David A. Rahn

Diagnosis of the Forcing and Structure of the Coastal Jet near Cape Mendocino Using In Situ Observations and Numerical Simulations

Several flights were conducted by the University of Wyoming King Air near Cape Mendocino, California, during June 2004 to examine finescale features of the coastal low-level jet (CJ) that frequently forms during summer over the ocean off the West Coast of the United States. The primary goal of these flights was to measure the horizontal pressure gradient force (PGF) and hence to determine the forcing of the CJ directly. By flying a series of redundant legs on an isobaric surface, heights of the pressure surface can be obtained from radar altimeter measurements and refined position estimates from an onboard global positioning system receiver. The slope of the isobaric surface height is proportional to the PGF. Results are shown for the 22 June 2004 case study conducted to the south of Cape Mendocino. The forcing of a CJ under weak synoptic forcing and the role of the elevated terrain near Cape Mendocino are explored. Ten isobaric legs approximately 70 km in length and directed east–west were conducted near the level of the maximum CJ wind speed. The vertical structure of the CJ was obtained from sawtooth legs conducted along an east–west flight leg. Numerical simulations have been performed for this case using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) for comparison with in situ measurements. Model simulations show pressure perturbations in the vicinity of the cape as the northerly winds associated with the CJ interact with the coastal topography. Close agreement is found between in situ measurements and MM5 analyses of the various state parameters and the PGF along the east–west flight track in the lee of Cape Mendocino. Strong variation in the PGF is observed along the flight path. Large ageostrophic accelerations are present in response to the adjustment of the CJ with Cape Mendocino, reflecting the force imbalance between the observed PGF and Coriolis force.

Assessing the Influence of the MJO on Strong Precipitation Events in Subtropical, Semi-Arid North-Central Chile (30°S)

AbstractAnnual precipitation in subtropical, semiarid north-central Chile (30°S) during rainy years comprises a few (3–5) strong events in the fall and winter, which are presumably modulated by the Madden–Julian oscillation (MJO). Precipitation from 1979–2009 was recorded daily at three stations along the Elqui Valley. The relationship between the MJO and precipitation is investigated from two perspectives: 1) examining a MJO index (MJOI) based on the actual precipitation events and 2) examining the likelihood of precipitation based on a favorable MJOI. About 80% of the strong precipitation events at the coast in La Serena are related to an active MJO near the central equatorial Pacific. These events are often typified by broad, slow moving synoptic systems in phase with the MJO propagation. Blocking in the far southeast Pacific is associated with precipitation 75% of the time, while deep troughs make up the rest.A relationship between a MJOI and strong rainfall suggests that, though it could be used as a...

The Low-Level Atmospheric Circulation near Tongoy Bay–Point Lengua de Vaca (Chilean Coast, 30°S)

AbstractStrong southerly, terrain parallel winds often occur along the coast of north-central Chile (25°–35°S) embedded in the marine atmospheric boundary layer and the lower part of the capping temperature inversion. Their offshore structure and variability have received considerable attention because of the effect on open-ocean processes and connection with the southeast Pacific cloud layer. Mesoscale low-level circulations linked to the coastal topography (e.g., coastal jets and sea breeze) are less studied in Chile, but are particularly relevant as they alter the upper-ocean circulation and the cloud pattern in the nearshore strip.Surface, radiosonde, and airborne meteorological observations near point Lengua de Vaca (LdV)–Tongoy Bay (TB) at 30°S are used alongside numerical modeling to understand the local circulation near a prominent upwelling center. Most observations were gathered during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Chilean Upwelling Exp...

Influence of large scale oscillations on upwelling-favorable coastal wind off central Chile

[1] Along the central coast of Chile is typically equatorward, upwelling-favorable wind associated with the southeast Pacific anticyclone. A coastal low-level jet often develops, and its wind speed is mostly controlled by the meridional pressure gradient. While the low-level jet is a mesoscale feature forced by an interaction between synoptic conditions and coastal topography, regional sea level pressure anomalies are associated with changes of the Antarctic, Madden Julian, and El Nino–Southern Oscillation. The connection between the alongshore wind and changes to the large-scale circulation is examined and quantified using 31 years of the Climate Forecast System Reanalysis, which resolves coastal features better than previous, coarser analyses. Composites based on each index reveal the modulation of the sea level pressure and significant alongshore wind anomalies of 0.5–1.5 m s 1 that correlate well to meridional surface pressure gradient changes and are centered near 35S. Constructive and destructive interference exists between the three indices that either enhance or cancel the alongshore wind anomaly. During favorable upwelling conditions the distribution of meridional wind is generally clustered around positive anomalies with a tail toward negative values, representing a stronger and persistent anticyclone. During unfavorable upwelling conditions the anomalies are generally more normally distributed, representing a weaker anticyclone and the passage of more cyclones.

Diurnal Climatology of the Boundary Layer in Southern California Using AMDAR Temperature and Wind Profiles

AbstractObservations from commercial aircraft [e.g., the Aircraft Meteorological Data Relay (AMDAR) automated weather reports] have been increasing dramatically. Two main applications of the aircraft data are use in short-term forecasts and assimilation into numerical weather prediction models. Now that more than 10 years of measurements exist, using this dataset to construct a description of the long-term climatological behavior (a “climatology”) of the lower atmosphere is explored with two main objectives. The first objective is to examine strengths and weaknesses of using the dataset to construct a climatology of the lower atmosphere. Unlike the traditional twice-daily radiosonde launches, the high frequency of observations at major airports allows for an unprecedented set of diurnal information at many locations globally. The second objective is to obtain a climatology of the lower atmosphere of Southern California, specifically at Los Angeles, San Diego, and Ontario, during the spring and summer when...

Airborne Observations of a Catalina Eddy

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 PrecisionAtmosphericMarineBoundaryLayerExperimentinJuneof2012.Measurementsweremadeofthe 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.

Airborne Measurements of Coastal Jet Transition around Point Conception, California

Low-level winds along the Californian coast during spring and early summer are typically strong and contained within the cool, well-mixed marine boundary layer (MBL). A temperature inversion separates the MBL from the warmer free troposphere. This setup is often represented by a two-layer shallow-water system with a lateral boundary. Near a prominent point such as Point Conception, California, the fast-moving MBL flow is supercritical and can exhibit distinct features including a compression bulge and an expansion fan. Measurements from the University of Wyoming King Air research aircraft on 19 May 2012 during the Precision Atmospheric MBL Experiment (PreAMBLE) captured wind in excess of 14ms 21 off of Point Conception under clear skies and wind ;2ms 21 east of San Miguel in the California Bight. A compression bulge was identified upwind of Point Conception. When the flow rounds the point, the MBL undergoes a near collapse and there is a spike in MBL height embedded in the general decrease of MBL height with greater turbulence just downwind that is associated with greater mixing through the inversion layer. Lidar and in situ measurements reveal that transport of continental aerosol is present near the pronounced MBL height change and that there is a complex vertical structure within the Santa Barbara Channel. Horizontal pressure gradients are obtained by measuring the slope of an isobaric surface. Observations of wind and pressure perturbations are able to be linked through a simple Bernoulli relationship. Variation of MBL depth explains most, but not all of the variation of the isobaric surface.

Research Aircraft Determination of D-Value Cross Sections

Useofanairborneplatformtodeterminethedynamicsofatmosphericmotionhasbeenongoingforoverthree decades. Much of the effort has been centered on the determination of the horizontal pressure gradient force along an isobaric surface, and with wind measurements the nongeostrophic components of motion can be obtained. Recent advances using differential GPS-based altitude measurements allow accurate assessment of the geostrophic wind. Porpoise or sawtooth maneuvers are used to determine the vertical cross section of the horizontal pressure gradient force. D-values, the difference of the height of a given pressure level from that in a reference atmosphere, are used to isolate the vertical structure of the horizontal component of the pressure gradient force from the vastly larger hydrostatic pressure gradient. Comparison of measured D-value cross sectionswith airborne measurements ofthehorizontal pressuregradientis shown.Comparisonof D-values with output from the WRF Model demonstrates that the airborne measurements are consistent with finescale numerical simulations. This technique provides a means of inferring the thermal wind, thereby enabling a detailed examination of the vertical structure of the forcing of mesoscale and synoptic-scale wind regimes.

Coastal Jet Adjustment near Point Conception, California, with Opposing Wind in the Bight

AbstractTypical spring and summer conditions offshore of California consist of strong northerly low-level wind contained within the cool, well-mixed marine boundary layer (MBL) that is separated from the warm and dry free troposphere by a sharp temperature inversion. This system is often represented by two layers constrained by a lateral boundary. Aircraft measurements near Point Conception, California, on 3 June 2012 during the Precision Atmospheric MBL Experiment (PreAMBLE) captured small-scale features associated with northerly flow approaching the point with the added complexity of encountering opposing wind in the Santa Barbara Channel. An extremely sharp cloud edge extends south-southwest of Point Conception and the flight strategy consisted of a spoke pattern to map the features across the cloud edge. Lidar and in situ measurements reveal a nearly vertical jump in the MBL from 500 to 100 m close to the coast and a sharp edge at least 70 km away from the coast. In this case, it is hypothesized that ...

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...