Thermodynamic structure of the Atmospheric Boundary Layer over the Arabian sea as revealed by MONSOON-77 data

The thermodynamic structure of the Atmospheric Boundary Layer (ABL) over the Arabian sea region has been studied with the help of 135 aerological observations obtained during MONSOON-77 in the region (10–14° N, 64–68° E) by USSR research vessels. Low-level inversions were observed over the western Arabian sea region (west of 66° E) in association with suppressed convection. The different sublayers of the ABL, viz. the mixed layer, the cloud layer and the inversion/isothermal/stable layer were identified. The low-level stability analysis indicated that in the region east of 66° E, conditions were favourable for deep convection. The thermodynamic transformation of the boundary layer after precipitation was documented.     

Characteristics of Boundary Layer Structure during a Persistent Haze Event in the Central Liaoning City Cluster,Northeast China

The characteristics of boundary layer structure during a persistent regional haze event over the central Liaoning city cluster of Northeast China from 16 to 21 December 2016 were investigated based on the measurements of particulate matter (PM) concentration and the meteorological data within the atmospheric boundary layer (ABL). During the observational period, the maximum hourly mean PM2.5 and PM10 concentrations in Shenyang, Anshan, Fushun, and Benxi ranged from 276 to 355 μg m^–3 and from 378 to 442 μg m^–3, respectively, and the lowest hourly mean atmospheric visibility (VIS) in different cities ranged from 0.14 to 0.64 km. The central Liaoning city cluster was located in the front of a slowly moving high pressure and was mainly controlled by southerly winds. Wind speed (WS) within the ABL (< 2 km) decreased significantly and WS at 10-m height mostly remained below 2 m s^–1 during the hazy episodes, which was favorable for the accumulation of air pollutants. A potential temperature inversion layer existed throughout the entire ABL during the earlier hazy episode [from 0500 Local Time (LT) 18 December to 1100 LT 19 December], and then a potential temperature inversion layer developed with the bottom gradually decreased from 900 m to 300 m. Such a stable atmospheric stratification further weakened pollutant dispersion. The atmospheric boundary layer height (ABLH) estimated based on potential temperature profiles was mostly lower than 400 m and varied oppositely with PM_2.5 in Shenyang. In summary, weak winds due to calm synoptic conditions, strong thermal inversion layer, and shallow atmospheric boundary layer contributed to the formation and development of this haze event. The backward trajectory analysis revealed the sources of air masses and explained the different characteristics of the haze episodes in the four cities.     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.     

Doppler sounder observations of trade winds and sea breezes along the African west coast near 34 ° S, 19 ° E

Summer weather conditions along the west coast of Africa near 34 ° S, 18 ° E are investigated using doppler acoustic sounder profiles. Case studies were selected from a two-year record to form composite analyses over the diurnal cycle. The SE trade wind exhibited a low level jet at the level of the temperature inversion due to a sharp reversal in the thermal wind vector aloft. Mean wind speeds reached 14 m s^–1 just before midnight as the surface and upper inversions strengthened. Seabreezes were categorised by the supporting gradient wind and found to have mean depths of 400 m, speeds of over 6 m s^–1 at the 200 m level, and advance/retreat times of 09 hr and 16–20 hr. During seabreezes and weak on-shore gradient flow conditions, the thermal internal boundary layer (TIBL) was monitored with sounder transects in the first 12 km of the coastal zone. The growth height was observed to be 1:20 in the first 5 km and 1:50 farther inland. The sounder climatology, together with surface network and aerial survey results, illustrate the four-dimensional characteristics of trade winds and seabreezes near Cape Town.     

VIMHEX 1972 revisited: The life cycles of two tropical convective mesosystems over land

Summary The evolutions of two slow-moving convective mesosystems observed in Venezuela during VIMHEX-II were studied using time changes in radar echo characteristics. The duration of the echo, maximum echo area, maximum echo height and mean echo velocity were about 6 hours, 5000km², 16km, and 95°/5.2m s^–1 for mesosystem A and about 11 hours, 12000km², 14km, and 90°/3.2m s^–1, for mesosystem B, respectively. The rainfall distributions associated with the mesosystems are described and correlated with the radar echo characteristics. The synoptic conditions are discussed, as well as the modification of the environment produced by the mesosystems.The relative wind profiles suggest that mesosystem A behaved more like a typical tropical squall line, with inflow at almost all levels at the front of the mesosystem, while mesosystem B behaved like a typical mid-latitude squall line, with inflow at lower levels and outflow at upper levels at the front of the mesosystem.With 12 Figures     

Tropical atmospheric stability and wind structures as inferred from the Indian MST radar observations

Summary The MST (Mesosphere-Stratosphere-Tropospher) Radar Facility at Gadanki (13.47° N, 79.18° E), near Tirupati, Andhra Pradesh, India has been operated over seven diurnal cycles—three in November 1994, one in September 1995 and three in January–February 1996 with an objective to study the wind and stability characteristics in the troposphere and lower-stratosphere. The radar-measured height profiles of both zonal (EW) and meridional (NS) wind components and near-simultaneous radiosonde measurements from Madras (13.04° N, 80.7° E) and Bangalore (12.85° N, 77.58° E), the two stations close to either side of the radar site, have been compared and they are found to be in gross agreement within the limitations of the sensing techniques.The results of the study also indicated multiple stable and turbulent structures/stratification throughout the height region from about 4 to 30 km. It is noticed that the stable layers are well marked around the altitudes 4 km, 12 km and the tropopause while the turbulent layers exist a few kilometers below the tropopause. These stable and turbulent layer structures showed good correspondence with the radar-measured wind gradients and also with the radiosonde-derived temperature and wind distributions over Madras. The maximum positive gradient in the signal-tonoise ratio (SNR) which corresponds to radar tropopause is found to coincide with the greater potential temperature gradient and smaller wind gradient. The time evolution of atmospheric stability structure, derived from the SNR, spectral width and vertical wind revealed a diffused tropopause or tropopause weakening which is found to be associated with broader spectral width and larger gradients of winds. This feature is considered to be due either to the instability associated with large vertical gradients in horizontal winds (dynamical instability) or to the instability generated by the convection (convective instability).With 6 Figures     

Evaluation of turbulent fluxes over Maitri,Antarctica

Turbulent fluxes have been evaluated for clear sunny days over the Indian Antarctic station, Maitri, using the basic meteorological data recorded at four levels of a 28 m tower. The data are supplemented with radiation data. The surface layer over Maitri remains thermally stratified during the hours of minimum solar insolation, the so-called nighttime period. The surface winds during this period are generally very strong resulting in high momentum fluxes. In particular, for high winds (>12 m s^–1), the temperature gradient is found to be less positive than for moderate winds (4 to 7 m s^–1). Solar insolation provided the daytime heating necessary for the diurnal variation of atmospheric stability, and hence, for the turbulent fluxes. Thus, on clear days daytime conditions are marked by upward transport of heat with reduced momentum flux, while stable nighttime conditions are marked by a downward heat flux with increased momentum fluxes.     

Topographic influence on surface winds

Using data collected during 1975–1976 from a meteorological network operating in the vicinity of the Columbia Generating Site approximately 8 km south of Portage, Wis., the influence of the Baraboo Hills on the surface wind field is determined. Half-hour means of wind speed and direction measured at 9 m at three sites were used to compute divergence and vorticity using Bellamy's method. The data were grouped into 18 sectors each 20 deg wide and averages computed for each quantity. Results indicate that for wind directions perpendicular to the eastern edge of the Baraboo Hills, the surface (9m) wind field is significantly perturbed up to 4 km from the bluffs. The largest convergence of 2.1 × 10^–4 s^–1 occurs with 160 deg wind direction and the largest divergence of 1.2 × 10^–4 s^–1 with 290 deg wind direction. The maximum anticyclonic vorticity was 1.6 × 10^–4 s^–1 at 210 deg and the maximum cyclonic vorticity was 1.6 × 10^–4 s^–1 at 330 deg.     

The structure of the stably stratified internal boundary layer in offshore flow over the sea

Observations obtained mainly from a research aircraft are presented of the mean and turbulent structure of the stably stratified internal boundary layer (IBL) over the sea formed by warm air advection from land to sea. The potential temperature and humidity fields reveal the vertical extent of the IBL, for fetches out to several hundred of kilometres, geostrophic winds of 20–25 m s^–1, and potential temperature differences between undisturbed continental air and the sea surface of 7 to 17 K. The dependence of IBL depth on these external parameters is discussed in the context of the numerical results of Garratt (1987), and some discrepancies are noted.Wind observations show the development of a low-level wind maximum (wind component normal to the coast) and rotation of the wind to smaller cross-isobar flow angles. Potential temperature () profiles within the IBL reveal quite a different structure to that found in the nocturnal boundary layer (NBL) over land. Over the sea, profiles have large positive curvature with vertical gradients increasing monotonically with height; this reflects the dominance of turbulent cooling within the layer. The behaviour is consistent with known behaviour in the NBL over land where curvature becomes negative (vertical gradients of decreasing with height) as radiative cooling becomes dominant.Turbulent properties are discussed in terms of non-dimensional quantities, normalised by the surface friction velocity, as functions of normalised height using the IBL depth. Vertical profiles of these and the normalised wavelength of the spectral maximum agree well with known results for the stable boundary layer over land (Caughey et al., 1979).     

A Large-Eddy Simulation Study of the Influence of Subsidence on the Stably Stratified Atmospheric Boundary Layer

The influence of the large-scale subsidence rate, S, on the stably stratified atmospheric boundary layer (ABL) over the Arctic Ocean snow/ice pack during clear-sky, winter conditions is investigated using a large-eddy simulation model. Simulations of two 24-h periods are conducted while varying S between 0, 0.001 and 0.002 ms⁻¹, and the resulting quasi-equilibrium ABL structures and evolutions are examined. Simulations conducted with S = 0 yield a boundary layer that is deeper, more strongly mixed and cools more rapidly than the observations. Simulations conducted with S > 0 yield improved agreement with the observations in the ABL height, potential temperature gradients and bulk heating rates. We also demonstrate that S > 0 limits the continuous growth of the ABL observed during quasi-steady conditions, leading to the formation of a nearly steady ABL of approximately uniform depth and temperature. Subsidence reduces the magnitudes of the stresses, as well as the implied eddy-diffusivity coefficients for momentum and heat, while increasing the vertical heat fluxes considerably. Subsidence is also observed to increases the Richardson number to values in excess of unity well below the ABL top.     

Sodar observations of the atmospheric boundary layer over the ocean during ASTEX-91

A complex marine experiment was conducted in autumn 1991 on the research vessel Dmitry Mendelev in association with the Atlantic Stratocumulus Transition Experiment (ASTEX). A three-axis Doppler sodar designed at the Institute of Atmospheric Physics, Moscow, was used in this experiment. Total observation time was about 770 hours from 6 October to 23 November. Besides facsimile records illustrating spatial and temporal structure of the turbulence distribution in the atmospheric boundary layer (ABL), routine quantitative measurements of profiles of wind and echo-signal strength were taken. Some main characteristics of the ABL behavior over the ocean were revealed through an analysis of these data as well as the results of other kinds of measurements. An important peculiarity of the ABL observed between the Canary Islands and the Azores was the presence of diurnal variation of convective turbulence strength having a maximum between 04:00 and 07:00 LT. A similar diurnal variation was observed for low-level cloud cover. Occurrence of various types of thermal stratification and their diurnal variation were obtained. Comparison of elevated stable layers and low-level cumulus showed that the lower boundary of clouds correlates well with the height of the bottom of elevated inversion layers (at heights of 200–600 m). Canary and Cabo Verde observations showed that islands strongly affect the ABL structure. The strong effect of a surface water temperature gradient on the ABL stability was observed when crossing the Canary, Azores, and Labrador currents and the Gulf Stream.     

Isoprene and Its Oxidation Products Methyl Vinyl Ketone, Methacrolein, and Isoprene Related Peroxides Measured Online over the Tropical Rain Forest of Surinam in March 1998

Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0–12 km altitude,2°–7° N, 54°–58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1–3 × 10⁵molecules cm^-3 averaged over 24 hours were calculated due to lossof OH and HO₂ in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer.     

Condensed water in tropical cyclone “Oliver”, 8 February 1993

On February 8, 1993, the NASA DC-8 aircraft profiled from 10,000 to 37,000 feet (3.1–11.3 km) pressure altitude in a stratified section of tropical cyclone “Oliver” over the Coral Sea northeast of Australia. Size, shape and phase of cloud and precipitation particles were measured with a 2-D Greyscale probe. Cloud/ precipitation particles changed from liquid to ice as soon as the freezing level was reached near 17,000 feet (5.2 km) pressure altitude. The cloud was completely glaciated at −5°C. There was no correlation between ice particle habit and ambient temperature. In the liquid phase, the precipitation-cloud drop concentration was 4.0 × 10³ m⁻³, the geometric mean diameter D_g=0.5−0.7 mm, and the liquid water content 0.7−1.9 g m⁻³. The largest particles anywhere in the cloud, dominated by fused dendrites at concentrations similar to that of raindrops (2.5 × 10³ m⁻³) but a higher condensed water content (5.4 g m⁻³ estimated) were found in the mixed phase; condensed water is removed very effectively from the mixed layer due to high settling velocities of the large mixed particles. The highest number concentration (4.9 × 10⁴ m⁻³), smallest size (D_g=0.3−0.4 mm), largest surface area (up to 2.6 × 10² cm² m⁻³ at 0.4−1.0 g m⁻³ of condensate) existed in the ice phase at the coldest temperature (−40°C) at 35,000 feet (10.7 km). Each cloud contained aerosol (haze particles) in addition to cloud particles. The aerosol total surface area exceeded that of the cirrus particles at the coldest temperature. Thus, aerosols must play a significant role in the upscattering of solar radiation. Light extinction (6.2 km⁻¹) and backscatter (0.8 sr⁻¹ km⁻¹) was highest in the coldest portion of the cirrus cloud at the highest altitude.     

Transient convective waves in the tropical SW Indian Ocean

Summary Wave-organized convective features in the southwest Indian Ocean are described using Hovmoller composites of satellite imagery, OLR anomalies and ECMWF precipitable water departures during the southern summer. Westward movement of large convective elements is noted in the 10–20°S latitude band in about half of the years between 1970 and 1984. A study of 47 convective systems from satellite imagery establishes the climatological features, including zonal propagation speeds for maritime systems in the range –2 to –4 m s^–1, wavelengths of 25–35° longitude (3,000 km), lifespans of 10–20 days and convective areas of 7–10° longitude (800 km). Transient convective waves over the tropical SW Indian Ocean are slower and more diverse than their northern hemisphere counterparts. Interannual tendencies in the frequency and mode are studied. Wet summers over SE Africa correspond with an increased frequency of westward moving convective systems, whereas in dry summers convective systems tend to be quasi-stationary. INSAT data composites provide additional insight into the convective structure and show that tropical waves penetrated into southern Africa in February 1988. A more quantitative assessment of transient convective waves is provided by Hovmoller composites of OLR anomalies and precipitable water departures. Both display westward moving systems in 1976 and 1984 and highlight the wide variety and mixed mode character of convective waves. A case study is analyzed which illustrates the deepening of a moist, unstable layer coincident with the westward passage of a convective wave.With 12 Figures     

Observations of the convective plume of a lake under cold-air advective conditions

Moderating effects of Lake Apopka, Florida on downwind surface temperatures were evaluated under cold-air advective conditions. Point temperature measurements north and south of the lake and data obtained from a thermal scanner flown at 1.6 km indicate that surface temperatures directly downwind may be higher than surrounding surface temperatures by as much as 5 °C under conditions of moderate winds (~4 m s^–1). No substantial temperature effects were observed with surface wind speed less than 1 m s^–1. Fluxes of sensible and latent heat from Lake Apopka were calculated from measurements of lake temperature, net radiation, relative humidity and air temperature above the lake. Bulk transfer coefficients and the Bowen ratio were calculated and found to be in agreement with reported data for non-advective conditions.IFAS Journal Series No. 1006.     

Mesoscale nocturnal jetlike winds within the planetary boundary layer over a flat,open coast

Mesoscale nocturnal jetlike winds have been observed over a flat, open coast. They occur within the planetary boundary layer between 100 and 600 m. At times the wind shear may reach 15 m s^-1 per 100 m. Unlike the common low-level jet that occurs most often at the top of the nocturnal inversion and only with a wind from the southerly quadrant, this second kind of jet exists between nocturnal ground-based inversion layers formed by the cool pool, or mesohigh, and the elevated mesoscale inversion layer over the coast. It occurs mostly when light % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikaiabgs% MiJkaaiwdacqGHsislcaaI2aGaaeyBaiaabccacaqGZbWaaWbaaSqa% beaacqGHsislcaaIXaaaaOGaaiykaaaa!3FCF!\[( \leqslant 5 - 6{\text{m s}}^{ - 1} )\] geostrophic winds blow from land to sea and when the air temperature over adjacent seas is more than 5 °C warmer than that over the coast. This phenomenon may be explained by combined Venturi and gravity-wind effects existing in a region from just above the area a few kilometres offshore to 100–600 m in height approximately 40–50 km inland because this region is sandwiched between the aforementioned two inversion layers.     

Temperature fluctuations inside a cloud-laden airstream flowing over a hill station

Observations of temperature in a cloud-laden monsoon airstream at a height of 12 m above ground at a hill station, Mahabaleshwar (elevation 1390 m asl), were obtained for ten hours using a linearised thermistor (YSI Part 44202) as a temperature sensor, and analysed. It was found that the frequency distributions of temperatures were multimodal. The horizontal temperature gradients in the air stream were symmetrically distributed. The temperature spectra in the frequency range (0.001 to 0.1 Hz) studied can be classified into a lower frequency regime and a higher frequency regime separated approximately at the frequency 0.01 Hz or the wave number 3 km^–1. The spectra in the higher range conform to Kolmogorov's power law for the inertial subrange and those in the lower range exhibit a steeper negative slope of the order of -1.3. There is very little variation of air temperature associated with rainfall intensities greater than 4 mm hr^–1; for lower intensities, the smaller the intensity, the greater the variation.     

Small-scale atmospheric structure deduced from measurements of temperature,humidity and refractive index

Measurements have been made with fast-response multi-channel temperature, humidity and refractive index sensors flown to 2000 m on a tethered balloon to investigate small-scale fluctuations important in radio-wave scattering, their relation to atmospheric parameters, and their spatial variation in both one and three dimensions. Data from the three types of sensors at one point were consistent for frequencies up to about 8 Hz. Power spectra of data at various heights were computed over 0.1 to 10 Hz and generally showed slopes (on a log-log plot) close to - 5/3 above 1 Hz but ranged from –1.5 to – 3.5 at lower frequencies; in this range (f < 1 Hz) slopes were close to – 5/3 for negative Richardson number (Ri), provided temperature gradients were steeper than –1.1 °C 100 m^–1 and wind shears > 1.4 x 10^–2 s^–1 approx. Steeper slopes were generally associated with stable atmospheric conditions but no precise relation to the above parameters was found. Spectral density was a maximum for Ri –0.75.Cross-correlations of 0.5 were frequently observed between sensors 1 m apart in orthogonal directions; in the vertical, examples of negative correlation of vapour pressure were occasionally found over this spacing. Using four sensors spaced in line over 9 m, cross-spectrum phase calculations of drift speeds were found to be consistent with measured wind speeds. The ratio of identification distance (coherence=0.6) to scale size of irregularities ranged from 0.25 to 0.5 with no apparent relation to height or meteorological parameters.     

Global distribution of natural freshwater wetlands and rice paddies,their net primary productivity,seasonality and possible methane emissions

A global data set on the geographic distribution and seasonality of freshwater wetlands and rice paddies has been compiled, comprising information at a spatial resolution of 2.5° by latitude and 5° by longitude. Global coverage of these wetlands total 5.7×10⁶ km² and 1.3×10⁶ km², respectively. Natural wetlands have been grouped into six categories following common terminology, i.e. bog, fen, swamp, marsh, floodplain, and shallow lake. Net primary productivity (NPP) of natural wetlands is estimated to be in the range of 4–9×10¹⁵ g dry matter per year. Rice paddies have an NPP of about 1.4×10¹⁵ g y^–1. Extrapolation of measured CH₄ emissions in individual ecosystems lead to global methane emission estimates of 40–160 Teragram (1 Tg=10¹² g) from natural wetlands and 60–140 Tg from rice paddies per year. The mean emission of 170–200 Tg may come in about equal proportions from natural wetlands and paddies. Major source regions are located in the subtropics between 20 and 30° N, the tropics between 0 and 10° S, and the temperate-boreal region between 50 and 70° N. Emissions are highly seasonal, maximizing during summer in both hemispheres. The wide range of possible CH₄ emissions shows the large uncertainties associated with the extrapolation of measured flux rates to global scale. More investigations into ecophysiological principals of methane emissions is warranted to arrive at better source estimates.     

Subcloud-layer kinematic and dynamic structures of a microburst-producing thunderstorm in Colorado determined from JAWS Dual-Doppler measurements

Some kinematic and dynamic structures of a microburst-producing storm in Colorado were investigated. Dual-Doppler data collected on 14 July, 1982 at 1647 MDT, during the Joint Airport Weather Studies (JAWS) project at Denver's Stapleton International Airport, were objectively analyzed to produce a three-dimensional wind field. The domain of interest had a horizontal dimension of 10 by 10 km centered on the microburst. Vertical velocities were computed by integrating the anelastic continuity equation downward from the storm's top with variational adjustment. Subsequently, fields of deviation perturbation pressure, density, and virtual temperature were retrieved from a detailed wind field using the three momentum equations.Results show that the microburst being investigated is embedded within a high-reflectivity region associated with heavy precipitation. A strong downflow impinges on the surface producing a stagnation mesohigh inside the microburst. This mesohigh is accompanied by mesolows in the strongest outflow regions, forming a pronounced horizontal perturbation pressure gradient outward from the high-pressure center. The outflow regions extend from the surface to approximately 1 km AGL with maximum divergence in excess of 10 ^–2 s^–1. Inclusion of friction in the pressure equation improves pressure recovery at all levels, especially in the atmospheric boundary layer (ABL). The microburst occurrence in the ABL enhances eddy transfer of momentum. Magnitudes of eddy viscosity and eddy stresses increase as a result of the microburst.