Leslie M. Hartten

The Flatland boundary layer experiments

Abstract This article describes the 1995 and 1996 Flatland boundary layer experiments, known as Flatland95 and Flatland96. A number of scientific and instrumental objectives were organized around the central theme of characterization of the convective boundary layer, especially the boundary layer top and entrainment zone. In this article the authors describe the objectives and physical setting of the experiments, which took place in the area near the Flatland Atmospheric Observatory, near Champaign–Urbana, Illinois, in August–September 1995 and June–August 1996. The site is interesting because it is extremely flat, has uniform land use, and is in a prime agricultural area. The instruments used and their performance are also discussed. The primary instruments were a triangle of UHF wind-profiling radars. Rawinsondes and surface meteorological and flux instruments were also included. Finally, some early results in terms of statistics and several case studies are presented.

Synoptic settings of westerly wind bursts

The morphology of westerly wind bursts (WWBs) during the 1980s is investigated using 1000-mbar winds analyzed by the European Center for Medium-Range Weather Forecasts (ECMWF). The variety of synoptic situations in which WWBs occur are explored so that the dynamics of WWB generation and maintenance can be studied and so that those interested in short- and long-term forecasts of conditions over the equatorial Pacific will have an increased knowledge of those elements associated with WWBs. WWBs are identified by searching for episodes in which the zonal wind was at least 5 m s -1 over at least 10° longitude for two or more days. This definition, based on surface winds rather than elevated winds or wind anomalies, reflects the impact WWBs have on air/sea interaction processes. The spatial and temporal criteria provide a synoptic-scale envelope for such interactions and mitigate against erroneous analyses. According to these criteria, 131 distinct WWBs occurred during the 1980s. A new two-dimensional classification scheme has been developed ; the nine patterns in it describe the near-surface flow during 90% of these WWBs. A single cyclone or a series of cyclones in one or both hemispheres and several different types of cross-equatorial flow are the major components of the patterns. Only 8% of the bursts were associated with concurrent cyclones in both the northern and southern hemispheres. Four bursts involving cross-equatorial flow are subjected to further study. The flow is decomposed into barotropic and baroclinic parts and into rotational and divergent baroclinic components. One of the bursts was predominantly barotropic and another predominantly baroclinic ; the cross-equatorial flow in all four cases was predominantly divergent in nature. The information from these cases, coupled with the typical vertical structures and small meridional length scales associated with the various synoptic patterns, suggests that simple baroclinic models of circulations driven by near-equatorial heating are not adequate to describe the dynamics during WWBs.

Impacts of Merging Profiler and Rawinsonde Winds on TOGA COARE Analyses

During the intensive observing period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment, a large number of collocated rawinsonde and profiler wind observations were taken at six Integrated Sounding System (ISS) sites and Biak, Indonesia. To mitigate limitations in the rawinsonde dataset due to missing and bad wind observations, a procedure was developed to combine profiler and sonde winds to produce an integrated, high-quality, upper-air sounding dataset. In addition to improving the overall quality of the winds, this procedure eliminates several data gaps in the sonde dataset. For example, below 800 hPa the amount of bad and missing wind data is reduced from about 45% to 20% at the land-based ISS sites. This paper describes the procedure for combining sonde and profiler winds into a single, coherent merged dataset. Examining the impact of this merger upon various atmospheric analyses, the authors find that inclusion of profiler winds results in some substantial changes in the analyses, particularly on daily to weekly timescales. To assess whether these changes are an improvement, budget-derived rainfall estimates from analyses with and without profiler winds are compared to Special Sensor Microwave/Imager satellite-based estimates. Overall, this comparison shows that inclusion of profiler winds into the sonde dataset has a beneficial impact upon the analyses. Information for accessing the merged datasets for the seven sites considered in this paper via the Internet is described.

Evaluation of Three-Beam and Four-Beam Profiler Wind Measurement Techniques Using a Five-Beam Wind Profiler and Collocated Meteorological Tower

In this paper a five-beam wind profiler and a collocated meteorological tower are used to estimate the accuracy of four-beam and three-beam wind profiler techniques in measuring horizontal components of the wind. In the traditional three-beam technique, the horizontal components of wind are derived from two orthogonal oblique beams and the vertical beam. In the less used four-beam method, the horizontal winds are found from the radial velocities measured with two orthogonal sets of opposing coplanar beams. In this paper the observations derived from the two wind profiler techniques are compared with the tower measurements using data averaged over 30 min. Results show that, while the winds measured using both methods are in overall agreement with the tower measurements, some of the horizontal components of the three-beam-derived winds are clearly spurious when compared with the tower-measured winds or the winds derived from the four oblique beams. These outliers are partially responsible for a larger 30-min, threebeam standard deviation of the profiler/tower wind speed differences (2. 2ms 1 ), as opposed to that from the four-beam method (1.2 m s 1 ). It was also found that many of these outliers were associated with periods of transition between clear air and rain, suggesting that the three-beam technique is more sensitive to small-scale variability in the vertical Doppler velocity because of its reliance on the point measurement from the vertical beam, while the four-beam method is surprisingly robust. Even after the removal of the rain data, the standard deviation of the wind speed error from the three-beam method (1.5 m s 1 ) is still much larger than that from the four-beam method. Taken together, these results suggest that the spatial variability of the vertical airflow in nonrainy periods or hydrometeor fall velocities in rainy periods makes the vertical beam velocities significantly less representative over the area across the three beams, and decreases the precision of the three-beam method. It is concluded that profilers utilizing the four-beam wind profiler technique have better reliability than wind profilers that rely on the three-beam wind profiler technique.

Range Errors in Wind Profiling Caused by Strong Reflectivity Gradients

Abstract Comparisons of data taken by collocated Doppler wind profilers using 100-, 500-, and 1000-m pulse lengths show that the velocity profiles obtained with the longer pulses are displaced in height from contemporaneous profiles measured with the shorter pulses. These differences are larger than can be expected from random measurement errors. In addition, there is evidence that the 500-m pulse may underestimate the wind speed when compared with the 100-m pulse. The standard radar equation does not adequately account for the conditions under which observations are made. In particular, it assumes that atmospheric reflectivity is constant throughout the pulse volume and that observations can be assigned to the peak of the range-weighting function. However, observations from several tropical profilers show that reflectivity gradients with magnitudes greater than 10 dB km−1 are common. Here, a more general radar equation is used to simulate the radar response to the atmosphere. The simulation shows that at...

Daily variability of lower tropospheric winds over the tropical western Pacific

Daily variability in lower tropospheric winds over the tropical western Pacific is examined using data from a network of wind-profiling radars deployed during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). The amplitude of the diurnal cycle in both u and v at the the westernmost sites, located just to the east of the large island of New Guinea, is of the order of 0.5 ms−1, significantly larger than the diurnal cycle observed east of the dateline in previous studies. The larger diurnal variability is attributed to the pronounced nocturnal maximum in deep convection over the island of New Guinea; the influence of this diurnal circulation extends (with smaller amplitude) to the more distant profiler sites 10°–15° E of New Guinea. The semidiurnal cycle of the zonal wind is smaller in amplitude but coherent in phase at all sites; the meridional wind exhibits a negligible semidiurnal cycle. These results are consistent with the solar semidiurnal tidal oscillation in pressure and entirely consistent with previous studies of eastern tropical Pacific winds. Two of the profilers were deployed on ships rather than islands, and we find no systematic differences between the ship data and the small island data attributable to the complete absence of land at the ship sites.

On Air–Sea Interaction at the Mouth of the Gulf of California

Surface flux, wind profiler, oceanic temperature and salinity, and atmospheric moisture, cloud, and wind observations gathered from the R/V Altair during the North American Monsoon Experiment (NAME) are presented. The vessel was positioned at the mouth of the Gulf of California halfway between La Paz and Mazatlan (23.5°N, 108°W), from 7 July to 11 August 2004, with a break from 22 to 27 July. Experimentmean findings include a net heat input from the atmosphere into the ocean of 70 Wm 2 . The dominant cooling was an experiment-mean latent heat flux of 108 W m 2 , equivalent to an evaporation rate of 0.16 mm h 1 . Total accumulated rainfall amounted to 42 mm. The oceanic mixed layer had a depth of approximately 20 m and both warmed and freshened during the experiment, despite a dominance of evaporation over local precipitation. The mean atmospheric boundary layer depth was approximately 410 m, deepening with time from an initial value of 350 m. The mean near-surface relative humidity was 66%, increasing to 73% at the top of the boundary layer. The rawinsondes documented an additional moist layer between 2and 3-km altitude associated with a land–sea breeze, and a broad moist layer at 5–6 km associated with land-based convective outflow. The observational period included a strong gulf surge around 13 July associated with the onset of the summer monsoon in southern Arizona. During this surge, mean 1000–700hPa winds reached 12 m s 1 , net surface fluxes approached zero, and the atmosphere moistened significantly but little rainfall occurred. The experiment-mean wind diurnal cycle was dominated by mainland Mexico and consisted of a near-surface westerly sea breeze along with two easterly return flows, one at 2–3 km and another at 5–6 km. Each of these altitudes experienced nighttime cloudiness. The corresponding modulation of the radiative cloud forcing diurnal cycle provided a slight positive feedback upon the sea surface temperature. Two findings were notable. One was an advective warming of over 1°C in the oceanic mixed layer temperature associated with the 13 July surge. The second was the high nighttime cloud cover fraction at 5–6 km, dissipating during the day. These clouds appeared to be thin, stratiform, slightly supercooled liquid-phase clouds. The preference for the liquid phase increases the likelihood that the clouds can be advected farther from their source and thereby contribute to a higher-altitude horizontal moisture flux into the United States.

A Minimum Threshold for Wind Profiler Signal-to-Noise Ratios

AbstractOne limiting factor in atmospheric radar observations is the inability to distinguish the often weak atmospheric signals from fluctuations of the noise. This study presents a minimum threshold of usability, SNRmin, for signal-to-noise ratios obtained from wind profiling radars. The basic form arises from theoretical considerations of radar noise; the final form includes empirical modifications based on radar observations. While SNRmin was originally developed using data from the 50-MHz profiler at Poker Flat, Alaska, it works well with data collected from a wide range of locations, frequencies, and parameter settings. It provides an objective criterion to accept or reject individual spectra, can be quickly applied to a large quantity of data, and has a false-alarm rate of approximately 0.1%. While this threshold’s form depends on the methods used to calculate SNR and spectral moments, variations of the threshold could be developed for use with data processed by other methods.

Stochastic forcing of north tropical Atlantic sea surface temperatures by the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is a rapidly decorrelating process that strongly affects the climate over the Atlantic and the surrounding continents. Although the NAO itself is basically unpredictable on seasonal timescales using statistical methods, NAO forcing is here shown to significantly affect sea surface temperatures (SSTs) evolving on those timescales. Results using linear inverse modeling (LIM) imply that the NAO index and its convolution with deterministic SST dynamics account for nearly half the unpredictable component of north tropical Atlantic SST at lead times greater than 9 months; adding this component to hindcasts at a lead of 48 weeks increases correlation with north tropical Atlantic SST from about 0.4 to about 0.6. Rapid fluctuations during boreal winter and spring, when the NAO is strongest, affect SST predictability throughout the entire year.

ENSO's impact on the annual cycle: The view from Galápagos

UHF profiler observations of lower tropospheric winds at San Cristobal in Ecuadors Galapagos Islands commenced in October 1994. The available data give a detailed picture of the lower troposphere‧s annual cycle over this portion of the East Pacific cold tongue. The profiler tracks the low-level southerly wind maximum from its weakest state in austral summer, when it may briefly rise above 1 km, to its strongest state mid-year, when it is often found below 400 m. The profiler operated during much of the 1997–98 El Nino and gives us an unprecedented look at dramatic changes in the flow over the cold tongue during a warm event. The seasonal deepening of the low-level flow was enhanced as the ITCZ moved into the Southern Hemisphere for a few months and the lower tropospheric winds became northerly.