Vertical Gradient Of Potential Temperature Research Articles

Identification of Convective Boundary Layer Depth over the Great Lakes Region Using Aircraft Observations: A Comparison of Various Methods

Abstract This study evaluates the methods of identifying the height zi of the top of the convective boundary layer (CBL) during winter (December and January) over the Great Lakes and nearby land areas using observations taken by the University of Wyoming King Air research aircraft during the Lake-Induced Convection Experiment (1997/98) and Ontario Winter Lake-effect Systems (2013/14) field campaigns. Since CBLs facilitate vertical mixing near the surface, the most direct measurement of zi is that above which the vertical velocity turbulent fluctuations are weak or absent. Thus, we use zi from the turbulence method as the “reference value” to which zi from other methods, based on bulk Richardson number (Rib), liquid water content, and vertical gradients of potential temperature, relative humidity, and water vapor mixing ratio, are compared. The potential temperature gradient method using a threshold value of 0.015 K m−1 for soundings over land and 0.011 K m−1 for soundings over lake provided the estimates of zi that are most consistent with the turbulence method. The Rib threshold-based method, commonly used in numerical simulation studies, underestimated zi. Analyzing the methods’ performance on the averaging window zavg we recommend using zavg = 20 or 50 m for zi estimations for lake-effect boundary layers. The present dataset consists of both cloudy and cloud-free boundary layers, some having decoupled boundary layers above the inversion top. Because cases of decoupled boundary layers appear to be formed by nearby synoptic storms, we recommend use of the more general term, elevated mixed layers. Significance Statement The depth zi of the convective atmospheric boundary layer (CBL) strongly influences precipitation rates during lake-effect snowstorms (LES). However, various zi approximation methods produce significantly different results. This study utilizes extensive concurrently collected observations by project aircraft during two LES field studies [Lake-Induced Convection Experiment (Lake-ICE) and OWLeS] to assess how zi from common estimation methods compare with “reference” zi derived from turbulent fluctuations, a direct measure of CBL mixing. For soundings taken both over land and lake; with cloudy or cloud-free conditions, potential temperature gradient (PTG) methods provided the best agreement with the reference zi. A method commonly employed in numerical simulations performed relatively poorly. Interestingly, the PTG method worked equally well for “coupled” and elevated decoupled CBLs, commonly associated with nearby cyclones.

Fine-scale mapping of daily minimum temperature in a cropland with complex terrains through the combination of a cold flow accumulation model with inversion strength

In complex terrains, the minimum temperature varies at fine spatial scales owing to cold air drainage. This leads to large spatial variabilities in phenological events and frost risk in plants. Various topographic variables have been used to model cold air drainage and its influence on temperature. In this study, we modified a conventional approach using flow accumulation (FA). First, we developed a new algorithm for FA by reconsidering effective areas of cold air drainage. This allowed us to increase the model's applicability. The FA model was then combined with the vertical gradient of potential temperature, which represents the inversion strength during stable, nighttime conditions. This process facilitated minimum temperature estimation at a given date and point. The model was developed using temperature observations in tea fields with complex terrains, where frost risk is increasing due to climate change. The maximum difference in daily minimum temperature across 15 observation points was 6.7°C, despite only a 73-m elevation difference. Compared with models using spatial interpolation alone, the proposed model produced a better estimate of the spatiotemporal variability of daily minimum temperature and improved the average residuals between estimated and measured temperatures in valley areas from 1.9 to 0.03°C. Moreover, compared to the conventional algorithm, the proposed algorithm improved temperature estimations in ridge areas and clarified the boundaries between ridges and valleys. Our approach could contribute to the spatiotemporal analysis of temperature variations and concomitant changes in phenological events and frost risk in complex terrains.

A Modern Approach to a Stability-Based Definition of the Tropopause

Abstract Definition of the tropopause has remained a focus of atmospheric science since its discovery near the beginning of the twentieth century. Few universal definitions (those that can be reliably applied globally and to both common observations and numerical model output) exist and many definitions with unique limitations have been developed over the years. The most commonly used universal definition of the tropopause is the temperature lapse-rate definition established by the World Meteorological Organization (WMO) in 1957 (the LRT). Despite its widespread use, there are recurrent situations where the LRT definition fails to reliably identify the tropopause. Motivated by increased availability of coincident observations of stability and composition, this study seeks to reexamine the relationship between stability and composition change in the tropopause transition layer and identify areas for improvement in a stability-based definition of the tropopause. In particular, long-term (40+ years) balloon observations of temperature, ozone, and water vapor from six locations across the globe are used to identify covariability between several metrics of atmospheric stability and composition. We found that the vertical gradient of potential temperature is a superior stability metric to identify the greatest composition change in the tropopause transition layer, which we use to propose a new universally applicable potential temperature gradient tropopause (PTGT) definition. Application of the new definition to both observations and reanalysis output reveals that the PTGT largely agrees with the LRT, but more reliably identifies tropopause-level composition change when the two definitions differ greatly. Significance Statement In this study we provide a review of existing tropopause definitions (and their limitations) and investigate potential improvement in the definition of the tropopause using balloon-based observations of stability and atmospheric composition. This work is motivated by the need for correct identification of the tropopause to accurately assess upper-troposphere–lower-stratosphere processes, which in turn has far-reaching implications for our understanding of Earth’s radiation budget and climate. The result of this research is the creation of a new, universally applicable stability-based definition of the tropopause: the potential temperature gradient tropopause (PTGT).

Performance Evaluation of Planetary Boundary Layer Schemes in Simulating Structures of Wintertime Lower Troposphere in Seoul Using One-Hour Interval Radiosonde Observation

We investigated the structures of the wintertime lower troposphere in Seoul, South Korea on 17 and 18 January 2017 by performing 1 h interval radiosonde observation and numerical simulations. In the daytime on 17 January, the height of the convective boundary layer (CBL) sharply and quickly increased when the residual layer became a part of the CBL. From the afternoon on 17 January, moist air with clouds began to substantially intrude in the lower troposphere in Seoul, and radiative heating/cooling weakened. As a result, the mixing of air in the lower troposphere was inhibited and the vertical gradients of potential temperature and water vapor mixing ratio changed little on 18 January. We evaluated the performance of four planetary boundary layer (PBL) parameterization schemes (the Yonsei University (YSU), Mellor–Yamada–Janjić (MYJ), Mellor–Yamada–Nakanishi–Niino (MYNN), and Asymmetric Convective Model version 2 (ACM2) schemes) coupled with the Weather Research and Forecasting model in simulating the structures of the lower troposphere against 1 h interval radiosonde observation. The general tendencies of the air temperature and wind speed in the lower troposphere were well-reproduced in the four simulations. However, the sharp increase in the CBL height did not appear in the four simulations, implying that the process of the residual layer becoming a part of the CBL in the daytime is not well-parameterized. Additionally, the simulated water vapor mixing ratio near the surface was smaller compared with the observation. We found that small-scale turbulence in the CBL, which mixes advected air and pre-existing air, was not reproduced well by the PBL parameterization schemes. Compared with the other simulations, the most accurate air temperature and wind speed were reproduced in the simulation with the MYJ scheme, while the CBL development and moisture advection were reproduced relatively well in the simulation with the MYNN scheme.

Using an Artificial Neural Network to improve operational wind prediction in a small unresolved valley

AbstractThis study improves surface wind predictions in an unresolved valley using an artificial neural network (ANN). Forecasting winds in complex terrain with a mesoscale model is challenging. This study assesses the quality of 3-km wind forecasts by the Weather Research and Forecasting (WRF) model and the potential of post-processing by an ANN within the 1-2 km wide Cadarache Valley in southeast France. Operational wind forecasts for 110m above ground level and the near-surface vertical potential temperature gradient with a lead time of 24-48h were used as ANN input. Observed horizontal wind components at 10m within the valley were used as targets during ANN training. We use the Directional ACCuracy (DACC45, wind direction error ≤ 45°) and mean absolute error to evaluate the WRF direct model output and the ANN results. By post-processing, the score for DACC45 improves from 56% in the WRF direct model output to 79% after applying the ANN. Furthermore, the ANN performed well during the day and night, but poorly during the morning and afternoon transitions. The ANN improves the DACC45 at 10m even for poor WRF forecasts (direction bias ≥ 45°) from 42% to 72%. A shorter lead time and finer grid spacing (1 km) showed negligible impact which suggests that a 3 km grid spacing and a 24-48h lead time is effective and relatively cheap to apply. We find that WRF performs well in near-neutral conditions and poorly in other atmospheric stability conditions. The ANN post-treatment consistently improves the wind forecast for all stability classes to a DACC45 of about 80%. The study demonstrates the ability to improve Cadarache valley wind forecasts using an ANN as post-processing for WRF daily forecasts.

Marine cold air outbreaks in the Russian Arctic: climatology, interannual variability, dependence on sea-ice concentration

In this study, we evaluated the climatology and interannual variability of marine cold-air outbreaks (MCAOs) in the Russian Arctic marginal seas (from the Barents to Chukchi seas). We used a simple index for identifying MCAOs based on the vertical potential temperature gradient between the sea surface and the 800 hPa level. We calculated the index using 6-hourly Era-Interim data for the 1979–2018 period. Given the index, we evaluated spatial and temporal variability of weak, medium, and strong MCAOs frequency as well as their dependence on sea-ice concentration using non-parametric tests. The most intense MCAOs were found in the Barents and Kara seas. The annual cycle maximum for the western Russian Arctic (WRA) were found in wintertime, while it was revealed in mid-late autumn for the eastern Russian Arctic (ERA). In the WRA, we found a statistically significant decrease in amount of strong MCAOs in winter and late autumn and a general strengthening of MCAOs in spring. Meanwhile, over the ERA region, increase of moderate and weak cold-air intrusions during October and November was revealed.

Processes Associated with Convection Initiation in the North American Mesoscale Forecast System, Version 3 (NAMv3)

Abstract In support of the Next Generation Global Prediction System (NGGPS) project, processes leading to convection initiation in the North American Mesoscale Forecast System, version 3 (NAMv3) are explored. Two severe weather outbreaks—occurring over the southeastern United States on 28 April 2014 and the central Great Plains on 6 May 2015—are forecast retrospectively using the NAMv3 CONUS (4 km) and Fire Weather (1.33 km) nests, each with 5-min output. Points of convection initiation are identified, and patterns leading to convection initiation in the model forecasts are determined. Results indicate that in the 30 min preceding convection initiation at a grid point, upward motion at low levels of the atmosphere enables a parcel to rise to its level of free convection, above which it is accelerated by the buoyancy force. A moist absolutely unstable layer (MAUL) typically is produced at the top of the updraft. However, when strong updrafts are collocated with large vertical gradients of potential temperature and moisture, noisy vertical profiles of temperature, moisture, and hydrometeor concentration develop beneath the rising MAUL. The noisy profiles found in this study are qualitatively similar to those that resulted in NAMv3 failures during simulations of Hurricane Joaquin in 2015. The CM1 cloud model is used to reproduce these noisy profiles, and results indicate that the noise can be mitigated by including explicit vertical diffusion in the model. Left unchecked, the noisy profiles are shown to impact convective storm features such as cold pools, precipitation, updraft helicity intensity and tracks, and the initiation of spurious convection.

Marginal sea surface temperature variation as a pre-cursor of heat waves over the Korean Peninsula

This study examines the role of the marginal sea surface temperature (SST) on heat waves over Korea. It is found that sea surface warming in the south sea of Korea/Japan (122-138°E, 24- 33°N) causes heat waves after about a week. Due to the frictional force, the positive geopotential height anomalies associated with the south sea warming induce divergent flows over the boundary layer. This divergent flow induces the southerly in Korea, which leads to a positive temperature advection. On the other hand, over the freeatmosphere, the geostrophic wind around high-pressure anomalies flows in a westerly direction over Korea during the south sea warming, which is not effective in temperature advection. Therefore, the positive temperature advection in Korea due to the south sea warming decreases with height. This reduces the vertical potential temperature gradient, which indicates a negative potential vorticity (PV) tendency over Korea. Therefore, the high-pressure anomaly over the south sea of Korea is propagated northward, which results in heat waves due to more incoming solar radiation.

Recirculation over complex terrain

AbstractThis study generated eddy covariance data to investigate atmospheric dynamics leeward of a small, forested hillside in upstate New York. The causes and effects of recirculation eddies were examined to support the larger goal of improving measurement of the exchange of energy, moisture, and trace gases between the terrestrial biosphere and the atmosphere over complex terrain. Sensors operated at five different altitudes on two separate towers—one at the top of the hill and one down the slope to the east—for approximately 8 weeks in the spring of 2013. During the experiment, the vertical potential temperature gradient was found to be the primary factor for determining whether winds interacting with the terrain features caused a recirculating eddy leeward of the hill. The study found evidence that the recirculation influenced carbon dioxide flux and caused the air column to be vertically well mixed.

Characteristics of the Heat Flux in the Unstable Atmospheric Surface Layer

Abstract The behavior of the heat flux H with respect to the stability parameter (=z/L, where z is the height above the ground, and L is the Obukhov length) in the unstable atmospheric surface layer is analyzed within the framework of Monin–Obukhov similarity (MOS) theory. Using MOS equations, H is expressed as a function of and vertical surface-layer potential temperature gradient . A mathematical analysis is carried out to analyze the theoretical nature of heat flux with the stability parameter by considering the vertical potential temperature gradient as (i) a constant and (ii) a power-law function of heat flux. For a given value of H, two values of associated with different stability regimes are found to occur in both the conditions, suggesting the nonuniqueness of MOS equations. Turbulent data over three different sites—(i) Ranchi, India; (ii) the Met Office’s Cardington, United Kingdom, monitoring facility; and (iii) 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99; United States—are analyzed to compare the observed nature of H with that predicted by MOS. The analysis of observational data over these three sites reveals that the observed variation of H with is consistent with that obtained theoretically from MOS equations when considering the vertical temperature gradient as a power-law function of heat flux having the exponent larger than 2/3. The existence of two different values of the stability parameter for a given value of heat flux suggests that the application of heat flux as a boundary condition involves some intricacies, and it should be applied with caution in convective conditions.

Atmospheric stability and diurnal patterns of aeolian saltation on the Llano Estacado

Abstract Aeolian transport is driven by aerodynamic surface stress imposed by turbulent winds in the Earth’s atmospheric boundary layer (ABL). ABL regime is influenced by stratification, which can either enhance or suppress production of turbulence by shear associated with the vertical gradient of streamwise velocity. During the day and night, surface heat fluxes induce a negative (unstable) and positive (stable) vertical gradient of potential temperature, respectively, which modifies the role of buoyancy in turbulence production. During the brief morning and evening transition periods, the vertical gradient of potential temperature vanishes (neutral stratification). The Monin–Obukhov similarity theory describes how the vertical gradient of streamwise velocity varies with stratification. Simultaneous field measurement of wind speed and aeolian activity were obtained over a 218-day period on a bare, sandy surface on the high plains of the Llano Estacado region of west Texas. Wind speed was measured at a height of 2 m with a propeller-type anemometer and aeolian activity was measured at the surface with a piezoelectric saltation sensor. We have used the wind speed measurements within the framework of the Monin–Obukhov similarity theory to estimate “typical” shear velocity, u ∗ , of the ABL as stratification is varied (characterized with the stability parameter). This approach results in a color flood contour of u ∗ against time of day and stability parameter: the procedure demonstrates that aeolian activity is most likely to occur during the day, when buoyancy acts in conjunction with mechanical shear to increase u ∗ .

Wet and dry deposition of mineral dust particles in Japan: factors related to temporal variation and spatial distribution

Abstract. Recent ground networks and satellite remote-sensing observations have provided useful data related to spatial and vertical distributions of mineral dust particles in the atmosphere. However, measurements of temporal variations and spatial distributions of mineral dust deposition fluxes are limited in terms of their duration, location, and processes of deposition. To ascertain temporal variations and spatial distributions of mineral dust deposition using wet and dry processes, weekly deposition samples were obtained at Sapporo, Toyama, Nagoya, Tottori, Fukuoka, and Cape Hedo (Okinawa) in Japan during October 2008–December 2010 using automatic wet and dry separating samplers. Mineral dust weights in water-insoluble residue were estimated from Fe contents measured using an X-ray fluorescence analyser. Wet and dry deposition fluxes of mineral dusts were both high in spring and low in summer, showing similar seasonal variations to frequency of aeolian dust events (Kosa) in Japan. For wet deposition, highest and lowest annual dust fluxes were found at Toyama (9.6 g m−2 yr−1) and at Cape Hedo (1.7 g m−2 yr−1) as average values in 2009 and 2010. Higher wet deposition fluxes were observed at Toyama and Tottori, where frequent precipitation (> 60% days per month) was observed during dusty seasons. For dry deposition among Toyama, Tottori, Fukuoka, and Cape Hedo, the highest and lowest annual dust fluxes were found respectively at Fukuoka (5.2 g m−2 yr−1) and at Cape Hedo (2.0 g m−2 yr−1) as average values in 2009 and 2010. The average ratio of wet and dry deposition fluxes was the highest at Toyama (3.3) and the lowest at Hedo (0.82), showing a larger contribution of the dry process at western sites, probably because of the distance from desert source regions and because of the effectiveness of the wet process in the dusty season. Size distributions of refractory dust particles were obtained using four-stage filtration: > 20, > 10, > 5, and > 1 μm diameter. Weight fractions of the sum of > 20 μm and 10–20 μm (giant fraction) were higher than 50% for most of the event samples. Irrespective of the deposition type, the giant dust fractions generally decreased with increasing distance from the source area, suggesting the selective depletion of larger giant particles during atmospheric transport. Based on temporal variations of PMc (2.5 < D < 10 μm), ground-based lidar, backward air trajectories, and vertical profiles of potential temperatures, transport processes of dust particles are discussed for events with high-deposition and low-deposition flux with high PMc. Low dry dust depositions with high PMc concentrations were observed under stronger (5 K km−1) stratification of potential temperature with thinner and lower (< 2 km) dust distributions because the PMc fraction of dust particles only survived after depletion of giant dust particles by rapid gravitational settling at the time they reach Japan. In contrast, transport through a thicker (> 2 km) dust layer with weak vertical gradient of potential temperature carry more giant dust particles to Japan. Because giant dust particles are an important mass fraction of dust accumulation, especially in the North Pacific, which is known as a high-nutrient, low-chlorophyll (HNLC) region, the transport height and fraction of giant dust particles are important factors for studying dust budgets in the atmosphere and their role in biogeochemical cycles.

In situ observations of the influence of a large onshore wind farm on near-surface temperature, turbulence intensity and wind speed profiles

Observations of wakes from individual wind turbines and a multi-megawatt wind energy installation in the Midwestern US indicate that directly downstream of a turbine (at a distance of 190 m, or 2.4 rotor diameters (D)), there is a clear impact on wind speed and turbulence intensity (TI) throughout the rotor swept area. However, at a downwind distance of 2.1 km (26 D downstream of the closest wind turbine) the wake of the whole wind farm is not evident. There is no significant reduction of hub-height wind speed or increase in TI especially during daytime. Thus, in high turbulence regimes even very large wind installations may have only a modest impact on downstream flow fields. No impact is observable in daytime vertical potential temperature gradients at downwind distances of >2 km, but at night the presence of the wind farm does significantly decrease the vertical gradients of potential temperature (though the profile remains stably stratified), largely by increasing the temperature at 2 m.

Comparison of Lorenz and Charney–Phillips vertical discretisations for dynamics–boundary layer coupling. Part I: Steady states

AbstractAccurate coupling between the resolved‐scale dynamics and the parametrised physics is essential for accurate modelling of the atmosphere. Previous emphasis has been on the temporal aspects of this so‐called physics–dynamics coupling problem, with little attention on the spatial aspects. When designing a model for numerical weather prediction there is a choice for how to vertically arrange the predicted variables, namely the Lorenz and Charney–Phillips grids, and there is ongoing debate as to which is the optimal. The Charney–Phillips grid is considered good for capturing the potential vorticity dynamics and wave propagation, whereas the Lorenz grid is more suitable for conservation. However the Lorenz grid supports a computational mode. It is argued here that the Lorenz grid is preferred for modelling the stably stratified boundary layer. This presents the question: which grid will produce more accurate results when coupling the large‐scale dynamics to the stably stratified planetary boundary layer? The question is addressed by examining the ability of both the Lorenz and Charney–Phillips grids to capture the steady state of a set of equations that simultaneously represents both large‐scale dynamics and the planetary boundary layer. The results show that the Charney–Phillips grid is able to capture accurately the steady boundary‐layer solution provided the Richardson number is calculated without vertically averaging the shear. Averaging the shear suppresses the negative feedback of the shear on the diffusion coefficient; the positive feedback, via the vertical gradient of potential temperature, then leads to the formation of unrealistic step‐like features. Copyright © 2012 Royal Meteorological Society

A Note on the Pure Katabatic Wind Maximum over Gentle Slopes

The wind maximum of pure katabatic winds over moderate slopes, the inclination varying between 3 and 6°, is studied using large-eddy simulation (LES) and further discussed in the light of the classical Prandtl model. The LES results show that both the maximum katabatic wind speed and its height decrease with increasing slope angle, and vice versa. However, in the Prandtl analytical, i.e. linear classical, solution, only the wind maximum height is affected by the slope angle, not the maximum wind speed. For the given range of slope inclinations, a linear relation between the height and the magnitude of the wind maximum is found in our simulations, which is supported by a limited dataset obtained by other researchers; these results are further discussed. The inability of the analytical Prandtl solution to give the maximum wind-speed dependency on the slope angle is associated with the assumed constancy of (1) the background vertical potential temperature gradient Γ, (2) the eddy diffusivity and (3) the Prandtl number.

Characterization of atmospheric optical turbulence for laser propagation

AbstractThe performance of imaging or transmission systems is limited by atmospheric inhomogeneities and fluctuations. Temperature and pressure variations associated with turbulent eddies cause random fluctuations of the refractive index structure constant (Cn2), which distort optical waves. The distortions may lead to significant blurring, scintillations, broadening and wander of the laser beam. High resolution nested numerical simulations are used to predict and characterize Optical Turbulence (OT) induced by jet streams and gravity waves under various local atmospheric conditions. Non‐homogeneous, anisotropic, non‐Kolmogorov patchy shear‐stratified turbulence in the Upper Troposphere and Lower Stratosphere (UTLS) requires that a fine mesh be used to resolve stiff velocity and temperature gradient profiles. Computational approach is based on nesting and refined vertical gridding in nested mesoscale/microscale codes. This methodology is applied to the analysis of field data from observations. Local distributions of simulated optical turbulence in the UTLS are obtained using very high resolution explicit simulations and parametrization formula. They show strongly laminated structures with thin layers of high values of the refractive index. These optical layers are characterized by steep vertical gradients of potential temperature and are located at the edges of relatively well mixed regions produced by shear instabilities and wave breaking.

Setup of tools and dataset selection for MBL investigation over a coastal Mediterranean site – preliminary results

Abstract. We propose the height zi of the inversion in the vertical profile of potential temperature, on the top of a mixed layer or of a residual layer as a relevant parameter for retrieving information related to water vapour in the lower layers of the atmosphere, from currently available operational satellites information. We estimated zi using an algorithm where a set of threshold values in the vertical gradients of potential temperature are used to analyse the available profiles in order to find relevant inversions. Thresholds are derived from the subjective analysis of a set of high vertical resolution soundings from the coastal site of the Italian Meteorological Service of Pratica di Mare (WMO #16245, lat. 41.66° N, lon. 12.45° E). The sensitivity of the method to vertical sampling is tested comparing results obtained by applying the method to two sets of about 6500 raw and sampled radiosoundings during the period 2002–2008. Differences in average seasonal and six-hourly values of zi from the two datasets are found within the range 5–10%. The method was then applied to a larger set of soundings from coastal and continental Mediterranean sites collected in a standard archive. It was found that mean summer values of zi at coastal sites are lower than in winter, conversely to continental sites. Furthermore, values of zi found using the proposed algorithm show a systematically higher inversion if compared to height found using the Richardson Number method, although there are cases when both identify the inversion at the same height.

Marine cold-air outbreaks in the North Atlantic: temporal distribution and associations with large-scale atmospheric circulation

The spatial and temporal distributions of marine cold air outbreaks (MCAOs) over the northern North Atlantic have been investigated using re-analysis data for the period from 1958 to 2007. MCAOs are large-scale outbreaks of cold air over a relatively warm ocean surface. Such conditions are known to increase the severity of particular types of hazardous mesoscale weather phenomena. We used a simple index for identifying MCAOs: the vertical potential temperature gradient between the sea surface and 700 hPa. It was found that atmospheric temperature variability is considerably more important than the sea surface temperature variability in governing both the seasonal and the inter-annual variability of MCAOs. Furthermore, a composite analysis revealed that a few well-defined and robust synoptic patterns are evident during MCAOs in winter. Over the Labrador and Irminger Seas the MCAO index was found to have a correlation of 0.70 with the North Atlantic Oscillation index, while over the Barents Sea a negative correlation of 0.42 was found.

Modulation of Small-Scale Turbulence by Ducted Gravity Waves in the Nocturnal Boundary Layer

Abstract Constant altitude measurements of temperature and velocity in the residual layer of the nocturnal boundary layer, collected by the Cooperative Institute for Research in Environmental Sciences (CIRES) Tethered Lifting System (TLS), exhibit fluctuations identified by previous work (Fritts et al.) as the signature of ducted gravity waves. The concurrent high-resolution TLS turbulence measurements (temperature structure constant C2T and turbulent kinetic energy dissipation rate ɛ) reveal the presence of patches of enhanced turbulence activity that are roughly synchronized with the troughs of the temperature and velocity fluctuations. To investigate the potentially dominant role ducted gravity waves might play on the modulation of atmospheric stability and therefore, on turbulence, time series of the wave-modulated gradient Richardson number (Ri) and of the vertical gradient of potential temperature ∂θ/∂z(t) are computed numerically and compared to the TLS small-scale turbulence measurements. The results of this study agree with the predictions of previous theoretical studies (i.e., wave-generated fluctuations of temperature and velocity modulate the gradient Richardson number), resulting in periodic enhancements of turbulence at Ri minima. The patches of turbulence observed in the TLS dataset are subsequently identified as convective instabilities generated locally within the unstable phase of the wave.

Principle of cross coupling between vertical heat turbulent transport and vertical velocity and determination of cross coupling coefficient

It has been proved that there exists a cross coupling between vertical heat turbulent transport and vertical velocity by using linear thermodynamics. This result asserts that the vertical component of heat turbulent transport flux is composed of both the transport of the vertical potential temperature gradient and the coupling transport of the vertical velocity. In this paper, the coupling effect of vertical velocity on vertical heat turbulent transportation is validated by using observed data from the atmospheric boundary layer to determine cross coupling coefficients, and a series of significant properties of turbulent transportation are opened out. These properties indicate that the cross coupling coefficient is a logarithm function of the dimensionless vertical velocity and dimensionless height, and is not only related to the friction velocity u *, but also to the coupling roughness height z W0 and the coupling temperature T W0 of the vertical velocity. In addition, the function relations suggest that only when the vertical velocity magnitude conforms to the limitation |W/u *| ≠ 1, and is above the level z W0, then the vertical velocity leads to the cross coupling effect on the vertical heat turbulent transport flux. The cross coupling theory and experimental results provide a challenge to the traditional turbulent K closure theory and the Monin-Obukhov similarity theory.