Shyh-Chin Chen

Project to Intercompare Regional Climate Simulations (PIRCS): Description and initial results

The first simulation experiment and output archives of the Project to Intercompare Regional Climate Simulations (PIRCS) is described. Initial results from simulations of the summer 1988 drought over the central United States indicate that limited-area models forced by large-scale information at the lateral boundaries reproduce bulk temporal and spatial characteristics of meteorological fields. In particular, the 500 hPa height field time average and temporal variability are generally well simulated by all participating models. Model simulations of precipitation episodes vary depending on the scale of the dynamical forcing. Organized synoptic-scale precipitation systems are simulated deterministically in that precipitation occurs at close to the same time and location as observed (although amounts may vary from observations). Episodes of mesoscale and convective precipitation are represented in a more stochastic sense, with less precise agreement in temporal and spatial patterns. Simulated surface energy fluxes show broad similarity with the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) observations in their temporal evolution and time average diurnal cycle. Intermodel differences in midday Bowen ratio tend to be closely associated with precipitation differences. Differences in daily maximum temperatures also are linked to Bowen ratio differences, indicating strong local, surface influence on this field. Although some models have bias with respect to FIFE observations, all tend to reproduce the synoptic variability of observed daily maximum and minimum temperatures. Results also reveal the advantage of an intercomparison in exposing common tendencies of models despite their differences in convective and surface parameterizations and different methods of assimilating lateral boundary conditions.

Regional simulation of the low‐level monsoon winds over the Gulf of California and southwestern United States

Using a fine-scale (10–20 km) nested regional modeling system, synoptic variations in climatological summertime low-level wind fields over the Gulf of California and the southwestern United States are described. Under appropriate synoptic conditions, strong surge events can develop. These are characterized by low-level southerly flow over the entire Gulf of California with southerly winds extending into Arizona, California, and southern Nevada. Vertically, these southerly winds are present through the bottom 1–2 km of the atmosphere. Southerly flow is persistent throughout the day with some local diurnal cycling over the foothills of the Sierra Madre Occidental and northwestern Mexico. Under nonsurge conditions the simulated low-level winds have a significantly different geographic structure. Nighttime southerly flow is limited to the northeastern Gulf and small portions of southwestern Arizona. Flow over the central and southern Gulf is northerly with weak, variable winds over the foothills of the Sierra Madre Occidental. During the day, southerly winds are present over the central and northern Gulf of California, extending into southwestern Arizona; however, this southerly wind pattern does not support continuous flow from the mouth of the Gulf into northwestern Mexico. Instead, there is a westerly component associated with predominantly upslope flow over the foothills of the Sierra Madre Occidental and the Sonora Desert.

Large‐scale forcing of summertime monsoon surges over the Gulf of California and the southwestern United States

Synoptic forcing of the Gulf of California summertime low-level wind field is described using a nested regional modeling system. Under appropriate synoptic conditions, strong surge events develop that are characterized by persistent, vertically extensive (1–2 km) southerly flow extending along the entire Gulf of California and into Arizona, California, and even southern Nevada. These surge periods are initiated either by tropical-cyclone activity to the south of Baja California or by the westward propagation of lower tropospheric troughs from over the Sierra Madre Occidental to the eastern Pacific. The flow over the Gulf is primarily geostrophic and is associated with the presence of these low-pressure centers to the south and west of Baja California.

Global to regional simulations of California wintertime precipitation

A global to regional modeling system, capable of making large-scale climate simulations and forecasts and then downscaling these forecasts and simulations to regional scales, is evaluated for the California region. Regional climate simulations of wet and dry winters compare well with available observations, especially in comparison to global simulations. While the global model paints a broad swath of precipitation across the western United States, the regional model, forced by the large-scale background flow produced by the global model, more accurately simulates the increased precipitation over the Sierra Nevada and coastal regions. The regional model is especially advantageous for intense precipitation events at regional spatial scales. Although the regional simulations do not significantly alter the imposed large-scale features, they do significantly enhance the spatial as well as temporal variations of the hydrologic variables. Basically, down-scaling by a regional model is a dynamically consistent method, which extends the usefulness of global climate forecasts. Still, current regional simulations are not perfect. Even higher resolution and further improvement in physical parameterizations are still needed.

Model dynamics of summertime low-level jets over northwestern Mexico

Using a fine-scale nested regional modeling system, the diurnal forcing of summertime low-level winds over the Gulf of California and northwestern Mexico is investigated. On diurnal timescales, simulated nocturnal low-level jets develop over the northern portion of the Gulf, the foothills of the Sierra Madre Occidental, and parts of southern Arizona. The southerly component of the nocturnal jet is the result of a geostrophic balance involving the Coriolis force and a cross-gulf pressure gradient force associated with nighttime slope cooling over the elevated Sierra Madre Occidental. Additionally, horizontal temperature gradients over the sloped orography produce vertical variations in this cross-gulf pressure gradient force, generating the jet-like vertical shear in wind components above the nocturnal boundary layer; frictional effects are responsible for producing shear in the wind profiles below the boundary layer. This balance is distinctly different from the inertial balance that is believed to be responsible for the low-level jet over the Great Plains region of the United States. Daytime winds are part of a directly driven wind field forced by horizontal pressure gradients associated with slope heating (up the Sierra Madre Occidental) and sea-land temperature gradients (north of the Gulf). During synoptically forced surge events, a similar diurnal cycle in low-level flow is still present; however, the local thermal forcing appears to be superimposed upon the large-scale synoptic forcing, resulting in weaker up-slope flow during the day and stronger along-slope flow at night.

ECPC's Weekly to Seasonal Global Forecasts

Abstract The Scripps Experimental Climate Prediction Center (ECPC) has been making experimental, near–real–time seasonal global forecasts since 26 September 1997 with the NCEP global spectral model used for the reanalysis. Images of these forecasts, at daily to seasonal timescales, are provided on the World Wide Web and digital forecast products are provided on the ECPC anonymous FTP site to interested researchers. These forecasts are increasingly being used to drive regional models at the ECPC and elsewhere as well as various application models. The purpose of this paper is to describe the forecast and analysis system, various biases and errors in the forecasts, as well as the significant skill of the forecasts. Forecast near–surface meteorological parameters, including temperature, precipitation, soil moisture, relative humidity, wind speed, and a fire weather index (a nonlinear combination of temperature, wind speed, and relative humidity) are skillful at weekly to seasonal timescales over much of the ...

Global aspects of the Los Alamos general circulation model hydrologic cycle

The global hydrologic cycle in the Los Alamos general circulation model (GCM) is compared to available global observations. Global observations of the water vapor, water-vapor flux and water-vapor flux divergence are derived from the National Meteorological Centers final analysis for the period 1986–1989. The new precipitation data set of Legates and Willmott (1990) is used for the global precipitation observations. Global evaporation is derived as a residual of the precipitation and water-vapor flux divergence. There are a number of similarities as well as discrepancies between the GCM and observations. The large-scale nondivergent and divergent GCM circulations are remarkably similar to the observed circulations; the large-scale GCM precipitation and evaporation patterns are also qualitatively similar to observations. Discrepancies are mainly quantitative and small-scale in nature: the GCM atmosphere is relatively dry which results in a slightly greater evaporation and precipitation rate than is observed; the GCM South Pacific convergence zone is displaced too far to the northwest.

GDAS’s GCIP Energy Budgets

Abstract The National Centers for Environmental Prediction’s operational global data assimilation system’s (GDAS) atmospheric and surface thermodynamic energy cycles are presented for the Mississippi River basin where the Global Energy and Water Cycle Experiment Continental-Scale International Project (GCIP) is under way. At the surface, during the winter, incoming solar radiation is balanced by longwave cooling. During the summer, latent and sensible cooling are equally important. In the atmosphere, thermodynamic energy convergence is also important, especially during the winter. In most places, precipitation is largely balanced by thermodynamic energy divergence. Anomalously high surface temperatures appear to be mainly related to decreased surface evaporation. Anomalously high (low) precipitation variations may also be related to anomalously high thermodynamic energy divergence (convergence). Unfortunately, residual terms, which are slightly noticeable for the GCIP climatological balances, are especial...

Variability and Predictability in an Empirically Forced Global Model

Abstract Climatological and predictability features of a simplified moist general circulation model are described herein. The simplified model is driven by empirical forcings designed to eliminate systematic errors and maintain computational efficiency. Resulting perpetual January climatological features of forced variables such as the tropospheric heights and rotational winds, as well as unforced variables such as the velocity potential, compare well with the observations. Unforced temporal variations in the midlatitude 500-mb geopotential, as well as the tropical circulations intraseasonal oscillation, are also simulated reasonably well. Short-range persistence and predictability features of this model replicate geographical persistence and predictability features from simpler models and from numerical weather prediction. The streamfunction is highly persistent in the extratropics, less so in the tropical regions; similarly, the streamfunction is predicted better in midlatitude regions than in the trop...