Horizontal Pressure Field Research Articles

Aircraft Observations and Numerical Simulations of the Developing Stage of a Southerly Surge near Southern California

Abstract Summertime low-level winds in the marine boundary layer off the California coast are predominantly from the north. This pattern is interrupted periodically by southerly winds and low stratus that can propagate for hundreds of kilometers northward along the coast. These events have been termed coastally trapped disturbances, coastally trapped wind reversals, and southerly surges; their forcing has been the subject of extensive study and debate. Southerly surges remain difficult to forecast, yet have a significant impact on coastal activities. The beginning stage of a southerly surge on 16 June 2012 was explored during the Precision Atmospheric Marine Boundary Layer Experiment. Measurements of the horizontal wind and pressure field in the marine layer offshore from Cape Arguello hours prior to the onset of the surge were made using the University of Wyoming King Air research aircraft. Airborne measurements show that a horizontal pressure field is established with higher pressure to the south just prior to the surge, supporting southerly ageostrophic winds and a south-to-north movement of marine stratus. Aircraft soundings and lidar returns confirm the existence of offshore flow of warm, continental air north of Point Arguello that alters the pressure field adjacent to the coast. The southerly surge originates near Point Arguello and propagates northward past San Francisco during the early morning hours on 17 June 2012. Results from the Weather Research and Forecasting Model are consistent with the King Air observations. Analyses and model output presented here confirm that the large-scale environment is critical to the initiation of these wind reversals.

Aircraft Observations of the Marine Boundary Layer Adjustment near Point Arguello, California

AbstractNortherly winds set up by synoptic conditions are persistent in the marine boundary layer (MBL) off the California coast from late spring through summer. Wind, pressure, and MBL height are modulated as the low-level flow impinges on the points and capes along the California coast. The Precision Atmospheric Marine Boundary Layer Experiment was conducted in May and June of 2012 with the primary goal to directly measure the dynamics responsible for the wind field near Point Arguello and Point Conception. Detailed measurements of the horizontal pressure field within the MBL were made using the University of Wyoming King Air research aircraft. Airborne measurements made during cases of strong northerly wind show an abrupt adjustment of the MBL near Point Arguello, including a modulation of the horizontal pressure gradient force and a near collapse of the MBL. Airborne lidar measurements complement measurements of the horizontal pressure field and help to elucidate the large changes in the MBL height in the vicinity of Point Arguello. The Weather Research and Forecasting Model was used to simulate the 20 May 2012 case at a high resolution. Model results showed large-amplitude height perturbations near Point Arguello, similar to those observed from the airborne platform. In this case, the offshore flow played an important role in the local forcing.

Airborne Measurements of Terrain-Induced Pressure Perturbations

Abstract Airborne measurement of the horizontal pressure field using differential GPS technology has been established during the last few years. Accurate aircraft measurement of the horizontal pressure gradient force requires an independent determination of the height of the airborne platform above some reference level. Here the authors demonstrate a differential GPS technique that uses data from a fixed reference station to refine the vertical position of the aircraft. A series of research flight legs by the University of Wyoming King Air research aircraft (UWKA) were conducted during the winter seasons of 2008 and 2009 over the Medicine Bow Mountains in southern Wyoming. Flight patterns consisted of a series of geographically fixed, parallel legs along a quasi-isobaric surface above the mountainous terrain, allowing the finescale mapping of the horizontal pressure (or geopotential height) field. The removal of the large-scale gradient and tendency isolates the terrain-induced pressure perturbation field. Results obtained using differential GPS measurements of aircraft height show that the Medicine Bow Range induces pronounced horizontal pressure perturbations, with a leeside region of low pressure downwind of the crest, in two cases: on 11 February 2008 and 20 February 2009. A wind maximum is found downwind of the elevated terrain consistent with this pressure gradient. Simulations of these two cases were performed using the Weather Research and Forecasting Model (WRF). The WRF height patterns for the time of the UWKA flight matched the general isobaric height patterns observed. Simulations and observations consistently show that the cross-mountain acceleration is stronger when the perturbation pressure gradient is larger.

Airborne Observations of a Catalina Eddy

Abstract Summertime low-level winds over the ocean adjacent to the California coast are typically from the north, roughly parallel to the coastline. Past Point Conception the flow often turns eastward, thereby generating cyclonic vorticity in the California Bight. Clouds are frequently present when the cyclonic motion is well developed and at such times the circulation is referred to as a Catalina eddy. Onshore flow south of the California Bight associated with the eddy circulation can result in a thickening of the low-level marine stratus adjacent to the coast. During nighttime hours the marine stratus typically expands over a larger area and moves northward along the coast with the cyclonic circulation. A Catalina eddy was captured during the Precision Atmospheric Marine Boundary Layer Experiment in June of 2012. Measurements were made of the cloud structure in the marine layer and the horizontal pressure field associated with the cyclonic circulation using the University of Wyoming King Air research aircraft. Airborne measurements show that the coastal mountains to the south of Los Angeles block the flow, resulting in enhanced marine stratus heights and a local pressure maximum near the coast. The horizontal pressure field also supports a south–north movement of marine stratus. Little evidence of leeside troughing south of Santa Barbara, California, was observed for this case, implying that the horizontal pressure field is forced primarily through topographic blocking by the coastal terrain south of Los Angeles, California, and the ambient large-scale circulation associated with the mean flow.

Measurement of Cloud Perturbation Pressures Using an Instrumented Aircraft

Abstract Vertical accelerations during the early stages of convective cloud formation are often the result of buoyancy and the perturbation vertical pressure gradient forces. Convection modifies the local pressure field surrounding the cloud. Measurement of the cloud perturbation pressure field is challenging over distance scales on the order of the convective elements, since the signals are often small and the turbulent environment complicates the measurement of static pressure. A technique is described that enables detection of the horizontal pressure perturbations associated with evolving convective clouds using global positioning system measurements on an airborne platform. Differential kinematic processing of data from dual-frequency, carrier-phase-tracking GPS receivers on research aircraft with static base station receivers enables the three-dimensional aircraft position to be resolved within decimeters. Vertical positioning and precise measurement of static pressure allow horizontal pressure perturbations to be determined to an accuracy of roughly 10 Pa. Errors in the static pressure measurement, rather than the GPS-derived altitude, are the largest source of error. A field experiment was conducted in May–June 2008 to demonstrate measurement of perturbations in the horizontal pressure field associated with summertime cumulus congestus clouds over the high plains. Observations of growing convective clouds show negative pressure perturbations on the order of 100 Pa near cloud base linked to updraft regions. Growing cumulus show a high degree of variability between subsequent passes that demonstrate that the horizontal pressure fields evolve rapidly along with attendant vertical circulations and cloud microphysical characteristics.

Isobaric Height Perturbations Associated with Mountain Waves Measured by Aircraft during the Terrain-Induced Rotor Experiment

Abstract It has only been in the last few years that accurate measurement of the horizontal pressure gradient has been possible over complex terrain using an airborne platform. To infer forcing mechanisms for the wind, an independent measure of the height of an isobaric surface is required. Differential GPS analyses have enabled determination of the aircraft height with sufficient accuracy to infer isobaric heights. When coupled with an accurate measurement of static pressure, the horizontal pressure field can be determined. To demonstrate this measurement technique, research flight legs by the University of Wyoming King Air (UWKA) conducted in support of the Terrain-Induced Rotor Experiment (T-REX) in March and April 2006 are examined. UWKA flights conducted on 14 and 25 March and 16 April 2006 encountered strong mountain waves in response to winds directed primarily normal to the Sierra Nevada ridgeline. Winds at flight level showed pronounced variation that suggested topographic influence. The magnitude of isobaric height perturbations along UWKA flight tracks obtained using differential GPS during case study days of 14 and 25 March and 16 April are shown to exceed 70 m, corresponding to horizontal pressure perturbations greater than 4 hPa. Measurements suggest that changes in wind speed are linked primarily to the perturbation height field and that the flow can be classified as Eulerian, implying that Coriolis accelerations are negligible and flows respond to the horizontal pressure gradient force.

On the Role of Sloping Terrain in the Forcing of the Great Plains Low-Level Jet

Abstract The summertime Great Plains low-level jet (LLJ) has been the subject of numerous investigations during the past several decades. Characteristics of the LLJ include nighttime development of a pronounced wind maximum of typically 15–20 m s−1 at levels 300–800 m above the surface and a clockwise rotation of the wind maximum during the course of the night. Maximum frequency of occurrence of the LLJ is found in the southern Great Plains. Theories proposed to explain the diurnal wind maximum of the Great Plains LLJ include inertial oscillation of the ageostrophic wind, the diurnal oscillation of the horizontal pressure field associated with heating and cooling of the sloping terrain, and the western boundary current interpretations. A simple equation system and output from the 12-km horizontal resolution Weather Research and Forecasting Nonhydrostatic Mesoscale Model (NAM) for July 2008 are used to provide evidence as to the importance of the Great Plains topography in driving the LLJ. Summertime heating of the sloping terrain is critical in establishing the climatological position for the Great Plains LLJ. Heating enhances the background geostrophic flow associated with the Bermuda high, resulting in a maximum low-level mean summertime flow over the Great Plains region. Maximum geostrophic winds in the NAM are found during late afternoon, providing a large background wind on which frictional decoupling can act. The nighttime LLJ maximum is the result of an inertial oscillation of the unbalanced components that arise fundamentally from frictional decoupling. Diurnal heating of the sloping terrain forces a cycle in the geostrophic wind that is out of phase with the wind maximum.

Cessation of the 22–25 June 2006 Coastally Trapped Wind Reversal

Abstract Coastally trapped wind reversals (CTWRs) occur periodically in the marine boundary layer off the western coast of the United States and dramatically change the low-level wind regime and coastal weather. Southerly flow becomes established with the passage of a CTWR along with cooler temperatures and a low stratus deck in a narrow band along the coast. CTWRs can propagate northward along the coast for hundreds of kilometers. A strong CTWR commenced in southern California on 22 June 2006 and moved north along the California coastline before stalling at Cape Mendocino on 24 June 2006. A transcritical Froude number differentiates the CTWR layer from the climatologically favored northern wind regime to the north of Cape Mendocino and indicates a barrier to the movement of a density current. A well-defined cloud boundary is present as detected by radar and satellite imagery and sharp gradients exist in the basic-state parameters as measured by instrumented aircraft. As the Pacific high migrates back offshore, the horizontal pressure field near Cape Mendocino becomes increasingly adverse to the continued northward movement of the CTWR layer and typical summertime conditions are reestablished. Observations and modeling results show that the cessation phase of this CTWR was characterized by a surprising lack of topographic blocking due to the mountainous coastal terrain and Cape Mendocino massif. The horizontal pressure field over the ocean to the north of the CTWR was the key impeding force to continued propagation of this event.

Measurement‐based estimation of the spatial gradient of aerosol radiative forcing

This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well‐mixed green‐house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate. For this, we diagnose the Normalized Gradient of Radiative Forcing (NGoRF), as a fraction of the present global heterogeneous insolation attributed to human activity. Although the GHG has a larger forcing (+1.7 Wm−2) as measured than those of aerosol direct (−1.59 Wm−2) and possible indirect effect (−1.38 Wm−2) in terms of a spatially averaged top‐of‐atmosphere value, the aerosol direct and indirect effects have far greater NGoRF values (∼0.18) than that of GHG (∼0.003).

An Automobile Platform for the Measurement of Foehn and Gap Flows

An instrument package to measure temperature, pressure, humidity, and position was designed to be quickly deployable on any automobile to be used for the study of gap and other orographically influenced flows. Differential GPS (global positioning system) measurements together with a distance counter gave the submeter accuracy of vertical position that was needed for observation of changes in the horizontal pressure field, which is an integral measure of the flow field aloft. A slantwise pressure reduction method was tailored for this application and verified with data from radio soundings. The automobile platform was successfully used during the field phase of the Mesoscale Alpine Programme (MAP) to classify flow states and observe hydraulic jumps in gap flows and to extend aircraft measurements to the ground.

On the Role of the Antarctic Continent in Forcing Large-Scale Circulations in the High Southern Latitudes

The Antarctic topography and attendant katabatic wind regime appear to play a key role in the climate of the high southern latitudes. During the nonsummer months, persistent and often times intense katabatic winds occur in the lowest few hundred meters of the Antarctic atmosphere. These slope flows transport significant amounts of cold air northward and thereby modify the horizontal pressure field over the high southern latitudes. Three-year seasonal cycle numerical simulations using the NCAR Community Climate Model Version 1 (CCM1) with and without representation of the Antarctic orography were performed to explore the role of the elevated terrain and drainage flows on the distribution and evolution of the horizontal pressure field. The katabatic wind regime is an important part of a clearly defined mean meridional circulation in the high southern latitudes. The position and intensity of the attendant sea level low pressure belt appears to be tied to the Antarctic orography. The seasonal movement of mass in the high southern latitudes is therefore constrained by the presence of the Antarctic ice sheet. The semiannual oscillation of pressure over Antarctica and the high southern latitutdes is well depicted in the CCM1 only when the Antarctic orography is included.

Relationship between horizontal spiralling wind field and pressure field

Pressure field associated with a spiralling wind field as in a cyclone is obtained by areal integration and also by integration along isogons. Further, expression for vertical motion is obtained. Vertical downward motion is shown to occur in the eye of the cyclone when the cyclone is relatively intense characterised by strong winds on the ring of maximum wind and/or contraction of ring of maximum wind.

An investigation of wind structure in the trades: anegada 1953

An investigation of wind structure in the trades: anegada 1953

Pressure Gradient Associated with Steeply Inclined Air-flow*

Abstract Steeply inclined air-flow, such as the flow over an escarpment, is of great importance in meteorology. The horizontal pressure field associated with such flow is investigated, and three components, associated respectively with hydrostatic effects, with accelerations and with friction, are evaluated. A suggestion for the relation between the hydrostatic and the accelerational components is given. The upper-level pressure gradient in the direction of flow is thus calculable from a simple formula, involving the height of the escarpment (or other obstacle), the lapse rate of the air and the temperature. Application of the theory to practical forecasting is briefly discussed.

A Re-examination of the “Leipzig Wind Profile” Considering some Relations between Wind and Turbulence in the Frictional Layer

The “Leipzig Wind Profile” is a unique example of a representative wind distribution in the frictional layer which resulted from MILDNER's set of 28 pilot-balloon observations with two theodolites on October 20, 1931, near Leipzig for a stable weather situation. It is shown, that the vectors of geostrophic motion and ground drag follow from the wind profile when a scalar austausch is assumed. The re-computed vertical austausch-distribution indicates that the energy for maintaining the turbulence in a steady frictional wind profile is taken from the potential energy of the horizontal pressure field by means of the steady flow of air across the isobars. DOI: 10.1111/j.2153-3490.1950.tb00321.x

PRESSURE GRADIENT ASSOCIATED WITH STEEPLY INCLINED AIR-FLOW

Abstract Steeply inclined air-flow, such as the flow over an escarpment, is of great importance in meteorology. The horizontal pressure field associated with such flow is investigated, and three components, associated respectively with hydrostatic effects, with accelerations and with friction, are evaluated. A suggestion for the relation between the hydrostatic and the accelerational components is given. The upper-level pressure gradient in the direction of flow is thus calculable from a simple formula, involving the height of the escarpment (or other obstacle), the lapse rate of the air and the temperature. Application of the theory to practical forecasting is briefly discussed.