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- Author or Editor: A. R. Robinson x
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Abstract
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Abstract
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Accurate monitoring of the large-scale dimensions of global snow cover is essential for understanding details of climate dynamics and climate change. Presently, such information is gathered individually from ground station networks and satellite platforms. Efforts are in progress to consolidate and analyze long-term station records from a number of countries. To gain truly global coverage, however, satellite-based monitoring techniques must be employed. A 27-year record of Northern Hemisphere continental snow cover produced by the National Oceanic and Atmospheric Administration (NOAA) is the longest such environmental record available. Records of Southern Hemisphere continental cover and snow on top of Arctic sea ice have been produced by similar means for a portion of this interval. The visible imagery charting technique used to generate these data provides information on snow extent but not on snow volume. Satellite microwave analyses over Northern Hemisphere lands show some promise in this regard, however, large-scale monitoring of snow extent with microwave data remains less accurate than visible charting.
This paper updates the status of global snow cover monitoring, concentrating on the weekly snow charts prepared by NOAA and discussing a new and consistent record of monthly snow cover generated from these weekly charts. The NOAA charts show a reduction of hemispheric snow cover over the past five years, particularly in spring. Snow areas from the NOAA product are then compared with values derived using passive microwave data. The latter consistently reports less snow cover than the more accurate visible product. Finally, future snow monitoring initiatives are recommended. These include continuing the consistent NOAA product until an all-weather all-surface product is developed. The latter would use multiple data sources and geographic information systems techniques. Such an integrative product would need extensive comparisons with the NOAA product to ensure the continued utility of the lengthy NOAA observations in studies of climate change. In a retrospective sense, satellite charts from the middle 1960s to early 1970s need reevaluation and techniques to merge satellite products with historic station time series must be developed.
Accurate monitoring of the large-scale dimensions of global snow cover is essential for understanding details of climate dynamics and climate change. Presently, such information is gathered individually from ground station networks and satellite platforms. Efforts are in progress to consolidate and analyze long-term station records from a number of countries. To gain truly global coverage, however, satellite-based monitoring techniques must be employed. A 27-year record of Northern Hemisphere continental snow cover produced by the National Oceanic and Atmospheric Administration (NOAA) is the longest such environmental record available. Records of Southern Hemisphere continental cover and snow on top of Arctic sea ice have been produced by similar means for a portion of this interval. The visible imagery charting technique used to generate these data provides information on snow extent but not on snow volume. Satellite microwave analyses over Northern Hemisphere lands show some promise in this regard, however, large-scale monitoring of snow extent with microwave data remains less accurate than visible charting.
This paper updates the status of global snow cover monitoring, concentrating on the weekly snow charts prepared by NOAA and discussing a new and consistent record of monthly snow cover generated from these weekly charts. The NOAA charts show a reduction of hemispheric snow cover over the past five years, particularly in spring. Snow areas from the NOAA product are then compared with values derived using passive microwave data. The latter consistently reports less snow cover than the more accurate visible product. Finally, future snow monitoring initiatives are recommended. These include continuing the consistent NOAA product until an all-weather all-surface product is developed. The latter would use multiple data sources and geographic information systems techniques. Such an integrative product would need extensive comparisons with the NOAA product to ensure the continued utility of the lengthy NOAA observations in studies of climate change. In a retrospective sense, satellite charts from the middle 1960s to early 1970s need reevaluation and techniques to merge satellite products with historic station time series must be developed.
Abstract
The skill with which amplitudes of quasi-geostrophic modes can be estimated is important in the analysis and modeling of data from mixed CTD/XBT surveys. Here, several methods for estimation of quasi-geostrophic vertical mode amplitudes (QGMs) are compared, both in the context of idealized estimation and (especially) in application to some recent CTD and XBT data from the California Current Systems (CCS). The methods compared are: 1) direct least-squares fitting by QGMs (LSF); 2) projection of “empirical orthogonal function” amplitudes onto QGM amplitudes at each station (EOF); 3) ridge regression (RR); 4) an “optimal estimate” using covariances between QGM amplitudes (OE); and 5) another optimal estimate using covariances between EOF amplitudes and QGM amplitudes (CEOF). For deep CTD casts (>1500 m), all methods perform well. For shallow CTD and XBT casts (<750 m), method five (CEOF) is recommended, using EOFs and amplitude covariances derived from just the deeper CTD casts. Since low-frequency internal waves have the same modal structure for density as the QGMS, they are not distinguishable from the QGMs in the present analysis. The analysis is applied to a recent survey to produce amplitude maps for the first few baroclinic modes. Comparisons with another survey indicate that the density analysis is transportable, but the T-S characteristics are so variable that the temperature analysis is not (the surveys are approximately three months apart).
Abstract
The skill with which amplitudes of quasi-geostrophic modes can be estimated is important in the analysis and modeling of data from mixed CTD/XBT surveys. Here, several methods for estimation of quasi-geostrophic vertical mode amplitudes (QGMs) are compared, both in the context of idealized estimation and (especially) in application to some recent CTD and XBT data from the California Current Systems (CCS). The methods compared are: 1) direct least-squares fitting by QGMs (LSF); 2) projection of “empirical orthogonal function” amplitudes onto QGM amplitudes at each station (EOF); 3) ridge regression (RR); 4) an “optimal estimate” using covariances between QGM amplitudes (OE); and 5) another optimal estimate using covariances between EOF amplitudes and QGM amplitudes (CEOF). For deep CTD casts (>1500 m), all methods perform well. For shallow CTD and XBT casts (<750 m), method five (CEOF) is recommended, using EOFs and amplitude covariances derived from just the deeper CTD casts. Since low-frequency internal waves have the same modal structure for density as the QGMS, they are not distinguishable from the QGMs in the present analysis. The analysis is applied to a recent survey to produce amplitude maps for the first few baroclinic modes. Comparisons with another survey indicate that the density analysis is transportable, but the T-S characteristics are so variable that the temperature analysis is not (the surveys are approximately three months apart).
Abstract
We present here a regional, eddy resolving, numerical study of the dynamics of Gulf Stream Meander and Ring (GSMR) interaction processes. We initialize the Harvard quasi-geostrophic open-boundary model with realistic meander and ring locations as indicated by remotely sensed sea surface temperature (SST) data and predict the flow evolution for the period 23 November to 19 December 1984. The methodology of Feature-Model initialization is introduced to extend the surface information to the thermocline and deep levels in terms of climatological structures, which are then dynamically adjusted by the model. Six numerical simulators are carried out to explore the influence of initial and boundary conditions on the flow evolution. All of the major events observed in the SST data are simulated, including the birth of new warm and cold core rings. The results show the relevance of quasi-geostrophic dynamics for the GSMR region on these time scales in the thermocline. A set of parameter and sensitivity experiments then elucidate the dependence on physical parameters; ring births are nonlinear baroclinic processes. The dynamics of these realistic cold and warm core formation events are quantified via local energy and vorticity budget analyses (EVA). The cold core case involves a process of nonlinear baroclinic cascades that convert available gravitational energy to kinetic energy and vice versa. The warm core case involves a differential horizontal advection process.
Abstract
We present here a regional, eddy resolving, numerical study of the dynamics of Gulf Stream Meander and Ring (GSMR) interaction processes. We initialize the Harvard quasi-geostrophic open-boundary model with realistic meander and ring locations as indicated by remotely sensed sea surface temperature (SST) data and predict the flow evolution for the period 23 November to 19 December 1984. The methodology of Feature-Model initialization is introduced to extend the surface information to the thermocline and deep levels in terms of climatological structures, which are then dynamically adjusted by the model. Six numerical simulators are carried out to explore the influence of initial and boundary conditions on the flow evolution. All of the major events observed in the SST data are simulated, including the birth of new warm and cold core rings. The results show the relevance of quasi-geostrophic dynamics for the GSMR region on these time scales in the thermocline. A set of parameter and sensitivity experiments then elucidate the dependence on physical parameters; ring births are nonlinear baroclinic processes. The dynamics of these realistic cold and warm core formation events are quantified via local energy and vorticity budget analyses (EVA). The cold core case involves a process of nonlinear baroclinic cascades that convert available gravitational energy to kinetic energy and vice versa. The warm core case involves a differential horizontal advection process.
Abstract
This is the first part of a three-part study on the circulation, dynamics, and mesoscale forecasting of the western North Atlantic. The overall objective of this series of studies is threefold: 1) to present a methodology for deriving a dynamically balanced regional climatology that maintains the synoptic structure of the permanent fronts embedded in a mean background circulation, 2) to present a methodology for using such a regional climatology for calibrating and validating dynamical models, and 3) to use similarly derived synoptic realizations as initialization and assimilation fields for mesoscale nowcasting and forecasting.
In this paper, a data-based, kinematically balanced circulation model for the western North Atlantic is developed and described. The various multiscale synoptic and general circulation structures in this region are represented by analytical and analytical/empirical functions based on dynamical considerations and using observational datasets. These include the jet-scale currents, namely, the Gulf Stream and the deep western boundary current, the subbasin-scale recirculating gyres called the southern and the northern recirculation gyres, and the slope water gyre. The inclusion of subbasin-scale gyres as the background circulation for the energetic jet and mesoscale activity in any limited oceanic region is a new paradigm of this multiscale regional modeling study. A generalized kinematical constraint that links the multiscale structures is derived in terms of their interaction scales. For synoptic realizations, the currents and gyres are distorted from their mean state with mass conserving constraints, and mesoscale structures are added thereon. The kinematically balanced linked system is then adjusted via quasigeostrophic dynamics and a regional water-mass model to obtain three-dimensional circulation fields to be used for initialization and assimilation in primitive equation models.
Abstract
This is the first part of a three-part study on the circulation, dynamics, and mesoscale forecasting of the western North Atlantic. The overall objective of this series of studies is threefold: 1) to present a methodology for deriving a dynamically balanced regional climatology that maintains the synoptic structure of the permanent fronts embedded in a mean background circulation, 2) to present a methodology for using such a regional climatology for calibrating and validating dynamical models, and 3) to use similarly derived synoptic realizations as initialization and assimilation fields for mesoscale nowcasting and forecasting.
In this paper, a data-based, kinematically balanced circulation model for the western North Atlantic is developed and described. The various multiscale synoptic and general circulation structures in this region are represented by analytical and analytical/empirical functions based on dynamical considerations and using observational datasets. These include the jet-scale currents, namely, the Gulf Stream and the deep western boundary current, the subbasin-scale recirculating gyres called the southern and the northern recirculation gyres, and the slope water gyre. The inclusion of subbasin-scale gyres as the background circulation for the energetic jet and mesoscale activity in any limited oceanic region is a new paradigm of this multiscale regional modeling study. A generalized kinematical constraint that links the multiscale structures is derived in terms of their interaction scales. For synoptic realizations, the currents and gyres are distorted from their mean state with mass conserving constraints, and mesoscale structures are added thereon. The kinematically balanced linked system is then adjusted via quasigeostrophic dynamics and a regional water-mass model to obtain three-dimensional circulation fields to be used for initialization and assimilation in primitive equation models.
Abstract
We present the results of a multi-level, constant depth, primitive equation general ocean numerical circulation simulation with mesoscale resolution. A single mid-latitude model gyre is driven by wind and heating. After 30 years of spin-up with a relatively coarse grid and large diffusion coefficients, the grid size and diffusion coefficients are reduced. The circulation then adjusts into a nonlinear and time-dependent flow with periods of tens of days and space scales of hundreds of kilometers. After a quasi-equilibrium state is achieved, two years of data are obtained which are separated into time-mean and time-dependent fluctuations, and analyzed. Dynamically distinct regions are intensified, momentum, heat and vorticity balances examined, and energy integrals calculated. Statistical measures of significance and of uncertainty are computed where possible. Eddy energy is produced primarily by Reynolds stress work (barotropic instability) on the mean circulation shear in the recirculation and near-field region of the northern current system. Mean fluctuation correlation terms are presented in some regions at order 1 in the mean heat and vorticity balance and can be the leading ageostrophic effect in the mean momentum balance. The flow is non-quasigeostrophic in some parts of the intense boundary currents.
Abstract
We present the results of a multi-level, constant depth, primitive equation general ocean numerical circulation simulation with mesoscale resolution. A single mid-latitude model gyre is driven by wind and heating. After 30 years of spin-up with a relatively coarse grid and large diffusion coefficients, the grid size and diffusion coefficients are reduced. The circulation then adjusts into a nonlinear and time-dependent flow with periods of tens of days and space scales of hundreds of kilometers. After a quasi-equilibrium state is achieved, two years of data are obtained which are separated into time-mean and time-dependent fluctuations, and analyzed. Dynamically distinct regions are intensified, momentum, heat and vorticity balances examined, and energy integrals calculated. Statistical measures of significance and of uncertainty are computed where possible. Eddy energy is produced primarily by Reynolds stress work (barotropic instability) on the mean circulation shear in the recirculation and near-field region of the northern current system. Mean fluctuation correlation terms are presented in some regions at order 1 in the mean heat and vorticity balance and can be the leading ageostrophic effect in the mean momentum balance. The flow is non-quasigeostrophic in some parts of the intense boundary currents.
Abstract
Spring snowmelt onset has occurred earlier across much of the Northern Hemisphere land area in the last four decades. Understanding the mechanisms driving spring melt has remained a challenge, particularly in its spatial and temporal variability. Here, melt onset dates (MOD) obtained from passive microwave satellite data are used, as well as energy balance and meteorological fields from NASA’s Modern-Era Retrospective Analysis for Research and Applications, to assess trends in the MOD and attribute melt onset across much of Arctic and sub-Arctic Eurasia and North America during the spring snowmelt season from 1979 to 2012. Across much of the Northern Hemisphere MOD has occurred 1–2 weeks earlier over this period, with the strongest trends in western and central Russia and insignificant trends across most of North America. Trends in MOD are reflected by those in energy balance terms, with energy advection providing an increasing proportion of melt energy in regions with the strongest MOD trends. Energy advection plays a larger role in melt onset in regions where snow begins melting in March and April, while insolation and longwave radiation drives melt where the MOD occurs in May and June. This implies that there is a potential shift in snowmelt drivers toward those involved in advective processes rather than radiative processes with an earlier MOD. As the high latitudes warm and terrestrial snow cover continues to melt and disappear earlier in the spring, it is valuable to elucidate regional snowmelt sensitivities to better understand regional responses to changing climatological processes.
Abstract
Spring snowmelt onset has occurred earlier across much of the Northern Hemisphere land area in the last four decades. Understanding the mechanisms driving spring melt has remained a challenge, particularly in its spatial and temporal variability. Here, melt onset dates (MOD) obtained from passive microwave satellite data are used, as well as energy balance and meteorological fields from NASA’s Modern-Era Retrospective Analysis for Research and Applications, to assess trends in the MOD and attribute melt onset across much of Arctic and sub-Arctic Eurasia and North America during the spring snowmelt season from 1979 to 2012. Across much of the Northern Hemisphere MOD has occurred 1–2 weeks earlier over this period, with the strongest trends in western and central Russia and insignificant trends across most of North America. Trends in MOD are reflected by those in energy balance terms, with energy advection providing an increasing proportion of melt energy in regions with the strongest MOD trends. Energy advection plays a larger role in melt onset in regions where snow begins melting in March and April, while insolation and longwave radiation drives melt where the MOD occurs in May and June. This implies that there is a potential shift in snowmelt drivers toward those involved in advective processes rather than radiative processes with an earlier MOD. As the high latitudes warm and terrestrial snow cover continues to melt and disappear earlier in the spring, it is valuable to elucidate regional snowmelt sensitivities to better understand regional responses to changing climatological processes.
Real-time operational shipboard forecasts of Iceland–Faeroe frontal variability were executed for the first time with a primitive equation model. High quality, intensive hydrographic surveys during August 1993 were used for initialization, updating, and validation of the forecasts. Vigorous and rapid synoptic events occurred over several-day timescales including a southeastward reorientation of the Iceland–Faeroe Front and the development of a strong, cold deep-sock meander. A qualitative and quantitative assessment of the skill of these forecasts shows they captured the essential features of both events. The anomaly pattern correlation coefficient and the rms error between forecast and observed fields are particularly impressive (and substantially superior to persistence) for the forecast of the cold meander.
Real-time operational shipboard forecasts of Iceland–Faeroe frontal variability were executed for the first time with a primitive equation model. High quality, intensive hydrographic surveys during August 1993 were used for initialization, updating, and validation of the forecasts. Vigorous and rapid synoptic events occurred over several-day timescales including a southeastward reorientation of the Iceland–Faeroe Front and the development of a strong, cold deep-sock meander. A qualitative and quantitative assessment of the skill of these forecasts shows they captured the essential features of both events. The anomaly pattern correlation coefficient and the rms error between forecast and observed fields are particularly impressive (and substantially superior to persistence) for the forecast of the cold meander.
Abstract
Altimeter data obtained from GEOS-3 during the three year period 1975–78 for a region of the western North Atlantic which includes a portion of the Gulf Stream system and part of the open ocean area of the subtropical gyre are analyzed by a new technique which utilizes all the points along the satellite tracks. The physical phenomenon studied are the time-variable but almost geostrophic currents, or mesoscale eddies, so that geoid errors contaminate the scientific signal minimally and the dynamical interpretation is direct. Results presented include the spatial distribution of geostrophic eddy kinetic energy and examples of a synoptic map of the eddy field (April 1977) and of a time series at a point. These results are compared to and synthesized with a diverse and selected set of existing measurements and observations obtained in situ by a variety of instrumental techniques. The agreement is generally good, and the altimeter data analyzed provides new information on features in the map of mean eddy kinetic energy. The implications are that satellite altimetry will serve as a powerful quantitative tool in eddy current research and that even presently archived data contains further useful scientific information.
Abstract
Altimeter data obtained from GEOS-3 during the three year period 1975–78 for a region of the western North Atlantic which includes a portion of the Gulf Stream system and part of the open ocean area of the subtropical gyre are analyzed by a new technique which utilizes all the points along the satellite tracks. The physical phenomenon studied are the time-variable but almost geostrophic currents, or mesoscale eddies, so that geoid errors contaminate the scientific signal minimally and the dynamical interpretation is direct. Results presented include the spatial distribution of geostrophic eddy kinetic energy and examples of a synoptic map of the eddy field (April 1977) and of a time series at a point. These results are compared to and synthesized with a diverse and selected set of existing measurements and observations obtained in situ by a variety of instrumental techniques. The agreement is generally good, and the altimeter data analyzed provides new information on features in the map of mean eddy kinetic energy. The implications are that satellite altimetry will serve as a powerful quantitative tool in eddy current research and that even presently archived data contains further useful scientific information.
Creation and Validation of a Comprehensive 1° by 1° Daily Gridded North American Dataset for 1900–2009: SnowfallAbstract
This study details the creation of a gridded snowfall dataset for North America, with focus on the quality of the interpolated product. Daily snowfall amounts from National Weather Service Cooperative Observer Program stations and Meteorological Service of Canada surface stations are interpolated to 1° by 1° grids from 1900 to 2009 and examined for data record length and quality. The interpolation is validated spatially and temporally through the use of stratified sampling and k-fold cross-validation analyses. Interpolation errors average around 0.5 cm and range from less than 0.01 to greater than 2.5 cm. For most locations, this is within the measurement sensitivity. Grid cells with large variations in elevation experience higher errors and should be used with caution. A new gridded snowfall climatology is presented based on in situ observations that capture seasonal and interannual variability in monthly snowfall over most of the North American land area from 1949 to 2009. The Community Collaborative Rain, Hail and Snow Network is used as an independent set of point data that is compared to the gridded product. Errors are mainly in the form of the gridded data underestimating snowfall compared to the point data. The spatial extent, temporal length, and resolution of the dataset are unprecedented with regard to observational snowfall products and will present new opportunities for examining snowfall across North America.
Abstract
This study details the creation of a gridded snowfall dataset for North America, with focus on the quality of the interpolated product. Daily snowfall amounts from National Weather Service Cooperative Observer Program stations and Meteorological Service of Canada surface stations are interpolated to 1° by 1° grids from 1900 to 2009 and examined for data record length and quality. The interpolation is validated spatially and temporally through the use of stratified sampling and k-fold cross-validation analyses. Interpolation errors average around 0.5 cm and range from less than 0.01 to greater than 2.5 cm. For most locations, this is within the measurement sensitivity. Grid cells with large variations in elevation experience higher errors and should be used with caution. A new gridded snowfall climatology is presented based on in situ observations that capture seasonal and interannual variability in monthly snowfall over most of the North American land area from 1949 to 2009. The Community Collaborative Rain, Hail and Snow Network is used as an independent set of point data that is compared to the gridded product. Errors are mainly in the form of the gridded data underestimating snowfall compared to the point data. The spatial extent, temporal length, and resolution of the dataset are unprecedented with regard to observational snowfall products and will present new opportunities for examining snowfall across North America.
