Chung-Chieng Lai

A Technique for Diagnosing Three-Dimensional Ageostrophic Circulations in Baroclinic Disturbances on Limited-Area Domains

Abstract The methodology developed by Keyser et al. for representing and diagnosing three-dimensional vertical circulations in baroclinic disturbances using a two-dimensional vector streamfunction, referred to as the psi vector, is restricted to f-plane channel-model geometry. The vertical circulation described by the psi vector consists of the irrotational (or divergent) part of the ageostrophic wind and the vertical velocity. A key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations, allowing separation of a three-dimensional vertical circulation into two two-dimensional components, and thus objective assessment of the extent to which a three-dimensional vertical circulation is oriented in a preferred direction. Here the methodology for determining the psi vector is modified to be suitable for real-data applications. The modifications consists of reformulating the diagnostic equations to apply to conformal map pr...

Initiation and Evolution of an Intense Upper-Level Front

Abstract Within confluent northwesterly flow of an intensifying baroclinic wave over North America in late October 1963, an intense frontal zone developed in 12 h near the inflection point in the middle and upper troposphere. By 24 h after its initial appearance, the zone extended roughly from 400 to 700 mb and from the inflection point to just beyond the downstream trough. Horizontal gradients of potential temperature reached 15–20 K (100 km)−1. Air within the frontal zone was extremely dry. As the accompanying trough approached the east coast of he United States, surface frontogenesis occurred offshore, remaining distinct from the upper-level front. A region of subsidence, elongated in the direction of the upper-level flow, displayed maximum descent on the warm edge of the frontal zone and played a frontogenetical role through tilting of the isentropic surfaces. Analysis of isentropic potential vorticity showed significant increase of this quantity near the cold base and a probable decrease near the top...

Data Assimilation and Model Evaluation Experiment Datasets

The Institute for Naval Oceanography, in cooperation with Naval Research Laboratories and universities, executed the Data Assimilation and Model Evaluation Experiment (DAMEE) for the Gulf Stream region during fiscal years 1991–1993. Enormous effort has gone into the preparation of several high-quality and consistent datasets for model initialization and verification. This paper describes the preparation process, the temporal and spatial scopes, the contents, the structure, etc., of these datasets. The goal of DAMEE and the need of data for the four phases of experiment are briefly stated. The preparation of DAMEE datasets consisted of a series of processes: 1) collection of observational data; 2) analysis and interpretation; 3) interpolation using the Optimum Thermal Interpolation System package; 4) quality control and reanalysis; and 5) data archiving and software documentation. The data products from these processes included a time series of 3D fields of temperature and salinity, 2D fields of surface dy...

The Synoptic and Subsynoptic Structure of a Long-Lived Severe Convective System

Abstract A long-lived severe convective system in the southern United States from 2–4 May 1978 is documented. The distinguishing feature of the convection was its origin in a region of deep synoptic scale ascent and its subsequent steady motion away from that ascent toward increasingly warmer and more moist, unstable boundary layer air. On 2 May 1978 a north-south oriented squall line originated above and within a shallow cold air mass in west central Texas north of a quasi-stationary east-west oriented frontal boundary. Potential instability was generated by a warm, moist airmass from the Gulf of Mexico that advected westward beneath a warm, dry plume of air moving northward ahead of the trough aloft. Convection first appeared along an inverted cough in western Texas that separated southward-flowing cool air, dammed up against the New Mexico mountains, from slightly warmer northeasterly and easterly flow to the east. Frontogenetical processes associated with the inverted trough played a crucial role in t...

ECODYNAMICS AND DISSOLVED GAS CHEMISTRY ROUTINES FOR OCEAN CIRCULATION MODELS

Abstract Interactions between oceanic nitrate ecology and circulation determine the marine distribution of dissolved, climate relevant trace gases such as dimethyl sulfide (DMS) and carbonyl sulfide (OCS), and a variety of hydrocarbons. Our group is constructing a suite of ecosystem/reaction/transport models, which link nitrate to the chemistry of volatiles near the sea–air interface. In this paper, we describe programs which will be inserted into the high resolution Parallel Ocean Program. Major features of the coding include: (1) ecodynamics represented in seven biological compartments (phytoplankton, zooplankton, bacteria, detritus, nitrate, ammonium and dissolved organic nitrogen). Light limited primary production is computed, along with nitrogen cycling among the bioentities. (2) Photochemistry for the volatile species DMS, OCS, the methyl halides, nonmethane hydrocarbons (NMHC) and ammonia. DMS and the halides are emitted by phytoplankton, while OCS and NMHC are produced by photolysis of dissolved organic material. Ammonia is exuded by animals and bacteria. Removal mechanisms for the gases include consumption by organisms, hydrolysis, chlorination and interfacial transfer. (3) Explicit, efficient and mass conserving numerical solutions for the biological and chemical continuity equations. Production and loss forms are generalized and automated so that they are readily applied to new constituents. Ecology and the chemical transformations are exposed qualitatively to begin, and are then expressed as differential and differencing equations. The structure of the program is described in terms of the major subroutines and their purposes. Results are provided from both one- and three-dimensional sample runs. Computational aspects such as performance and code availability are discussed.

A Case Study of Heavy Rainfall Associated with Weak Cyclogenesis in the Northwest Gulf of Mexico

Abstract This paper describes a case of unexpected weak cyclogenesis over the northwestern Gulf of Mexico from 16 to 19 September 1984 based upon manually prepared and European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses. Noteworthy aspects of the case include: 1) upward of 50 cm of rain along the extreme southern coast of Texas and 2) the brief occurrence of minimal strength tropical-storm conditions in a weak baroclinic marine environment. A crucial antecedent condition to rainstorm formation was the creation of a low-level baroclinic zone over the northwestern Gulf of Mexico due to the southward advance of drier and slightly cooler air behind a cold front that penetrated into northeastern Mexico. Four factors were responsible for rainfall concentration along the coast: 1) a northward-moving 700-mb trough and embedded vorticity maximum in the easterlies over the western Gulf of Mexico, 2) an eastward-propagating upper-tropospheric disturbance in the midlatitude westerlies over the...

A Case Study of Trough Merger In Split Westerly Flow

Abstract Trough merger, defined as the amalgamation of two separate vorticity centers in distinct branches of westerlies into one coherent vorticity center, is studied for a case In November 1980. The analysis approach takes advantage of the failure of the operational Limited-Area Fine-Mesh Model (LFM) to simulate the mow process. A diagnostic analysis of the observed and forecast vorticity and thermodynamic structure is used to help isolate physical processes occurring during trough merger. During the period 17–19 November 1980 major cyclogenesis occurred along the coast of North America as a deepening 500 mb trough in the northern branch of the westerlies merged with a weaker trough in the southern branch of the westerlies. Cyclonic vorticity in the middle troposphere ahead of the northern trough was generated by: 1) horizontal convergence in the lower and middle troposphere and the upward advection of this vorticity and, 2) horizontal advection in the middle and upper troposphere. Frontogenesis along t...