Jason E. Nachamkin

Mesoscale Verification Using Meteorological Composites

Abstract Mesoscale models are often used to explicitly predict discrete, highly structured phenomena. Information regarding the ability of the model to predict events as coherent entities is thus a useful statement of performance. Observational constraints are a significant problem, though, as the shape, size, and intensity of any given event are often only partially known. Composite techniques offer an attractive approach because the full deterministic information about any one event need not be known. If enough quasi-random observations of a distribution of events exist, bulk properties of the distributions of forecasts and observations can be estimated. Composites are also useful in that the verification measures are based on conditional samples of events. Sample distributions contingent on event existence in either the forecasts or the observations can be compared to one another. A verification technique in which meteorological events are located and composited on a relative grid centered on each even...

Simulating convective events using a high-resolution mesoscale model

Four multiscale numerical simulations of convective events are analyzed to determine the essential characteristics of a numerical model which lead to useful simulations of convective events. Although several universities and weather forecasting centers are currently running high-resolution forecast models, the predictability of convective events, especially in the warm season, is still an issue among researchers and forecasters in the meteorological community. This study shows that explicit simulations of convection depend on the high spatial resolution of physiography (particularly topography and top soil moisture), efficient communication between grids of different scales, and initialization procedures that incorporate mesoscale storm features.

Upscale Evolution of MCSs: Doppler Radar Analysis and Analytical Investigation

The development of two small mesoscale convective systems (MCSs) in northeastern Colorado is investigated via dual-Doppler radar analysis. The first system developed from several initially isolated cumulonimbi, which gradually coalesced into a minimal MCS with relatively little stratiform precipitation. The second system developed more rapidly along an axis of convection and generated a more extensive and persistent stratiform echo and MCS cloud shield. In both systems, the volumetric precipitation rate exhibited an early meso-b-scale convective cycle (a maximum and subsequent minimum), followed by reintensification into a modest mature stage. This sequence is similar to that noted previously in the developing stage of larger MCSs by McAnelly and Cotton. They speculated that the early meso-b convective cycle is a characteristic feature of development in many MCSs that is dynamically linked to a rather abrupt transition toward mature stage structure. This study presents kinematic evidence in support of this hypothesis for these cases, as derived from dual-Doppler radar analyses over several-hour periods. Mature stage MCS characteristics such as deepened low- to midlevel convergence and mesoscale descent developed fairly rapidly, about 1 h after the early meso-b convective maximum. The dynamic linkage between the meso-b convective cycle and evolution toward mature structure is examined with a simple analytical model of the linearized atmospheric response to prescribed heating. Heating functions that approximate the temporal and spatial characteristics of the meso-b convective cycle are prescribed. The solutions show that the cycle forces a response within and near the thermally forced region that is consistent with the observed kinematic evolution in the MCSs. The initial response to an intensifying convective ensemble is a self-suppressing mechanism that partially explains the weakening after a meso-b convective maximum. A lagged response then favors reintensification and areal growth of the weakened ensemble. A conceptual model of MCS development is proposed whereby the early meso-b convective cycle and the response to it are hypothesized to act as a generalized forcing‐feedback mechanism that helps explain the upscale growth of a convective ensemble into an organized MCS.

The Impact of Ice Phase Cloud Parameterizations on Tropical Cyclone Prediction

AbstractThe impact of ice phase cloud microphysical processes on prediction of tropical cyclone environment is examined for two microphysical parameterizations using the Coupled Ocean–Atmosphere Mesoscale Prediction System–Tropical Cyclone (COAMPS-TC) model. An older version of microphysical parameterization is a relatively typical single-moment scheme with five hydrometeor species: cloud water and ice, rain, snow, and graupel. An alternative newer method uses a hybrid approach of double moment in cloud ice and rain and single moment in the other three species. Basin-scale synoptic flow simulations point to important differences between these two schemes. The upper-level cloud ice concentrations produced by the older scheme are up to two orders of magnitude greater than the newer scheme, primarily due to differing assumptions concerning the ice nucleation parameterization. Significant (1°–2°C) warm biases near the 300-hPa level in the control experiments are not present using the newer scheme. The warm bi...

Interactions between a Developing Mesoscale Convective System and Its Environment. Part II: Numerical Simulation

The 19 July 1993 mesoscale convective system (MCS), discussed in Part I, was simulated using the Regional Atmospheric Modeling System (RAMS). The model was initialized with variable physiographic and atmospheric data with the goal of reproducing the convective system and its four-dimensional environment. Four telescopically nested, moving grids allowed for horizontal grid spacings down to 1.67 km on the cloud resolving grid. Comparisons with the analysis show that the propagation, evolution, and structure of this MCS were well simulated. The simulation is used to further investigate the interactions between this MCS and its surrounding environment. In Part I, the Doppler-derived winds indicated that upshear (westward) propagating gravity waves left uppertropospheric front-to-rear and midtropospheric rear-to-front flow perturbations in their wake. A similar flow structure developed in the simulated MCS, and unlike the Doppler results, the low-frequency waves that produced it were resolved in the data. In the simulation, much of the convectively generated temperature and momentum perturbations propagated westward with the waves, leaving a warm wake in the clear air trailing the system. Although the gravity waves traveled rearward, the perturbation flow in their wake was not strong enough to reverse the upper-tropospheric storm-relative winds. Thus, most of the anvil condensate advected ahead of the convective line. As the MCS encountered the low-level jet, the midtropospheric upward mass flux increased, but gravity wave motions became less detectable. The upper-tropospheric anvil pushed westward into the strong flow as the system expanded into a characteristically oval shape. Temperature and momentum perturbations propagated rearward along with the anvil in a disturbance that resembled an advective outflow. Unlike the gravity waves, this disturbance became almost stationary with respect to the ground, and it retained its continuity through the rest of the simulation. Vertical cross sections indicate that a large slab of convectively processed air had detrained into the upper troposphere. Prior to this event, much of the warm temperature anomalies generated within the convective towers either remained in the updrafts, or propagated outward with the gravity waves. Early on, individual updrafts were relatively erect and although condensate did detrain eastward in the forward anvil, the temperature anomalies did not propagate with it. In contrast, convective updrafts associated with the expanding oval anvil disturbance were more continuous, and they tilted strongly westward with height.

Regional Ensemble Forecasts Using the Ensemble Transform Technique

Abstract A computationally inexpensive ensemble transform (ET) method for generating high-resolution initial perturbations for regional ensemble forecasts is introduced. The method provides initial perturbations that (i) have an initial variance consistent with the best available estimates of initial condition error variance, (ii) are dynamically conditioned by a process similar to that used in the breeding technique, (iii) add to zero at the initial time, (iv) are quasi-orthogonal and equally likely, and (v) partially respect mesoscale balance constraints by ensuring that each initial perturbation is a linear sum of forecast perturbations from the preceding forecast. The technique is tested using estimates of analysis error variance from the Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS) and the Navy’s regional Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) over a 3-week period during the summer of 2005. Lateral boundary conditions are provided by...

Evaluation of Heavy Precipitation Forecasts Using Composite-Based Methods: A Distributions-Oriented Approach

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.

On the Improvement of COAMPS Weather Forecasts Using an Advanced Radiative Transfer Model

Abstract Fu–Liou’s delta-four-stream (with a two-stream option) radiative transfer model has been implemented in the U.S. Navy’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)1 to calculate solar and thermal infrared fluxes in 6 shortwave and 12 longwave bands. The model performance is evaluated at high resolution for clear-sky and overcast conditions against the observations from the Southern Great Plains of the Atmospheric Radiation Measurement Program. In both cases, use of the Fu–Liou model provides significant improvement over the operational implementation of the standard Harshvardhan radiation parameterization in both shortwave and longwave fluxes. A sensitivity study of radiative flux on clouds reveals that the choices of cloud effective radius schemes for ice and liquid water are critical to the flux calculation due to the effects on cloud optical properties. The sensitivity study guides the selection of optimal cloud optical properties for use in the Fu–Liou parameterization as im...

An Observational Analysis of a Developing Mesoscale Convective Complex

Abstract Dual-Doppler radar, surface mesonet, satellite, and upper-air sounding data from the 1985 Preliminary Regional Experiment for STORM-Central field experiment are used to analyze the early growth stages of a mesoscale convective complex (MCC) that developed in the network on 3 June 1985. This MCC was characterized by a complex distribution of convective clusters and intervening stratiform echo as it grew from its initial stage to the typical meso-α-scale cloud shield structure at its mature stage. The MCC exhibited two very different states of organization as it grew. The early state was characterized by a relatively weak and disorganized surface pressure pattern and a highly variable three-dimensional mesoscale flow structure. The later state was characterized by a well-developed mesohigh-wake-low surface pressure pattern and more organized mososcale flow fields. The evolution between these two regimes occurred about 1 h after the upper-level cloud shield reached MCC proportions and manifested its...

Third COMPARE Workshop: A Model Intercomparison Experiment of Tropical Cyclone Intensity and Track Prediction

The Third Comparison of Mesoscale Prediction and Research Experiment (COMPARE) workshop was held in Tokyo, Japan, on 13–15 December 1999, cosponsored by the Japan Meteorological Agency (JMA), Japan Science and Technology Agency, and the World Meteorological Organization. The third case of COMPARE focuses on an event of explosive tropical cyclone [Typhoon Flo (9019)] development that occurred during the cooperative three field experiments, the Tropical Cyclone Motion experiment 1990, Special Experiment Concerning Recurvature and Unusual Motion, and TYPHOON-90, conducted in the western North Pacific in August and September 1990. Fourteen models from nine countries have participated in at least a part of a set of experiments using a combination of four initial conditions provided and three horizontal resolutions. The resultant forecasts were collected, processed, and verified with analyses and observational data at JMA. Archived datasets have been prepared to be distributed to participating members for use in further evaluation studies. In the workshop, preliminary conclusions from the evaluation study were presented and discussed in the light of initiatives of the experiment and from the viewpoints of tropical cyclone experts. Initial conditions, depending on both large-scale analyses and vortex bogusing, have a large impact on tropical cyclone intensity predictions. Some models succeeded in predicting the explosive deepening of the target typhoon at least qualitatively in terms of the time evolution of central pressure. Horizontal grid spacing has a very large impact on tropical cyclone intensity prediction, while the impact of vertical resolution is less clear, with some models being very sensitive and others less so. The structure of and processes in the eyewall clouds with subsidence inside as well as boundary layer and moist physical processes are considered important in the explosive development of tropical cyclones. Follow-up research activities in this case were proposed to examine possible working hypotheses related to the explosive development. New strategies for selection of future COMPARE cases were worked out, including seven suitability requirements to be met by candidate cases. The VORTEX95 case was withdrawn as a candidate, and two other possible cases were presented and discussed.