Abstract

A detailed ice-phase bulk microphysical scheme has been developed for simulating the hydrometeor distributions of convective and stratiform precipitation in different large-scale environmental conditions. The proposed scheme involves 90 distinct microphysical processes, which predict the mixing ratios and the number concentrations of small ice crystals, snow, graupel, and frozen drops/hail, as well as the mixing ratios of liquid water on wet precipitation ice (snow, graupel, frozen drops). The number of adjustable coefficients has been significantly reduced in comparison with other bulk schemes. Additional improvements have been made to the parameterization in the following areas: 1) representing small ice crystals with nonzero terminal fall velocities and dispersive size distributions, 2) accurate and computationally efficient calculations of precipitation collection processes, 3) reformulating the collection equation to prevent unrealistically large accretion rates, 4) more realistic conversion by riming between different classes of precipitation ice, 5) preventing unrealistically large rates of raindrop freezing and freezing of liquid water on ice, 6) detailed treatment of various rime-splintering ice multiplication mechanisms, 7) a simple representation of the Hobbs-Rangno ice enhancement process, 8) aggregation of small ice crystals and snow, and 9) allowing explicit competition between cloud water condensation and ice deposition rates rather than using saturation adjustment techniques. For the purposes of conserving the higher moments of the particle distributions, preserving the spectral widths (or slopes) of the particle spectra is shown to be more important than strict conservation of particle number concentration when parameterizing changes in ice-particle number concentrations due to melting, vapor transfer processes (sublimation of dry ice, evaporation from wet ice), and conversion between different hydrometeor species.

The microphysical scheme is incorporated into a nonhydrostatic cloud model in Part II of this study. The model performed well in simulating the radar and microphysical structures of a midlatitude–continental squall line and a tropical–maritime squall system with minimal tuning of the parameterization, even though the vertical profiles of radar reflectivity differed substantially between these storms.

Abstract

A vertical distribution formulation of liquid water content (LWC) for steady radiation fog was obtained and examined through the singular perturbation method. The asymptotic LWC distribution is a consequential balance among cooling, droplet gravitational settling, and turbulence in the liquid water budget of radiation fog. The cooling produces liquid water, which is depleted by turbulence near the surface. The influence of turbulence on the liquid water budget decreases with height and is more significant for shallow fogs than for deep fogs. The depth of the region of surface-induced turbulence can be characterized with a fog boundary layer (FBL). The behavior of the FBL bears some resemblance to the surface mixing layer in radiation fog. The characteristic depth of the FBL is thinner for weaker turbulence and stronger cooling, whereas if turbulence intensity increases or cooling rate decreases then the FBL will develop from the ground. The asymptotic formulation also reveals a critical turbulent exchange coefficient for radiation fog that defines the upper bound of turbulence intensity that a steady fog can withstand. The deeper a fog is, the stronger a turbulence intensity it can endure. The persistence condition for a steady fog can be parameterized by either the critical turbulent exchange coefficient or the characteristic depth of the FBL. If the turbulence intensity inside a fog is smaller than the turbulence threshold, the fog persists, whereas if the turbulence intensity exceeds the turbulence threshold or the characteristic depth of the FBL dominates the entire fog bank then the balance will be destroyed, leading to dissipation of the existing fog. The asymptotic formulation has a first-order approximation with respect to turbulence intensity. Verifications with numerical solutions and an observed fog event showed that it is more accurate for weak turbulence than for strong turbulence and that the computed LWC generally agrees with the observed LWC in magnitude.

Abstract

Different definitions of storm precipitation efficiency were investigated from numerical simulators of convective systems in widely varying ambient conditions using a two-dimensional cloud model with sophisticated ice microphysics. The model results indicate that the vertical orientation of the updrafts, which is controlled by the vertical wind shear, and the ambient moisture content are important in determining storm efficiency.

In terms of rainfall divided by condensation, simulated efficiencies ranged from 20%–35% for convective systems that tilted strongly against the low-level shear (upshear), to 40%–50% for erect storms. Changes in environmental moisture produced smaller variations in efficiency that were less than 10%. Upright convection allows for effective collection of cloud condensate by precipitation, whereas lower efficiencies in upshear storms are due to greater evaporation of cloud at middle levels and evaporation of rain at lower levels. Development of trailing stratiform precipitation is promoted by the rearward transport of moisture and condensate in upshear-tilted updrafts with evaporation moistening the ambient air as it passes through the convection. The fraction of rainfall from stratiform processes increases with upshear tilt of the convection and is inefficient. Rainfall from convection tilting downshear is efficient in terms of the total condensation, but is inefficient in terms of the flux of vapor into the storm because the gust fronts are too weak to completely block the low-level inflow.

Different closure assumptions in cumulus parameterization schemes that use functional relationships for precipitation efficiency were evaluated. None of them showed consistent agreement with the efficiency parameters diagnosed from the simulations.

Detailed diagnostics over various temporal and spatial scales indicate that storm efficiency determined by total condensation varied much less than that obtained from moisture convergence. The former definition should be more useful in cumulus parameterizations. Spatial variations in moisture convergence were dominated by changes in net condensation within the area of the storm, while variability at larger scales resulted from the advection of dry air in downdraft wakes.

Abstract

A one-dimensional time-dependent cumulonimbus model is designed that, unlike in previous one-dimensional models, simulates cloud-top heights, vertical velocities, and water contents that are reasonably consistent with those observed in real convective cores. The model successfully simulates deep tropical oceanic cumulonimbus with results that are in agreement with aircraft observations of vertical velocity, observations of radar reflectivity, and three-dimensional model simulations. These results are achieved by improving the parameterizations of the following physical processes: vertical mixing through the inclusion of an overturning thermal circulation near cloud top, lateral entrainment by modifying the assumed shape of the cloud, initiating convection with sustained boundary-layer forcing that resembles the lifting by gust fronts associated with tropical oceanic cumulonimbus, and making the pressure perturbation internally consistent with the horizontal distribution of vertical velocity in the cloud. The effect of a tilted updraft on precipitation fallout and enhanced cloud growth are also examined.

Abstract

Central Florida is the ideal test laboratory for studying convergence zone–induced convection. The region regularly experiences sea-breeze fronts and rainfall-induced outflow boundaries. The focus of this study is convection associated with the commonly occurring convergence zone established by the interaction of the sea-breeze front and an outflow boundary. Previous studies have investigated mechanisms primarily affecting storm initiation by such convergence zones. Few have focused on rainfall morphology, yet these storms contribute a significant amount of precipitation to the annual rainfall budget. Low-level convergence and midtropospheric moisture have been shown to be correlated with rainfall amounts in Florida. Using 2D and 3D numerical simulations, the roles of low-level convergence and midtropospheric moisture in rainfall evolution are examined.

The results indicate that area- and time-averaged, vertical moisture flux (VMF) at the sea-breeze front–outflow convergence zone is directly and linearly proportional to initial condensation rates. A similar relationship exists between VMF and initial rainfall. The VMF, which encompasses depth and magnitude of convergence, is better correlated to initial rainfall production than surface moisture convergence. This extends early observational studies that linked rainfall in Florida to surface moisture convergence. The amount and distribution of midtropospheric moisture affects how much rainfall associated with secondary cells develop. Rainfall amount and efficiency varied significantly over an observable range of relative humidities in the 850–500-mb layer even though rainfall evolution was similar during the initial or “first cell” period. Rainfall variability was attributed to drier midtropospheric environments inhibiting secondary cell development through entrainment effects. Observationally, a 850–500-mb moisture structure exhibits wider variability than lower-level moisture, which is virtually always present in Florida. A likely consequence of the variability in 850–500-mb moisture is a stronger statistical correlation to rainfall as noted in previous observational studies.

The VMF at convergence zones is critical in determining rainfall in the initial stage of development but plays a decreasing role in rainfall evolution as the system matures. The midtropospheric moisture (e.g., environment) plays an increasing role in rainfall evolution as the system matures. This suggests the need to improve measurements of depth and magnitude of convergence and midtropospheric moisture distribution. It also highlights that the influence of the environment needs to be better represented in convective parameterizations of larger-scale models to account for entrainment effects.

Abstract

Two-dimensional experiments using the Goddard Cumulus Ensemble model are performed in order to examine the influence of environmental profiles of wind and humidity on the dynamical and microphysical structure of mesoscale convective systems (MCSs) over the tropical oceans. The initial environments used in this study are derived from the results of a cluster analysis of the TOGA COARE sounding data. The model data are analyzed with methods and measurements similar to those used in observational studies.

Experiments to test the sensitivity of MCSs to the thermodynamic profile focus on the role of humidity in the free troposphere. In the experiments, a constant amount of relative humidity is added to every level above the boundary layer. As humidity is increased, model storms transition from weak, unsteady systems with little precipitation to strong, upshear-tilted systems with copious rainfall. This behavior is hypothesized to be the result of the entrainment of environmental air into the updraft cores.

Experiments to test the sensitivity of MCSs to the kinematic profile focus on the amount of vertical wind shear in the midlevels, between approximately 2 and 10 km. Five kinematic profiles are used. The dynamical and microphysical characteristics of the runs changed dramatically in different shear environments. Shear in the midlevels affects the convective systems by altering the perturbation pressure field. Stronger shear results in a broader and deeper mesolow below the updraft and a more intense dynamic high above the leading edge.

Abstract

Part I of this study described a detailed four-class bulk ice scheme (4ICE) developed to simulate the hydro-meteor profiles of convective and stratiform precipitation associated with mesoscale convective systems. In Part II, the 4ICE scheme is incorporated into the Goddard Cumulus Ensemble (GCE) model and applied without any “tuning” to two squall lines occurring in widely different environments, namely, one over the “Pica) ocean in the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) and the other over a midlatitude continent in the Cooperative Huntsville Meteorological Experiment (COHMEX). Comparisons were made both with earlier three-class ice formulations and with observations. In both cases, the 4ICE scheme interacted with the dynamics so as to resemble the observations much more closely than did the model runs with either of the three-class ice parameterizations. The following features were well simulated in the COHMEX case: a lack of stratiform rain at the surface ahead of the storm, reflectivity maxima near 60 dBZ in the vicinity of the melting level, and intense radar echoes up to near the tropopause. These features were in strong contrast with the GATE simulation, which showed extensive trailing stratiform precipitation containing a horizontally oriented radar bright band. Peak reflectivities were below the melting level, rarely exceeding 50 dBz, with a steady decrease in reflectivity with height above. With the other bulk formulations, the large stratiform rain areas were not reproduced in the GATE conditions.

The microphysical structure of the model clouds in both environments were more realistic than that of earlier modeling efforts. Number concentrations of ice of O(100 L⁻¹) occurred above 6 km in the GATE model clouds as a result of ice enhancement and rime splintering in the 4ICE runs. These processes were more effective in the GATE simulation, because near the freezing level the weaker updrafts were comparable in magnitude to the fall speeds of newly frozen drops. Many of the ice crystals initiated at relatively warm temperatures (above −15°C) grew rapidly by deposition into sizes large enough to be converted to snow. In contrast, in the more intense COHMEX updrafts, very large numbers of small ice crystals were initiated at colder temperatures (below −15°C) by nucleation and stochastic freezing of droplets, such that relatively few ice crystals grew by deposition to sizes large enough to be converted to snow. In addition, the large number of frozen drops of O(5 L⁻¹) in the 4ICE run am consistent with airborne microphysical data in intense COHMEX updrafts.

Numerous sensitivity experiments were made with the four-class and three-class ice schemes, varying fall speed relationships, particle characteristics, and ice collection efficiencies. These tests provide strong support to the conclusion that the 4ICE scheme gives improved resemblance to observations despite present uncertainties in a number of important microphysical parameters.

Abstract

The Ferrier–Aligo (FA) microphysics scheme has been running operationally in the National Centers for Environmental Prediction (NCEP) North American Mesoscale Forecast System (NAM) since August 2014. It was developed to improve forecasts of deep convection in the NAM contiguous United States (CONUS) nest, and it replaces previous versions of the NAM microphysics. The FA scheme is the culmination of extensive microphysical scheme sensitivity experiments made over nearly a dozen warm- and cool-season severe weather cases, as well as an extensive real-time testing in a full, system-wide developmental version of the NAM. While the FA scheme advects each hydrometeor species separately, it was the mass-weighted rime factor (RF) that allowed rimed ice to be advected to very cold temperatures aloft and improved the vertical structure of deep convection. Rimed ice fall speeds were reduced in order to offset an increase in bias of heavy precipitation as a consequence of the mass-weighted RF advection. The FA scheme also incorporated findings from 3-km model runs using the Thompson scheme, including 1) improved closure assumptions for large precipitating ice that targeted the convective and anvil regions of storms, 2) a new diagnostic calculation of radar reflectivity from rimed ice in association with intense convection, and 3) a variable rain intercept parameter that reduced widespread spurious weak reflectivity from shallow boundary layer clouds and increased stratiform rainfall.

Abstract

An active day during the Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Observation Period (IOP) is examined in which nine convective systems evolved and moved eastward across the region of shipboard radar coverage in the Intensive Flux Array (IFA) within westerly wind burst conditions. The detailed genesis, morphology, and interactions between these cloud systems are documented from a radar and satellite perspective. One of these systems was a large and complex elliptical cluster, among the largest observed during the Tropical Ocean Global Atmosphere COARE. Multiple, parallel deep convective lines spaced 20–30 km apart and embedded within this system were initially oriented from north-northwest to south-southeast, oblique to the storm motion. Furthermore, the lines underwent counterclockwise realignment as the system moved eastward. The influence of strong lower-tropospheric directional and speed shear on these convective system properties is examined in the context of a dynamic, large-scale near-equatorial trough/transequatorial flow regime. A daily analysis of flow conditions during the 119-day IOP revealed that this type of synoptic regime was present in the IFA at least 40% of the time.

Radar-derived rainfall statistics are examined throughout the life cycles of each individual convective system. Spatial mapping of accumulated rainfall reveals long, linear swaths produced by the most intense cells embedded within convective lines. The evolution of rainfall properties includes an increase in the stratiform rainfall fraction and areal coverage in later generations of systems, with a peak in total rainfall production after local midnight. These trends can be explained by anvil cloud interactions originating within the sequence of closely spaced disturbances, including the effects of both enhanced midtropospheric moisture and also strong reversing (easterly) shear. The issue of boundary layer recovery between the frequent, intense convective systems on this day is also examined.