ABSTRACT This empirical study demonstrates the feasibility of using 89 GHz Advanced Microwave Sca... more ABSTRACT This empirical study demonstrates the feasibility of using 89 GHz Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) passive microwave brightness temperature data to detect heavily drizzling cells within marine stratocumulus. A binary heavy drizzle product is described that can be used to determine areal and feature statistics of drizzle cells within the major marine stratocumulus regions. Current satellite liquid water path (LWP) and cloud radar products capable of detecting drizzle are either lacking in resolution (AMSR-E LWP), diurnal coverage (MODIS LWP), or spatial coverage (CloudSat). The AMSR-E 89 GHz data set at 6 × 4 km spatial resolution is sufficient for resolving individual heavily drizzling cells. Radiant emission at 89 GHz by liquid-water cloud and precipitation particles from drizzling cells in marine stratocumulus regions yields local maxima in brightness temperature against an otherwise cloud-free background brightness temperature. The background brightness temperature is primarily constrained by column-integrated water vapor and sea surface temperature. Clouds containing ice are screened out. Once heavily drizzling pixels are identified, connected pixels are grouped into discrete drizzle cell features. The identified drizzle cells are used in turn to determine several spatial statistics for each satellite scene, including drizzle cell number and size distribution. The identification of heavily drizzling cells within marine stratocumulus regions with satellite data facilitates analysis of seasonal and regional drizzle cell occurrence and the interrelation between drizzle and changes in cloud fraction.
ABSTRACT Forecasts of the amount and geographic distribution of snow are highly sensitive to a mo... more ABSTRACT Forecasts of the amount and geographic distribution of snow are highly sensitive to a model's parameterization of hydrometeor fallspeed. Riming is generally thought to lead to particles with a higher mass and terminal velocity. Yet models commonly assume that heavily rimed particles such as graupel have a fixed density and that their settling speed is unaffected by turbulence in storms. Here we show automated measurements of photographed hydrometeor shape and fallspeed using a new instrument placed in Utah's Wasatch Mountain Range. The data show that graupel in low turbulence conditions has a size-dependent fallspeed distribution with a mode near 1 m s–1, a result that is generally consistent with prior observations. However, the distributions are broadened by turbulence and there is a correspondence between particle density and air temperature. In high turbulence and at low temperatures, any sensitivity of fallspeed to particle size disappears.
ABSTRACT Photographs of nearly 73,000 snowflakes in free fall are used to determine the aspect ra... more ABSTRACT Photographs of nearly 73,000 snowflakes in free fall are used to determine the aspect ratio and orientation of aggregates, moderately rimed particles, and graupel. Observations indicate that there can be a much broader range of orientation angles, with a larger median value, than has been indicated by previous observational and theoretical studies. The data show that aspect ratio depends on riming extent but that orientation is only weakly dependent on the degree of riming and on particle size. Instead, more vertical orientations for frozen particles become increasingly common with higher turbulence. The results suggest that distributions of size, fall speed, orientation, and aspect ratio may each need to be considered in order to optimize the accuracy of precipitation retrievals using microwave sensors.
Recent studies evaluating the bulk microphysical schemes (BMPs) within cloud resolving models (CR... more Recent studies evaluating the bulk microphysical schemes (BMPs) within cloud resolving models (CRMs) have indicated large uncertainties and errors in the amount and size distributions of snow and cloud ice aloft. The snow prediction is sensitive to the snow densities, habits, and degree of riming within the BMPs. Improving these BMPs is a crucial step toward improving both weather forecasting and climate predictions. Several microphysical schemes in the Weather Research and Forecasting (WRF) model down to 1.33-km grid spacing are evaluated using aircraft, radar, and ground in situ data from the Global Precipitation Mission Cold-season Precipitation Experiment (GCPEx) experiment, as well as a few years (15 winter storms) of surface measurements of riming, crystal habit, snow density, and radar measurements at Stony Brook, NY (SBNY on north shore of Long Island) during the 2009-2012 winter seasons. Surface microphysical measurements at SBNY were taken every 15 to 30 minutes using a st...
Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport within extratropi... more Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport within extratropical cyclones. Although focused research during the last few years has yielded quantitative linkages between ARs and both the regional water supply and extreme precipitation events, questions remain regarding the modification and redistribution of precipitation in ARs by California's coastal mountains and Sierra Nevada. For example, rain rates are strongly correlated with the magnitude of the cross-barrier flow (and hence water vapor flux and uplift) at upper windward slope locations. However, at lowland locations they are poorly correlated due to the influence of low-level barrier airflows within the atmosphere. The most important of these barrier airflows is likely the Sierra Barrier Jet (SBJ), which is a dominant feature in Sierra Nevada storms. Winds reaching the mountain range often are partially stalled and, in part, diverted along the range rather than directed up and over. Kno...
Propagating disturbances in the tropical atmosphere which exhibit characteristics of linear equat... more Propagating disturbances in the tropical atmosphere which exhibit characteristics of linear equatorial waves have been shown to be ``coupled" to convection. Considering the difference in dynamical characteristics between these equatorial wave types, it follows that the ``coupled" convective dynamics might differ depending on wave type. Rain events spanning three years at Kwajalein Atoll, RMI, were classified by associated wave type (i.~e. Kelvin or mixed Rossby--gravity (MRG)) using space--time spectral--filtered outgoing longwave radiation (OLR). Contoured frequency by altitude diagrams (CFADs) of radar for the classified dates were compared between the two groups. Kelvin wave--associated rain days show a distinctly different vertical structure from MRG wave--associated rain days with a significant shift towards smaller reflectivity values at all heights. The mean rain--rate for Kelvin wave--associated storms was also higher than the MRG events despite a shift towards sma...
This paper investigates the structural and dynamical evolution of an intense mesoscale snowband o... more This paper investigates the structural and dynamical evolution of an intense mesoscale snowband occur-ring 25–26 December 2002 over the northeastern United States. Dual-Doppler, wind profiler, aircraft, and water vapor observations in concert with the fifth-generation Pennsylvania State University–NCAR Me-soscale Model run at 4-km grid spacing are used to highlight evolutionary aspects of a snowband unresolved by previous studies. The high-resolution observations and model simulations show that band formation was coincident with a sharpening of a midlevel trough and associated increase in frontogenesis in an environ-ment of conditional and inertial instability. Band maturity was marked by increasing conditional stability and a threefold increase in frontogenetical forcing. Band dissipation occurred as the midlevel trough and associated frontogenetical forcing weakened, while the conditional stability continued to increase. The effect of changing ascent is shown to dominate over chan...
The role drizzle plays in the dynamics of the stratocumulus (Sc) topped boundary layer is uncerta... more The role drizzle plays in the dynamics of the stratocumulus (Sc) topped boundary layer is uncertain and its parameterization difficult. Observations obtained during the 2001 East Pacific Investigation of Climate stratocumulus study (EPIC Sc) were designed to provide insight into the Sc physical processes with an eventual goal of improving model parameterizations. The unique EPIC data set combines comprehensive surface
Journal of Geophysical Research: Atmospheres, 2013
ABSTRACT Negative societal impacts can result from intense individual downpours, the accumulation... more ABSTRACT Negative societal impacts can result from intense individual downpours, the accumulation of rainfall over a day or more, or a combination of these. Accumulation is reasonably well captured by daily reporting rain gauges, but rainfall intensity is not. Ten years of data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) are used to describe the spatial and seasonal distributions of instantaneous rainfall intensity with an emphasis on how these differ from the distributions of mean daily accumulation. Over tropical land, the rainy season, when rainfall is most frequent, does not coincide with the highest mean intensity. Rather, intensity peaks just before the rainy season. This offset is most obvious in the pre-onset and post-onset months in monsoon regions and it is also evident in equatorial regions without a well-defined dry and rainy season. Most seasonal variations in rainfall intensity can be explained as parallel variations in the occurrence of convective, relative to stratiform, precipitation. However, regional differences in rainfall intensity are related to differences in the intensity of convection itself. Compared with seasonal changes in intensity over land, variations in convective precipitation fraction over tropical oceans are trivial, and the modest seasonal changes in the intensity of rainfall parallel those of frequency. These findings suggest that studies of precipitation extremes under global warming should (1) explicitly tackle the question of changes in the intensity of rainfall separately from changes in daily rainfall accumulation and (2) consider the different qualities of extreme precipitation events over ocean and over land.
ABSTRACT This empirical study demonstrates the feasibility of using 89 GHz Advanced Microwave Sca... more ABSTRACT This empirical study demonstrates the feasibility of using 89 GHz Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) passive microwave brightness temperature data to detect heavily drizzling cells within marine stratocumulus. A binary heavy drizzle product is described that can be used to determine areal and feature statistics of drizzle cells within the major marine stratocumulus regions. Current satellite liquid water path (LWP) and cloud radar products capable of detecting drizzle are either lacking in resolution (AMSR-E LWP), diurnal coverage (MODIS LWP), or spatial coverage (CloudSat). The AMSR-E 89 GHz data set at 6 × 4 km spatial resolution is sufficient for resolving individual heavily drizzling cells. Radiant emission at 89 GHz by liquid-water cloud and precipitation particles from drizzling cells in marine stratocumulus regions yields local maxima in brightness temperature against an otherwise cloud-free background brightness temperature. The background brightness temperature is primarily constrained by column-integrated water vapor and sea surface temperature. Clouds containing ice are screened out. Once heavily drizzling pixels are identified, connected pixels are grouped into discrete drizzle cell features. The identified drizzle cells are used in turn to determine several spatial statistics for each satellite scene, including drizzle cell number and size distribution. The identification of heavily drizzling cells within marine stratocumulus regions with satellite data facilitates analysis of seasonal and regional drizzle cell occurrence and the interrelation between drizzle and changes in cloud fraction.
ABSTRACT Forecasts of the amount and geographic distribution of snow are highly sensitive to a mo... more ABSTRACT Forecasts of the amount and geographic distribution of snow are highly sensitive to a model's parameterization of hydrometeor fallspeed. Riming is generally thought to lead to particles with a higher mass and terminal velocity. Yet models commonly assume that heavily rimed particles such as graupel have a fixed density and that their settling speed is unaffected by turbulence in storms. Here we show automated measurements of photographed hydrometeor shape and fallspeed using a new instrument placed in Utah's Wasatch Mountain Range. The data show that graupel in low turbulence conditions has a size-dependent fallspeed distribution with a mode near 1 m s–1, a result that is generally consistent with prior observations. However, the distributions are broadened by turbulence and there is a correspondence between particle density and air temperature. In high turbulence and at low temperatures, any sensitivity of fallspeed to particle size disappears.
ABSTRACT Photographs of nearly 73,000 snowflakes in free fall are used to determine the aspect ra... more ABSTRACT Photographs of nearly 73,000 snowflakes in free fall are used to determine the aspect ratio and orientation of aggregates, moderately rimed particles, and graupel. Observations indicate that there can be a much broader range of orientation angles, with a larger median value, than has been indicated by previous observational and theoretical studies. The data show that aspect ratio depends on riming extent but that orientation is only weakly dependent on the degree of riming and on particle size. Instead, more vertical orientations for frozen particles become increasingly common with higher turbulence. The results suggest that distributions of size, fall speed, orientation, and aspect ratio may each need to be considered in order to optimize the accuracy of precipitation retrievals using microwave sensors.
Recent studies evaluating the bulk microphysical schemes (BMPs) within cloud resolving models (CR... more Recent studies evaluating the bulk microphysical schemes (BMPs) within cloud resolving models (CRMs) have indicated large uncertainties and errors in the amount and size distributions of snow and cloud ice aloft. The snow prediction is sensitive to the snow densities, habits, and degree of riming within the BMPs. Improving these BMPs is a crucial step toward improving both weather forecasting and climate predictions. Several microphysical schemes in the Weather Research and Forecasting (WRF) model down to 1.33-km grid spacing are evaluated using aircraft, radar, and ground in situ data from the Global Precipitation Mission Cold-season Precipitation Experiment (GCPEx) experiment, as well as a few years (15 winter storms) of surface measurements of riming, crystal habit, snow density, and radar measurements at Stony Brook, NY (SBNY on north shore of Long Island) during the 2009-2012 winter seasons. Surface microphysical measurements at SBNY were taken every 15 to 30 minutes using a st...
Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport within extratropi... more Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport within extratropical cyclones. Although focused research during the last few years has yielded quantitative linkages between ARs and both the regional water supply and extreme precipitation events, questions remain regarding the modification and redistribution of precipitation in ARs by California's coastal mountains and Sierra Nevada. For example, rain rates are strongly correlated with the magnitude of the cross-barrier flow (and hence water vapor flux and uplift) at upper windward slope locations. However, at lowland locations they are poorly correlated due to the influence of low-level barrier airflows within the atmosphere. The most important of these barrier airflows is likely the Sierra Barrier Jet (SBJ), which is a dominant feature in Sierra Nevada storms. Winds reaching the mountain range often are partially stalled and, in part, diverted along the range rather than directed up and over. Kno...
Propagating disturbances in the tropical atmosphere which exhibit characteristics of linear equat... more Propagating disturbances in the tropical atmosphere which exhibit characteristics of linear equatorial waves have been shown to be ``coupled" to convection. Considering the difference in dynamical characteristics between these equatorial wave types, it follows that the ``coupled" convective dynamics might differ depending on wave type. Rain events spanning three years at Kwajalein Atoll, RMI, were classified by associated wave type (i.~e. Kelvin or mixed Rossby--gravity (MRG)) using space--time spectral--filtered outgoing longwave radiation (OLR). Contoured frequency by altitude diagrams (CFADs) of radar for the classified dates were compared between the two groups. Kelvin wave--associated rain days show a distinctly different vertical structure from MRG wave--associated rain days with a significant shift towards smaller reflectivity values at all heights. The mean rain--rate for Kelvin wave--associated storms was also higher than the MRG events despite a shift towards sma...
This paper investigates the structural and dynamical evolution of an intense mesoscale snowband o... more This paper investigates the structural and dynamical evolution of an intense mesoscale snowband occur-ring 25–26 December 2002 over the northeastern United States. Dual-Doppler, wind profiler, aircraft, and water vapor observations in concert with the fifth-generation Pennsylvania State University–NCAR Me-soscale Model run at 4-km grid spacing are used to highlight evolutionary aspects of a snowband unresolved by previous studies. The high-resolution observations and model simulations show that band formation was coincident with a sharpening of a midlevel trough and associated increase in frontogenesis in an environ-ment of conditional and inertial instability. Band maturity was marked by increasing conditional stability and a threefold increase in frontogenetical forcing. Band dissipation occurred as the midlevel trough and associated frontogenetical forcing weakened, while the conditional stability continued to increase. The effect of changing ascent is shown to dominate over chan...
The role drizzle plays in the dynamics of the stratocumulus (Sc) topped boundary layer is uncerta... more The role drizzle plays in the dynamics of the stratocumulus (Sc) topped boundary layer is uncertain and its parameterization difficult. Observations obtained during the 2001 East Pacific Investigation of Climate stratocumulus study (EPIC Sc) were designed to provide insight into the Sc physical processes with an eventual goal of improving model parameterizations. The unique EPIC data set combines comprehensive surface
Journal of Geophysical Research: Atmospheres, 2013
ABSTRACT Negative societal impacts can result from intense individual downpours, the accumulation... more ABSTRACT Negative societal impacts can result from intense individual downpours, the accumulation of rainfall over a day or more, or a combination of these. Accumulation is reasonably well captured by daily reporting rain gauges, but rainfall intensity is not. Ten years of data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) are used to describe the spatial and seasonal distributions of instantaneous rainfall intensity with an emphasis on how these differ from the distributions of mean daily accumulation. Over tropical land, the rainy season, when rainfall is most frequent, does not coincide with the highest mean intensity. Rather, intensity peaks just before the rainy season. This offset is most obvious in the pre-onset and post-onset months in monsoon regions and it is also evident in equatorial regions without a well-defined dry and rainy season. Most seasonal variations in rainfall intensity can be explained as parallel variations in the occurrence of convective, relative to stratiform, precipitation. However, regional differences in rainfall intensity are related to differences in the intensity of convection itself. Compared with seasonal changes in intensity over land, variations in convective precipitation fraction over tropical oceans are trivial, and the modest seasonal changes in the intensity of rainfall parallel those of frequency. These findings suggest that studies of precipitation extremes under global warming should (1) explicitly tackle the question of changes in the intensity of rainfall separately from changes in daily rainfall accumulation and (2) consider the different qualities of extreme precipitation events over ocean and over land.
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Papers by Sandra Yuter