John D. Horel

Planetary-scale atmospheric phenomena associated with the Southern Oscillation.

Abstract Atmospheric phenomena associated with the Southern Oscillation are examined, with emphasis on vertical structure and teleconnections to middle latitudes. This paper is specifically concerned with the interannual variability of seasonal means for the Northern Hemisphere winter during the period 1951–78. Among the variables considered are sea surface temperature in the equatorial Pacific, precipitation at selected equatorial Pacific stations, a “Southern Oscillation Index” of sea level pressure, 200 mb height and tropospheric mean temperature at stations throughout the tropics, and Northern Hemisphere geopotential height fields. Selected statistics derived from surface data also are examined for the period 1910–45. Results are presented in the form of time series and correlation statistics for the variables listed above. Results concerning the relationships between sea surface temperature, sea level pressure and rainfall are consistent with the major conclusions of previous studies by J. Bjerknes a...

A Rotated Principal Component Analysis of the Interannual Variability of the Northern Hemisphere 500 mb Height Field

Abstract The principal components derived by Wallace and Gutzler (1981) from a 500 mb height data set are linearly transformed using the varimax method. Their data set consists of 45 winter months of National Meteorological Center analyses of Northern Hemisphere 500 mb height. The linear transformation (or rotation) of the principal components emphasizes the strongest relationships within the 500 mb height data set; hence, spatial patterns associated with the rotated principal components are simpler to interpret than the spatial patterns associated with the unrotated components. The teleconnection patterns identified by Wallace and Gutzler (1981) on the basis of the negative extrema approach closely resemble several of the spatial patterns of the rotated principal components. In order to show the seasonal dependence of the rotated principal components, an expanded data set consisting of 30 years of 500 mb height data is used. Most of the teleconnection patterns derived from the 90 winter month data set ar...

Complex Principal Component Analysis: Theory and Examples

Abstract Complex principal component (CPC) analysis is shown to be a useful method for identifying traveling and standing waves in geophysical data sets. Combinations of simple progressive and standing oscillations are used to examine the properties of this technique. These examples illustrate that although CPC analysis allows for the identification of traveling waves, many of the drawbacks associated with conventional principal component analysis remain, and sometimes become worse; e.g. the interpretation of CPC solutions is more difficult since both amplitude and phase relationships must be considered. A method for linearly transforming complex principal components was devised in order to identify regional relationships within large geophysical data sets. The errors in CPC analysis resulting from limited sample sizes are discussed.

Mesowest: Cooperative Mesonets in the Western United States

Abstract Meteorological data from over 2800 automated environmental monitoring stations in the western United States are collected, processed, archived, integrated, and disseminated as part of the MesoWest program. MesoWest depends upon voluntary access to provisional observations from environmental monitoring stations installed and maintained byfederal, state, and local agencies and commercial firms. In many cases, collection and transmission of these observations are facilitated by NWS forecast offices, government laboratories, and universities. MesoWest augments the Automated Surface Observing System (ASOS) network maintained by the NWS, Federal Aviation Administration, and Department of Defense. MesoWest increases the coverage of observations in remote locations and helps capture many of the localand mesoscale weather phenomena that impact the public. The primary goal of MesoWest is to improve timely access to automated observations for NWS forecasters at offices throughout the western United States. ...

An investigation of the Annual Cycle of Convective Activity over the Tropical Americas

Abstract Outgoing longwave radiation (OLR) is used to describe the annual cycle of convection that resides over the Amazon Basin during austral summer and over Central America and the adjacent waters of the Pacific during austral winter. The preferred locations of the convective activity during the wet season in the respective hemispheres are determined, and the beginning and ending of these seasons is specified. The onset of the wet season over Amazonia usually occurs within a single month, while the onset of the wet season over Central America typically requires from one to three months. The annual cycle of convective activity in this regime is shown to exhibit a seasonal regularity and degree of symmetry with respect to the equator which exceeds those characterizing the other two annually varying regimes in the tropical belt. Analyses produced by the European Centre for Medium-range Weather Forecasts (ECMWF) are superimposed upon OLR fields to illustrate features of the atmospheric circulation in the v...

Sensitivity of a Spectrally Filtered and Nudged Limited-Area Model to Outer Model Options

Abstract Numerical filters required to control spatial computational modes in a limited-area model (LAM) that uses the unstaggered. A grid are developed and tested over the complex topography of the Great Basin of the western United States. The filters are founded upon Fourier expansions of forecast deviation fields and function equally effectively for both periodic and aperiodic local structures. Unlike other spatial filters, the approach used here avoids any direct contamination of larger scales. Provided that the shortest resolved wavelength of two grid intervals is removed, the results do not depend strongly on the range of filtered short waves or on the type and order of horizontal space difference approximations. This approach leads naturally to methods in which the large scales predicted by an ambient outer model can be directly incorporated within the complete domain of the inner LAM, rather than just through conditions applied at the lateral boundaries of the LAM. This technique has some similari...

On the Annual Cycle of the Tropical Pacific Atmosphere and Ocean

Abstract The annual cycle in sea surface temperature (SST), surface wind and other atmospheric variables in the tropical Pacific are described. The primary data sets of SST and surface wind are derived from ship observations in the Pacific between 29°N and 29°S during the period 1946–76. The annual cycle in SST away from the equator can be attributed to the annual cycle in solar heating. However, in the eastern equatorial Pacific, the annual cycle in SST undergoes systematic longitudinal changes in phase and amplitude. Near the coast of Peru, the warmest temperatures occur during March, while further west along the equator, the warmest temperatures occur progressively later and with diminished amplitude. The annual cycle in surface wind convergence along the equator displays similar changes with longitude. The annual cycle in surface wind is dominated by the meridional migrations of the trade wind belts. Near the equator, the amplitude of the annual cycle in meridional wind is larger than that in zonal wi...

Cold-Air-Pool Structure and Evolution in a Mountain Basin: Peter Sinks, Utah

Abstract The evolution of potential temperature and wind structure during the buildup of nocturnal cold-air pools was investigated during clear, dry, September nights in Utahs Peter Sinks basin, a 1-km-diameter limestone sinkhole that holds the Utah minimum temperature record of −56°C. The evolution of cold-pool characteristics depended on the strength of prevailing flows above the basin. On an undisturbed day, a 30°C diurnal temperature range and a strong nocturnal potential temperature inversion (22 K in 100 m) were observed in the basin. Initially, downslope flows formed on the basin sidewalls. As a very strong potential temperature jump (17 K) developed at the top of the cold pool, however, the winds died within the basin and over the sidewalls. A persistent turbulent sublayer formed below the jump. Turbulent sensible heat flux on the basin floor became negligible shortly after sunset while the basin atmosphere continued to cool. Temperatures over the slopes, except for a 1–2-m-deep layer, became war...

The Persistent Cold-Air Pool Study

The Persistent Cold-Air Pool Study (PCAPS) was conducted in Utahs Salt Lake valley from 1 December 2010 to 7 February 2011. The field campaigns primary goal was to improve understanding of the physical processes governing the evolution of multiday cold-air pools (CAPs) that are common in mountain basins during the winter. Meteorological instrumentation deployed throughout the Salt Lake valley provided observations of the processes contributing to the formation, maintenance, and destruction of 10 persistent CAP episodes. The close proximity of PCAPS field sites to residences and the University of Utah campus allowed many undergraduate and graduate students to participate in the study. Ongoing research, supported by the National Science Foundation, is using the PCAPS dataset to examine CAP evolution. Preliminary analyses reveal that variations in CAP thermodynamic structure are attributable to a multitude of physical processes affecting local static stability: for example, synoptic-scale processes impact ...

Convection along the Coast of Northern Peru during 1983: Spatial and Temporal Variation of Clouds and Rainfall

Abstract Streamflow and historical records indicate that flooding in northern Peru was more severe during 1983 than during any year since 1891. A case study of the meteorological conditions along the northwest coast of South America from 10°S to 10°N during 1982–83 is presented. Station rainfall and satellite-derived outgoing infrared observations are used to deduce the structure and time evolution of convection in this region. Substantial rainfall amounts were first observed along the western slopes of the Andes Mountains and coastal plain of southern Ecuador during November and December 1982, and it continued to rain in this region through June 1983. In northern Peru, the onset of the rains along the coastal plain was delayed until January 1983 and ended abruptly during mid-June 1983. Convective activity was much greater along the coastal strip than over the eastern equatorial Pacific to the cast of 100°W until May-June 1983. In addition, cloudiness was strongly modulated on the diurnal time scale, with...