Joseph A. Zehnder

Urban Modifications in a Mesoscale Meteorological Model and the Effects on Near-Surface Variables in an Arid Metropolitan Region

Abstract A refined land cover classification for the arid Phoenix (Arizona) metropolitan area and some simple modifications to the surface energetics were introduced in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The single urban category in the existing 24-category U.S. Geological Survey land cover classification used in MM5 was divided into three classes to account for heterogeneity of urban land cover. Updated land cover data were derived from 1998 Landsat Thematic Mapper satellite images. The composition of the urban land use classes in terms of typical fractions of vegetation and anthropogenic surfaces was determined from ground-truth information, allowing a variety of moisture availability for evaporation by land cover class. Bulk approaches for characteristics of the urban surface energy budget, such as heat storage, the production of anthropogenic heat, and radiation trapping, were introduced in MM5’s Medium Range Forecast boun...

Contribution of land use changes to near-surface air temperatures during recent summer extreme heat events in the Phoenix Metropolitan Area

Abstract The impact of 1973–2005 land use–land cover (LULC) changes on near-surface air temperatures during four recent summer extreme heat events (EHEs) are investigated for the arid Phoenix, Arizona, metropolitan area using the Weather Research and Forecasting Model (WRF) in conjunction with the Noah Urban Canopy Model. WRF simulations were carried out for each EHE using LULC for the years 1973, 1985, 1998, and 2005. Comparison of measured near-surface air temperatures and wind speeds for 18 surface stations in the region show a good agreement between observed and simulated data for all simulation periods. The results indicate consistent significant contributions of urban development and accompanying LULC changes to extreme temperatures for the four EHEs. Simulations suggest new urban developments caused an intensification and expansion of the area experiencing extreme temperatures but mainly influenced nighttime temperatures with an increase of up to 10 K. Nighttime temperatures in the existing urban c...

The interaction of planetary-scale tropical easterly waves with topography : a mechanism for the initiation of tropical cyclones

Abstract The interaction of a basic state flow consisting of a planetary-scale easterly wave superimposed on a uniform easterly zonal wind with an isolated topographic feature is examined by numerical integration of the shallow water equations in an equatorial beta-plane channel. The topographic parameters are chosen to represent the Sierra Madre Mountains of Mexico. The zonal wavelength and phase speeds of the basic state easterly waves are chosen to correspond with previously published observations. The topographic modification of a slow Rossby mode in which the basic state vorticity maximum is located near 15° latitude is characterized by the generation of a cyclonic vorticity maximum in the lee of the mountain, which extends the entire length of the ridge, and a secondary maximum which propagates downstream. A fast mixed Rossby–gravity mode in which the vorticity maximum is located at the equator interacts less strongly with the mountain, with a lee vorticity maximum that is confined near the southern...

Simple Modifications to Improve Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Performance for the Phoenix, Arizona, Metropolitan Area

Abstract The diurnal temperature cycle in the Phoenix, Arizona, metropolitan area, as represented in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), is examined using a high-resolution 2-km grid spacing to simulate the dry portion of the summer. The model is run for a 48-h period with negligible synoptic forcing and a sufficiently dry atmosphere so that moist convection does not occur. The standard version of MM5 underestimates the magnitude of the diurnal maximum and also causes the maximum to occur too early. This behavior is due to an overestimate of the latent heat flux in the model, resulting from a poor specification of the land cover category and corresponding physical parameters. Adjusting the available moisture improves the modeled temperature maximum at both rural and urban sites, but the model temperatures are still cooler than those observed at the urban site at night because of a poorly represented urban heat island effect. It...

The Influence of Large-Scale Topography on Barotropic Vortex Motion

Abstract The motion of a barotropic vortex in the vicinity of a large-scale topographic feature is examined through numerical integration of the shallow-water equations in an equatorial beta-plane channel. The topographic parameters are chosen to represent the Sierra Madre mountains of Mexico. The motion of the vortex is affected through modification of the asymmetric circulation by vortex stretching associated with divergence of the horizontal velocity field as the air flows across the mountain. A vortex that approaches the topography from the east is accelerated and deflected toward the south and a vortex initially west of the mountain recurves and moves toward the cut. This motion is in contrast to the northwestward motion of a vortex over level terrain. The vortex motion is also compared to the direction of the imposed potential vorticity gradient. The topographic effects described in this paper may be important in forecasting hurricane motion in the eastern North Pacific and western Gulf of Mexico.

The Cumulus, Photogrammetric, In Situ, and Doppler Observations Experiment of 2006

The finescale structure and dynamics of cumulus, evolving from shallow to deep convection, and the accompanying changes in the environment and boundary layer over mountainous terrain were the subjects of a field campaign in July–August 2006. Few measurements exist of the transport of boundary layer air into the deep troposphere by the orographic toroidal circulation and orographic convection. The campaign was conducted over the Santa Catalina Mountains in southern Arizona, a natural laboratory to study convection, given the spatially and temporally regular development of cumulus driven by elevated heating and convergent boundary layer flow. Cumuli and their environment were sampled via coordinated observations from the surface, radiosonde balloons, and aircraft, along with airborne radar data and stereophotogrammetry from two angles. The collected dataset is expected to yield new insights in the boundary layer processes leading to orographic convection, in the cumulus-induced transport of boundary layer a...

Orographic Influence on the Synoptic-Scale Circulations Associated with the Genesis of Hurricane Guillermo (1991)

Abstract The early stages of tropical cyclogenesis in the eastern Pacific Ocean are investigated in this case study, which is focused on the development of the initial circulation that eventually intensified into Hurricane Guillermo (1991). The authors document the synoptic and mesoscale winds from upper-air soundings, satellite imagery, and a gridded analysis. In addition, flight-level observations taken during the Tropical Experiment in Mexico are used. These winds reveal that, prior to the formation of the tropical cyclone, an easterly wave moved over the Caribbean Sea and that the initial circulation developed while the easterly wave was located over the central Caribbean, east of the mountains in Central America. Numerical simulations with the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model are used to examine the details of the structure of the flow that resulted in the formation of the eastern Pacific circulation. In these simulations, the model is initialized...

Simulations of the Urban Planetary Boundary Layer in an Arid Metropolitan Area

Abstract A modified version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was applied to the arid Phoenix, Arizona, metropolitan region. The ability of the model to simulate characteristics of the summertime urban planetary boundary layer (PBL) was tested by comparing model results with observations from two field campaigns conducted in May/June 1998 and June 2001. The modified MM5 included a refined land use/cover classification and updated land use data for Phoenix and bulk approaches of characteristics of the urban surface energy balance. PBL processes were simulated by a version of MM5’s Medium-Range Forecast Model (MRF) scheme that was enhanced by new surface flux and nonlocal mixing approaches. Simulated potential temperature profiles were tested against radiosonde data, indicating that the modified MRF scheme was able to simulate vertical mixing and the evolution and height of the PBL with good accuracy and better than the origi...

A stereo photogrammetric technique applied to orographic convection

Abstract This paper describes a technique for photogrammetric analysis of stereo pairs of images that is applied to the study of orographic convection. The technique is designed for use with digital images and assumes detailed knowledge of the camera properties (focal length and imaging chip) and that the position and orientation are known as a first guess. An iterative scheme using known landmarks on the frame is used to determine the camera orientation. The scheme is accurate to 10–100 m at a distance of 15 km from the camera pair. The transition from shallow to deep convection over the Santa Catalina Mountains in southern Arizona on 26 July 2005 is presented. The three-dimensional structure of the visible portion of the cloud is determined and compared with the composite reflectivity from the National Weather Service Weather Surveillance Radar-1988 Doppler radar and the tropopause height from the 1200 UTC sounding in Tucson, Arizona, providing additional validation of the scheme. The shallow to deep tr...

Frontogenesis over a Mountain Ridge

Abstract The interaction Of developing two-dimensional cold and warm frontal systems with a mesoscale mountain ridge is examined. The flow of the rotating model atmosphere is assumed to be inviscid, adiabatic, and Boussinesq. The geostrophic momentum approximation is made. An imposed horizontal deformation field forces the frontogenesis. The nonlinear model equations are solved numerically in physical space using terrain-following Coordinates which incorporate the fully nonlinear, lower boundary condition. Comparison of the model results with and without topography enables assessment of the impact of the mountain on the frontogenesis. A scale analysis indicates that the most general problem of frontal interaction with an infinite mountain ridge encompasses a seven-dimensional parameter space. The scale analysis provides justification for die two-dimensional geostrophic momentum approximation and defines an inverse Richardson number as a measure of the importance of the ageostrophic advection. Sensitivity ...