Search Results

You are looking at 1 - 10 of 35 items for

  • Author or Editor: Gregory J. Tripoli x
  • Refine by Access: All Content x
Clear All Modify Search
Gregory J. Tripoli

Abstract

A three-dimensional nonhydrostatic mesoscale model is presented that is designed to optimally represent the scale-interaction process among inertially balanced and unbalanced modes occurring within convective weather systems. Because scale-interaction simulations are long-term integrations that emphasize the evolution of the three-dimensional kinetic energy spectrum, the model is built to conserve enstrophy, as well as kinetic energy against numerical sources and sinks in three dimensions. A non-Boussinesq and quasi-compressible framework is employed to maintain applicability on meso-α and larger scales as well as for situations of relatively large local density variation.

Sample integrations in two and three dimensions are presented that show enstrophy conservation to be effective in improving the prediction of nonlinear evolution as truncation errors act to force anomalous bifurcations from the physical solution.

Full access
Gregory J. Tripoli

Abstract

This paper presents the results of a series of idealized cloud resolving simulations of the evolution of moist roll convection observed as part of the Lake-Induced Convection Experiment (Lake-ICE) that took place during the 1997/98 winter over central Lake Michigan. Satellite and radar observations of the roll convection depict striking linear rolls stretching from 10 km off the western shore of the lake, across to the eastern shore, and then continuing across Michigan. The spacing of the primary rolls was observed to be 6 km, giving a ratio of spacing to depth of about 5:1, which is consistent with theory. In addition, a longer wavelength (13 km) of stationary banding was observed parallel to the shoreline.

In an earlier study of this case, multiply nested simulations of the convective rolls based on real data variable initialization were successful in producing banded structures with similar spacing and location over the water to those observed using fine grid resolution of about 500 m. Unfortunately, the initial locations of simulated bands were organized primarily by numerical effects of grid interpolation. This suggested that the spacing of the bands was robust, but that their initial location was highly sensitive to subtle systematic forcings. In this paper, a set of idealized model experiments, designed to isolate the role that physically realistic local forcing plays in the organization of the rolls, was performed. Because externally generated upstream turbulence was suppressed in these tests so as not to bias the result, the generation of rolls was delayed until 20–30 km downwind of the observed location and the location simulated in the previous grid nesting experiments. It was shown that the subtle effects of the shoreline geometry were sufficient to spawn a near-surface streamwise vorticity that became the primary seed for roll development at the most efficient mode of roll convection. These results suggest that previous structures evolved in the upstream shear-driven land-based mixed layer were likely also important in determining where the nonlocal overturning was first triggered. It is not clear from these results whether the shear-driven structures that evolved over the land also played a significant role in organizing the structural geometry of the lake rolls. Results also suggested that the shore parallel bands were a robust feature of the atmospheric structure resulting from resonant gravity wave trapping in the frontal layer.

Full access
Yoshio Kurihara and Gregory J. Tripoli

Abstract

A two-step iterative time integration scheme is formulated, by which the amplitude of a low-frequency wave in a primitive equations model is preserved fairly well for a period of short-range weather prediction while the high-frequency noises are damped. The desired computational characteristics are obtained by separating the terms of the equations at the corrector step. Numerical examples are presented which show the damping property of the proposed scheme. The new scheme does not require more data space in a computer than the amount used in the Euler-backward method.

Full access
Tempei Hashino and Gregory J. Tripoli

Abstract

This paper proposes a framework to predict polycrystals along with hexagonal monocrystals in a cloud-resolving model and discusses validity of the scheme and physical processes important for habit distribution by implementing 2D cloud-resolving simulations of an orographic winter storm. The vapor depositional growth is simulated based on a habit frequency map constructed in the laboratory and based on a predicted growth history under evolving atmospheric conditions. Differences brought by predicting polycrystals in spatial distribution were apparent, and sedimentation of polycrystals from middle and upper levels was significant. The immersion freezing process appeared to be a key process resulting in the creation of planar polycrystals, while the homogeneous freezing process was found important for columnar and irregular polycrystals. The ice nuclei (IN) concentration was shown to affect habit distribution by changing the supersaturation, tendency of the freezing nucleation processes, and sedimentation. Preliminary comparison of simulated habit distributions and frequencies with observations indicates usefulness of the framework for polycrystals. Implications of these modeling studies on the needs of future observational campaigns are discussed.

Full access
Tempei Hashino and Gregory J. Tripoli

Abstract

The purpose of this paper is to assess the prediction of particle properties of aggregates and particle size distributions with the Spectral Ice Habit Prediction System (SHIPS) and to investigate the effects of crystal habits on aggregation process. Aggregation processes of ice particles are critical to the understanding of precipitation and the radiative signatures of cloud systems. Conventional approaches taken in cloud-resolving models (CRMs) are not ideal to study the effects of crystal habits on aggregation processes because the properties of aggregates have to be assumed beforehand. As described in Part III, SHIPS solves the stochastic collection equation along with particle property variables that contain information about crystal habits and maximum dimensions of aggregates. This approach makes it possible to simulate properties of aggregates explicitly and continuously in CRMs according to the crystal habits.

The aggregation simulations were implemented in a simple model setup, assuming seven crystal habits and several initial particle size distributions (PSDs). The predicted PSDs showed good agreement with observations after rescaling except for the large-size end. The ice particle properties predicted by the model, such as the mass–dimensional (m-D) relationship and the relationship between diameter of aggregates and number of component crystals in an aggregate, were found to be quantitatively similar to those observed. Furthermore, these predictions were dependent on the initial PSDs and habits. A simple model for the growth of a particle’s maximum dimension was able to simulate the typically observed fractal dimension of aggregates when an observed value of the separation ratio of two particles was used. A detailed analysis of the collection kernel indicates that the m-D relationship unique to each crystal habit has a large impact on the growth rate of aggregates through the cross-sectional area or terminal velocity difference, depending on the initial equivalent particle distribution. A significant decrease in terminal velocity differences was found in the inertial flow regime for all the habits but the constant-density sphere. It led to formation of a local maximum in the collection kernel and, in turn, formed an identifiable mode in the PSDs. Remaining issues that must be addressed in order to improve the aggregation simulation with the quasi-stochastic model are discussed.

Full access
Tempei Hashino and Gregory J. Tripoli

Abstract

The purpose of this paper is to describe a numerical scheme of the Spectral Ice Habit Prediction System (SHIPS) that simulates the dependency of aggregation process explicitly on crystal habit and size in cloud-resolving models (CRMs). The sizes and shapes of ice crystals are known to modulate the aggregation process, which is a critical part of physical processes leading to precipitation in addition to vapor deposition and riming processes. A problem with conventional formulation of aggregation process in CRMs is that it is not designed to predict the aggregates’ properties based on the information on crystals. To simulate such dependency, SHIPS solves a quasi-stochastic model that describes growth tendency for a group of particles, together with particle property variables (PPVs) that carry information on habit and types of ice particles.

SHIPS diagnoses the ice particle properties based on the PPVs for each mass bin at given a time and space, which are used to calculate the collision cross-sectional area and terminal velocity differences based on crystal habits explicitly. To achieve prediction of properties of aggregates and rimed particles, SHIPS introduces 1) the use of a conceptually based, circumscribing shape, called the ice particle model, and 2) the explicit prediction of the circumscribing sphere volume based on a simple growth model of the maximum dimension. Based on these properties, SHIPS is able to predict the mass–dimensional relationships of aggregates so that they are physically consistent with the growth history of the particles. In addition, the information about the crystals making up the aggregates is predicted. Part IV of this series will present the results and evaluation of the application of this formulation to idealized tests in a Lagrangian “box model” setup.

Full access
Gregory J. Tripoli and William R. Cotton

Abstract

Previous studies have shown liquid water potential temperature to be an inappropriate choice for a thermodynamic variable in a deep cumulus convection model. In this study, an alternate form of this variable called ice-liquid water potential temperature (θu) is derived. Errors resulting from approximations made are discussed, and an empirical form of the θu equation is introduced which eliminates much of this error. Potential temperature lapse rates determined in saturated updrafts and unsaturated downdrafts by various θu approximations, an equivalent potential temperature approximation and a conventional irreversible moist thermodynamic approximation are then compared to the potential temperature lapse rate determined from a rigorously derived reversible thermodynamic energy equation. These approximations are then extended to a precipitating system where comparisons are again made. It is found that the errors using the empirical form of the θu equation are comparable to those made using conventional irreversible moist thermodynamic approximations. The advantages of using θu as an alternative to θ in deep convection and second-order closure models also are discussed.

Full access
Gregory J. Tripoli and T. N. Krishnamurti

Abstract

An assimilation of satellite low-cloud vector data and conventional meteorological data is presented in this paper. The domain of the study is the GATE A-scale area. The period is the summer months of 1972. Objective analysis of the data for 93 individual days was carried out for this entire domain. One of the important climatological findings of this study is the presence of a velocity maximum in the southeast trades along the Brazilian coast. Mean speeds for three months exceed 10 m/s in this region; daily values occasionally are as large as 25 m/s. Besides showing the monthly mean motion field, we have examined in detail one-level barotropic energy exchanges and fluxes in the GATE A-scale domain. The number of conventional plus non-conventional wind observations are about 400 per day. This is more than has been used in most previous studies. Some of these results of the energetics, especially with regards to the period when Hurricane Agnes formed, are thus of considerable interest.

Full access
John R. Mecikalski and Gregory J. Tripoli

Abstract

Tropical plumes are identified in satellite data as elongated cloud bands originating from convective activity along the intertropical convergence zone (ITCZ), often extending far into the subtropics and middle latitudes. Many previous studies consider tropical plumes as a product of quasigeostrophic or convergent forcing. Here the authors consider the view that a tropical plume is the upper branch of an enhanced thermally direct circulation driven by latent heat released along the ITCZ. In this way, tropical plume formation is strongly tied to deep cumulus convection and inertial processes.

Observations of plume development show that as a midlatitude wave nears a subsequent plume genesis region, a northward advection of upper-tropospheric, low potential vorticity (potential vorticity unit ≪ 1) occurs as anticyclonic flow intensifies southeast of the midlatitude wave. As this low potential vorticity (PV) ridges over and straddles the ITCZ, plume genesis occurs. Plume development occurs about 1–2 days prior to the midlatitude wave’s more direct impact on the ITCZ environment as it moves to within 5°–10° of the ITCZ. However, as the midlatitude wave nears the ITCZ, an equatorward advection of high PV occurs to end plume development. Thus, a midlatitude wave both indirectly causes tropical plume formation and appears directly responsible for plume demise.

As the low PV advects across the ITCZ, the meridional inertial stability gradient equilibrates. Under these conditions, it is hypothesized that the work requirements of deep ITCZ convection to spread its outflow and force compensating subsidence ease as inertial stability lowers. In the event that convection transports easterly boundary layer momentum to a level of strong convective outflow, it is found that regions poleward of the ITCZ become dynamically preferred for outflow as convectively generated (negative) PV lowers inertial stability there more than equatorward. Thus, convective-scale processes are suggested to be critical to plume formation.

The diagnostic parameter “inertial available kinetic energy” (IAKE), computed on the 340-K isentrope surface, reveals much reduced inertial stability as PV lowers across the ITCZ in conjunction with tropical plume formation. With an easterly (downgradient for the ITCZ environment) convective momentum transport, IAKE becomes positive in the poleward direction in the plume genesis region, suggesting an inertial instability relative to convective updrafts. Theoretically, ITCZ convection in these instances may use convective available potential energy in the presence of IAKE to explosively develop, forming a tropical plume.

Full access
Lindsey E. Nytes and Gregory J. Tripoli

Abstract

This study employs 40 years of ERA-Interim data to quantify the periodic behavior of the integral amount of potential energy stored in the tropical upper UTLS (upper troposphere and lower stratosphere) originally placed there by high-entropy outflow of deep convection and tropical cyclones (TCs). The upper UTLS is defined to be isentropic layers above the level of zero net radiation (LZNR) in the tropics. Once there, the trapped high-entropy air mass must flow into the extratropics where radiative loss will allow it to subside back to lower-entropy levels. Mean poleward fluxes of isentropic mass are prevented by an inertial wall associated with Earth’s rotation. This causes the mass of these isentropic layers to build up, creating a bubble of high potential energy. Periodic releases of this mass into the extratropics in the form of tropical plumes (TPs) help drain the bubble of its mass buildup over time. We present a metric to quantify the energy trapped and stored in this bubble to be the jet available potential energy (JAPE), in reference to the availability of this energy to fuel the kinetic energy of the subtropical jet stream (STJ), which bounds the tropical JAPE bubble (TJB) on its poleward extremities. We also calculate the isentropic mass M, which measures the isentropic thickness. Results of a 40-yr-time-series analysis of upper-UTLS JAPE and M show that the TJB features periodicity in the buildup and release of JAPE and M.

Full access