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- Author or Editor: M. Decker x
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Abstract
During the 1978 PHOENIX experiment at the Boulder Atmospheric Observatory in Colorado, the presence of atmospheric gravity waves was detected by various independent remote sensing instruments. Fluctuations in the zenith atmospheric radiation were measured at 22.235 and 55.45 GHz in the water vapor and oxygen absorption bands and compared with corresponding fluctuations of surface pressure and the height of FM-CW radar echo returns. These fluctuations are explained, qualitatively and quantitatively, in terms of an internal gravity wave generated by wind shear above the boundary layer. The analysis shows that the oscillations at 22.235 GHz are essentially due to fluctuations of water vapor in the antenna beam while those at 55.45 GHz are due to temperature variations.
Abstract
During the 1978 PHOENIX experiment at the Boulder Atmospheric Observatory in Colorado, the presence of atmospheric gravity waves was detected by various independent remote sensing instruments. Fluctuations in the zenith atmospheric radiation were measured at 22.235 and 55.45 GHz in the water vapor and oxygen absorption bands and compared with corresponding fluctuations of surface pressure and the height of FM-CW radar echo returns. These fluctuations are explained, qualitatively and quantitatively, in terms of an internal gravity wave generated by wind shear above the boundary layer. The analysis shows that the oscillations at 22.235 GHz are essentially due to fluctuations of water vapor in the antenna beam while those at 55.45 GHz are due to temperature variations.
Abstract
Profiles of atmospheric temperature and water vapor derived from ground-based microwave radiometric measurements are compared with concurrent rawinsonde profiles including both clear and cloudy cases. Accuracies of the temperature profiles including the cloudy cases are quite close to predicted accuracies. Mean virtual temperatures between commonly used pressure levels are also compared and resulting rms accuracies are 1.1, 1.6, 2.0 and 2.8°C for the 1000–850, 850–700, 700–500 and 500–300 mb layers, respectively. The microwave technique is potentially useful in applications requiring high time resolution or in data-sparse regions of the oceans that might be covered by an ocean data buoy system.
Abstract
Profiles of atmospheric temperature and water vapor derived from ground-based microwave radiometric measurements are compared with concurrent rawinsonde profiles including both clear and cloudy cases. Accuracies of the temperature profiles including the cloudy cases are quite close to predicted accuracies. Mean virtual temperatures between commonly used pressure levels are also compared and resulting rms accuracies are 1.1, 1.6, 2.0 and 2.8°C for the 1000–850, 850–700, 700–500 and 500–300 mb layers, respectively. The microwave technique is potentially useful in applications requiring high time resolution or in data-sparse regions of the oceans that might be covered by an ocean data buoy system.
Abstract
This study validates the predicted association between frequency of dry microburst occurrence and large temperature lapse rate. In applying lapse rate trend data and high time resolution data from remote sensors, we first compared lapse rates from the Denver rawinsonde with the thermodynamic profiler and obtained linear correlation coefficients ranging from .63 to .94. Continuous 20-minute radiometer samples of lapse rate were available throughout the experiment period. The data indicate a critical value of 700–500 mb lapse rate ≥8°C km−1 for dry microburst occurrence. Also, we found dry microburst occurrence in the Denver area better correlated with late afternoon lapse rates than with early morning lapse rates: 67% of dry microbursts occurred with 1200 UTC lapse rates ≥8°C km−1, while 89% of dry microbursts occurred with 2200 UTC lapse rates ≥8°C km−1. We recommend that remote sensor temperature retrievals such as with Radio Acoustic Sounding Systems (RASS) extend to at least 3 km AGL to aid dry microburst nowcasting and forecast verification.
Abstract
This study validates the predicted association between frequency of dry microburst occurrence and large temperature lapse rate. In applying lapse rate trend data and high time resolution data from remote sensors, we first compared lapse rates from the Denver rawinsonde with the thermodynamic profiler and obtained linear correlation coefficients ranging from .63 to .94. Continuous 20-minute radiometer samples of lapse rate were available throughout the experiment period. The data indicate a critical value of 700–500 mb lapse rate ≥8°C km−1 for dry microburst occurrence. Also, we found dry microburst occurrence in the Denver area better correlated with late afternoon lapse rates than with early morning lapse rates: 67% of dry microbursts occurred with 1200 UTC lapse rates ≥8°C km−1, while 89% of dry microbursts occurred with 2200 UTC lapse rates ≥8°C km−1. We recommend that remote sensor temperature retrievals such as with Radio Acoustic Sounding Systems (RASS) extend to at least 3 km AGL to aid dry microburst nowcasting and forecast verification.
Abstract
Climate model simulations of daily precipitation statistics from the third phase of the Coupled Model Intercomparison Project (CMIP3) were evaluated against precipitation observations from North America over the period 1979–99. The evaluation revealed that the models underestimate the intensity of heavy and extreme precipitation along the Pacific coast, southeastern United States, and southern Mexico, and these biases are robust among the models. The models also overestimate the intensity of light precipitation events over much of North America, resulting in fairly realistic mean precipitation in many places. In contrast, heavy precipitation is simulated realistically over northern and eastern Canada, as is the seasonal cycle of heavy precipitation over a majority of North America. An evaluation of the simulated atmospheric dynamics and thermodynamics associated with extreme precipitation events was also conducted using the North American Regional Reanalysis (NARR). The models were found to capture the large-scale physical mechanisms that generate extreme precipitation realistically, although they tend to overestimate the strength of the associated atmospheric circulation features. This suggests that climate model deficiencies such as insufficient spatial resolution, inadequate representation of convective precipitation, and overly smoothed topography may be more important for biases in simulated heavy precipitation than errors in the large-scale circulation during extreme events.
Abstract
Climate model simulations of daily precipitation statistics from the third phase of the Coupled Model Intercomparison Project (CMIP3) were evaluated against precipitation observations from North America over the period 1979–99. The evaluation revealed that the models underestimate the intensity of heavy and extreme precipitation along the Pacific coast, southeastern United States, and southern Mexico, and these biases are robust among the models. The models also overestimate the intensity of light precipitation events over much of North America, resulting in fairly realistic mean precipitation in many places. In contrast, heavy precipitation is simulated realistically over northern and eastern Canada, as is the seasonal cycle of heavy precipitation over a majority of North America. An evaluation of the simulated atmospheric dynamics and thermodynamics associated with extreme precipitation events was also conducted using the North American Regional Reanalysis (NARR). The models were found to capture the large-scale physical mechanisms that generate extreme precipitation realistically, although they tend to overestimate the strength of the associated atmospheric circulation features. This suggests that climate model deficiencies such as insufficient spatial resolution, inadequate representation of convective precipitation, and overly smoothed topography may be more important for biases in simulated heavy precipitation than errors in the large-scale circulation during extreme events.
Abstract
A modular design for a ground-based thermodynamic profiler is presented, based on experience with a six-channel microwave radiometer that has provided temperature, pressure, and moisture measurements continuously, unattended, since 1981. Each module consists of one pair of microwave channels, whose frequencies are chosen to facilitate the joint use of radio-frequency (RF) components, thus reducing hardware costs by nearly half. The number of modules included in a given system can be chosen to suit the altitude and accuracy requirements for that particular application. The accuracy of temperatures and pressure heights retrieved from simulated profilers with 4 to 18 channels is presented to illustrate the tradeoff between cost and accuracy.
Abstract
A modular design for a ground-based thermodynamic profiler is presented, based on experience with a six-channel microwave radiometer that has provided temperature, pressure, and moisture measurements continuously, unattended, since 1981. Each module consists of one pair of microwave channels, whose frequencies are chosen to facilitate the joint use of radio-frequency (RF) components, thus reducing hardware costs by nearly half. The number of modules included in a given system can be chosen to suit the altitude and accuracy requirements for that particular application. The accuracy of temperatures and pressure heights retrieved from simulated profilers with 4 to 18 channels is presented to illustrate the tradeoff between cost and accuracy.
Abstract
A land surface scheme with and without groundwater–vegetation interactions is used to explore the impact of rainfall variability on transpiration over drought-vulnerable regions of southeastern Australia. The authors demonstrate that if groundwater is included in the simulations, there is a low correlation between rainfall variability and the response of transpiration to this variability over forested regions. Groundwater reduces near-surface water variability, enabling forests to maintain transpiration through several years of low rainfall, in agreement with independent observations of vegetation greenness. If groundwater is not included, the transpiration variability matches the rainfall variability independent of land cover type. The authors’ results suggest that omitting groundwater in regions where groundwater sustains forests will 1) probably overestimate the likelihood of forest dieback during drought, 2) overestimate a positive feedback linked with declining transpiration and a drying boundary layer, and 3) underestimate the impact of land cover change due to inadequately simulating the different responses to drought for different land cover types.
Abstract
A land surface scheme with and without groundwater–vegetation interactions is used to explore the impact of rainfall variability on transpiration over drought-vulnerable regions of southeastern Australia. The authors demonstrate that if groundwater is included in the simulations, there is a low correlation between rainfall variability and the response of transpiration to this variability over forested regions. Groundwater reduces near-surface water variability, enabling forests to maintain transpiration through several years of low rainfall, in agreement with independent observations of vegetation greenness. If groundwater is not included, the transpiration variability matches the rainfall variability independent of land cover type. The authors’ results suggest that omitting groundwater in regions where groundwater sustains forests will 1) probably overestimate the likelihood of forest dieback during drought, 2) overestimate a positive feedback linked with declining transpiration and a drying boundary layer, and 3) underestimate the impact of land cover change due to inadequately simulating the different responses to drought for different land cover types.
Abstract
An instrument that remotely senses the integrated amounts of water vapor and liquid on a path through the atmosphere is discussed. The vapor and liquid are measured simultaneously but independently by microwave radiometers. Comparison of the accuracy in measurement of vapor is made with radiosondes, and of liquid with an independent method employing transmission from a geosynchronous satellite. The instrument is designed for unattended operation; examples of measured data are given. Applications including observations for weather forecasting, weather modification, solar-radiation studies, and instrumentation for geodetic metrology are also discussed.
Abstract
An instrument that remotely senses the integrated amounts of water vapor and liquid on a path through the atmosphere is discussed. The vapor and liquid are measured simultaneously but independently by microwave radiometers. Comparison of the accuracy in measurement of vapor is made with radiosondes, and of liquid with an independent method employing transmission from a geosynchronous satellite. The instrument is designed for unattended operation; examples of measured data are given. Applications including observations for weather forecasting, weather modification, solar-radiation studies, and instrumentation for geodetic metrology are also discussed.
Abstract
This paper describes the results of a three-week experiment in which ground-based microwave radiometricmeasurements were combined with VHF radar measurements of tropopause height to yield vertical temperature profiles. Several algorithms to derive tropopause height are presented and their results are comparedwith radiosondes. The best of the algorithms yields radar versus radiosonde rms differences of 0.65 km.By the use of the combined radar-radiometric method, improvements were obtained in rms temperatureaccuracy of as much as 2.0 K rms over the pure radiometric technique.
Abstract
This paper describes the results of a three-week experiment in which ground-based microwave radiometricmeasurements were combined with VHF radar measurements of tropopause height to yield vertical temperature profiles. Several algorithms to derive tropopause height are presented and their results are comparedwith radiosondes. The best of the algorithms yields radar versus radiosonde rms differences of 0.65 km.By the use of the combined radar-radiometric method, improvements were obtained in rms temperatureaccuracy of as much as 2.0 K rms over the pure radiometric technique.
Abstract
Global climate models play an important role in quantifying past and projecting future changes in drought. Previous studies have pointed to shortcomings in these models for simulating droughts, but systematic evaluation of their level of agreement has been limited. Here, historical simulations (1950–2004) for 20 models from the latest Coupled Model Intercomparison Project (CMIP5) were analyzed for a variety of drought metrics and thresholds using a standardized drought index. Model agreement was investigated for different types of drought (precipitation, runoff, and soil moisture) and how this varied with drought severity and duration. At the global scale, climate models were shown to agree well on most precipitation drought metrics, but systematically underestimated precipitation drought intensity compared to observations. Conversely, simulated runoff and soil moisture droughts varied significantly across models, particularly for intensity. Differences in precipitation simulations were found to explain model differences in runoff and soil moisture drought metrics over some regions, but predominantly with respect to drought intensity. This suggests it is insufficient to evaluate models for precipitation droughts to increase confidence in model performance for other types of drought. This study shows large but metric-dependent discrepancies in CMIP5 for modeling different types of droughts that relate strongly to the component models (i.e., atmospheric or land surface scheme) used in the coupled modeling systems. Our results point to a need to consider multiple models in drought impact studies to account for high model uncertainties.
Abstract
Global climate models play an important role in quantifying past and projecting future changes in drought. Previous studies have pointed to shortcomings in these models for simulating droughts, but systematic evaluation of their level of agreement has been limited. Here, historical simulations (1950–2004) for 20 models from the latest Coupled Model Intercomparison Project (CMIP5) were analyzed for a variety of drought metrics and thresholds using a standardized drought index. Model agreement was investigated for different types of drought (precipitation, runoff, and soil moisture) and how this varied with drought severity and duration. At the global scale, climate models were shown to agree well on most precipitation drought metrics, but systematically underestimated precipitation drought intensity compared to observations. Conversely, simulated runoff and soil moisture droughts varied significantly across models, particularly for intensity. Differences in precipitation simulations were found to explain model differences in runoff and soil moisture drought metrics over some regions, but predominantly with respect to drought intensity. This suggests it is insufficient to evaluate models for precipitation droughts to increase confidence in model performance for other types of drought. This study shows large but metric-dependent discrepancies in CMIP5 for modeling different types of droughts that relate strongly to the component models (i.e., atmospheric or land surface scheme) used in the coupled modeling systems. Our results point to a need to consider multiple models in drought impact studies to account for high model uncertainties.
