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Abstract
Wind channeling effects in a major strait have been studied using pressure data from seven meteorological stations in and around Cook Strait, New Zealand. The data were analyzed using pairs of stations to obtain pressure differences and groups of three stations to give vector gradients. Wind data from a hill site in the channel have been used as a measure of the wind in Cook Strait. Wind directions have a range of orientations relative to the pressure gradient but are almost parallel to it for a substantial amount of the time, rather than approximately perpendicular to it as is the case in the open ocean. The balance of terms in the equation of motion perpendicular to the wind direction indicates that flow curvature is important in Cook Strait. In the balance of terms along the wind direction, the acceleration term is usually larger than the friction term in the entrance regions. With southerly wind directions, the pressure gradient direction in the southern entrance zone to the strait has an unusual relation to the wind direction and varies over a wide range. This latter feature is attributed to the reactions set up between the mountains and the varying orientations of the incident airstream.
Abstract
Wind channeling effects in a major strait have been studied using pressure data from seven meteorological stations in and around Cook Strait, New Zealand. The data were analyzed using pairs of stations to obtain pressure differences and groups of three stations to give vector gradients. Wind data from a hill site in the channel have been used as a measure of the wind in Cook Strait. Wind directions have a range of orientations relative to the pressure gradient but are almost parallel to it for a substantial amount of the time, rather than approximately perpendicular to it as is the case in the open ocean. The balance of terms in the equation of motion perpendicular to the wind direction indicates that flow curvature is important in Cook Strait. In the balance of terms along the wind direction, the acceleration term is usually larger than the friction term in the entrance regions. With southerly wind directions, the pressure gradient direction in the southern entrance zone to the strait has an unusual relation to the wind direction and varies over a wide range. This latter feature is attributed to the reactions set up between the mountains and the varying orientations of the incident airstream.
Abstract
The existence of an annual variation in height and temperature of the tropopause over tropical regions has long been recognized, but has not been fully explained. In this paper it is proposed that the variation is a fairly direct response to the annual variation in average tropical surface insolation. The variation in insolation causes a corresponding annual cycle in average tropical sea surface temperature with a total range of order 1 K. The consequent variation in absolute humidity in turn produces an annual variation in upper tropospheric potential temperatures, and hence in the height and temperature of the tropopause.
The physical link between the surface and the tropopause is provided by convection in the cores of the giant cumulonimbus clouds (hot towers) of the tropical oceanic regions, in which air parcels can achieve the maximum possible heating by release of latent heat. The process is modeled quantitatively in a simplified way, and excellent agreement is found between the predicted and observed phase and amplitude of the annual variation in tropopause potential temperature.
Since the regular seasonal variation in insolation is relatively small in the tropics, the annual variation in sun-earth distance is an important factor in the variation of surface insolation. The annual cycle in the properties of the tropical tropopause thus provides the first identifiable effect of the earth's orbital eccentricity on climate parameters.
Abstract
The existence of an annual variation in height and temperature of the tropopause over tropical regions has long been recognized, but has not been fully explained. In this paper it is proposed that the variation is a fairly direct response to the annual variation in average tropical surface insolation. The variation in insolation causes a corresponding annual cycle in average tropical sea surface temperature with a total range of order 1 K. The consequent variation in absolute humidity in turn produces an annual variation in upper tropospheric potential temperatures, and hence in the height and temperature of the tropopause.
The physical link between the surface and the tropopause is provided by convection in the cores of the giant cumulonimbus clouds (hot towers) of the tropical oceanic regions, in which air parcels can achieve the maximum possible heating by release of latent heat. The process is modeled quantitatively in a simplified way, and excellent agreement is found between the predicted and observed phase and amplitude of the annual variation in tropopause potential temperature.
Since the regular seasonal variation in insolation is relatively small in the tropics, the annual variation in sun-earth distance is an important factor in the variation of surface insolation. The annual cycle in the properties of the tropical tropopause thus provides the first identifiable effect of the earth's orbital eccentricity on climate parameters.
Abstract
Regional climate trends are of interest both for understanding natural climate processes and as tests of anthropogenic climate change signatures. Relative to global trends, however, their detection is hampered by smaller datasets and the influence of regional climate processes such as the Southern Oscillation. Regional trends are often presented by authors without demonstration of statistical significance. In this paper, regional-average annual series of air temperature and sea surface temperature for the New Zealand region are analyzed using a systematic statistical approach that selects the optimum statistical model (with respect to serial correlation, linearity, etc.), explicitly models the interannual variability associated with observable regional climate processes, and tests significance. It is found that the residuals are stationary and are a red noise process [ARMA(1,0)] for all the series examined. The SOI and a meridional circulation anomaly index (both high-pass filtered) are “explanatory variables” for interannual variability. For national-average air temperature (AT), linear and exponential trend models are equally valid but for simplicity the linear model is preferred. Failure to model the serial correlation in AT would result in an estimated confidence interval for trend that is too small by 36%. The use of the explanatory variables tightens the confidence interval by 15%.
Significant trends were detected. The trend in AT for 1896–1994 is 0.11 ± 0.035°C decade−1 (95% confidence interval). This is about double the trend reported for global data, which may be due to the relative absence of sulfate aerosols in the South Pacific region. The trends in maximum and minimum temperature over this period are not statistically different. However, for the later period of 1951–90, the trend in maximum temperature reduces to an insignificant value, while the trend in minimum temperature remains high, resulting in a significant downward trend in diurnal range of 0.10°C decade−1. Similar diurnal range behavior in other regions has been tentatively attributed to increasing cloudiness. The trend in a regional SST series for 1928–94, 0.07°C decade−1, is about half the trend in AT for the same period. The trend in the difference, SST–AT, −0.06°C decade−1, is statistically significant. This implies the existence of an atmospheric warming source for the additional air temperature trend, and may mean that the heat fluxes between the atmosphere and ocean in the New Zealand region are subject to a large trend, with the direction of flux change being toward the ocean. The results of the study are consistent with the IPCC predictions of climate change.
Abstract
Regional climate trends are of interest both for understanding natural climate processes and as tests of anthropogenic climate change signatures. Relative to global trends, however, their detection is hampered by smaller datasets and the influence of regional climate processes such as the Southern Oscillation. Regional trends are often presented by authors without demonstration of statistical significance. In this paper, regional-average annual series of air temperature and sea surface temperature for the New Zealand region are analyzed using a systematic statistical approach that selects the optimum statistical model (with respect to serial correlation, linearity, etc.), explicitly models the interannual variability associated with observable regional climate processes, and tests significance. It is found that the residuals are stationary and are a red noise process [ARMA(1,0)] for all the series examined. The SOI and a meridional circulation anomaly index (both high-pass filtered) are “explanatory variables” for interannual variability. For national-average air temperature (AT), linear and exponential trend models are equally valid but for simplicity the linear model is preferred. Failure to model the serial correlation in AT would result in an estimated confidence interval for trend that is too small by 36%. The use of the explanatory variables tightens the confidence interval by 15%.
Significant trends were detected. The trend in AT for 1896–1994 is 0.11 ± 0.035°C decade−1 (95% confidence interval). This is about double the trend reported for global data, which may be due to the relative absence of sulfate aerosols in the South Pacific region. The trends in maximum and minimum temperature over this period are not statistically different. However, for the later period of 1951–90, the trend in maximum temperature reduces to an insignificant value, while the trend in minimum temperature remains high, resulting in a significant downward trend in diurnal range of 0.10°C decade−1. Similar diurnal range behavior in other regions has been tentatively attributed to increasing cloudiness. The trend in a regional SST series for 1928–94, 0.07°C decade−1, is about half the trend in AT for the same period. The trend in the difference, SST–AT, −0.06°C decade−1, is statistically significant. This implies the existence of an atmospheric warming source for the additional air temperature trend, and may mean that the heat fluxes between the atmosphere and ocean in the New Zealand region are subject to a large trend, with the direction of flux change being toward the ocean. The results of the study are consistent with the IPCC predictions of climate change.
Abstract
Monte Carlo and Morris screening techniques are used to examine the relative sensitivity of deep convective simulations to changes in initial conditions (IC) versus changes to microphysical parameters (MP). IC are perturbed using a set of empirical orthogonal function–principal component pairs obtained from a database of tropical soundings, while MP are perturbed consistent with their range of realistic values. Monte Carlo experiments provide a broad overview of parameter–output response, while Morris screening techniques identify the most significant influences on specific model output variables. Changes to MP produce a similar order-of-magnitude response in convective hydrologic cycle, dynamics, and latent heating as changes to IC. Changes in IC appear to have a larger effect on radiative fluxes than perturbations to MP. The distribution of low-level latent heating reveals that changes in MP have a larger influence on cold pool properties than changes to the environment. The dominant effects are produced by a subset of cloud MP and thermodynamic structure functions, indicating perturbation of a subset of the control factors may be used to produce most of the variability in a short-term convective-scale ensemble forecast. The most influential MP are the autoconversion threshold, the rain particle size distribution intercept, and the ice particle fall speed parameters. The most influential EOFs are those that correspond to variability in lower- to midtropospheric temperature and water vapor, as well as zonal low-level shear. The results have implications for both the understanding of what influences convective development and the design of ensemble-based prediction and data assimilation systems.
Abstract
Monte Carlo and Morris screening techniques are used to examine the relative sensitivity of deep convective simulations to changes in initial conditions (IC) versus changes to microphysical parameters (MP). IC are perturbed using a set of empirical orthogonal function–principal component pairs obtained from a database of tropical soundings, while MP are perturbed consistent with their range of realistic values. Monte Carlo experiments provide a broad overview of parameter–output response, while Morris screening techniques identify the most significant influences on specific model output variables. Changes to MP produce a similar order-of-magnitude response in convective hydrologic cycle, dynamics, and latent heating as changes to IC. Changes in IC appear to have a larger effect on radiative fluxes than perturbations to MP. The distribution of low-level latent heating reveals that changes in MP have a larger influence on cold pool properties than changes to the environment. The dominant effects are produced by a subset of cloud MP and thermodynamic structure functions, indicating perturbation of a subset of the control factors may be used to produce most of the variability in a short-term convective-scale ensemble forecast. The most influential MP are the autoconversion threshold, the rain particle size distribution intercept, and the ice particle fall speed parameters. The most influential EOFs are those that correspond to variability in lower- to midtropospheric temperature and water vapor, as well as zonal low-level shear. The results have implications for both the understanding of what influences convective development and the design of ensemble-based prediction and data assimilation systems.
No abstract available.
No abstract available.
Abstract
Handheld sun photometers are typically used to make aerosol optical depth measurements while on the ground. Various investigators, in unrelated efforts, have used handheld sun photometers to make aerosol optical depth measurements from light aircraft, but the strengths and weakness of this approach have not been characterized until now. While the ease and relatively low cost of an aircraft manual sun photometer are attractive, determining if the sun photometer was correctly pointed at the sun for each measurement is the biggest challenge. This problem can be partially addressed by collecting a large number of measurements at each altitude, then manually removing the largest optical depths (misalignment always results in erroneous larger values). Examples of past aircraft manual sun photometer measurements are demonstrating that it is possible to obtain quantitative measurements if sufficient sun photometer measurements are made at each elevation. In order to improve on manual sun photometer measurements, a small webcam was attached to the side of a Microtops sun photometer, and the Microtops sun photometer was triggered by computer control. By detecting the position of the sun in the webcam image, it is possible to determine whether the sun photometer was pointed at the sun correctly when the aerosol optical depth measurement was made. Unfortunately, it was found that the Microtops sun photometer takes ∼1.1 s to scan over the five wavelength channels. This 1.1-s delay proved to be too long, preventing the proposed approach from working as the aircraft was bouncing around.
Abstract
Handheld sun photometers are typically used to make aerosol optical depth measurements while on the ground. Various investigators, in unrelated efforts, have used handheld sun photometers to make aerosol optical depth measurements from light aircraft, but the strengths and weakness of this approach have not been characterized until now. While the ease and relatively low cost of an aircraft manual sun photometer are attractive, determining if the sun photometer was correctly pointed at the sun for each measurement is the biggest challenge. This problem can be partially addressed by collecting a large number of measurements at each altitude, then manually removing the largest optical depths (misalignment always results in erroneous larger values). Examples of past aircraft manual sun photometer measurements are demonstrating that it is possible to obtain quantitative measurements if sufficient sun photometer measurements are made at each elevation. In order to improve on manual sun photometer measurements, a small webcam was attached to the side of a Microtops sun photometer, and the Microtops sun photometer was triggered by computer control. By detecting the position of the sun in the webcam image, it is possible to determine whether the sun photometer was pointed at the sun correctly when the aerosol optical depth measurement was made. Unfortunately, it was found that the Microtops sun photometer takes ∼1.1 s to scan over the five wavelength channels. This 1.1-s delay proved to be too long, preventing the proposed approach from working as the aircraft was bouncing around.
Abstract
Aerosol radiative forcing in the Persian Gulf region is derived from data collected during the United Arab Emirates (UAE) Unified Aerosol Experiment (UAE2). This campaign took place in August and September of 2004. The land–sea-breeze circulation modulates the diurnal variability of the aerosol properties and aerosol radiative forcing at the surface. Larger aerosol radiative forcing is observed during the land breeze in comparison to the sea breeze. The aerosol optical properties change as the onshore wind brings slightly cleaner air. The mean diurnal value of the surface aerosol forcing during the UAE2 campaign is about −20 W m−2, which corresponds to large aerosol optical thickness (0.45 at 500 nm). The aerosol forcing efficiency [i.e., broadband shortwave forcing per unit optical depth at 550 nm, W m−2 (τ 500)−1] is −53 W m−2 (τ 500)−1 and the average single scattering albedo is 0.93 at 550 nm.
Abstract
Aerosol radiative forcing in the Persian Gulf region is derived from data collected during the United Arab Emirates (UAE) Unified Aerosol Experiment (UAE2). This campaign took place in August and September of 2004. The land–sea-breeze circulation modulates the diurnal variability of the aerosol properties and aerosol radiative forcing at the surface. Larger aerosol radiative forcing is observed during the land breeze in comparison to the sea breeze. The aerosol optical properties change as the onshore wind brings slightly cleaner air. The mean diurnal value of the surface aerosol forcing during the UAE2 campaign is about −20 W m−2, which corresponds to large aerosol optical thickness (0.45 at 500 nm). The aerosol forcing efficiency [i.e., broadband shortwave forcing per unit optical depth at 550 nm, W m−2 (τ 500)−1] is −53 W m−2 (τ 500)−1 and the average single scattering albedo is 0.93 at 550 nm.
No Abstract available.
No Abstract available.
Abstract
When unaccounted for in numerical weather prediction (NWP) models, heavy aerosol events can cause significant unrealized biases in forecast meteorological parameters such as surface temperature. To improve near-surface forecasting accuracies during heavy aerosol loadings, we demonstrate the feasibility of incorporating aerosol fields from a global chemical transport model as initial and boundary conditions into a higher-resolution NWP model with aerosol–meteorological coupling. This concept is tested for a major biomass burning smoke event over the northern Great Plains region of the United States that occurred during summer of 2015. Aerosol analyses from the global Navy Aerosol Analysis and Prediction System (NAAPS) are used as initial and boundary conditions for Weather Research and Forecasting Model with Chemistry (WRF-Chem) simulations. Through incorporating more realistic aerosol direct effects into the WRF-Chem simulations, errors in WRF-Chem simulated surface downward shortwave radiative fluxes and near-surface temperature are reduced when compared with surface-based observations. This study confirms the ability to decrease biases induced by the aerosol direct effect for regional NWP forecasts during high-impact aerosol episodes through the incorporation of analyses and forecasts from a global aerosol transport model.
Abstract
When unaccounted for in numerical weather prediction (NWP) models, heavy aerosol events can cause significant unrealized biases in forecast meteorological parameters such as surface temperature. To improve near-surface forecasting accuracies during heavy aerosol loadings, we demonstrate the feasibility of incorporating aerosol fields from a global chemical transport model as initial and boundary conditions into a higher-resolution NWP model with aerosol–meteorological coupling. This concept is tested for a major biomass burning smoke event over the northern Great Plains region of the United States that occurred during summer of 2015. Aerosol analyses from the global Navy Aerosol Analysis and Prediction System (NAAPS) are used as initial and boundary conditions for Weather Research and Forecasting Model with Chemistry (WRF-Chem) simulations. Through incorporating more realistic aerosol direct effects into the WRF-Chem simulations, errors in WRF-Chem simulated surface downward shortwave radiative fluxes and near-surface temperature are reduced when compared with surface-based observations. This study confirms the ability to decrease biases induced by the aerosol direct effect for regional NWP forecasts during high-impact aerosol episodes through the incorporation of analyses and forecasts from a global aerosol transport model.
Abstract
A parameterization of runway visual range (RVR) has been developed using relevant meteorological parameters such as visibility (Vk ), relative humidity (RH), temperature (T), precipitation intensity (PI), and precipitation type (PT) measured in years between 2009 and 2011 at Toronto Pearson International Airport during the Canadian Airport Nowcasting Project. The FD12P probe measured PI, Vk , and PT. The observed Vk and PT were tested against data reported by hourly surface observations (SAs). The measured Vk has correlated well with the SA with a correlation coefficient (r) of 0.76 for Vk < 5 km, but the FD12P underestimated visibility by about 20% with a mean difference (MD) of about 196 m. For Vk < 2 km, the FD12P overestimated visibility by about 7% with an MD of 60 m. The SA reported slightly more snow events—22% as compared to 17%—but the FD12P reported many more snow grain cases than the SA. Both the SA and the FD12P reported rain at similar frequency—4% and 5%, respectively. Using a theoretical approach, a parameterization that can be used to determine RVR as a function of Vk has been developed. Using the observed T, RH, and dewpoint temperature (Td ), a new parameterization for predicting Vk /RVR in fog has been also developed. These parameterizations agreed with observations (r ≈ 0.8). The parameterizations have been tested using the Canadian Environmental Multiscale Regional model. The results show that when PI, RH, and T are reasonably predicted and the fog events are correctly diagnosed, the model can be used to forecast RVR.
Abstract
A parameterization of runway visual range (RVR) has been developed using relevant meteorological parameters such as visibility (Vk ), relative humidity (RH), temperature (T), precipitation intensity (PI), and precipitation type (PT) measured in years between 2009 and 2011 at Toronto Pearson International Airport during the Canadian Airport Nowcasting Project. The FD12P probe measured PI, Vk , and PT. The observed Vk and PT were tested against data reported by hourly surface observations (SAs). The measured Vk has correlated well with the SA with a correlation coefficient (r) of 0.76 for Vk < 5 km, but the FD12P underestimated visibility by about 20% with a mean difference (MD) of about 196 m. For Vk < 2 km, the FD12P overestimated visibility by about 7% with an MD of 60 m. The SA reported slightly more snow events—22% as compared to 17%—but the FD12P reported many more snow grain cases than the SA. Both the SA and the FD12P reported rain at similar frequency—4% and 5%, respectively. Using a theoretical approach, a parameterization that can be used to determine RVR as a function of Vk has been developed. Using the observed T, RH, and dewpoint temperature (Td ), a new parameterization for predicting Vk /RVR in fog has been also developed. These parameterizations agreed with observations (r ≈ 0.8). The parameterizations have been tested using the Canadian Environmental Multiscale Regional model. The results show that when PI, RH, and T are reasonably predicted and the fog events are correctly diagnosed, the model can be used to forecast RVR.
