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Abstract
Laboratory and airborne observations show that the protective hemispheres of aircraft pyrgeometers are partly covered by water when used in cloud. This cover can reduce the incident longwave flux by as much as 60%. Improved agreement between observations and theory is obtained when a parameterization of water cover in terms of cloud liquid water content is used to correct flux divergence and cooling rate data. It is suggested that all previous in-cloud pyrgeometer measurements may suffer cloud-water contamination.
Abstract
Laboratory and airborne observations show that the protective hemispheres of aircraft pyrgeometers are partly covered by water when used in cloud. This cover can reduce the incident longwave flux by as much as 60%. Improved agreement between observations and theory is obtained when a parameterization of water cover in terms of cloud liquid water content is used to correct flux divergence and cooling rate data. It is suggested that all previous in-cloud pyrgeometer measurements may suffer cloud-water contamination.
Abstract
The potential thermal effects of traffic on road surface thermal energy balance under frost/snow cover conditions have been largely ignored in meteorological evaluations of road ice deposit conditions. Preliminary exploration of these effects, particularly for heavy traffic scenarios with calm wind conditions and an ambient temperature of 0°C, is provided in this study using a conceptual model. Observational data were used to constrain the model, and parameterizations were employed to estimate the various heat transfer processes involved. The results indicate that, for heavy traffic situations, as well as for stopped traffic at intersections, the traffic thermal flux contribution at the surface is noticeable in a wide range of possible frost/snow-covered road conditions. The sensitivity to variation in traffic density, speed, and the emissivity of vehicle radiative surfaces, among others, is evaluated. Simple quantification of these traffic thermal effects, which might be considered in operational meteorological model forecasting of icy road conditions, is offered.
Abstract
The potential thermal effects of traffic on road surface thermal energy balance under frost/snow cover conditions have been largely ignored in meteorological evaluations of road ice deposit conditions. Preliminary exploration of these effects, particularly for heavy traffic scenarios with calm wind conditions and an ambient temperature of 0°C, is provided in this study using a conceptual model. Observational data were used to constrain the model, and parameterizations were employed to estimate the various heat transfer processes involved. The results indicate that, for heavy traffic situations, as well as for stopped traffic at intersections, the traffic thermal flux contribution at the surface is noticeable in a wide range of possible frost/snow-covered road conditions. The sensitivity to variation in traffic density, speed, and the emissivity of vehicle radiative surfaces, among others, is evaluated. Simple quantification of these traffic thermal effects, which might be considered in operational meteorological model forecasting of icy road conditions, is offered.
Abstract
Evaluation of an earlier raingage design based on counting drops formed on the tip of a small-diameter stainless-steel tube shows a defect due to resonant oscillation of the water column in the dropper unit. The defect causes nonlinearity in the drop rate-flow rate relationship, precluding useful integration of the gage output. An improved design is presented which eliminates this defect. Laboratory tests on the new design show linear performance over two orders of magnitude of rainfall intensity with time resolution of better than 5 s and accuracy limited by sampling errors. The new gage is also shown to perform well in field testing.
Abstract
Evaluation of an earlier raingage design based on counting drops formed on the tip of a small-diameter stainless-steel tube shows a defect due to resonant oscillation of the water column in the dropper unit. The defect causes nonlinearity in the drop rate-flow rate relationship, precluding useful integration of the gage output. An improved design is presented which eliminates this defect. Laboratory tests on the new design show linear performance over two orders of magnitude of rainfall intensity with time resolution of better than 5 s and accuracy limited by sampling errors. The new gage is also shown to perform well in field testing.
Abstract
A two-dimensional cloud model is used to study a tropical oceanic squall-line system. The dynamical and microphysical structures of the simulated squall-line system and the impact of environmental wind profiles on these structures are presented. The influence of the microphysics treatment on cloud radiative properties and the sensitivity of this simulated system to radiation is also investigated. In addition, partitioned heat, moisture and water budgets, and two radiative transfer schemes are used to assess the role of anvil clouds on the simulated system and on the assumption used in a bulk parameterization for cloud radiative properties. The comparison with a midlatitude study is also made to show its climatic implication. The major conclusions are as follows.
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The simulated tropical squall-line system replicates many observed features. A transition zone in the simulated multicellular storm is primarily caused by the jetlike wind profile, while it is due to longwave radiation in the midlatitude system.
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The effect of a jetlike wind profile is to weaken/strengthen the convective/anvil portion of the simulated system, which leads to an overall decrease of total surface precipitation by 17%.
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The moisture budgets indicate that tropical deep convection serves as a more efficient engine, pumping low-level moisture upward to form the upper-level anvil cloud, than its midlatitude counterpart although the convective instability is lower in the tropical environment.
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Microphysical production is the primary source of the water budget (∼3/5) in the simulated tropical anvil, and rest (∼2/5) is contributed by horizontal transport of hydrometeors from deep convection. This is just the reverse of the midlatitude case.
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The simulated tropical oceanic anvil has a stronger shortwave radiative forcing than the midlatitude continental anvil, although they have comparable longwave forcings.
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The small difference in total precipitation of the simulated system caused by different radiation transfer schemes appears to justify the assumption of using a bulk parameterization for cloud radiative properties.
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Comparisons of water budges and cloud radiative properties between simulated tropical and midlatitude anvils suggest the need to parameterize the tilting structure of mesoscale convective systems for improving the representation of cloud processes in general circulation models.
Abstract
A two-dimensional cloud model is used to study a tropical oceanic squall-line system. The dynamical and microphysical structures of the simulated squall-line system and the impact of environmental wind profiles on these structures are presented. The influence of the microphysics treatment on cloud radiative properties and the sensitivity of this simulated system to radiation is also investigated. In addition, partitioned heat, moisture and water budgets, and two radiative transfer schemes are used to assess the role of anvil clouds on the simulated system and on the assumption used in a bulk parameterization for cloud radiative properties. The comparison with a midlatitude study is also made to show its climatic implication. The major conclusions are as follows.
-
The simulated tropical squall-line system replicates many observed features. A transition zone in the simulated multicellular storm is primarily caused by the jetlike wind profile, while it is due to longwave radiation in the midlatitude system.
-
The effect of a jetlike wind profile is to weaken/strengthen the convective/anvil portion of the simulated system, which leads to an overall decrease of total surface precipitation by 17%.
-
The moisture budgets indicate that tropical deep convection serves as a more efficient engine, pumping low-level moisture upward to form the upper-level anvil cloud, than its midlatitude counterpart although the convective instability is lower in the tropical environment.
-
Microphysical production is the primary source of the water budget (∼3/5) in the simulated tropical anvil, and rest (∼2/5) is contributed by horizontal transport of hydrometeors from deep convection. This is just the reverse of the midlatitude case.
-
The simulated tropical oceanic anvil has a stronger shortwave radiative forcing than the midlatitude continental anvil, although they have comparable longwave forcings.
-
The small difference in total precipitation of the simulated system caused by different radiation transfer schemes appears to justify the assumption of using a bulk parameterization for cloud radiative properties.
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Comparisons of water budges and cloud radiative properties between simulated tropical and midlatitude anvils suggest the need to parameterize the tilting structure of mesoscale convective systems for improving the representation of cloud processes in general circulation models.
An automated weather station was installed in October 1996 at the summit of Nevado Sajama, located in the western Andean Cordillera of Bolivia (6542 m, 18°06′S, 68°53′W). Meteorological conditions on the mountain are being observed to improve the calibration of geochemical variations within tropical ice cores. This article documents the design and operation of the station and presents a discussion of measurements made through the first annual cycle. Variables analyzed include pressure, incoming solar radiation, air temperature, humidity, wind, and snow accumulation. Large diurnal fluctuations were recorded in most variables, which is not unexpected given the location at 18°S; the data also reveal substantial day-to-day variability and rapid seasonal changes in weather and circulation. As a result, snowfall events and periods of evaporation are more episodic in nature than previously believed. Measurement of atmospheric conditions during and between snowfall events will therefore greatly facilitate the interpretation of geochemical variations in each resultant snowpack layer.
An automated weather station was installed in October 1996 at the summit of Nevado Sajama, located in the western Andean Cordillera of Bolivia (6542 m, 18°06′S, 68°53′W). Meteorological conditions on the mountain are being observed to improve the calibration of geochemical variations within tropical ice cores. This article documents the design and operation of the station and presents a discussion of measurements made through the first annual cycle. Variables analyzed include pressure, incoming solar radiation, air temperature, humidity, wind, and snow accumulation. Large diurnal fluctuations were recorded in most variables, which is not unexpected given the location at 18°S; the data also reveal substantial day-to-day variability and rapid seasonal changes in weather and circulation. As a result, snowfall events and periods of evaporation are more episodic in nature than previously believed. Measurement of atmospheric conditions during and between snowfall events will therefore greatly facilitate the interpretation of geochemical variations in each resultant snowpack layer.
Abstract
Results are presented from a set of experiments designed to investigate factors that may influence proxy-based reconstructions of large-scale temperature patterns in past centuries. The factors investigated include 1) the method used to assimilate proxy data into a climate reconstruction, 2) the proxy data network used, 3) the target season, and 4) the spatial domain of the reconstruction. Estimates of hemispheric-mean temperature are formed through spatial averaging of reconstructed temperature patterns that are based on either the local calibration of proxy and instrumental data or a more elaborate multivariate climate field reconstruction approach. The experiments compare results based on the global multiproxy dataset used by Mann and coworkers, with results obtained using the extratropical Northern Hemisphere (NH) maximum latewood tree-ring density set used by Briffa and coworkers. Mean temperature reconstructions are compared for the full NH (Tropics and extratropics, land and ocean) and extratropical continents only, withvarying target seasons (cold-season half year, warm-season half year, and annual mean). The comparisons demonstrate dependence of reconstructions on seasonal, spatial, and methodological considerations, emphasizing the primary importance of the target region and seasonal window of the reconstruction. The comparisons support the generally robust nature of several previously published estimates of NH mean temperature changes in past centuries and suggest that further improvements in reconstructive skill are most likely to arise from an emphasis on the quality, rather than quantity, of available proxy data.
Abstract
Results are presented from a set of experiments designed to investigate factors that may influence proxy-based reconstructions of large-scale temperature patterns in past centuries. The factors investigated include 1) the method used to assimilate proxy data into a climate reconstruction, 2) the proxy data network used, 3) the target season, and 4) the spatial domain of the reconstruction. Estimates of hemispheric-mean temperature are formed through spatial averaging of reconstructed temperature patterns that are based on either the local calibration of proxy and instrumental data or a more elaborate multivariate climate field reconstruction approach. The experiments compare results based on the global multiproxy dataset used by Mann and coworkers, with results obtained using the extratropical Northern Hemisphere (NH) maximum latewood tree-ring density set used by Briffa and coworkers. Mean temperature reconstructions are compared for the full NH (Tropics and extratropics, land and ocean) and extratropical continents only, withvarying target seasons (cold-season half year, warm-season half year, and annual mean). The comparisons demonstrate dependence of reconstructions on seasonal, spatial, and methodological considerations, emphasizing the primary importance of the target region and seasonal window of the reconstruction. The comparisons support the generally robust nature of several previously published estimates of NH mean temperature changes in past centuries and suggest that further improvements in reconstructive skill are most likely to arise from an emphasis on the quality, rather than quantity, of available proxy data.
Abstract
A new compilation of monthly mean surface air temperature for the Northern Hemisphere for 1851–1984 is presented based on land-based meteorological station data and fixed-position weather ship data. This compilation differs from others in two ways. First, a considerable amount of new data, previously hidden away in archives, has been included, thus improving both spatial and temporal coverage. Second, the station data have been analyzed to assess their homogeneity. Only reliable or corrected station data have been used in calculating area averages. Grid point temperature estimates have been made by interpolating onto a 5° latitude by 10° longitude grid for each month of the 134 years. In the period of best data coverage, 58% of the area of the Northern Hemisphere is covered by the available data network. (The remaining area is mainly ocean too far from land-based stations to warrant extrapolation.) The reliability of hemispheric estimates is assessed for earlier periods when coverage is less than this maximum. Year-to-year estimates are considered reliable back to about 1875. Estimates earlier than this are judged sufficiently good to indicate trends back to 1851. This new land-based hemispheric temperature curve is compared with recent estimates of Northern Hemisphere temperatures based on marine data. The two independent estimates agree well on the decadal time scale back to the start of the century, but important discrepancies exist for earlier times.
Abstract
A new compilation of monthly mean surface air temperature for the Northern Hemisphere for 1851–1984 is presented based on land-based meteorological station data and fixed-position weather ship data. This compilation differs from others in two ways. First, a considerable amount of new data, previously hidden away in archives, has been included, thus improving both spatial and temporal coverage. Second, the station data have been analyzed to assess their homogeneity. Only reliable or corrected station data have been used in calculating area averages. Grid point temperature estimates have been made by interpolating onto a 5° latitude by 10° longitude grid for each month of the 134 years. In the period of best data coverage, 58% of the area of the Northern Hemisphere is covered by the available data network. (The remaining area is mainly ocean too far from land-based stations to warrant extrapolation.) The reliability of hemispheric estimates is assessed for earlier periods when coverage is less than this maximum. Year-to-year estimates are considered reliable back to about 1875. Estimates earlier than this are judged sufficiently good to indicate trends back to 1851. This new land-based hemispheric temperature curve is compared with recent estimates of Northern Hemisphere temperatures based on marine data. The two independent estimates agree well on the decadal time scale back to the start of the century, but important discrepancies exist for earlier times.
The Southern Alps Experiment is being mounted to study the influence of New Zealand's Southern Alps on local weather and climate. This paper describes these alpine influences and outlines proposed field and modeling experiments. Experiment goals include understanding and quantifying factors that govern the intensity and spatial distribution of heavy rainfall, the west to east distribution of precipitation across the mountains, and the intensity of lee wind storms and warming. Linked research will explore the use of deterministic rainfall models to predict river flows from mountain watersheds.
The Southern Alps Experiment is being mounted to study the influence of New Zealand's Southern Alps on local weather and climate. This paper describes these alpine influences and outlines proposed field and modeling experiments. Experiment goals include understanding and quantifying factors that govern the intensity and spatial distribution of heavy rainfall, the west to east distribution of precipitation across the mountains, and the intensity of lee wind storms and warming. Linked research will explore the use of deterministic rainfall models to predict river flows from mountain watersheds.
Abstract
An accurate platinum RTD-based (resistive temperature device) system has been developed to measure the vertical temperature profile in the region of the atmosphere-ocean interface. TASITA, the towed air–sea interaction temperature analyzer, continuously measures the vertical temperature profile using 17 fixed temperature probes mounted on the instrument: 9 in the uppermost meter of the ocean, and 8 in the lowest 2 m of the atmosphere. The absolute accuracy is better than ±0.05°C, and the relative accuracy between RTDs is ±0.01°C. The instrument is designed to be towed beside a research vessel in undisturbed water outside the ship's wake. Towing speeds between 4 and 8 kt are possible. Instrument operational use is aimed specifically at low wind conditions when the sea surface is smooth to slight and mixing in the top few meters of the ocean is inhibited. Under these conditions a diurnal warm surface water layer is often present in which the surface temperature of the water is markedly different to that tens of centimeters below. Data collected in the western equatorial Pacific show variations in the temperature structure of the surface mixed layer caused by solar beating of the ocean surface and freshwater lenses resulting from heavy precipitation.
Abstract
An accurate platinum RTD-based (resistive temperature device) system has been developed to measure the vertical temperature profile in the region of the atmosphere-ocean interface. TASITA, the towed air–sea interaction temperature analyzer, continuously measures the vertical temperature profile using 17 fixed temperature probes mounted on the instrument: 9 in the uppermost meter of the ocean, and 8 in the lowest 2 m of the atmosphere. The absolute accuracy is better than ±0.05°C, and the relative accuracy between RTDs is ±0.01°C. The instrument is designed to be towed beside a research vessel in undisturbed water outside the ship's wake. Towing speeds between 4 and 8 kt are possible. Instrument operational use is aimed specifically at low wind conditions when the sea surface is smooth to slight and mixing in the top few meters of the ocean is inhibited. Under these conditions a diurnal warm surface water layer is often present in which the surface temperature of the water is markedly different to that tens of centimeters below. Data collected in the western equatorial Pacific show variations in the temperature structure of the surface mixed layer caused by solar beating of the ocean surface and freshwater lenses resulting from heavy precipitation.
Abstract
The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the Rapid Radiative Transfer Model (RRTM), from 55 radiosondes ascents in cloud-free conditions during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment field program, gave mean agreement within 2 W m−2 of those measured with a shipborne PIR. PIR data from two sets of instrument intercomparisons were used to demonstrate ways of detecting inconsistencies in thermopile-sensitivity coefficients and dome-heating correction coefficients. These comparisons indicated that pairs of PIRs are easily corrected to yield mean differences of 1 W m−2 and rms differences of 2 W m−2. Data from a previous field program over the ocean indicate that pairs of PIRs can be used to deduce the true surface skin temperature to an accuracy of a few tenths of a kelvin.
Abstract
The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the Rapid Radiative Transfer Model (RRTM), from 55 radiosondes ascents in cloud-free conditions during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment field program, gave mean agreement within 2 W m−2 of those measured with a shipborne PIR. PIR data from two sets of instrument intercomparisons were used to demonstrate ways of detecting inconsistencies in thermopile-sensitivity coefficients and dome-heating correction coefficients. These comparisons indicated that pairs of PIRs are easily corrected to yield mean differences of 1 W m−2 and rms differences of 2 W m−2. Data from a previous field program over the ocean indicate that pairs of PIRs can be used to deduce the true surface skin temperature to an accuracy of a few tenths of a kelvin.
