11 research outputs found
NOTES AND CORRESPONDENCE On the Use of Slow Ascent Meter-Scale Sampling (SAMS) Radiosondes for Observing Overturning Events in the Free Atmosphere
ABSTRACT This note describes the development of a method for obtaining high vertical resolution (meter scale) measurements of basic meteorological quantities and turbulent overturns, using radiosondes with slow ascent rates. Although the method has some limitations, it can provide profiles of standard atmospheric variables from the surface to more than 20 km, with significantly improved vertical resolution. It can also help better identify regions of turbulent overturns. This correspondence presents some initial results demonstrating the occurrence of relatively small-vertical-scale overturns throughout the entire atmospheric column
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The Very Stable Boundary Layer on Nights with Weak Low-Level Jets
The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk
Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong
temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during
the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds
were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent
profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The
near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the
vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent
turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very
weak turbulence and negligible turbulent mixing. The focus of this paper is on the effects of this quiescent
layer just above the shallow BL, and the impacts of this quiescent layer on turbulent transport and numerical
modeling. High-frequency time series of temperature T on a 60-m tower showed that 1) the
amplitudes of the T fluctuations were dramatically suppressed at levels above 30 m in contrast to the
relatively larger intermittent T fluctuations in the shallow BL below, and 2) the temperature at 40- to 60-m
height was nearly constant for several hours, indicating that the very cold air near the surface was not being
mixed upward to those levels. The presence of this quiescent layer indicates that the atmosphere above the
shallow BL was isolated and detached both from the surface and from the shallow BL.
Although some of the nights studied had modestly stronger winds and traveling disturbances (density
currents, gravity waves, shear instabilities), these disturbances seemed to pass through the region without
having much effect on either the SBL structure or on the atmosphere–surface decoupling. The decoupling
suggests that under very stable conditions, the surface-layer lower boundary condition for numerical
weather prediction models should act to decouple and isolate the surface from the atmosphere, for example,
as a free-slip, thermally insulated layer.
A multiday time series of ozone from an air quality campaign in Tennessee, which exhibited nocturnal
behavior typical of polluted air, showed the disappearance of ozone on weak low-level jets (LLJ) nights.
This behavior is consistent with the two-stratum structure of the vSBL, and with the nearly complete
isolation of the surface and the shallow BL from the rest of the atmosphere above, in contrast to cases with
stronger LLJs, where such coupling was stronger
Tedlar bag sampling technique for vertical profiling of carbon dioxide through the atmospheric boundary layer with high precision and accuracy
Tedlar Bag Sampling Technique for Vertical Profiling of Carbon Dioxide through the Atmospheric Boundary Layer with High Precision and Accuracy
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Simultaneous observations of structure function parameter of refractive index using a high-resolution radar and the DataHawk small airborne measurement system
The SOUSY (SOUnding SYstem) radar was relocated to the Jicamarca Radio Observatory (JRO) near Lima, Peru, in 2000, where the radar controller and acquisition system were upgraded with state-of-the-art parts to take full advantage of its potential for high-resolution atmospheric sounding. Due to its broad bandwidth (4MHz), it is able to characterize clear-air backscattering with high range resolution (37.5m). A campaign conducted at JRO in July 2014 aimed to characterize the lower troposphere with a high temporal resolution (8.1Hz) using the DataHawk (DH) small unmanned aircraft system, which provides in situ atmospheric measurements at scales as small as 1m in the lower troposphere and can be GPS-guided to obtain measurements within the beam of the radar. This was a unique opportunity to make coincident observations by both systems and to directly compare their in situ and remotely sensed parameters. Because SOUSY only points vertically, it is only possible to retrieve vertical radar profiles caused by changes in the refractive index within the resolution volume. Turbulent variations due to scattering are described by the structure function parameter of refractive index Cn2. Profiles of Cn2 from the DH are obtained by combining pressure, temperature, and relative humidity measurements along the helical trajectory and integrated at the same scale as the radar range resolution. Excellent agreement is observed between the Cn2 estimates obtained from the DH and SOUSY in the overlapping measurement regime from 1200m up to 4200m above sea level, and this correspondence provides the first accurate calibration of the SOUSY radar for measuring Cn2