phillips, Backus–Gilbert and non-linear regression techniques have been evaluated and compared. Practical and empirical rules for efficient use have been obtained.
Abstract
A new way to assess the resolution of inversion methods is defined. The dependence of resolution on accuracy is broken into three parts, each of which has a distinct practical significance. Representatives of four commonly used inversion techniques are evaluated in this context.
Abstract
A new way to assess the resolution of inversion methods is defined. The dependence of resolution on accuracy is broken into three parts, each of which has a distinct practical significance. Representatives of four commonly used inversion techniques are evaluated in this context.
Abstract
A dust prediction system, developed earlier at the University of Athens within the framework of the Mediterranean Dust Experiment (MEDUSE) project, was enhanced at Tel Aviv University to support the Israeli–American Mediterranean Israeli Dust Experiment (MEIDEX) project. These enhancements include development of a dust initialization approach using Total Ozone Mapping Spectrometer (TOMS) aerosol index (AI) data and improved specification of the dust sources. The skill of the model against the TOMS AI measurements was tested during two periods in March and June 2000 using four different scores. It is shown that the TOMS-based initialization has a significant positive impact on all the scores. For instance, the average distance between the predicted and TOMS-observed dust plumes drops from 350–485 to less than 200 km. Verification of model forecasts against surface dust measurements in Tel Aviv shows correlations of up to 0.69 based on 27 predictions, for both 24 and 48 h. One example of a narrow dust plume over Israel, successfully forecast with the current system, is presented. This event occurred in midsummer (4 July) when dust bursts are rare over the Eastern Mediterranean.
Abstract
A dust prediction system, developed earlier at the University of Athens within the framework of the Mediterranean Dust Experiment (MEDUSE) project, was enhanced at Tel Aviv University to support the Israeli–American Mediterranean Israeli Dust Experiment (MEIDEX) project. These enhancements include development of a dust initialization approach using Total Ozone Mapping Spectrometer (TOMS) aerosol index (AI) data and improved specification of the dust sources. The skill of the model against the TOMS AI measurements was tested during two periods in March and June 2000 using four different scores. It is shown that the TOMS-based initialization has a significant positive impact on all the scores. For instance, the average distance between the predicted and TOMS-observed dust plumes drops from 350–485 to less than 200 km. Verification of model forecasts against surface dust measurements in Tel Aviv shows correlations of up to 0.69 based on 27 predictions, for both 24 and 48 h. One example of a narrow dust plume over Israel, successfully forecast with the current system, is presented. This event occurred in midsummer (4 July) when dust bursts are rare over the Eastern Mediterranean.
Abstract
The authors first review a system for specifying monthly mean anomalies of midday temperature (T), dew-point (D), and wind speed (W) at a large network of surface stations across the United States. Multiple regression equations containing approximately three terms were derived for each element and month from concurrent fields of 700-mb height anomaly (H) observed over North America and vicinity, plus the previous month's observed local anomaly of T, D, or W, during the 20-year period 1964–1983. Results of testing this forecast system on prognostic values of H prepared twice a month at the National Weather Service from 1973 to 1990 and on observed values of H during the independent period from 1984 to 1990 are presented.
The authors pooled the data for all months and 122 stations to compute Heidke skill scores (HSS) for two, three, four, and five classes for each weather element. All scores showed skill that dropped steadily as the number of classes increased. In all cases skill was highest for T and lowest for W, the same result as that obtained in the original derivation. In order to examine the distribution of skill, we computed the HSS separately for each class. The skill scores for the first and last classes were greater than those for the middle class for all elements and months. It is encouraging that forecasts above the 90th percentile, the most critical category for fire potential, consistently showed greater skill than forecasts for any other class. Geographical and seasonal variations of skill based on prognostic height was also examined. The results showed that skill is strongly dependent on location, month, and weather element. Mean annual scores were positive for each element in all parts of the country, while monthly scores were highest in January and lowest in October.
Although the forecasts based on prognostic heights were more skillful than climatology for all weather elements, they improved over month-to-month persistence only for T and D. If the height forecasts had been perfect, the skill scores for these elements would have been about two to three times as large as those for prognostic heights and four to five times as large as the persistence scores. When the test period was divided into two subperiods, values of HSS were considerably higher in the later period, despite a drop in persistence scores from the early to the later period. The authors attribute this improvement to increased accuracy of medium-range numerical model predictions during the 1980s.
Abstract
The authors first review a system for specifying monthly mean anomalies of midday temperature (T), dew-point (D), and wind speed (W) at a large network of surface stations across the United States. Multiple regression equations containing approximately three terms were derived for each element and month from concurrent fields of 700-mb height anomaly (H) observed over North America and vicinity, plus the previous month's observed local anomaly of T, D, or W, during the 20-year period 1964–1983. Results of testing this forecast system on prognostic values of H prepared twice a month at the National Weather Service from 1973 to 1990 and on observed values of H during the independent period from 1984 to 1990 are presented.
The authors pooled the data for all months and 122 stations to compute Heidke skill scores (HSS) for two, three, four, and five classes for each weather element. All scores showed skill that dropped steadily as the number of classes increased. In all cases skill was highest for T and lowest for W, the same result as that obtained in the original derivation. In order to examine the distribution of skill, we computed the HSS separately for each class. The skill scores for the first and last classes were greater than those for the middle class for all elements and months. It is encouraging that forecasts above the 90th percentile, the most critical category for fire potential, consistently showed greater skill than forecasts for any other class. Geographical and seasonal variations of skill based on prognostic height was also examined. The results showed that skill is strongly dependent on location, month, and weather element. Mean annual scores were positive for each element in all parts of the country, while monthly scores were highest in January and lowest in October.
Although the forecasts based on prognostic heights were more skillful than climatology for all weather elements, they improved over month-to-month persistence only for T and D. If the height forecasts had been perfect, the skill scores for these elements would have been about two to three times as large as those for prognostic heights and four to five times as large as the persistence scores. When the test period was divided into two subperiods, values of HSS were considerably higher in the later period, despite a drop in persistence scores from the early to the later period. The authors attribute this improvement to increased accuracy of medium-range numerical model predictions during the 1980s.
Abstract
Composite analyses of marine surface observations from 37 hurricanes between 1975 and 1998 show that the difference between the sea surface temperature and the surface air temperature significantly increases just outside the hurricane inner core. This increase in the sea–air contrast is primarily due to a reduction in surface air temperature and is more likely to occur when sea temperatures are at least 27°C. Results show that 90% of the observed cooling occurs 3.25°–1.25° latitude from the hurricane center, well outside the region of strongest surface winds. Since surface pressure only decreases 3 mb over this interval, the ∼2°C drop in air temperature is not a result of adiabatic expansion.
For the subset of observations that contained moisture measurements, surface specific humidity decreased 1.2 g kg−1 4.5°–1.75° latitude from the storm center. This finding suggests that the observed reduction in surface air temperature is not simply a result of near-surface evaporation from sea spray or precipitation. An alternate explanation may be that outside the hurricane inner core, unsaturated convective downdrafts act to dry and evaporatively cool the near-surface environment.
Between 3.25° and 1.25° radius, composite analyses show that low-level inflow is not isothermal, surface moisture is not constant, and the near-surface environment is not in thermodynamic equilibrium with the sea. Calculations based on these observations show that θ e decreases between 4.0° and 1.25° radius and then quickly rises near the inner core as surface pressures fall and specific humidity increases. Surface fluxes of heat and moisture are also observed to significantly increase near the inner core. The largest increase in surface sensible heat flux occurs radially inward of 1.5°, where surface winds are strong and sea–air temperature contrasts are greatest. As a result, the average Bowen ratio is 0.20∼0.5° radius from the composite storm center. This increase in sensible heat flux (in conjunction with near-saturated conditions at low to midlevels) may help explain why average surface air temperatures inside 1.25° radius remain relatively constant, despite the potential for additional cooling from evaporation and adiabatic expansion within the high wind inner core.
Abstract
Composite analyses of marine surface observations from 37 hurricanes between 1975 and 1998 show that the difference between the sea surface temperature and the surface air temperature significantly increases just outside the hurricane inner core. This increase in the sea–air contrast is primarily due to a reduction in surface air temperature and is more likely to occur when sea temperatures are at least 27°C. Results show that 90% of the observed cooling occurs 3.25°–1.25° latitude from the hurricane center, well outside the region of strongest surface winds. Since surface pressure only decreases 3 mb over this interval, the ∼2°C drop in air temperature is not a result of adiabatic expansion.
For the subset of observations that contained moisture measurements, surface specific humidity decreased 1.2 g kg−1 4.5°–1.75° latitude from the storm center. This finding suggests that the observed reduction in surface air temperature is not simply a result of near-surface evaporation from sea spray or precipitation. An alternate explanation may be that outside the hurricane inner core, unsaturated convective downdrafts act to dry and evaporatively cool the near-surface environment.
Between 3.25° and 1.25° radius, composite analyses show that low-level inflow is not isothermal, surface moisture is not constant, and the near-surface environment is not in thermodynamic equilibrium with the sea. Calculations based on these observations show that θ e decreases between 4.0° and 1.25° radius and then quickly rises near the inner core as surface pressures fall and specific humidity increases. Surface fluxes of heat and moisture are also observed to significantly increase near the inner core. The largest increase in surface sensible heat flux occurs radially inward of 1.5°, where surface winds are strong and sea–air temperature contrasts are greatest. As a result, the average Bowen ratio is 0.20∼0.5° radius from the composite storm center. This increase in sensible heat flux (in conjunction with near-saturated conditions at low to midlevels) may help explain why average surface air temperatures inside 1.25° radius remain relatively constant, despite the potential for additional cooling from evaporation and adiabatic expansion within the high wind inner core.
