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Abstract
A nonlinear balance condition, which permits the diagnosis of both balanced divergent and nondivergent flows, is presented. This analysis approach is applied to the results of a numerical simulation of a midlatitude mesoscale convective complex (MCC) to assess the degree of balance of these and similar convective weather systems.
It is found that, to a large extent, the simulated MCC represents a highly balanced fluid system. The nondivergent component of the MCC wind field was found to be largely balanced from the time of initial convection to dissipation. Perhaps more surprisingly, the storm-induced divergent model winds are also balanced to a fair degree, though certainly less so than the nondivergent flow. Further, the balanced divergent flow makes up a significant portion of the total balanced flow in some regions of the MCC. System-scale divergence profiles of the model and balanced winds are compared and found to agree reasonably well, especially in the growth and mature stages of the MCC.
Within a stationary averaging volume enclosing the MCC, the greatest disparity between the model and balanced circulations is found in the downward vertical motion. The model downward mass flux significantly exceeds the balanced downward flux at most times during the simulation, suggesting that the process of mass adjustment due to convective heating is largely dominated by unbalanced fast-manifold processes, such as inertia–gravity waves. The unbalanced flow is found to be composed largely of divergent circulations of periodic nature (i.e., gravity waves). The appearance and characteristics of these features are found to be in good agreement with current theoretical predictions regarding the atmospheric response to convective heating and associated compensating subsidence.
The (modified) Rossby radii (λ R ) for two lowest-order gravity wave modes are calculated. The mesoscale convective vortex (MCV) within the storm is larger than λ R for all but the gravest mode. The λ R n for the mature storm as an ensemble also indicates a good degree of balance with λ R n=1 scaling similar to the MCC and larger values of n scaling smaller than the system as a whole. These λ R values strongly suggest that this simulated MCC represents an inertially stable balanced mesoscale convective system.
Abstract
A nonlinear balance condition, which permits the diagnosis of both balanced divergent and nondivergent flows, is presented. This analysis approach is applied to the results of a numerical simulation of a midlatitude mesoscale convective complex (MCC) to assess the degree of balance of these and similar convective weather systems.
It is found that, to a large extent, the simulated MCC represents a highly balanced fluid system. The nondivergent component of the MCC wind field was found to be largely balanced from the time of initial convection to dissipation. Perhaps more surprisingly, the storm-induced divergent model winds are also balanced to a fair degree, though certainly less so than the nondivergent flow. Further, the balanced divergent flow makes up a significant portion of the total balanced flow in some regions of the MCC. System-scale divergence profiles of the model and balanced winds are compared and found to agree reasonably well, especially in the growth and mature stages of the MCC.
Within a stationary averaging volume enclosing the MCC, the greatest disparity between the model and balanced circulations is found in the downward vertical motion. The model downward mass flux significantly exceeds the balanced downward flux at most times during the simulation, suggesting that the process of mass adjustment due to convective heating is largely dominated by unbalanced fast-manifold processes, such as inertia–gravity waves. The unbalanced flow is found to be composed largely of divergent circulations of periodic nature (i.e., gravity waves). The appearance and characteristics of these features are found to be in good agreement with current theoretical predictions regarding the atmospheric response to convective heating and associated compensating subsidence.
The (modified) Rossby radii (λ R ) for two lowest-order gravity wave modes are calculated. The mesoscale convective vortex (MCV) within the storm is larger than λ R for all but the gravest mode. The λ R n for the mature storm as an ensemble also indicates a good degree of balance with λ R n=1 scaling similar to the MCC and larger values of n scaling smaller than the system as a whole. These λ R values strongly suggest that this simulated MCC represents an inertially stable balanced mesoscale convective system.
Abstract
A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23–24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nested-grid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically reported in the mature and decaying stages of the life cycle. Several hours later, a second MCV formed near the opposite end of the convective line, and by the mature phase of the MCC, these MCVs were embedded within a more complex system-wide vortical flow in the lower troposphere.
Analysis of the first MCV during its incipient phase indicates that the vortex initially formed near the surface by convergence/stretching of the large low-level ambient vertical vorticity in this region. Vertical advection appeared largely responsible for the upward extension of this MCV to about 3.5 km above the surface, with tilting of horizontal vorticity playing a secondary role. This mechanism of MCV formation is in contrast to recent idealized high-resolution squall line simulations, where MCVs were found to result from the tilting into the vertical of storm-induced horizontal vorticity formed near the top of the cold pool.
Another interesting aspect of the simulation was the development of a banded vorticity structure at midtropospheric levels. These bands were found to be due to the apparent vertical transport of zonal momentum by the descending rear-to-front circulation, or rear-inflow jet. An equivalent alternative viewpoint of this process, deformation of horizontal vorticity filaments by the convective updrafts and rear-inflow jet, is discussed.
Part II of this work presents a complementary approach to the analysis presented here, demonstrating that the circulations seen in this MCC simulation are, to a large degree, contained within the nonlinear balance approximation, the related balanced omega equation, and the PV as analyzed from the PE model results.
Abstract
A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23–24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nested-grid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically reported in the mature and decaying stages of the life cycle. Several hours later, a second MCV formed near the opposite end of the convective line, and by the mature phase of the MCC, these MCVs were embedded within a more complex system-wide vortical flow in the lower troposphere.
Analysis of the first MCV during its incipient phase indicates that the vortex initially formed near the surface by convergence/stretching of the large low-level ambient vertical vorticity in this region. Vertical advection appeared largely responsible for the upward extension of this MCV to about 3.5 km above the surface, with tilting of horizontal vorticity playing a secondary role. This mechanism of MCV formation is in contrast to recent idealized high-resolution squall line simulations, where MCVs were found to result from the tilting into the vertical of storm-induced horizontal vorticity formed near the top of the cold pool.
Another interesting aspect of the simulation was the development of a banded vorticity structure at midtropospheric levels. These bands were found to be due to the apparent vertical transport of zonal momentum by the descending rear-to-front circulation, or rear-inflow jet. An equivalent alternative viewpoint of this process, deformation of horizontal vorticity filaments by the convective updrafts and rear-inflow jet, is discussed.
Part II of this work presents a complementary approach to the analysis presented here, demonstrating that the circulations seen in this MCC simulation are, to a large degree, contained within the nonlinear balance approximation, the related balanced omega equation, and the PV as analyzed from the PE model results.
Abstract
Rainfall-generated floods in the Arctic are rare and seldom documented. The authors were fortunate in July 1999 to monitor such a flood on the Upper Kuparuk River in response to a 50-h duration rainfall event that produced a watershed average in excess of 80 mm. Atmospheric conditions prevailed that allowed moist air to move northward over areas of little or no vertical relief from the North Pacific Ocean to the Arctic Ocean. Cyclogenesis occurred along the quasi-stationary front separating maritime and continental air masses along the arctic coast. This low-pressure system propagated southward (inland) over the 142-km2 headwater basin of the Kuparuk River in the northern foothills of the Brooks Range; a treeless area underlain by continuous permafrost. This research catchment was instrumented with a stream gauging station, two major and six minor meteorological stations, for a total of eight shielded rain gauges. The peak instantaneous flow was estimated at 100 m3 s−1 and was about 3 times greater than any previously measured flood peak. Historically in the Arctic, annual peak floods occur following snowmelt when the snowpack that has accumulated for 8–9 months typically melts in 7–14 days. The shallow active layer, that surficial layer that freezes and thaws each year over the continuous permafrost, has limited subsurface storage when only thawed to a depth of 40 cm (at the time of the flood). Typically for this area, the ratio of runoff volume to snowmelt volume is near 0.67 or greater and the ratio for cumulative summer runoff and rainfall averages around 0.5 or greater. For the storm discussed here the runoff ratio was 0.73. These high runoff ratios are due to the role of permafrost limiting the potential subsurface storage and the steep slopes of this headwater basin.
Abstract
Rainfall-generated floods in the Arctic are rare and seldom documented. The authors were fortunate in July 1999 to monitor such a flood on the Upper Kuparuk River in response to a 50-h duration rainfall event that produced a watershed average in excess of 80 mm. Atmospheric conditions prevailed that allowed moist air to move northward over areas of little or no vertical relief from the North Pacific Ocean to the Arctic Ocean. Cyclogenesis occurred along the quasi-stationary front separating maritime and continental air masses along the arctic coast. This low-pressure system propagated southward (inland) over the 142-km2 headwater basin of the Kuparuk River in the northern foothills of the Brooks Range; a treeless area underlain by continuous permafrost. This research catchment was instrumented with a stream gauging station, two major and six minor meteorological stations, for a total of eight shielded rain gauges. The peak instantaneous flow was estimated at 100 m3 s−1 and was about 3 times greater than any previously measured flood peak. Historically in the Arctic, annual peak floods occur following snowmelt when the snowpack that has accumulated for 8–9 months typically melts in 7–14 days. The shallow active layer, that surficial layer that freezes and thaws each year over the continuous permafrost, has limited subsurface storage when only thawed to a depth of 40 cm (at the time of the flood). Typically for this area, the ratio of runoff volume to snowmelt volume is near 0.67 or greater and the ratio for cumulative summer runoff and rainfall averages around 0.5 or greater. For the storm discussed here the runoff ratio was 0.73. These high runoff ratios are due to the role of permafrost limiting the potential subsurface storage and the steep slopes of this headwater basin.
Mixed-phase stratus clouds are ubiquitous in the Arctic and play an important role in climate in this region. However, climate and regional models have generally proven unsuccessful at simulating Arctic cloudiness, particularly during the colder months. Specifically, models tend to underpredict the amount of liquid water in mixed-phase clouds. The Mixed-Phase Arctic Cloud Experiments (M-PACE), conducted from late September through October 2004 in the vicinity of the Department of Energy's Atmospheric Radiation Measurement (ARM) North Slope of Alaska field site, focused on characterizing low-level Arctic stratus clouds. Ice nuclei (IN) measurements were made using a continuous-flow ice thermal diffusion chamber aboard the University of North Dakota's Citation II aircraft. These measurements indicated IN concentrations that were significantly lower than those used in many models. Using the Regional Atmospheric Modeling System (RAMS), we show that these low IN concentrations, as well as inadequate parameterizations of the depletion of IN through nucleation scavenging, may be partially responsible for the poor model predictions. Moreover, we show that this can lead to errors in the modeled surface radiative energy budget of 10–100 Wm−2. Finally, using the measured IN concentrations as input to RAMS and comparing to a mixed-phase cloud observed during M-PACE, we show excellent agreement between modeled and observed liquid water content and net infrared surface flux.
Mixed-phase stratus clouds are ubiquitous in the Arctic and play an important role in climate in this region. However, climate and regional models have generally proven unsuccessful at simulating Arctic cloudiness, particularly during the colder months. Specifically, models tend to underpredict the amount of liquid water in mixed-phase clouds. The Mixed-Phase Arctic Cloud Experiments (M-PACE), conducted from late September through October 2004 in the vicinity of the Department of Energy's Atmospheric Radiation Measurement (ARM) North Slope of Alaska field site, focused on characterizing low-level Arctic stratus clouds. Ice nuclei (IN) measurements were made using a continuous-flow ice thermal diffusion chamber aboard the University of North Dakota's Citation II aircraft. These measurements indicated IN concentrations that were significantly lower than those used in many models. Using the Regional Atmospheric Modeling System (RAMS), we show that these low IN concentrations, as well as inadequate parameterizations of the depletion of IN through nucleation scavenging, may be partially responsible for the poor model predictions. Moreover, we show that this can lead to errors in the modeled surface radiative energy budget of 10–100 Wm−2. Finally, using the measured IN concentrations as input to RAMS and comparing to a mixed-phase cloud observed during M-PACE, we show excellent agreement between modeled and observed liquid water content and net infrared surface flux.
