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Abstract
Mesoscale water and energy budgets are diagnosed for a squall line during the Convection and Precipitation Electrification Experiment and combined with the results of the two-dimensional Goddard Cumulus Ensemble Model. The fine temporal and spatial resolution of cloud-scale processes contained in the model is used to reduce uncertainty in the diagnosed water budget residual and, thus, to arrive at a good estimate of storm-total rainfall. Profiles of cumulus heating (Q 1) and drying (Q 2) inferred from the sounding observations are in turn compared with the cloud-scale energy budget terms calculated from the model. This comparison reveals near-agreement in the magnitude and vertical distribution of the peak Q 1 and Q 2, and also the relative size of the heating and drying at different levels in the column.
When the size of the mesoscale convective disturbance is approximately the same as the sounding observation network, it may be wrong to assume that the diagnosed vertical eddy heat transport accounts for most of the total eddy transport of moist static energy, F. The cloud model is used to resolve the relative contribution of the horizontal and vertical eddy flux convergence of heat and moisture, and thus it serves as a guide to interpreting the sounding-diagnosed total flux. The model results suggest that although the mean column vertical flux convergence is significantly larger than the column-mean horizontal flux convergence, the horizontal flux convergence does play a significant role in midlevels of the convective region. This flux convergence may be associated with a strong front-to-rear inflow that develops during the mature stage of the squall line.
This study suggests that when combined with the independent results of a mesoscale cloud model, the sounding diagnostics can provide a sensitivity test for the Tropical Rainfall Measuring Mission measurements of rainfall and diabatic heating over the life cycle of an entire mesoscale convective system.
Abstract
Mesoscale water and energy budgets are diagnosed for a squall line during the Convection and Precipitation Electrification Experiment and combined with the results of the two-dimensional Goddard Cumulus Ensemble Model. The fine temporal and spatial resolution of cloud-scale processes contained in the model is used to reduce uncertainty in the diagnosed water budget residual and, thus, to arrive at a good estimate of storm-total rainfall. Profiles of cumulus heating (Q 1) and drying (Q 2) inferred from the sounding observations are in turn compared with the cloud-scale energy budget terms calculated from the model. This comparison reveals near-agreement in the magnitude and vertical distribution of the peak Q 1 and Q 2, and also the relative size of the heating and drying at different levels in the column.
When the size of the mesoscale convective disturbance is approximately the same as the sounding observation network, it may be wrong to assume that the diagnosed vertical eddy heat transport accounts for most of the total eddy transport of moist static energy, F. The cloud model is used to resolve the relative contribution of the horizontal and vertical eddy flux convergence of heat and moisture, and thus it serves as a guide to interpreting the sounding-diagnosed total flux. The model results suggest that although the mean column vertical flux convergence is significantly larger than the column-mean horizontal flux convergence, the horizontal flux convergence does play a significant role in midlevels of the convective region. This flux convergence may be associated with a strong front-to-rear inflow that develops during the mature stage of the squall line.
This study suggests that when combined with the independent results of a mesoscale cloud model, the sounding diagnostics can provide a sensitivity test for the Tropical Rainfall Measuring Mission measurements of rainfall and diabatic heating over the life cycle of an entire mesoscale convective system.
Abstract
Several sensitivity tests are performed to assess the effect of melting processes on the development of a midlatitude continental squall line and a tropical oceanic squall line. It is found that melting processes play an important role in the structure of a midlatitude continental squall system. For the maritime tropical case, squall development is not as sensitive to the presence of melting, due to the dominance of warm rain processes.
Melting processes exert an influence on midlatitude cloud system development through the conversion of ice particles to rain. The simulated convective system was found to be much weaker in the absence of evaporative cooling by rain. For a given vertical shear of horizontal wind, cooling by evaporation in the convective region was found to be essential for maintaining a long-lived cloud system. Diabatic cooling by melting played only a secondary role in this respect. In the absence of melting processes, the simulated mildlatitude squall system acquired the characteristics of unicell-type (erect and steady) convection rather than the observed multicellular (upsher tilt) structure. This suggests that the diabatic cooling by melting can have significant impact on the structure (dynamics) of a simulated midlatitude squall system. In addition, results from air parcel trajectory analyses indicate that jump-type downdrafts that originate either from the convective region or from above the melting level in the stratiform region are not simulated for convection that develops in the absence of melting.
The horizontal momentum transport associated with the midlatitude squall system simulation were quite different in the presence and absence of melting. Significant horizontal momentum transport by convection was not observed in the absence of melting. However, an upper-level jet was simulated in the case where melting processes were active. It is also found that the horizontal perturbed pressure gradient force is comparable in magnitude yet almost always opposite in sign to the vertical transport effect by clouds.
Abstract
Several sensitivity tests are performed to assess the effect of melting processes on the development of a midlatitude continental squall line and a tropical oceanic squall line. It is found that melting processes play an important role in the structure of a midlatitude continental squall system. For the maritime tropical case, squall development is not as sensitive to the presence of melting, due to the dominance of warm rain processes.
Melting processes exert an influence on midlatitude cloud system development through the conversion of ice particles to rain. The simulated convective system was found to be much weaker in the absence of evaporative cooling by rain. For a given vertical shear of horizontal wind, cooling by evaporation in the convective region was found to be essential for maintaining a long-lived cloud system. Diabatic cooling by melting played only a secondary role in this respect. In the absence of melting processes, the simulated mildlatitude squall system acquired the characteristics of unicell-type (erect and steady) convection rather than the observed multicellular (upsher tilt) structure. This suggests that the diabatic cooling by melting can have significant impact on the structure (dynamics) of a simulated midlatitude squall system. In addition, results from air parcel trajectory analyses indicate that jump-type downdrafts that originate either from the convective region or from above the melting level in the stratiform region are not simulated for convection that develops in the absence of melting.
The horizontal momentum transport associated with the midlatitude squall system simulation were quite different in the presence and absence of melting. Significant horizontal momentum transport by convection was not observed in the absence of melting. However, an upper-level jet was simulated in the case where melting processes were active. It is also found that the horizontal perturbed pressure gradient force is comparable in magnitude yet almost always opposite in sign to the vertical transport effect by clouds.
Abstract
A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to estimate the heating (Q 1, moisture (Q 2), and water budgets in the convective and stratiform regions for a tropical and a midlatitude squall line (EMEX and PRE-STORM). The model is anelastic and includes a parameterized three-class ice-phase microphysical scheme and longwave radiative transfer processes. A quantitative estimate of the impact of the longwave radiative cooling on the total surface precipitation as well as on the development and structure of these two squall lines is presented.
It was found that the vertical eddy moisture fluxes are a major contribution to the model-derived Q 2 budgets in both squall cases. A distinct midlevel minimum in the Q 2 profile for the EMEX case is due to vertical eddy transport in the convective region. On the other hand, the contribution to the Q 1 budget by the cloud-scale fluxes is minor for the EMEX case. In contrast, the vertical eddy heat flux is relatively important for the PRE-STORM case due to the stronger vertical velocities present in the PRE-STORM convective cells. It was found that the convective region plays an important role in the generation of stratiform rainfall for both cases. Although the EMEX case has more stratiform rainfall than its PRE-STORM counterpart, the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is less. But the transfer of condensate from the convective region became relatively less important with time in the stratiform water budget of the PRE-STORM system as it developed from its initial stage, such that the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is similar at the mature stages of both systems.
Longwave radiative cooling enhanced the total surface precipitation about 14% and 31% over a 16-h simulation time for the PRE-STORM and EMEX cases, respectively. The relative contribution to the stratiform water budget from the convective region is, however, more sensitive to the longwave radiative cooling for the PRE-STORM case than for the EMEX case. These results are due to the relatively moist environment and comparatively earlier development of the stratiform cloud in the EMEX squall system. Nevertheless, the effect of radiative cooling is shown to increase as systems age in both cases. It was also determined that the Q 1 and Q 2 budgets in the convective and stratiform regions are only quantitatively, not qualitatively, altered by the inclusion or exclusion of longwave radiative transfer processes.
Abstract
A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to estimate the heating (Q 1, moisture (Q 2), and water budgets in the convective and stratiform regions for a tropical and a midlatitude squall line (EMEX and PRE-STORM). The model is anelastic and includes a parameterized three-class ice-phase microphysical scheme and longwave radiative transfer processes. A quantitative estimate of the impact of the longwave radiative cooling on the total surface precipitation as well as on the development and structure of these two squall lines is presented.
It was found that the vertical eddy moisture fluxes are a major contribution to the model-derived Q 2 budgets in both squall cases. A distinct midlevel minimum in the Q 2 profile for the EMEX case is due to vertical eddy transport in the convective region. On the other hand, the contribution to the Q 1 budget by the cloud-scale fluxes is minor for the EMEX case. In contrast, the vertical eddy heat flux is relatively important for the PRE-STORM case due to the stronger vertical velocities present in the PRE-STORM convective cells. It was found that the convective region plays an important role in the generation of stratiform rainfall for both cases. Although the EMEX case has more stratiform rainfall than its PRE-STORM counterpart, the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is less. But the transfer of condensate from the convective region became relatively less important with time in the stratiform water budget of the PRE-STORM system as it developed from its initial stage, such that the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is similar at the mature stages of both systems.
Longwave radiative cooling enhanced the total surface precipitation about 14% and 31% over a 16-h simulation time for the PRE-STORM and EMEX cases, respectively. The relative contribution to the stratiform water budget from the convective region is, however, more sensitive to the longwave radiative cooling for the PRE-STORM case than for the EMEX case. These results are due to the relatively moist environment and comparatively earlier development of the stratiform cloud in the EMEX squall system. Nevertheless, the effect of radiative cooling is shown to increase as systems age in both cases. It was also determined that the Q 1 and Q 2 budgets in the convective and stratiform regions are only quantitatively, not qualitatively, altered by the inclusion or exclusion of longwave radiative transfer processes.
A severe 5-day lake-effect storm resulted in eight deaths, hundreds of injuries, and over $3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30–90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic-scale, mesoscale, and microscale processes.
An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake-Induced Convection Experiment (Lake-ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake-effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of these collaborative projects. Preliminary findings are given with illustrative examples of new state-of-the-art research observations collected. Analyses associated with Lake-ICE and SNOWBAND hold the promise of greatly improving our scientific understanding of processes involved in these important wintertime phenomena.
A severe 5-day lake-effect storm resulted in eight deaths, hundreds of injuries, and over $3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30–90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic-scale, mesoscale, and microscale processes.
An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake-Induced Convection Experiment (Lake-ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake-effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of these collaborative projects. Preliminary findings are given with illustrative examples of new state-of-the-art research observations collected. Analyses associated with Lake-ICE and SNOWBAND hold the promise of greatly improving our scientific understanding of processes involved in these important wintertime phenomena.
