Mesosphere-stratosphere-troposphere Research Articles

Initial MST radar observations of upper tropospheric-lower stratospheric duct-like structures over Jicamarca, Peru

Abstract. Persistent wind jet structures along zonal and meridional fields, believed to be caused by stationary gravity waves, were detected in February 1999 in mesosphere-stratosphere-troposphere (MST) radar wind measurements of the troposphere and lower stratosphere over Jicamarca, Peru. Over a continuous seven day span of MST-data analyzed in this study, two days of observations showed signatures of wave-like structures in the upper troposphere/lower stratosphere wind jets associated with the phases of the stationary gravity waves. We believe these wave-like structures are ducted gravity waves. We present these initial observations, their characteristics, and the results of simple numerical simulations used in an attempt to mimic these observed features. Although a fair replication of the observed ducted structure in the numerical model is found, the observed period of ~90 min is nonetheless much longer than what is traditionally observed. As a result, the specific physical nature of the observed structures is not fully established. Nevertheless, given the high quality of the observations, we demonstrate here that continued analysis of this data set and concurrent modeling efforts will allow for a better understanding of Doppler ducts at high spatial and temporal resolution, and the results presented here can ultimately be applied to studies of middle atmospheric fronts, ducts, and bores.

MST radar observations of short‐period gravity wave during overhead tropical cyclone

Short‐period gravity waves associated with the passage of tropical cyclone using mesosphere‐stratosphere‐troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E) has been discussed. The observed stratospheric gravity wave is found to have a periodicity of ∼42 min, vertical and horizontal wavelength of ∼3.5 km and 14 km, respectively. Maximum amplitude of gravity wave is observed in the upper troposphere and lower stratosphere (UTLS) region due to which periodic updrafts and downdrafts are observed. This weakens the stability of tropopause, which is observed in radar signal‐to‐noise ratio (SNR). The enhancement of vertical momentum flux of order ∼−0.6 m2/s2 observed in the lower stratosphere is attributed to the cyclone generated gravity waves. The obstacle effect is found to be the generative mechanism for the observed gravity waves associated with the tropical cyclone.

On the Vertical Distribution of Mean Vertical Velocities in the Convective Regions during the Wet and Dry Spells of the Monsoon over Gadanki

The Indian Mesosphere–Stratosphere–Troposphere (MST) radar observations of vertical distribution of mean vertical velocities w in convective regions during the wet and dry spells of the Indian summer monsoon over a tropical station at Gadanki, India (13.5°N, 79.2°E) are discussed. The composite w profile during the wet spell consistently shows a single peak at ~13 km whereas during the dry spell it shows two peaks, one at 5 km and another at 11–13 km. The characteristics of this altitudinal distribution in w are discussed in terms of background wind and thermal structure during both spells of the monsoon. Background w obtained from NCEP–NCAR reanalysis shows subsidence throughout the depth of the troposphere during the dry spell of the monsoon over Gadanki. Analysis of background wind and thermal structure clearly reveal that wind shear and temperature inversion in the midtroposphere are different in the dry spell compared to that of the wet spell, which may be the possible reason for the observed double-peak w structure during the dry spell of the monsoon. The present analysis for the first time brought out the distinct vertical distribution in w and the background meteorological conditions during the wet and dry spells of the monsoon over Gadanki, which may have implications in understanding the monsoon convective systems during the wet and dry spells of the Indian summer monsoon.

Low-latitude mesospheric vertical winds observed using VHF radar

[1] Using long-term data (January 1998 to December 2009) collected from Mesosphere-Stratosphere-Troposphere (MST) radar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, variability of low-latitude mesospheric vertical wind is investigated for the first time. The emphasis is on sub-daily, monthly, seasonal and annual variabilities in vertical wind in the altitude region 65–85 km. Possible sources of errors in the vertical wind measurement at mesospheric altitudes from the MST radar are discussed. The observed mesospheric vertical wind is generally upward in all the seasons. During winter and summer, significant sub-daily variations are noticed followed by spring and fall equinoxes. The vertical wind can reach occasionally values as high as 5 ms−1 but most of the time (95%) it is, in general, less than ∼2.63 ms−1. The present observations are consistent with the general circulation features in recent models for low latitude locations with northward and upward wind prevailing throughout the year representing part of meridional circulation.

Vertical and lateral propagation characteristics of intraseasonal oscillation from the tropical lower troposphere to upper mesosphere

[1] Using a mesosphere-stratosphere-troposphere (MST) radar and a Rayleigh lidar collocated at the Indian tropical station, Gadanki (13.5°N, 79.2°E), European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis data and the horizontal wind velocities measured by the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED Doppler Interferometer (TIDI) instrument onboard the TIMED satellite, the present study makes a report on the lateral and vertical propagation characteristics of an intraseasonal oscillation (ISO, 20–40 day period band), propagating from the lower troposphere to the upper mesosphere region during the winter (November, December, January, and February) months of the years 2004–2005. It is found that the ISO had its origin in the tropical lower troposphere and propagated laterally and vertically to the upper mesosphere through the subtropical latitudes by partial refraction and reflection processes about the subtropical westerly jet. Corresponding enhancements in the wavelet spectrum of outgoing long-wave-radiation (OLR, NOAA interpolated) indicate that convective forces in the lower troposphere are one of the main sources for the generation of the ISO. From the phase of the ISO in the horizontal winds, as observed by the MST radar, it is found that the ISO propagated upward from the lower troposphere up to near the tropopause, where it got sharply attenuated. From the ECMWF data, it is observed that the ISO was refracted to the subtropical latitudes, through the tropical tropopause, from where it got radiated upward into the stratosphere. The phase propagation of the filtered winds shows that the ISO arched back toward the tropical latitudes and propagated to the mesospheric region.

Signal Processing of Radar Echoes Using Wavelets and Hilbert Huang Transform

Atmospheric Radar Signal Processing is one field of Signal Processing where there is a lot of scope for development of new and efficient tools for spectrum cleaning, detection and estimation of desired parameters. The wavelet transform and HHT (Hilbert-Huang transform) are both signal processing methods. This paper is based on comparing HHT and Wavelet transform applied to Radar signals. Wavelet analysis is one of the most important methods for removing noise and extracting signal from any data. The de-noising application of the wavelets has been used in spectrum cleaning of the atmospheric radar signals. HHT can be used for processing non-stationary and nonlinear signals. HHT is one of the timefrequency analysis techniques which consists of two parts: Empirical Mode Decomposition (EMD) and instantaneous frequency solution. EMD is a numerical sifting process to decompose a signal into its fundamental intrinsic oscillatory modes, namely intrinsic mode functions (IMFs). A series of IMFs can be obtained after the application of EMD. In this paper wavelets and EMD has been applied to the time series data obtained from the mesosphere-stratosphere-troposphere (MST) region near Gadanki, Tirupati for 6 beam directions. The Algorithm is developed and tested using Matlab. Moments were estimated and analysis has brought out improvement in some of the characteristic features like SNR, Doppler width, Noise power of the atmospheric signals. The results showed that the proposed algorithm is efficient for dealing non-linear and non- stationary signals contaminated with noise. The results were compared using ADP (Atmospheric Data Processor) and plotted for validation of the proposed algorithm.

Classifying fronts in data from a VHF wind-profiling radar

Many fronts over the UK do not fit the traditional conceptual model of a single maximum of vertical shear of the horizontal wind sloped over the cold air. A 2-year climatology of 296 cold, warm, and occluded fronts from a mesosphere–stratosphere–troposphere radar near Aberystwyth, Wales, reveals that 74% of warm fronts were associated with multiple linear bands representing maxima of vertical wind shear, radar return signal power, or both. In contrast, 51% of cold fronts lacked any such maxima. Similarly, the warm frontal segments of occluded fronts exhibited more banding than the cold frontal segments. Copyright © 2011 Royal Meteorological Society

MST radar observation of inertia-gravity waves generated from tropical cyclones

Using a Mesosphere Stratosphere Troposphere (MST) radar, operating at 53 MHz at Gadanki (13.5°N, 79.2°E), India, the present study reports on the temporal and spatial characteristics of inertia-gravity waves (IGWs) generated from four tropical cyclones that formed over the Bay of Bengal. IGWs are observed with intrinsic frequencies, vertical and horizontal wavelengths in the ranges of 1.2 f–3.0 f, 2.5–5, and 300–1600 km, respectively, where f is the Coriolis frequency at the Gadanki site. It is found that both the convective and geostrophic adjustment processes in the troposphere play a major role in the generation of IGWs. Also it is found that the horizontal propagation direction of IGWs is aligned along the motion of convective rain bands.

Further investigation on stratospheric air intrusion into the troposphere during the episode of tropical cyclone: Numerical simulation and MST radar observations

For the first time, spatiotemporal structure of stratospheric air intrusion into the troposphere during the episode of tropical cyclone is simulated using Advanced Research-Weather Research and Forecast (WRF-ARW) model. The WRF-ARW simulation effectively reproduces the spatiotemporal evolution of vertical velocity associated with tropical cyclone and shows good agreement with the mesosphere–stratosphere–troposphere (MST) radar observations at Gadanki (13.5°N, 79.2°E). The observed intrusion in the upper-troposphere has a narrow band with the width of about 50 km and stretch over 200–250 km and vertical structure of 5–6 km. This study shows that the intrusion occurs in the periphery of the cyclone and confirms that mesoscale simulation using WRF model can be used during disturbed weather conditions and for studying TSE processes.

Sub-daily variations observed in Tropical Easterly Jet (TEJ) streams

To investigate the sub-daily variation in Tropical Easterly Jet (TEJ), a few special experiments have been conducted with mesosphere–stratosphere–troposphere (MST) radar located at Gadanki(13.5°N, 79.2°E) under Study on Atmospheric Forcing and Responses (SAFAR) campaign during 2008–2009. Large sub-daily changes in the TEJ characteristics are observed within a day in addition to day-to-day changes. Based on this observational study, three possible mechanisms for the sub-daily variations are proposed i.e., (1) motion of TEJ core, (2) large updrafts and downdrafts, and (3) strong wave activity. Interestingly, TEJ peak altitude is seen above the Cold Point Tropopause altitude for about 42% of the days.

First observations of Southern Hemisphere polar mesosphere winter echoes including conjugate occurrences at ∼69°S latitude

[1] We present the first account of polar mesosphere winter echoes (PMWE) observed from the Southern Hemisphere (SH), using measurements from Davis, Antarctica (68.6°S; 78.0°E). PMWE were observed by mesosphere-stratosphere-troposphere (MST) radar during solar proton events (SPEs) and auroral substorms. We supplement the MST radar measurements with profiles of temperature and turbulent velocity from co-located Rayleigh lidar and medium frequency (MF) radar, respectively, as well as temperature data from the Microwave Limb Sounder (MLS) onboard the Aura satellite. We establish that SH PMWE exhibit similar characteristics to their well-studied northern hemisphere counterpart. Significantly our observations reveal that these radar echoes in the lower mesosphere (∼50–80 km) can occur year-round, and on occasions simultaneously in both hemispheres. We report the seasonal occurrence distribution of SH PMWE is linked to the seasonal distribution of atmospheric turbulence in the lower mesosphere. We hypothesise that given sufficient turbulent velocities in the neutral atmosphere and co-located gradients in electron density, PMWE can occur throughout the year. However they are more likely in winter when turbulent velocities in the lower mesosphere maximise.

Signal Processing of Radar Echoes Using Wavelets and Hilbert Huang Transform

Atmospheric Radar Signal Processing is one field of Signal Processing where there is a lot of scope for development of new and efficient tools for spectrum cleaning, detection and estimation of desired parameters. The wavelet transform and HHT (Hilbert-Huang transform) are both signal processing methods. This paper is based on comparing HHT and Wavelet transform applied to Radar signals. Wavelet analysis is one of the most important methods for removing noise and extracting signal from any data. The de-noising application of the wavelets has been used in spectrum cleaning of the atmospheric radar signals. HHT can be used for processing non-stationary and nonlinear signals. HHT is one of the timefrequency analysis techniques which consists of two parts: Empirical Mode Decomposition (EMD) and instantaneous frequency solution. EMD is a numerical sifting process to decompose a signal into its fundamental intrinsic oscillatory modes, namely intrinsic mode functions (IMFs). A series of IMFs can be obtained after the application of EMD. In this paper wavelets and EMD has been applied to the time series data obtained from the mesosphere-stratosphere-troposphere (MST) region near Gadanki, Tirupati for 6 beam directions. The Algorithm is developed and tested using Matlab. Moments were estimated and analysis has brought out improvement in some of the characteristic features like SNR, Doppler width, Noise power of the atmospheric signals. The results showed that the proposed algorithm is efficient for dealing non-linear and non- stationary signals contaminated with noise. The results were compared using ADP (Atmospheric Data Processor) and plotted for validation of the proposed algorithm.

A review of Mesosphere–Stratosphere–Troposphere (MST) radar developments and studies, circa 1997–2008

Studies of the Mesosphere, Stratosphere and Troposphere by radar have shown great growth in the last 10–15 years. The growth has occurred in both diversity and depth in all areas of technology, fundamental research, and application. New technology and faster computers have permitted real-time analysis routines that were previously impossible, and allowed better spatial and temporal resolution. Interferometric and imaging techniques, and spectral processing, have been a particular focus of new technological advances, and systems with multiple receivers and multiple radar frequencies have been developed. Better computer-processing capability, coupled with multiple frequency capability, has allowed procedures like Capon's method to be applied, for example. With regard to fundamental processes, new diffusive mechanisms have been detected, such as the slow diffusion associated with “dressed aerosols” in the Polar Mesosphere, leading to Polar Mesosphere Summer Echoes. Higher resolution studies of turbulent breakdown have allowed better examination of wave breakdown processes. Some old techniques have had a substantial rebirth, such as applications of meteor radars, which can now measure both winds and temperatures in the mesopause region. New techniques have also allowed smaller and less powerful instruments to make significant contributions in important areas. An example is development of new momentum flux measurement techniques, which have allowed small compact meteor radars to make measurements of this parameter, thereby increasing the global coverage of radars that can do this. In regard to application, windprofilers (a significant offshoot of MST radars) have become key components in many meteorological networks, and provide important input to numerical forecast models in many countries. These developments, and many more, are discussed within this review. It is of course impossible to give due discussion to all new areas, and some selectivity is necessary within the review, but hopefully we will at least touch on most of the key new areas.

Mesosphere‐stratosphere‐troposphere radar observations of characteristics of lower atmospheric high‐frequency gravity waves passing through the tropical easterly jet

In this study, we have examined the characteristics of high‐frequency gravity waves (5–50 min periodicity) over a tropical region using the mesosphere‐stratosphere‐troposphere (MST) radar installed at Gadanki (13.5°N, 79.2°E), India. The MST radar (53 MHz) was operated continuously for ∼15.5 h during 1700–0840 LT on 2–3 June 2005. During this period, a strong unstable wind shear region existed above the tropical easterly jet at the height of tropopause. This has provided an excellent opportunity to study the characteristics of internal atmospheric high‐frequency (∼15–40 min periodicity) gravity waves, generated in the boundary layer and passing through the shear layer. The study reveals the generation of higher‐frequency (5–15 min periodicity) gravity waves from this strong shear region and their vertical propagation both below up to a few kilometers and above the shear layer (∼0.5 km thick). These waves showed upward propagation even above 20 km in the lower stratosphere, indicating that unstable shear layers are the important source of momentum and energy fluxes that contribute significantly to the middle atmospheric dynamics in terms of gravity waves. Further, a close association was also observed between the dissipating gravity waves and the distinctly enhanced signal‐to‐noise ratio and Doppler spectral width of the MST radar echoes. The present observation of radiation of high‐frequency gravity waves that propagate vertically upward from a strong wind shear region located immediately below a highly stratified layer is in accordance with the “direct mechanism” explained by nonlinear numerical simulation studies. For the first time, the present study illustrates the existence of layers of polarized refractive index structures in the heights of 10–15 km.

MST radar investigation on inertia-gravity waves associated with tropical depression in the upper troposphere and lower stratosphere over Gadanki (13.5°N, 79.2°E)

Continuous measurements of 3-dimensional winds, spectral parameters, and tropopause height for ∼114 h during the passage of a tropical depression using mesosphere–stratosphere–troposphere (MST) radar at Gadanki (13.5°N, 79.2°E) are discussed. The spectral analysis of zonal and meridional winds shows the presence of inertia-gravity wave (IGW) with the dominant periodicity of 56 h and intrinsic period of 27 h in the upper troposphere and lower stratosphere (UTLS). The strengthening of easterly jet and associated wind shears during the passage of the depression is one of the causative mechanisms for exciting the IGW. A well-established radar method is used to identify the tropopause and to study its response to the propagating atmospheric disturbances. The significance of the present study lies in showing the response of tropopause height to the IGW during tropical depression for the first time, which will have implications in stratosphere–troposphere exchange processes.

Characteristics of atmospheric turbulence in terms of background atmospheric parameters inferred using MST radar at Gadanki (13.5°N, 79.2°E)

[1] Indian mesosphere-stratosphere-troposphere (MST) radar is located at Gadanki (13.5°N, 79.2°E) in a tropical zone of India. In this region the background atmospheric parameters such as atmospheric winds, temperature, and humidity show a distinct change from monsoon to winter season. For example, the monsoon season is dominated by tropical easterly jet winds in the upper troposphere (i.e., 14–17 km), and the winter season is dominated by westerly winds with peak value at about 12–14 km. The basic wind patterns in the two seasons differ significantly, and this is expected to give rise to differences in the characteristics of atmospheric turbulence parameters in the two seasons. In this paper, various atmospheric turbulence parameters such as eddy dissipation rate (ɛ), eddy diffusivity (Kh), and horizontal scale length (l0 and LB) are obtained for the first time for the monsoon and winter seasons by using the available data sets from MST radar. An attempt is made to understand and explain the seasonal variability of these turbulence parameters in terms of the observed changes in the background atmospheric conditions.

A climatology of mid-tropospheric mesoscale strong wind events as observed by the MST radar, Aberystwyth

Continuous sounding of the troposphere by the NERC Mesosphere-Stratosphere-Troposphere (MST) wind profiling radar, mid-Wales, since late 1997 provides a unique dataset with which to study the mesoscale wind field of the mid-troposphere over the United Kingdom. A wind speed probability density function (PDF) generated from 7 years of data is used to design a feature-finding algorithm to identify mesoscale strong wind events in the radar dataset. The resulting 117 events form a climatology of mesoscale strong wind features at mid-tropospheric heights over the British Isles, defined as peak wind speeds at or above the top 1% of the PDF, wind speeds exceeding the top 5% for at least 1 h, and extending over at least 1 km in depth. Plotting the location of the peak winds of these features on schematic charts of the Norwegian and Shapiro–Keyser models of cyclogenesis show these to be mainly warm sector and cold frontal in nature, with smaller numbers of tropopause folds and sting jets. Wind roses indicate a prevailing westerly direction for mid-tropospheric winds generally, but the strong wind events are distinctly different: cold frontal and warm sector events prevail from the south to southwest, tropopause folds from the northwest and sting jets from the west. The wider applicability of the results to the British Isles is established through comparison with radiosonde data from five sites covering the British Isles. Copyright © 2010 Royal Meteorological Society

An Autonomous Interference Detection and Filtering Approach Applied to Wind Profilers

An autonomous interference detection and filtering algorithm has been developed for removing the interference bands generated in the Doppler spectra of mesosphere-stratosphere-troposphere (MST) radar signals. The technique, implemented with the MST radar at Gadanki (13.5° N, 79° E), is based on identifying interferencelike band signals using a statistical signal variance approach for fixing the amplitude threshold, through which detecting the interference frequency and designing an adaptable notch filter to filter the undesired frequency bands are done. Signals received from the MST radar are sometimes contaminated with interference signals received from other objects or generated within the system through arcing of high-power devices. Multiple interference bands with different characteristics are observed in the power spectra, which contaminate the wind information and other atmospheric signals. The autonomous interference detection and filtering algorithm is applied to various cases, and it is found that the interference signals could effectively be removed, leaving behind the original signals. By this approach, the effective number of signal samples obtained is increased, which helps one to improve the temporal resolution.

Differences in Draft Core Statistics from the Wet to Dry Spell over Gadanki, India (13.5°N, 79.2°E)

AbstractThe Indian mesosphere–stratosphere–troposphere (MST) radar observations during the passage of 37 convective systems are utilized to investigate the characteristics of vertical air velocity w in different convection categories (shallow, deep, and decaying) and also the differences in draft core statistics from the wet to dry spell. The radar and optical rain gauge measurements show pronounced differences in core statistics (in terms of their vertical structure, draft strength, size, number, and the elevation angle) and surface rainfall characteristics from the dry to wet spell. The shallow convective cores are preponderant in the dry spell. Composite w profiles, retrieved from all deep cases and also from individual convection cases, depict an upper-tropospheric peak in the wet spell and a bimodal distribution (peaks at 5 and 11–13 km) in the dry spell, illustrating that they are characteristic features of wet and dry spells. The average vertical extents of the cores are nearly equal (about 8 km) in both spells of the monsoon; however, the core-base (and top) altitudes are different. In both wet and dry spells, the composite w profile for all cores show similar vertical variation to that of for updraft cores, while the composite w for downdraft cores do not show much variation with altitude, indicating that the updraft cores dictate the vertical structure of composite w. The core size varies considerably (a factor of 2) with altitude in both spells of the monsoon. Although nearly equal in the lower troposphere in both phases of the monsoon, the core size is larger by 1–2 km in the dry spell in the middle and upper troposphere. Consistent with the longer lifetime (bigger core size) of cores in the dry spell, the cores are more inclined (with a mean elevation angle of 30°) in the dry spell. The surface rainfall distribution is wider and has large number of intense rainfall rates in the wet spell. The mean rainfall rate for the wet spell is also larger by a factor of 2, consistent with earlier studies.

Spectral analysis of atmospheric radar signal using filter banks — polyphase approach

This paper proposes a new non-parametric method to estimate the power spectral density of atmospheric radar signals using uniform filter banks through polyphase approach. The performance of proposed method is investigated for a simulated broadband and narrowband signals. This method reduces the variance without affecting the resolution and improves the signal to noise ratio significantly when compared to the existing method (periodogram/modified periodogram). The variance reduction is observed to be more in case of narrowband signals. The method has the computational cost similar to modified periodogram approach. This method is tested for practical atmospheric data collected at NARL, Gadanki, India, through the mesosphere-stratosphere-troposphere (MST) radar, back scattered echoes, in particular from high altitudes, which has low signal to noise ratio. Results have been validated using simultaneous Global Positioning System (GPS) Sonde data.