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Abstract
A two-week prediction experiment was performed with the GISS atmospheric model on a global data set beginning 20 December 1972 to test the sensitivity of the model to sea-surface temperature (SST) variations. Use of observed SST's in place of climatological monthly mean sea temperatures for surface flux calculations in the model was found to have a marked local effect on predicted precipitation over the ocean, with enhanced convection over warm SST anomalies. However, use of observed SST's did not lead to any detectable general improvement in forecast skill. The influence of the SST anomalies on daily predicted fields of pressure and geopotential was small up to about one week compared with the growth of prediction error, and no greater over a two-week period than that resulting from random errors in the initial meteorological state. The 14-day average fields of sea-level pressure and 500 mb height predicted by the model were similarly insensitive to the SST anomalies.
Abstract
A two-week prediction experiment was performed with the GISS atmospheric model on a global data set beginning 20 December 1972 to test the sensitivity of the model to sea-surface temperature (SST) variations. Use of observed SST's in place of climatological monthly mean sea temperatures for surface flux calculations in the model was found to have a marked local effect on predicted precipitation over the ocean, with enhanced convection over warm SST anomalies. However, use of observed SST's did not lead to any detectable general improvement in forecast skill. The influence of the SST anomalies on daily predicted fields of pressure and geopotential was small up to about one week compared with the growth of prediction error, and no greater over a two-week period than that resulting from random errors in the initial meteorological state. The 14-day average fields of sea-level pressure and 500 mb height predicted by the model were similarly insensitive to the SST anomalies.
Abstract
In the case of a nadir-looking spaceborne or aircraft radar in the presence of rain the return power corresponding to secondary surface scattering may provide information on the properties of the surface and the precipitation. The object of the study is to evaluate a method for determining simultaneously the rainfall rate and the back-scattering coefficient of the surface, σ0. The method is based upon the mirror-reflected power, Pm , which corresponds to the portion of the incident power scattered from the surface to the precipitation, intercepted by the precipitation, and again returned to the surface where it is scattered a final time back to the antenna. Two approximations are obtained for Pm depending on whether the held of view at the surface is either much greater or much less than the height of the reflection layer. Since the dependence of Pm on the backscattering coefficient of the surface differs in the two cases, two algorithms are given by which the path-averaged rain rate and σ0 can be deduced. We also discuss the delectability of Pm , the relative strength of other contributions to the return power arriving simultaneously with Pm , and the validity of the approximations used in deriving Pm .
Abstract
In the case of a nadir-looking spaceborne or aircraft radar in the presence of rain the return power corresponding to secondary surface scattering may provide information on the properties of the surface and the precipitation. The object of the study is to evaluate a method for determining simultaneously the rainfall rate and the back-scattering coefficient of the surface, σ0. The method is based upon the mirror-reflected power, Pm , which corresponds to the portion of the incident power scattered from the surface to the precipitation, intercepted by the precipitation, and again returned to the surface where it is scattered a final time back to the antenna. Two approximations are obtained for Pm depending on whether the held of view at the surface is either much greater or much less than the height of the reflection layer. Since the dependence of Pm on the backscattering coefficient of the surface differs in the two cases, two algorithms are given by which the path-averaged rain rate and σ0 can be deduced. We also discuss the delectability of Pm , the relative strength of other contributions to the return power arriving simultaneously with Pm , and the validity of the approximations used in deriving Pm .
Abstract
In this article two subsynoptic-scale cyclones that developed between 3 and 10 October 1996 over the western-central Mediterranean, causing floods, strong winds, and severe damage, are analyzed. Surface observations reveal that the accumulated rainfall at Santuario di Polsi (southern Calabria, Italy) is more than 480 mm for the first event (cyclone 9610A). The second cyclone (9610B) was characterized by a storm track predominantly over the sea, thus causing less recorded precipitation, but stronger wind. Satellite imagery shows two intensely convective vortices with a scale of 200–400 km and a spiral structure, with the cyclone 9610B displaying a well-defined eyelike feature.
The corresponding National Centers for Environmental Prediction analyses, although limited by 1° resolution, confirm the cyclones’ positions and intensities, as they can be inferred from satellite imagery, SSM/I data, and observations, but display also the “signature” of two tropical cyclone–like vortices, including a perfect alignment between the cutoffs at all levels with the surface center, and a warm core. The wind speed cross sections in the meridional and zonal directions through the eyelike feature reveal a virtually motionless column of air. A comparison with the cross sections taken in the same analyses across a named tropical storm in the Atlantic show a strong analogy between the gridded representation of these events.
Other remarkable features include very strong horizontal shear in the midtroposphere, and simultaneous lack of vertical shear; increasing low-level vorticity at the expenses of upper-level vorticity; creation of a low-level vorticity maximum; and finally strong low-level convergence and upper-level divergence during the onset and development of each cyclone.
Abstract
In this article two subsynoptic-scale cyclones that developed between 3 and 10 October 1996 over the western-central Mediterranean, causing floods, strong winds, and severe damage, are analyzed. Surface observations reveal that the accumulated rainfall at Santuario di Polsi (southern Calabria, Italy) is more than 480 mm for the first event (cyclone 9610A). The second cyclone (9610B) was characterized by a storm track predominantly over the sea, thus causing less recorded precipitation, but stronger wind. Satellite imagery shows two intensely convective vortices with a scale of 200–400 km and a spiral structure, with the cyclone 9610B displaying a well-defined eyelike feature.
The corresponding National Centers for Environmental Prediction analyses, although limited by 1° resolution, confirm the cyclones’ positions and intensities, as they can be inferred from satellite imagery, SSM/I data, and observations, but display also the “signature” of two tropical cyclone–like vortices, including a perfect alignment between the cutoffs at all levels with the surface center, and a warm core. The wind speed cross sections in the meridional and zonal directions through the eyelike feature reveal a virtually motionless column of air. A comparison with the cross sections taken in the same analyses across a named tropical storm in the Atlantic show a strong analogy between the gridded representation of these events.
Other remarkable features include very strong horizontal shear in the midtroposphere, and simultaneous lack of vertical shear; increasing low-level vorticity at the expenses of upper-level vorticity; creation of a low-level vorticity maximum; and finally strong low-level convergence and upper-level divergence during the onset and development of each cyclone.
Abstract
A procedure is proposed to expand the diagnostic capabilities of the pressure tendency equation of a primitive equation NWP model by computing the pressure tendency in physical coordinates. The advantage of isolating the density advection as a diagnostic tool to understand pressure changes is shown.
By simple thermodynamic arguments it is demonstrated that in areas of synoptic-scale cyclonic development, the vertically integrated density advection is more than sufficient to explain the depletion of mass over a growing depression. Consequently, the joint contribution of the net divergence and vertical motion opposes the pressure fall. This is illustrated for a case of rapid cyclogenesis in southern South America.
Abstract
A procedure is proposed to expand the diagnostic capabilities of the pressure tendency equation of a primitive equation NWP model by computing the pressure tendency in physical coordinates. The advantage of isolating the density advection as a diagnostic tool to understand pressure changes is shown.
By simple thermodynamic arguments it is demonstrated that in areas of synoptic-scale cyclonic development, the vertically integrated density advection is more than sufficient to explain the depletion of mass over a growing depression. Consequently, the joint contribution of the net divergence and vertical motion opposes the pressure fall. This is illustrated for a case of rapid cyclogenesis in southern South America.
Abstract
The GISS general circulation model is used to compute global monthly mean forecasts for January 1973, 1974 and 1975 from initial conditions on the first day of each month, with ocean surface fluxes based on climatological mean January sea-surface temperatures. Forecasts are evaluated in terms of global and hemispheric energetics, zonally averaged meridional and vertical profiles, forecast error statistics, and monthly mean synoptic fields. Although it generates a realistic mean meridional structure for the month of January, the model does not adequately reproduce the observed interannual variations in the large-scale monthly mean energetics and zonally averaged circulation. The model exhibits no general skill in predicting the monthly mean sea-level pressure field, but it does simulate observed changes in the intensity of the Icelandic low from year to year. For each January the model produces a prognostic monthly mean 500 mb height field that is superior to climatology and persistence.
The impact of temporal sea-surface temperature variations on monthly mean global forecasts with the GISS model is investigated by comparing two parallel forecasts for January 1974, one using climatological ocean temperatures for the surface flux computations and the other observed daily ocean temperatures. In the one case studied, the use of daily-updated sea-surface temperatures produced no discernable beneficial effect on the forecasts, and the total impact on the large-scale pressure and temperature fields was small.
Abstract
The GISS general circulation model is used to compute global monthly mean forecasts for January 1973, 1974 and 1975 from initial conditions on the first day of each month, with ocean surface fluxes based on climatological mean January sea-surface temperatures. Forecasts are evaluated in terms of global and hemispheric energetics, zonally averaged meridional and vertical profiles, forecast error statistics, and monthly mean synoptic fields. Although it generates a realistic mean meridional structure for the month of January, the model does not adequately reproduce the observed interannual variations in the large-scale monthly mean energetics and zonally averaged circulation. The model exhibits no general skill in predicting the monthly mean sea-level pressure field, but it does simulate observed changes in the intensity of the Icelandic low from year to year. For each January the model produces a prognostic monthly mean 500 mb height field that is superior to climatology and persistence.
The impact of temporal sea-surface temperature variations on monthly mean global forecasts with the GISS model is investigated by comparing two parallel forecasts for January 1974, one using climatological ocean temperatures for the surface flux computations and the other observed daily ocean temperatures. In the one case studied, the use of daily-updated sea-surface temperatures produced no discernable beneficial effect on the forecasts, and the total impact on the large-scale pressure and temperature fields was small.
Abstract
Q-vector partitioning has proven to be a useful tool for the understanding of the frictionless, adiabatic processes responsible for the generation of synoptic-scale vertical motion in the extratropical atmosphere. Partitioning of Q into components parallel and normal to the isotherms on an isobaric surface is standard practice in studies dealing with vertical motion and frontogenesis. This paper is concerned with vertical motion only and examines the consequences of projecting Q onto isohypses, instead of isotherms, on an isobaric surface. Specifically, the Q vector is partitioned in the natural coordinate system that follows the geostrophic wind. The novelty with this partitioning is that it naturally leads to the evaluation of different vertical motion forcing mechanisms, among which are those related to flow curvature and to confluence or diffluence. This evaluation is illustrated by applying the new Q-vector partition to a gridded analysis of a real weather situation. An important conclusion is that the thermal advection by horizontal geostrophic shear is as significant to the forcing of vertical motion as the geostrophic confluence/diffluence. While this result has previously been obtained in the study of frontal dynamics, this is the first application of this finding to the synoptic scale.
Abstract
Q-vector partitioning has proven to be a useful tool for the understanding of the frictionless, adiabatic processes responsible for the generation of synoptic-scale vertical motion in the extratropical atmosphere. Partitioning of Q into components parallel and normal to the isotherms on an isobaric surface is standard practice in studies dealing with vertical motion and frontogenesis. This paper is concerned with vertical motion only and examines the consequences of projecting Q onto isohypses, instead of isotherms, on an isobaric surface. Specifically, the Q vector is partitioned in the natural coordinate system that follows the geostrophic wind. The novelty with this partitioning is that it naturally leads to the evaluation of different vertical motion forcing mechanisms, among which are those related to flow curvature and to confluence or diffluence. This evaluation is illustrated by applying the new Q-vector partition to a gridded analysis of a real weather situation. An important conclusion is that the thermal advection by horizontal geostrophic shear is as significant to the forcing of vertical motion as the geostrophic confluence/diffluence. While this result has previously been obtained in the study of frontal dynamics, this is the first application of this finding to the synoptic scale.
Abstract
As operational forecast and data assimilation (DA) systems evolve, observing system simulation experiment (OSSE) systems must evolve in parallel. Expected development of operational systems—especially the use of data that are currently not used or are just beginning to be used, such as all-sky and surface-affected microwave radiances—will greatly challenge our ability to construct realistic OSSE systems. An additional set of challenges will arise when future DA systems strongly couple the different Earth system components. In response, future OSSE systems will require coupled models to simulate nature and coupled observation simulators. The requirements for future evolving OSSE systems and potential solutions to satisfy these requirements are discussed. It is anticipated that in the future the OSSE technique will be applied to diverse and coupled domains with the use of increasingly advanced and sophisticated simulations of nature and observations.
Abstract
As operational forecast and data assimilation (DA) systems evolve, observing system simulation experiment (OSSE) systems must evolve in parallel. Expected development of operational systems—especially the use of data that are currently not used or are just beginning to be used, such as all-sky and surface-affected microwave radiances—will greatly challenge our ability to construct realistic OSSE systems. An additional set of challenges will arise when future DA systems strongly couple the different Earth system components. In response, future OSSE systems will require coupled models to simulate nature and coupled observation simulators. The requirements for future evolving OSSE systems and potential solutions to satisfy these requirements are discussed. It is anticipated that in the future the OSSE technique will be applied to diverse and coupled domains with the use of increasingly advanced and sophisticated simulations of nature and observations.
Abstract
This study demonstrates that Global Hawk unmanned aircraft system dropwindsondes and Atmospheric Infrared Sounder (AIRS) observations can be complementary in sampling a tropical cyclone (TC). The assimilation of both datasets in a regional ensemble data assimilation system shows that the cumulative impact of both datasets is greater than either one alone because of the presence of mutually independent information content. The experiment that assimilates both datasets has smaller position and intensity errors in the mean analysis than those with individual datasets. The improvements in track and intensity forecasts that result from combining both datasets also indicate synergistic benefits. Overall, superior track and intensity forecasts are evident. This study suggests that polar-orbiting satellite spatial coverage should be considered in operational reconnaissance mission planning in order to achieve further improvements in TC analyses and forecasts.
Abstract
This study demonstrates that Global Hawk unmanned aircraft system dropwindsondes and Atmospheric Infrared Sounder (AIRS) observations can be complementary in sampling a tropical cyclone (TC). The assimilation of both datasets in a regional ensemble data assimilation system shows that the cumulative impact of both datasets is greater than either one alone because of the presence of mutually independent information content. The experiment that assimilates both datasets has smaller position and intensity errors in the mean analysis than those with individual datasets. The improvements in track and intensity forecasts that result from combining both datasets also indicate synergistic benefits. Overall, superior track and intensity forecasts are evident. This study suggests that polar-orbiting satellite spatial coverage should be considered in operational reconnaissance mission planning in order to achieve further improvements in TC analyses and forecasts.
Abstract
Quasi-Lagrangian diagnostics of mass, angular momentum, water vapor, and kinetic energy are evaluated for four different Goddard Laboratory for Atmospheres model simulations of the Queen Elizabeth II storm of 9–11 September 1978 to study the impact of Seasat-A satellite Scatterometer (SASS) winds and horizontal resolution in numerical prediction. In a four-way comparison, the diagnostics investigate the impact of including dealiased SASS winds in the initial conditions of the model and doubling the horizontal resolution on 36 h simulations of the QE II storm. The largest impact on the simulation stemmed from doubling the model's horizontal resolution from 4° × 5° to 2° × 2.5°. The increased resolution resulted in a storm track much closer to that observed, a much deeper surface development, a stronger mass circulation, stronger heating, and stronger increase of angular momentum. The inclusion of SASS data resulted in an approximately 2–3-mb-deeper surface cyclone for both the 2° × 2.5° and 4° × 5° resolution simulations. The inclusion also led to substantial increases in the horizontal mass circulation and heating for the 2° × 2.5° simulation. During the early explosive deepening phase of the cyclone, the inward lateral transport of water vapor in lower layers was larger in the 2° × 2.5° SASS than in the 2° × 2.5° NOSASS (exclusion of SASS surface winds) simulation. During the period of most rapid development, the results from the SASS simulation revealed a larger generation of kinetic energy throughout the troposphere and increased outward transport of kinetic energy in upper layers.
Abstract
Quasi-Lagrangian diagnostics of mass, angular momentum, water vapor, and kinetic energy are evaluated for four different Goddard Laboratory for Atmospheres model simulations of the Queen Elizabeth II storm of 9–11 September 1978 to study the impact of Seasat-A satellite Scatterometer (SASS) winds and horizontal resolution in numerical prediction. In a four-way comparison, the diagnostics investigate the impact of including dealiased SASS winds in the initial conditions of the model and doubling the horizontal resolution on 36 h simulations of the QE II storm. The largest impact on the simulation stemmed from doubling the model's horizontal resolution from 4° × 5° to 2° × 2.5°. The increased resolution resulted in a storm track much closer to that observed, a much deeper surface development, a stronger mass circulation, stronger heating, and stronger increase of angular momentum. The inclusion of SASS data resulted in an approximately 2–3-mb-deeper surface cyclone for both the 2° × 2.5° and 4° × 5° resolution simulations. The inclusion also led to substantial increases in the horizontal mass circulation and heating for the 2° × 2.5° simulation. During the early explosive deepening phase of the cyclone, the inward lateral transport of water vapor in lower layers was larger in the 2° × 2.5° SASS than in the 2° × 2.5° NOSASS (exclusion of SASS surface winds) simulation. During the period of most rapid development, the results from the SASS simulation revealed a larger generation of kinetic energy throughout the troposphere and increased outward transport of kinetic energy in upper layers.
Abstract
In the absence of a current capability for global routine daily soil moisture observation, an infrared technique using existing instrumentation is sought. Numerical modeling results are reported from a pilot study, the purpose of which was to develop such a technique and to determine the quality and reliability of soil moisture information which it can produce.
In order to determine which physical parameters observable from GOES are most sensitive to soil moisture and which are less prone to interference by seasonal changes, atmospheric effects, vegetation cover, etc., a detailed one-dimensional boundary layer-surface-soil model was employed. The model is described briefly. Results of sensitivity tests are presented which show that the mid-morning differential of surface temperature with respect to absorbed solar radiation is optimally sensitive to soil moisture. A case study comparing model results with GOES infrared data confirms the sensitivity of this parameter to soil moisture and also establishes the applicability of the model to predicting area-averaged surface temperature changes.
A series of model runs were then used to develop a simulated surface temperature dataset from which a soil moisture algorithm was developed. This algorithm uses only GOES observations to separate the soil moisture signal from the interfering effects on the surface temperature. It is shown that soil moisture can be most accurately estimated by this method in dry or marginal agricultural areas where drought is a frequent threat. Sources of error, including the effects of advection and clouds, are examined and methods of minimizing errors are discussed.
Abstract
In the absence of a current capability for global routine daily soil moisture observation, an infrared technique using existing instrumentation is sought. Numerical modeling results are reported from a pilot study, the purpose of which was to develop such a technique and to determine the quality and reliability of soil moisture information which it can produce.
In order to determine which physical parameters observable from GOES are most sensitive to soil moisture and which are less prone to interference by seasonal changes, atmospheric effects, vegetation cover, etc., a detailed one-dimensional boundary layer-surface-soil model was employed. The model is described briefly. Results of sensitivity tests are presented which show that the mid-morning differential of surface temperature with respect to absorbed solar radiation is optimally sensitive to soil moisture. A case study comparing model results with GOES infrared data confirms the sensitivity of this parameter to soil moisture and also establishes the applicability of the model to predicting area-averaged surface temperature changes.
A series of model runs were then used to develop a simulated surface temperature dataset from which a soil moisture algorithm was developed. This algorithm uses only GOES observations to separate the soil moisture signal from the interfering effects on the surface temperature. It is shown that soil moisture can be most accurately estimated by this method in dry or marginal agricultural areas where drought is a frequent threat. Sources of error, including the effects of advection and clouds, are examined and methods of minimizing errors are discussed.
