Motivated by the need to obtain accurate objective analysis of field experimental data to force physical parameterizations in numerical models, this paper first reviews the existing objective analysis methods and interpolation schemes that are used to derive atmospheric wind divergence, vertical velocity, and advective tendencies. Advantages and disadvantages of different methods are discussed. It is shown that considerable uncertainties in the analyzed products can result from the use of different analysis. The paper then describes a hybrid approach to combine the strengths of the regular grid and the line-integral methods, together with a variational constraining procedure for the analysis of field experimental data. In addition to the use of upper-air data, measurements at the surface and at the top of the atmosphere (TOA) are used to constrain the upper-air analysis to conserve column-integrated mass, water, energy, and momentum. Analyses are shown for measurements taken in the Atmospheric Ra...

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Objective Analysis of ARM IOP Data: Method and Sensitivity

Nearly 50 years of observations of hook echoes and their associated rear-flank downdrafts (RFDs) are reviewed. Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis remains poorly understood. Despite many strong similarities between simulated and observed storms, some possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional numerical models.

http://tornado.sfsu.edu/geosciences/classes/m835/pdfs/HookEcho.pdf

Hook Echoes and Rear-Flank Downdrafts: A Review

In the present study, hourly infrared (IR) brightness temperatures observed by the Geostationary Meteorological Satellite (GMS) over the region 20°–40°N, 95°–145°E in May–August 1998–2001 are used to compile a climatology of warm-season cloud/precipitation episodes over east Asia. With a goal to better understand the characteristics of warm-season convection and the behavior of these episodes, results are compared with those obtained in North America using radar-derived data. The convection in east Asia, similar to that in North America, is shown to also exhibit coherent patterns and characteristics of propagating events in the longitude–time (Hovmoller) space, with a preferred phase speed of ∼10–25 m s−1, considerably faster than warm-season synoptic-scale waves. Near the eastern edge of the Tibetan Plateau, convection was most active with a strong diurnal signal, peaking in late afternoon or early evening then propagating eastward. The zonal span and duration of episodes could reach 3000 km and...

A Climatology of Warm-Season Cloud Patterns over East Asia Based on GMS Infrared Brightness Temperature Observations

The Taiwan Area Mesoscale Experiment (TAMEX) is a research program conducted jointly by scientists of Taiwan, the Republic of China, and the United States to improve, through better understanding, the forecasting of heavy precipitation events that lead to flash floods. In order to achieve this objective the field phase of TAMEX was launched to collect the data necessary to study the mesoscale circulation associated with the Mei-Yu front; the evolution of the mesoscale convective systems in the vicinity of the Mei-Yu front; and the effects of orography on the Mei-Yu front and on mesoscale convective systems. The field phase extended from 1 May to 29 June 1987, covering 13 operational missions. In addition, soundings were taken every 6 h within the special network for a 1-month period from 15 May to 15 June. Meteorological phenomena on which special observations were collected include the Mei-Yu front, low-level jet, prefrontal squall lines, open-ocean mesoscale convective systems, mountain convection, terr...

The Taiwan Area Mesoscale Experiment (TAMEX): An Overview

The seasonal variations of heavy rainfall days over Taiwan are analyzed using 6-yr (1997–2002) hourly rainfall data from about 360 rainfall stations, including high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System stations and 25 conventional stations. The seasonal variations and spatial variations of nontyphoon and typhoon heavy rainfall occurrences (i.e., the number of rainfall stations with rainfall rate >15 mm h−1 and daily accumulation >50 mm) are also analyzed. From mid-May to early October, with abundant moisture, potential instability, and the presence of mountainous terrain, nontyphoon heavy rainfall days are frequent (>60%), but only a few stations recorded extremely heavy rainfall (>130 mm day−1) during the passage of synoptic disturbances or the drifting of mesoscale convective systems inland. During the mei-yu season, especially in early June, these events are more widespread than in other seasons. The orographic effects are important in determining the spati...

Statistics of Heavy Rainfall Occurrences in Taiwan

In this study, structural features of a subtropical squall line that occurred on 17 May 1987 over the Taiwan Straits, were investigated using dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the storm-relative wind and reflectivity were derived in a horizontal domain of 45 km × 25 km using an objective analysis scheme with 1 km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.3 to 8.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Results show that many structural features of a subtropical squall line are similar to those for a fast-moving tropical squall line. A low-level jet (LLJ) associated with the frontal system provides the necessary strong shear at lower levels. On the front side of the squall line front-to-rear flow prevails at all levels and is accompanied by shallow rear-to-front flow on the back of the line. Ther...

Characteristics of a Subtropical Squall Line Determined from TAMEX Dual-Doppler Data. Part I: Kinematic Structure

In Part I of this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. The dynamic and thermodynamic structures and momentum budgets of the convective rainband will be reported in Part II. Dual-Doppler data were collected from CP-4 and TOGA Doppler radars positioned along the northwest coast of Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 40 km × 36 km using the objective analysis scheme with a 1-km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.8 to 9.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Results show that the low-level jet (LLJ) in the boundary layer provides large vertical shear ahead of the cold front. The orientation of the jet is in the direction almost...

The Structure of a Subtropical Prefrontal Convective Rainband. Part I: Mesoscale Kinematic Structure Determined from Dual-Doppler Measurements

A thermodynamic retrieval method was used to study the dynamical and thermodynamical structure of a subtropical squall line, which occurred on 17 May 1987 over the Taiwan Straits. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology presented in Part I of this paper. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations. Results show that the overall structural features of this subtropical squall line are similar to those of a tropical squall line. The orientation of the squall line is almost in a north–south direction. In the lowest layer, the gust front is located to the immediate east of the main convective updrafts. High pressure occurs behind the gust front with low pressure to its east. A buoyancy-induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. In the middle and upper layers, high pressure forms ...

Characteristics of a Subtropical Squall Line Determined from TAMEX Dual-Doppler Data. Part II: Dynamic and Thermodynamic Structures and Momentum Budgets.

Some dynamic and thermodynamic properties of a convective cell within a squall line that occurred on 6 June 1979 were studied based on dual-Doppler observations. The domain under investigation had a horizontal dimension of 27 km × 27 km with 12 levels in the vertical. The grid spacing used was 1 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Fields of deviation perturbation pressure, density and virtual temperature were recovered from a three-dimensional wind field using the thermodynamic retrieval method. These retrieved fields were then subjected to internal consistency checks to determine the level of confidence. Our findings demonstrate that thermodynamic retrieval is feasible when random errors inherent in the radial wind components are minimized by proper smoothing. Errors in the computation of vertical velocity can be substantially reduced when a variational approach is used with the anelastic continuity equ...

Pressure and temperature perturbations within a squall-line thunderstorm derived from SESAME dual-Doppler data

Perturbation pressure typical of the lowest 5 km of supercell severe thunderstorms was calculated from a composite of single Doppler radar and conventional data from various storms. Most of the data was taken from the 24 May 1973 Union City, Okla., case and gaps were filed with data from the 19 April 1972 Davis, Okla., case and others. Horizontal flow, equivalent radar reactivity factor, vertical velocity and perturbation pressure were compared in three dimensions to explain how updrafts and downdrafts can cooperate in the storm's lower portion (0–6 km) during its mature stage to maintain supercell structure.

Yeong-Jer Lin

Papers

Characteristics of a Subtropical Squall Line Determined from TAMEX Dual-Doppler Data. Part I: Kinematic Structure

The Structure of a Subtropical Prefrontal Convective Rainband. Part I: Mesoscale Kinematic Structure Determined from Dual-Doppler Measurements

Characteristics of a Subtropical Squall Line Determined from TAMEX Dual-Doppler Data. Part II: Dynamic and Thermodynamic Structures and Momentum Budgets.

Pressure and temperature perturbations within a squall-line thunderstorm derived from SESAME dual-Doppler data

A Study of Updraft-Downdraft Interaction Based on Perturbation Pressure and Single-Doppler Radar Data