Abstract

The fractions skill score (FSS) belongs to a class of spatial neighborhood techniques that measures forecast skill from samples of gridded forecasts and observations at increasing spatial scales. Each sample contains the fraction of the predicted and observed quantities that exist above a threshold value. Skill is gauged by the rate that the observed and predicted fractions converge with increasing scale. In this study, neighborhood sampling is applied to diagnose the performance of high-resolution (1.67 km) precipitation forecasts over central Florida. Reliability diagrams derived from the spatial fractions indicate that the FSS can be influenced by small, low-predictability events. Further tests indicate the FSS is subtly affected by samples from points on and near the grid boundaries. Inclusion of these points tends to reduce the magnitude and sensitivity of the FSS, especially at large scales. An attempt to mine data from the set of neighborhood fractions was moderately successful at obtaining descriptive information about the precipitation fields. The width of the distribution of the fractions at each scale provided information concerning forecast resolution and sharpness. The rate at which the distribution of the fractions converged toward the domain mean with increasing scale was found to be sensitive to the uniformity of coverage of precipitation through the domain. Generally, the 6-h forecasts possessed greater spatial skill than those at 12 h. High-FSS values at 12 h were mostly associated with evenly distributed precipitation patterns, while the 6-h forecasts also performed well for several nonuniform cases.

Abstract

Past microphysical investigations, including Part I of this study, have noted that the collection equation, when applied to the interaction between different hydrometeor species, can predict large mass transfer rates, even when an exact solution is used. The fractional depletion in a time step can even exceed unity for the collected species with plausible microphysical conditions and time steps, requiring “normalization” by a microphysical scheme. Although some of this problem can be alleviated through the use of more moment predictions and hydrometeor categories, the question as to why such “overdepletion” can be predicted in the first place remains insufficiently addressed. It is shown through both physical and conceptual arguments that the explicit time discretization of the bulk collection equation for any moment is not consistent with a quasi-stochastic view of collection. The result, under certain reasonable conditions, is a systematic overprediction of collection, which can become a serious error in the prediction of higher-order moments in a bulk scheme. The term numerical bounding is used to refer to the use of a quasi-stochastically consistent formula that prevents fractional collections exceeding unity for any moments. Through examples and analysis it is found that numerical bounding is typically important in mass moment prediction for time steps exceeding approximately 10 s. The Poisson-based numerical bounding scheme is shown to be simple in application and conceptualization; within a straightforward idealization it completely corrects overdepletion while even being immune to the rediagnosis error of the time-splitting method. The scheme’s range of applicability and utility are discussed.

Abstract

The collection equation is analyzed for the case of two spherical hydrometeors with collection efficiency unity and exponential size distributions. When the fall velocities are significantly different a more general form of the conventional Wisner approximation can be formulated. The accuracy of the new formula exceeds that of the Wisner approximation for all cases considered, except for the collection of a faster species by a slower species if the amount of the faster species is relatively small compared with that of the slower species. The exact solution of the collection equation is then rederived and cast into the form of a power series involving the ratio of the two characteristic fall velocities. It is shown that the new formulation is a first-order correction to the continuous collection equation for hydrometeors with finite diameters and fall velocities. Based on the analysis, the implications for the behavior of both the exact collection equation and its representation in numerical models are discussed.

Abstract

Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation.

We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate that the most vigorous and long-lived squall lines arise with a deep layer of strong vertical wind shear. This result is dependent on the changes in the phase speed and magnitude of the stable layer waves that occur in the sheared versus nonsheared environments. Without flow, waves generated by an initial heat pulse split into symmetric leftward and rightward moving disturbances. Waves generated in the upper tropospheric stable layer are found to move relative to the lower tropospheric waves resulting in a decoupling of deep tropospheric vertical motion and a decrease in strength of the simulated system. With vertical wind shear, the magnitude of the simulated waves is enhanced and an opportunity for sustained coupling between the upper and lower waves exists. It is shown that the upper and lower tropospheric waves in a sheared environment account for many of the circulation features typically associated with two-dimensional squall lines.

A simple mechanism for the rear-to-front middle-level jet and surface wake low is also presented.

Abstract

The characteristics of a severe squall line are examined using data acquired from the 1981 Cooperative Convective Precipitation Experiment (CCOPE). The case is unusual in that the squall line was decoupled from an immediate, surface-based inflow source due to a mesoβ-scale (20–200 km) outflow pool produced by a separate mesoscale convective system. Both systems participated in the development of a mesoscale convective complex which subsequently produced sustained, severe surface winds throughout its entire life cycle. Despite the absolutely stable, presquall atmospheric boundary layer, the squall line produced radar reflectivity values of 70 dBZ and storm-induced outflow of 30 m s⁻¹. Aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high-valued θ_c air overriding the cold dome produced by the developing MCC.

The strongest surface winds were located upshear from the convective line and consisted of a northerly (alongline) component. Because the middle-level environmental flow was from the southwest, a simple vertical transport of middle-level momentum cannot account for the observed surface flow. The strong surface winds were primarily a result of the local surface pressure gradient associated with a mesohigh–mesolow couplet that accompanied the squall line.

The squall line also maintained a strong, quasi-steady, supercell-like cell that could be tracked by radar for several hours. The kinematic structure, derived from a multiple Doppler radar analysis, shows that significant alongline flow was also generated by the rotational characteristics of the supercell. This feature distinguishes this system from tropical squall lines and many midlatitude squall lines which are composed of transient ordinary cells.

Abstract

Numerical forecasts of heavy warm-season precipitation events are verified using simple composite collection techniques. Various sampling methods and statistical measures are employed to evaluate the general characteristics of the precipitation forecasts. High natural variability is investigated in terms of its effects on the relevance of the resultant statistics. Natural variability decreases the ability of a verification scheme to discriminate between systematic and random error. The effects of natural variability can be mitigated by compositing multiple events with similar properties. However, considerable sample variance is inevitable because of the extreme diversity of mesoscale precipitation structures.

The results indicate that forecasts of heavy precipitation were often correct in that heavy precipitation was observed relatively close to the predicted area. However, many heavy events were missed due in part to the poor prediction of convection. Targeted composites of the missed events indicate that a large percentage of the poor forecasts were dominated by convectively parameterized precipitation. Further results indicate that a systematic northward bias in the predicted precipitation maxima is related to the deficits in the prediction of subsynoptically forced convection.

Abstract

Operational cloud forecasts generated by the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) were verified over the eastern Pacific Ocean. The study focused on the accuracy of cloud forecasts associated with extratropical cyclone and convective activity during the late winter and spring of 2007. The condensed total water (liquid and solid) path was used as a proxy for cloud cover to compare the forecasts with retrievals from the Geostationary Operational Environmental Satellites (GOES). Analyses of the GOES retrievals indicate that deep cloud systems were generally well represented during daylight hours. Thus, the bulk of the verification focused on the general aspects of quality and timing of these deep systems. Multiple statistics were collected, ranging from simple correlations and histograms to more sophisticated fuzzy and composite statistics. The results show that synoptic-scale systems were generally well predicted to at least two days, with the primary error being an overestimation of deep cloud occurrence. Smaller subsynoptic-scale systems were subject to spatial and timing biases in that a number of the forecasts were lagged by 3–6 h. Despite the bias, 60%–70% of the forecasts of the mesoscale phenomena displayed useful skill.

Abstract

Very-high-resolution Doppler radar observations are used together with aircraft measurements to document the dynamic and thermodynamic structure of a dissipating altocumulus cloud system associated with a deep virga layer. The cloud layer circulation is shown to consist of shallow vertical velocity couplets near cloud top and a series of subkilometer-scale Rayleigh–Bénard-like cells that extend vertically through the depth of the cloud layer. The subcloud layer was observed to contain a number of narrow virga fall streaks that developed below the more dominant Rayleigh–Bénard updraft circulations in the cloud layer. These features were discovered to be associated with kilometer-scale horizontally orientated rotor circulations that formed along the lateral flanks of the streaks collocated downdraft circulation. The Doppler analysis further reveals that a layer mean descent was present throughout both the cloud and subcloud layers. This characteristic of the circulation is analyzed with regard to the diabatic and radiative forcing on horizontal length scales ranging from the Rayleigh–Bénard circulations to the overall cloud layer width. In particular, linear analytical results indicate that a deep and broad mesoscale region of subsidence is quickly established in middle-level cloud layers of finite width when a layer-wide horizontal gradient in the cloud-top radiative cooling rate is present. A conceptual model summarizing the primary observed and inferred circulation features of the altocumulus layer is presented.

Abstract

The structure of a melting layer associated with a mesoconvective system is examined using a combination of in situ aircraft measurements and a unique Doppler radar operated by the U.S. Navy that has a range resolution as fine as 0.5 m. Interest in this case was motivated by ground-based all-sky camera images that captured the transient development of midlevel billow cloud structures within a precipitating trailing stratiform cloud shield associated with a passing deep convective system. A sequence of high-fidelity time–height radar measurements taken of this storm system reveal that the movement of the billow cloud structure over the radar site corresponded with abrupt transitions in the observed low-level precipitation structure. Of particular note is an observed transition from stratiform to more periodic and vertically slanted rain shaft structures that both radar and aircraft measurements indicate have the same temporal periodicity determined to arise visually between successive billow cloud bands. Doppler, balloon, and aircraft measurements reveal these transient bands are associated with a shallow circulation field that resides just above the melting level in a layer of moist neutral stability and strong negative vertical wind shear. The nature of these circulations and their impact on the evolving precipitation field are described in the context of known nimbostratus cloud types.

Abstract

Interactions between the upper-level outflow of a sheared, rapidly intensifying tropical cyclone (TC) and the background environmental flow in an idealized model are presented. The most important finding is that the divergent outflow from convection localized by the tilt of the vortex serves to divert the background environmental flow around the TC, thus reducing the local vertical wind shear. We show that this effect can be understood from basic theoretical arguments related to Bernoulli flow around an obstacle. In the simulation discussed, the environmental flow diversion by the outflow is limited to 2 km below the tropopause in the 12–14-km (250–150 hPa) layer. Synthetic water vapor satellite imagery confirms the presence of upshear arcs in the cloud field, matching satellite observations. These arcs, which exist in the same layer as the outflow, are caused by slow-moving wave features and serve as visual markers of the outflow–environment interface. The blocking effect where the outflow and the environmental winds meet creates a dynamic high pressure whose pressure gradient extends nearly 1000 km upwind, thus causing the environmental winds to slow down, to converge, and to sink. We discuss these results with respect to the first part of this three-part study, and apply them to another atypical rapid intensification hurricane: Matthew (2016).