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Abstract
We examine the potential role of aerosol pollution on the rainfall and intensity of Hurricane Harvey. For this study, we use the global model, Ocean–Land–Atmosphere Model (OLAM), with aerosol estimates from the global atmospheric chemistry model GEOS-Chem. Two sets of simulations of Hurricane Harvey were performed. Simulations in the first set cover the intensification phase of Harvey until initial landfall in Texas and focus on the sensitivity of storm track and intensity, while simulations in the second set examine the sensitivity of storm track and precipitation during the period after initial landfall when record flooding occurred near Houston. During each period, simulations were performed with no anthropogenic sources of aerosol, with both natural and anthropogenic aerosol sources, and with both sources enhanced 10 times. During the rapid intensification phase, the results indicate that aerosol amounts had very little impact on storm motion. Moreover, very little difference was found on the intensity of the simulated storm to aerosol amounts for the no-anthropogenic versus the GEOS-Chem estimated amounts with anthropogenic sources. However, when both natural and anthropogenic aerosol amounts were enhanced 10 times, the simulated storm intensity was enhanced appreciably in terms of minimum sea level pressure. During the second period of the simulation, through which Harvey remained a tropical storm, the main result was that very little sensitivity was found in precipitation or any other tropical cyclone (TC) characteristic to aerosol concentrations. We cannot definitively state why the individual convective cells did not respond to high aerosol concentrations during this phase of the storm. However, the abundant precipitation in all three simulations scavenged the vast majority of aerosols as it flowed radially inward, and we speculate that this modulated the potential impact of aerosols on the inner TC and eyewall. Overall, the simulated response of Hurricane Harvey to aerosols was far less spectacular than what has been simulated in the past. We conclude that this is because Hurricane Harvey was a strongly dynamically driven storm system that as a result was relatively impervious to the effects of aerosols.
Abstract
We examine the potential role of aerosol pollution on the rainfall and intensity of Hurricane Harvey. For this study, we use the global model, Ocean–Land–Atmosphere Model (OLAM), with aerosol estimates from the global atmospheric chemistry model GEOS-Chem. Two sets of simulations of Hurricane Harvey were performed. Simulations in the first set cover the intensification phase of Harvey until initial landfall in Texas and focus on the sensitivity of storm track and intensity, while simulations in the second set examine the sensitivity of storm track and precipitation during the period after initial landfall when record flooding occurred near Houston. During each period, simulations were performed with no anthropogenic sources of aerosol, with both natural and anthropogenic aerosol sources, and with both sources enhanced 10 times. During the rapid intensification phase, the results indicate that aerosol amounts had very little impact on storm motion. Moreover, very little difference was found on the intensity of the simulated storm to aerosol amounts for the no-anthropogenic versus the GEOS-Chem estimated amounts with anthropogenic sources. However, when both natural and anthropogenic aerosol amounts were enhanced 10 times, the simulated storm intensity was enhanced appreciably in terms of minimum sea level pressure. During the second period of the simulation, through which Harvey remained a tropical storm, the main result was that very little sensitivity was found in precipitation or any other tropical cyclone (TC) characteristic to aerosol concentrations. We cannot definitively state why the individual convective cells did not respond to high aerosol concentrations during this phase of the storm. However, the abundant precipitation in all three simulations scavenged the vast majority of aerosols as it flowed radially inward, and we speculate that this modulated the potential impact of aerosols on the inner TC and eyewall. Overall, the simulated response of Hurricane Harvey to aerosols was far less spectacular than what has been simulated in the past. We conclude that this is because Hurricane Harvey was a strongly dynamically driven storm system that as a result was relatively impervious to the effects of aerosols.
Abstract
Idealized large-eddy simulations (LESs) are performed of deep convective clouds over south Florida to examine the relative role of aerosol-induced condensational versus mixed-phase invigoration to convective intensity and rainfall. Aerosol concentrations and chemistry are represented by using output from the GEOS-Chem global atmospheric chemistry model run with and without anthropogenic aerosol sources. The results clearly show that higher aerosol concentrations result in enhanced precipitation, larger amounts of cloud liquid water content, enhanced updraft velocities during the latter part of the simulation, and a modest enhancement of the latent heating of condensation. Overall, our results are consistent with the concept that convective cloud invigoration is mainly due to condensational invigoration and not primarily to mixed-phase invigoration. Furthermore, our results suggest that condensational invigoration can result in appreciable precipitation enhancement of ordinary warm-based convective clouds such as are common in locations like south Florida.
Abstract
Idealized large-eddy simulations (LESs) are performed of deep convective clouds over south Florida to examine the relative role of aerosol-induced condensational versus mixed-phase invigoration to convective intensity and rainfall. Aerosol concentrations and chemistry are represented by using output from the GEOS-Chem global atmospheric chemistry model run with and without anthropogenic aerosol sources. The results clearly show that higher aerosol concentrations result in enhanced precipitation, larger amounts of cloud liquid water content, enhanced updraft velocities during the latter part of the simulation, and a modest enhancement of the latent heating of condensation. Overall, our results are consistent with the concept that convective cloud invigoration is mainly due to condensational invigoration and not primarily to mixed-phase invigoration. Furthermore, our results suggest that condensational invigoration can result in appreciable precipitation enhancement of ordinary warm-based convective clouds such as are common in locations like south Florida.
Abstract
The manner in which the deep western boundary current (DWBC) crosses the Gulf Stream is investigated using data from a hydrographic survey conducted in 1990. Absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level. Three density layers are considered in detail: two mid-depth layers, which together make up the shallowest water mass component of the DWBC (500–1200 m), and a deep layer consisting of the Norwegian–Greenland overflow water (2500–3500 m). The shallowest layer does not make it through the crossover and is completely entrained by the Gulf Stream; however, the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover. In the intermediate layer, which is denser than the Gulf Stream coming off the shelf, part of the DWBC recirculates to the northeast while the onshoremost portion continues equatorward. In the deep layer only a small amount of recirculation occurs. The lateral fields of potential vorticity (Q) reveal a Q barrier associated with the Gulf Stream in the two mid-depth layers, which is partially lessened in the intermediate one allowing the equatorward continuation of flow. In the deep layer, the DWBC maintains its potential vorticity through the crossover.
Abstract
The manner in which the deep western boundary current (DWBC) crosses the Gulf Stream is investigated using data from a hydrographic survey conducted in 1990. Absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level. Three density layers are considered in detail: two mid-depth layers, which together make up the shallowest water mass component of the DWBC (500–1200 m), and a deep layer consisting of the Norwegian–Greenland overflow water (2500–3500 m). The shallowest layer does not make it through the crossover and is completely entrained by the Gulf Stream; however, the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover. In the intermediate layer, which is denser than the Gulf Stream coming off the shelf, part of the DWBC recirculates to the northeast while the onshoremost portion continues equatorward. In the deep layer only a small amount of recirculation occurs. The lateral fields of potential vorticity (Q) reveal a Q barrier associated with the Gulf Stream in the two mid-depth layers, which is partially lessened in the intermediate one allowing the equatorward continuation of flow. In the deep layer, the DWBC maintains its potential vorticity through the crossover.
Revised and complete verification statistics for mainland United States long-range forecasts made for the period 1976–80 by the 1976 version of the University of Wisconsin model are presented. Corrections to earlier published values are given, as well as skill scores obtained using a much more complete set of stations for which forecasts were made.
The overall skill score for the pentad temperature forecasts made for January, April, July, and October is negative (−0.14), while those for pentad precipitation and individual year July precipitation forecasts are positive (0.12 and 0.04, respectively). The individual year January temperature forecast skill score was unchanged at −0.08 overall.
Revised and complete verification statistics for mainland United States long-range forecasts made for the period 1976–80 by the 1976 version of the University of Wisconsin model are presented. Corrections to earlier published values are given, as well as skill scores obtained using a much more complete set of stations for which forecasts were made.
The overall skill score for the pentad temperature forecasts made for January, April, July, and October is negative (−0.14), while those for pentad precipitation and individual year July precipitation forecasts are positive (0.12 and 0.04, respectively). The individual year January temperature forecast skill score was unchanged at −0.08 overall.
Abstract
No abstract available.
Abstract
No abstract available.
The 1976 version of the University of Wisconsin model's ultra long-range forecasts of monthly mean temperature and precipitation were verified for selected United States stations over the period 1976–80. In an overall sense, neither the pentad category forecasts for four months, nor the individual year forecasts for two months, showed significant skill relative to random chance expectation. Slight positive skill was found for the July precipitation forecasts. Considerable variability of skill scores were seen from one month type to another, and from year to year. The lack of demonstrated significant skill overall for the 1976–80 period contrasts with the positive results reported by the modelers for independent sample forecasts made for the period 1961–75.
The 1976 version of the University of Wisconsin model's ultra long-range forecasts of monthly mean temperature and precipitation were verified for selected United States stations over the period 1976–80. In an overall sense, neither the pentad category forecasts for four months, nor the individual year forecasts for two months, showed significant skill relative to random chance expectation. Slight positive skill was found for the July precipitation forecasts. Considerable variability of skill scores were seen from one month type to another, and from year to year. The lack of demonstrated significant skill overall for the 1976–80 period contrasts with the positive results reported by the modelers for independent sample forecasts made for the period 1961–75.
Abstract
The conservation of moisture requirement used in a hybrid Kuo-type cumulus parameterization scheme is generalized so that the source of moisture for the cumulus process originates from all layers below the level of condensation, including the subcloud layer(s). This conservation scheme is distinctly different than those used with the traditional Kuo-type cumulus parameterizations, which do not include convective-scale vertical transport involving the subcloud layer(s). Numerical forecasts with the modified conservation scheme are compared with those obtained using the conventional approach that extracts the moisture from the grid-scale moisture field at the level of condensation. Radiosonde observations and Geostationary Operational Environmental Satellite (GOES) observed brightness temperatures for water vapor channel 3 (6.7 μm) are used to verify the lower- and upper-tropospheric moisture fields, respectively.
Forecast statistics, including precipitation as measured against rain gauge reports, are all improved by using the generalized moisture conservation approach. Removing moisture from the subcloud layer(s) helps stabilize the sounding and promotes self-regulation of the convection. Including the subcloud layer(s) also alters the evolution and duration of some moist convective events. In contrast, an unregulated subcloud layer encourages the moist parameterization to produce excessive precipitation.
Abstract
The conservation of moisture requirement used in a hybrid Kuo-type cumulus parameterization scheme is generalized so that the source of moisture for the cumulus process originates from all layers below the level of condensation, including the subcloud layer(s). This conservation scheme is distinctly different than those used with the traditional Kuo-type cumulus parameterizations, which do not include convective-scale vertical transport involving the subcloud layer(s). Numerical forecasts with the modified conservation scheme are compared with those obtained using the conventional approach that extracts the moisture from the grid-scale moisture field at the level of condensation. Radiosonde observations and Geostationary Operational Environmental Satellite (GOES) observed brightness temperatures for water vapor channel 3 (6.7 μm) are used to verify the lower- and upper-tropospheric moisture fields, respectively.
Forecast statistics, including precipitation as measured against rain gauge reports, are all improved by using the generalized moisture conservation approach. Removing moisture from the subcloud layer(s) helps stabilize the sounding and promotes self-regulation of the convection. Including the subcloud layer(s) also alters the evolution and duration of some moist convective events. In contrast, an unregulated subcloud layer encourages the moist parameterization to produce excessive precipitation.
Abstract
The National Oceanic and Atmospheric Administration (NOAA) is transitioning the primary water level sensor at the majority of tide stations in the National Water Level Observation Network (NWLON) from an acoustic ranging system to a microwave radar system. Field comparison of the acoustic and microwave systems finds statistically equivalent performance when temperature gradients between the acoustic sensor and water surface are small and when significant wave height is less than roughly 0.5 m. When significant wave height is greater than approximately 0.5–1 m, the acoustic system consistently reports lower water levels. An analysis of 2 months of acoustic and microwave water level data at Duck, North Carolina, finds that the majority of differences between the two sensors can be attributed to systemic errors in the acoustic system and that the microwave system captures water level variability with higher fidelity than the acoustic system. NWLON real-time data products include the water level standard deviation, a statistic that can serve as a proxy for significant wave height. This study identifies 29 coastal water level stations that are candidates for monitoring wave height based on water level standard deviation, potentially adding a significant source of data for the sparsely sampled coastal wave fields around the United States, and finds that the microwave sensor is better suited than the acoustic system for wave height estimates.
Abstract
The National Oceanic and Atmospheric Administration (NOAA) is transitioning the primary water level sensor at the majority of tide stations in the National Water Level Observation Network (NWLON) from an acoustic ranging system to a microwave radar system. Field comparison of the acoustic and microwave systems finds statistically equivalent performance when temperature gradients between the acoustic sensor and water surface are small and when significant wave height is less than roughly 0.5 m. When significant wave height is greater than approximately 0.5–1 m, the acoustic system consistently reports lower water levels. An analysis of 2 months of acoustic and microwave water level data at Duck, North Carolina, finds that the majority of differences between the two sensors can be attributed to systemic errors in the acoustic system and that the microwave system captures water level variability with higher fidelity than the acoustic system. NWLON real-time data products include the water level standard deviation, a statistic that can serve as a proxy for significant wave height. This study identifies 29 coastal water level stations that are candidates for monitoring wave height based on water level standard deviation, potentially adding a significant source of data for the sparsely sampled coastal wave fields around the United States, and finds that the microwave sensor is better suited than the acoustic system for wave height estimates.
