Observational Study of Nisarg Cyclone Winds using Phased Array Doppler Sodar at Atigre, Kolhapur, India

One of the most important parameters in meteorology is the mean wind profile in the tropical cyclone boundary layer. The signature of the Nisarg cyclone is reported in the Phased Array Doppler Sound Detection and Ranging (SODAR) data installed at the Center for Space and Atmospheric Science (CSAS), Sanjay Ghodawat University, Kolhapur (16.74° N, 74.37° E; near India's western coast). The vertical profile of wind speed and wind direction measured from the sodar system clearly reveals the signature of Nisarg cyclone during 2- 3 June 2020. Our analysis revealed that, the maximum mean wind speed was 17 m/s on 3rd June 2020 at 10:00 IST. It also shows the change in the wind direction from southwest to southeast on 2nd June 2020 and 3rd June 2020. Daily high-resolution reanalysis in the domain, 0-25°N, 65-110°E, during the period from 31st May-5th June 2020 shown the variation in atmospheric pressure of the Nisarg cyclone from 1000 to 1008 hPa, sea surface tremperature (SST) between 30 and 31°C, outgoing longwave radiation (OLR) varied between 100 and 240 Wm−2, wind speed between 3 and 15 m/s and low values of vertical wind shear (VWS) was observed to the north of the track Nisarg. These findings could aid in better understanding and forecasting in this region. The present results are initial measurements of sodar system.

Turbulence, Low-Level Jets, and Waves in the Tyrrhenian Coastal Zone as Shown by Sodar

The characteristics of the vertical and temporal structure of the coastal atmospheric boundary layer are variable for different sites and are often not well known. Continuous monitoring of the atmospheric boundary layer was carried out close to the Tyrrhenian Sea, near Tarquinia (Italy), in 2015–2017. A ground-based remote sensing instrument (triaxial Doppler sodar) and in situ sensors (meteorological station, ultrasonic anemometer/thermometer, and net radiometer) were used to measure vertical wind velocity profiles, the thermal structure of the atmosphere, the height of the turbulent layer, turbulent heat and momentum fluxes in the surface layer, atmospheric radiation, and precipitation. Diurnal alternation of the atmospheric stability types governed by the solar cycle coupled with local sea/land breeze circulation processes is found to be variable and is classified into several main regimes. Low-level jets (LLJ) at heights of 100–300 m above the surface with maximum wind speed in the range of 5–18 m s−1 occur in land breezes, both during the night and early in the morning. Empirical relationships between the LLJ core wind speed characteristics and those near the surface are obtained. Two separated turbulent sub-layers, both below and above the LLJ core, are often observed, with the upper layer extending up to 400–600 m. Kelvin–Helmholtz billows associated with internal gravity–shear waves occurring in these layers present opposite slopes, in correspondence with the sign of vertical wind speed gradients. Our observational results provide a basis for the further development of theoretical and modelling approaches, taking into account the wave processes occurring in the atmospheric boundary layer at the land–sea interface.

Fixed Echo Rejection in Sodar Using Noncoherent Matched Filter Detection and Gaussian Mixture Model–Based Postprocessing

Doppler sodar is a technology used for acoustic-based remote sensing of the lower planetary boundary layer. Sodars are often used to measure wind profiles; however, they suffer from problems caused by noise (both acoustic and electrical) and echoes from fixed objects, which can bias radial velocity estimates. An experimental bistatic sodar was developed with 64 independent channels. The device enables flexible beamforming; beams can be tilted at the same angle irrelevant of frequency, a limitation in most commercial devices. This paper presents an alternative sodar signal-processing algorithm for wind profiling using a multifrequency stepped-chirp pulse. A noncoherent matched filter was used to analyze returned signals. The noncoherent matched filter combines radial velocity estimates from multiple frequencies into a single optimization. To identify and separate sources of backscatter, noise, and fixed echoes, a stochastic pattern-recognition technique, Gaussian mixture modeling, was used to postprocess the noncoherent matched filter data. This method allowed the identification and separation of different stochastic processes. After identification, noise and fixed echo components were removed and a clean wind profile was produced. This technique was compared with traditional spectrum-based radial velocity estimation methods, and an improvement in the rejection of fixed echo components was demonstrated; this is one of the major limitations of sodar performance when located in complex terrain and urban environments.

A Comparison of Vertical Atmospheric Wind Profiles Obtained from Monostatic Sodar and Unmanned Aerial Vehicle–Based Acoustic Tomography

The natural sound generated by an unmanned aerial vehicle is used in conjunction with tomography to remotely sense the virtual temperature and wind profiles of the atmosphere in a horizontal plane up to an altitude of 1200 m and over a baseline of 600 m. Sound fields recorded on board the aircraft and by an array of microphones on the ground are compared and converted to sound speed estimates for the ray paths intersecting the intervening medium. Tomographic inversion is then used to transform these sound speed values into two-dimensional profiles of virtual temperature and wind vector, which enables the atmosphere to be visualized and monitored over time. The wind vector and temperature estimates are compared to measurements taken by a collocated midrange Doppler sodar and sensors on board the aircraft. Large-eddy simulations of daytime atmospheric boundary layers and error models of the tomographic inversion and sodar are also used to assess the magnitude and nature of anticipated differences. Both the simulations and field trials data show similar levels of correspondence between the tomographically derived and independently observed measurements.

Evaluation of WRF-Predicted Near-Hub-Height Winds and Ramp Events over a Pacific Northwest Site with Complex Terrain

One challenge with wind-power forecasts is the accurate prediction of rapid changes in wind speed (ramps). To evaluate the Weather Research and Forecasting (WRF) model's ability to predict such events, model simulations, conducted over an area of complex terrain in May 2011, are used. The sensitivity of the model's performance to the choice among three planetary boundary layer (PBL) schemes [Mellor–Yamada–Janjić (MYJ), University of Washington (UW), and Yonsei University (YSU)] is investigated. The simulated near-hub-height winds (62 m), vertical wind speed profiles, and ramps are evaluated against measurements obtained from tower-mounted anemometers, a Doppler sodar, and a radar wind profiler deployed during the Columbia Basin Wind Energy Study (CBWES). The predicted winds at near–hub height have nonnegligible biases in monthly mean under stable conditions. Under stable conditions, the simulation with the UW scheme better predicts upward ramps and the MYJ scheme is the most successful in simulating downward ramps. Under unstable conditions, simulations using the YSU and UW schemes show good performance in predicting upward ramps and downward ramps, with the YSU scheme being slightly better at predicting ramps with durations longer than 1 h. The largest differences in mean wind speed profiles among simulations using the three PBL schemes occur during upward ramps under stable conditions, which were frequently associated with low-level jets. The UW scheme has the best overall performance in ramp prediction over the CBWES site when evaluated using prediction accuracy and capture-rate statistics, but no single PBL parameterization is clearly superior to the others when all atmospheric conditions are considered.

Evidence of Very Shallow Summertime Katabatic Flows in Dronning Maud Land, Antarctica

The three-axis “Latan-3” Doppler sodar was operated near the Finnish Antarctic station Aboa in Dronning Maud Land (73.04°S, 13.40°W) in the austral summer of 2010/11. The measuring site is located at a practically flat, slightly sloped (about 1%) surface of the glacier. The sodar was operated in multiple-frequency parallel mode with 20–800-m sounding range, 20-m vertical resolution, and 10-s temporal resolution. To reveal the wind and temperature profiles below the sounding range as well as turbulent fluxes at 2 and 10 m, the data from a 10-m meteorological mast were used. During the measurements, the atmospheric boundary layer was within the sounding range of the sodar most of the time. Despite a large variety of observed sodar echo patterns and wind speed profiles, several cases of clear steady katabatic flows were observed. Practically all of them were easterly, whereas the uphill direction is southern. The thickness of the katabatic flow varied from a few tens to several hundreds of meters; the wind speed maximum could be as low as 5 m. Thin katabatic flows had lower wind speed and much stronger temperature gradients (up to 1 K m−1) but had smaller surface heat flux than did the thicker ones.