Mean Horizontal Temperature Gradient Research Articles

Characteristics of the North Pacific Oscillation in CMIP5 Models in Relation to Atmospheric Mean States

AbstractThe North Pacific Oscillation (NPO), the second leading atmospheric mode in the North Pacific Ocean, is known to be responsible for climate variability and extremes in adjacent regions. The reproducibility of the NPO in climate models is thus a topic of interest for the more accurate prediction of climate extremes. By investigating the spatial characteristics of the NPO in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), this study reveals the intimate relationship between the NPO structure and the atmospheric mean states over the North Pacific. The majority of the models reasonably capture the meridional contrast of pressure anomalies, but the detailed horizontal characteristics of the NPO are found to differ among the models. Diagnostic analysis of 30 climate models and long-term observations suggest that systematic bias in the mean atmospheric baroclinicity over the North Pacific crucially affects the horizontal shape and zonal position of the NPO. In the models in which the climatological continental trough over the western North Pacific extends farther to the east, the NPO tends to be simulated farther to the east, strengthening its impact on the downstream climate. In contrast, when the climatological continental trough is reduced in size toward the west, the growth of the NPO is limited to the west, and its influence is weakened downstream. This relationship can be understood via the altered available potential and kinetic energy conversions that feed the total energy of the NPO, primarily stemming from the difference in the mean horizontal temperature gradient and stretching deformation of the mean horizontal wind.

Improving Understanding of Microclimate Heterogeneity within a Contemporary Plant Growth Facility to Advance Climate Control and Plant Productivity

Greenhouse crop production is maximized by maintaining optimal growing conditions. Accurate management of climate conditioning equipment based on measurements of the internal greenhouse microclimate is necessary to optimize crop production. Traditionally, greenhouse microclimate is monitored by a single suite of sensors located at a fixed (often central) location that is considered representative of the entire greenhouse climate. To advance greenhouse crop production additional sensors may better represent greenhouse microclimate heterogeneity and improve performance of climate conditioning equipment. However, elucidating the proper number and distribution of additional sensors requires investigation. Distributed high resolution air temperature (n = 63), relative humidity (n=63), and incoming solar radiation data were collected between May 9th, 2012 and September 5th, 2012 to test the efficacy of conventional centrally located sensors to characterize the spatial and temporal climate variability inside three contemporary greenhouse facilities. Results indicate substantial microclimate heterogeneity with mean horizontal temperature gradients of as much as 5.0°C/m, and mean horizontal VPD gradients of 1.5 kPa/m. Most substantially, the maximum vertical temperature gradient was 11.65°C/m. Results indicate that as few as five properly deployed sensor assemblages (e.g. temperature, humidity, solar radiation) may be necessary to more accurately monitor horizontal and vertical microclimate heterogeneity in a typical greenhouse room. This would improve climate conditioning accuracy and improve the homogeneity of the internal greenhouse climate, which may result in increased productivity and profits for greenhouse managers.

Spectral properties of magnetohydrodynamic convection with mean horizontal temperature gradient

AbstractSpectral properties of convective magnetohydrodynamic (MHD) turbulence in two and three dimensions are studied by means of direct numerical simulations (Skandera D. & Müller W.-C. 2006). The investigated system is set up with a mean horizontal temperature gradient in order to avoid a development of elevator instabilities in a fully periodic box. All simulations are performed without mean magnetic field. The applied resolution is 5123 and 20482. The MHD equation are solved by a numerical code (Müller & Biskamp 2000) that uses a standard pseudospectral scheme. For removing of aliasing errors a spherical truncation method is employed. Obtained results are compared with predictions of various existing phenomenological theories for magnetohydrodynamic and convective turbulence (Müller & Biskamp 2000). While the three-dimensional system is found to operate in a Kolmogorov-like regime where buoyant forces have a negligible impact on the turbulence dynamics (relatively low Rayleigh number achieved in the simulation; Ra ∼106), the two-dimensional system exhibits interesting irregular quasi-oscillations between a buoyancy dominated Bolgiano-Obukhov-like regime of turbulence and a standard Iroshnikov-Kraichnan-like regime of turbulence (Müller & Biskamp 2000). The most important parameter determining the turbulent regime of 2D magnetoconvection, apart from a high Rayleigh number, seems to be the mutual alignment of velocity and magnetic fields. The non-linear dynamics and the interplay between individual fields are examined with different transfer functions that confirm basic assumptions about directions of energy transfer in spectral space. Kinetic, magnetic and temperature energy are transported by a turbulent cascade from large to smaller scales. The local/nonlocal character of the transport is tested for several individual terms in the governing equations. Moreover, other statistical quantities, e.g. probability density functions, are computed as well. A passive character of the temperature field in the investigated three-dimensional magnetoconvection is supported by computations of intermittency using extended self-similarity. The intermittency of the Elsasser field z+ is in agreement with results from numerical simulations of isotropic MHD turbulence (Müller & Biskamp 2000). The intermittency of the temperature field is found to approximately agree with results of passive scalar measurements in hydrodynamic turbulence (Ruiz-Chavarria, Baudet & Ciliberto 1996).

Three‐dimensional model study of organized mesoscale circulations induced by vegetation

Organized mesoscale circulations induced by vegetation discontinuities and their effects on the vertical transport of heat and moisture are investigated under realistic atmospheric conditions. A regional climate model (RegCM2) is used to simulate local circulations which result from contrasts in surface thermal forcing over land. The limited area domain, with a grid resolution of 10 km, is forced at the boundaries by analyses of observations for the 30‐day period of June 1990. Surface vegetation and soil moisture are idealized to provide strong, geometric contrasts in surface heating. Control (homogeneous surface) and perturbation (surface thermal contrast) simulations are required to isolate the circulation signal. Differences in the wind, temperature, and humidity fields show the diurnal evolution of coherent circulations during the simulated month. On three days, circulations having strength similar to a land‐sea breeze occur. Vertical fluxes of heat and moisture, which result from the circulations, show a different vertical structure and diurnal development than turbulent fluxes. The vertical transport of moisture is found to be comparable to or larger than transport by turbulent processes. Vertical transport of heat by the circulations is small. An increase is seen in the monthly total large‐scale precipitation downwind of the ascending region of the circulations. Of the large‐scale forcings studied, the circulation is most closely correlated to the mean horizontal temperature gradient.

Winter observations of the atmosphere over Antarctic sea ice

During the Winter Weddell Sea Project 1986 (WWSP 86), meteorological data were gathered above Antarctic sea ice in winter and spring. This paper discusses near‐surface data from 1986 and 1987, which were provided by weather stations deployed on drifting sea ice in October 1986. The drift tracks of these buoys for the subsequent 18 months document the sea ice motion of a larger fraction of the complete Weddell gyre than historical vessel drifts and previous buoy programs. The wind roses calculated from the buoys wind measurements show the large‐scale rotational wind field over the Weddell Sea. The northward components of near‐surface horizontal temperature gradients in winter vary between 1.3 and 2.6 K/100 km. Horizontal temperature gradients have a component toward the center of the gyre, which varies between 1 and 1.6 K/100 km in the central Weddell Sea. The marginal ice zone advances in autumn and retreats in spring with a speed significantly larger than the actual ice speed. Enhanced meridional temperature contrasts in the troposphere were found each time Polarstern passed the marginal ice zone. Significant mean horizontal temperature gradients of 1 to 5 K/100 km in the troposphere have also been detected in the continental marginal zone from Polarstern data and related aerological data of the Georg‐von‐Neumayer Antarctic station.

Large-Scale Dynamical Response to Differential Heating: Statistical Equilibrium States and Amplitude Vacillation

Abstract This is a study of the statistical behavior of a very low order “general circulation model,” consisting of a single finite-amplitude baroclinic wave interacting with a mean zonal shear flow, maintained against dissipation by differential heating. Starting with the equations for two-level quasi-geostrophic flow in a β-plane channel, we derive a closed system of evolution equations for five zonally-averaged quantities at 45° latitude—including net poleward heat transport, meridional kinetic energy and mean vertical shear (or mean horizontal temperature gradient). This system of equations enables us to relate the equilibrium values of mean vertical shear between 250 and 750 mb and rms northward component of velocity at 500 mb to the rate of differential heating. The former is estimated to be 10.7 m s−1 at 45°S: Oort's observed statistics show that it is actually about 12 m s−1. The theoretical estimate of the rms northward component of velocity at 45°S is 11.0 m s−1. Oort's and Trenberth's statistic...

Large aspect ratio convection cells in the upper mantle

Summary. Numerical experiments which model a possible large-scale flow in the upper mantle are described. The lithospheric plates are represented by a constant velocity boundary condition on the upper surface of a twodimensional convecting layer. The transition from small-scale multiple-cell flow to single cell flow is described in terms of the Rayleigh number and the Peclet number. The mean flow field is successfully compared with a simple parameterized model in which flow is driven by the moving upper boundary and by a mean horizontal temperature gradient which must be present in any large aspect ratio cell. Bathymetry and gravity profiles are calculated for the long cells, and it is shown that in the absence of a low viscosity zone, a realistic variation of bathymetry and gravity is only obtained if the flow in the cell is being driven predominantly by the horizontal temperature gradient rather than the moving plate. Mean horizontal temperature gradients in the upper mantle of 0.025C km-' are suggested beneath a fast moving plate such as the Pacific. Transverse boundary layer instabilities in the large cells are shown to be governed by a critical boundary layer Rayleigh number, which linearly increases with the boundary layer PCclet number.

Statistical analysis of fluctuations in the Iceland-Scotland frontal zone

The kinematic structure of current and temperature fluctuations from two moorings in the Iceland-Scotland frontal zone is investigated. On the Iceland-Faroe ridge the current fluctuations at periods longer than 2 days can be described by randomly superposed linear free waves. For higher frequencies friction needs to be invoked. This is in contrast to the Faroe-Shetland area, where the channel geometry violates the assumption of a random phase relationship. A simple stochastic advection model is developed to describe the observed temperature fluctuations. The model allows one to determine from a single current meter the mean horizontal temperature gradient, the horizontal eddy scale and a diffusive time scale. The eddy heat transport across the polar front is estimated and found to be of the same order as the mean advective transport. A discussion of energy conversion rates suggests that baroclinic instability is a major source of the low-frequency fluctuations. The average input of energy from fluctuating winds is estimated to be an order of magnitude less effective.

Importance of eddy heat flux in a heat budget for Oregon coastal waters

A heat budget for Oregon coastal waters during the summer upwelling season in 1973 is estimated from current and temperature measurements at a distance of 15 km from the Oregon coast. In this heat budget, the time‐averaged onshore‐offshore circulation transports heat offshore at an estimated rate of 4.5×104 cal cm−1 s−1. This offshore heat transport is compensated by a combination of onshore eddy heat flux, alongshore variation of alongshore heat transport, radiative heat gain by coastal waters, and cooling of coastal waters during summer. Of these mechanisms, the eddy heat flux is most important, balancing 40% of the offshore heat transport by the mean circulation. The eddy heat flux is directed down the mean horizontal temperature gradient and thus removes potential energy from the mean circulation. Removal of energy by eddies is estimated to be 70% of the energy put into the water column by the wind stress acting on the surface current. By removing energy from the mean circulation, eddies provide a mechanism limiting the magnitude of the alongshore circulation in Oregon coastal waters.

An observed relation between the macroscale local eddy flux of heat and the mean horizontal temperature gradient

AbstractFrequent and detailed upper air observations at Laverton (38°S) for 30 days during September/October 1966 are analysed to determine the local horizontal eddy flux of heat and the mean horizontal potential temperature gradient. Components in the meridional and zonal directions are studied, but a more meaningful relation emerges when the vector eddy flux is considered in relation to the vector temperature gradient. the angle, δ, between these two vectors, and the down‐gradient and cross‐gradient exchange coefficients, K and R, are derived. A 3‐tiered structure between the surface and 30 km emerges, with down‐gradient flux (K positive) in most of the troposphere and mid‐stratosphere, and counter‐gradient flux (K negative) in the lower stratosphere. the cross‐gradient flux is in the direction of the thermal wind (R positive) at nearly all heights except in the lowest layers and between 20 and 24 km. δ exhibits a fairly steady and systematic change throughout the height range analysed (2–28 km). Computations using standard upper air observations at selected Australian stations for 1970 show a mean annual pattern which is remarkably coherent with latitude (10–55°) and height (0–22km), except in the tropical troposphere. Typical derived mid‐latitude values for 1970 are: Only part of the vertical variation in the lowest layers can be accounted for by surface frictional influences.

An observed relation between the macroscale local eddy flux of heat and the mean horizontal temperature gradient

An observed relation between the macroscale local eddy flux of heat and the mean horizontal temperature gradient

The forced flow due to heating of a rotating liquid

An investigation is made of the forced liquid motion in a rotating cylindrical vessel with a horizontal base when a temperature difference exists between the outer and inner cylindrical boundaries of the liquid. It has been observed experimentally that at critical values of a certain non-dimensional parameter, known as the Rossby number, the flow patterns change abruptly in character. The present investigation derives a stability criterion which agrees qualitatively with the experimental results and also gives reasonable quantitative results. In the derivation of this result it is shown that there exists a relation between the mean vertical temperature gradient, the mean horizontal temperature gradient and the angular velocity of rotation of the system.