scholarly journals The Use of Semigeostrophic Theory to Diagnose the Behaviour of an Atmospheric GCM

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 72
Author(s):  
Mike Cullen
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Atmospheric Model ◽  
Static Stability ◽  
Rate Of Change ◽  
Model Response ◽  
Layer Heating ◽  
Ageostrophic Circulation ◽  
The Tropics ◽  
Hydrostatic Balance

A diagnostic method is presented for analysing the large-scale behaviour of the Met Office Unified Model, which is a comprehensive atmospheric model used for weather and climate prediction. Outside the boundary layer, on scales larger than the radius of deformation, semi-geostrophic theory will give an accurate approximation to the model evolution. In particular, the ageostrophic circulation required to maintain geostrophic and hydrostatic balance against prescribed forcing and a rate of change of the geostrophic pressure can be calculated. In the tropics, the balance condition degenerates to the weak temperature gradient approximation. Within the boundary layer, the semi-geotriptic approximation has to be used because friction and rotation are equally important. Assuming the calculated pressure tendency and ageotriptic circulation match the observed model behaviour, the influence of the large-scale state and the nature of the forcing on the model response can be deduced in a straightforward way. The capabilities of the diagnostic are illustrated by comparing predictions of the ageotriptic circulation from the theory and the model. It is then used to show that the effects of latent heat release can be included by modifying the static stability, and to show the effect of an idealised tropical heat source on the subtropical jet. Finally, the response of the ageotriptic flow to boundary layer heating in the tropics is demonstrated. These illustrations show that the model behaviour on large scales conforms with theoretical expectations, so that the results of the diagnostic can be used to aid the development of further improvements to the model, in particular investigating systematic errors and understanding the large-scale atmospheric response to forcing.

scholarly journals The use of Semigeostrophic Theory to Diagnose the Behaviour of an Atmospheric GCM

2018 ◽  
Author(s):  
Mike Cullen
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Atmospheric Model ◽  
Static Stability ◽  
Rate Of Change ◽  
Model Response ◽  
Layer Heating ◽  
Ageostrophic Circulation ◽  
The Tropics ◽  
Hydrostatic Balance

A diagnostic method is presented for analysing the large-scale behaviour of the Met Office Unified Model, which is a comprehensive atmospheric model used for weather and climate prediction. Outside the boundary layer, on scales larger than the radius of deformation, semigeostrophic theory will give an accurate approximation to the model evolution. In particular, the ageostrophic circulation required to maintain geostrophic and hydrostatic balance against prescribed forcing and a rate of change of the geostrophic pressure can be calculated. In the tropics the balance condition degenerates to the weak temperature gradient approximation. Within the boundary layer the semigeostriptic approximation has to be used because friction and rotation are equally important. Assuming the calculated pressure tendency and ageotriptic circulation match the observed model behaviour, the influence of the large-scale state and the nature of the forcing on the model response can be deduced in a straightforward way. This process is illustrated by comparing predictions of the ageotriptic circulation from the theory and the model. It is then used to show that the effects of latent heat release can be included by modifying the static stability, and to show the effect of an idealised tropical heat source on the subtropical jet. Finally the response of the ageotriptic flow to boundary layer heating in the tropics is demonstrated. These illustrations show that the model behaviour on large scales conforms with theoretical expectations, so that the results of the diagnostic can be used to aid the development of further improvements to the model.

An MJO Simulated by the NICAM at 14- and 7-km Resolutions

2009 ◽  
Vol 137 (10) ◽  
pp. 3254-3268 ◽  
Author(s):  
Ping Liu ◽  
Masaki Satoh ◽  
Bin Wang ◽  
Hironori Fudeyasu ◽  
Tomoe Nasuno ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Atmospheric Model ◽  
Easterly Wind ◽  
Low Level ◽  
Central Indian Ocean ◽  
Simulated Event ◽  
The Indian Ocean ◽  
Substantial Bias ◽  
The Tropics

Abstract This study discloses detailed Madden–Julian oscillation (MJO) characteristics in the two 30-day integrations of the global cloud-system-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) using the all-season real-time multivariate MJO index of Wheeler and Hendon. The model anomaly is derived by excluding the observed climatology because the simulation is sufficiently realistic. Results show that the MJO has a realistic evolution in amplitude pattern, geographical locations, eastward propagation, and baroclinic- and westward-tilted structures. In the central Indian Ocean, convection develops with the low-level easterly wind anomaly then matures where the low-level easterly and westerly anomalies meet. Anomalous moisture tilts slightly with height. In contrast, over the western Pacific, the convection grows with a low-level westerly anomaly. Moisture fluctuations, leading convection in eastward propagation, tilt clearly westward with height. The frictional moisture convergence mechanism operates to maintain the MJO. Such success can be attributed to the explicit representation of the interactions between convection and large-scale circulations. The simulated event, however, grows faster in phases 2 and 3, and peaks with 30% higher amplitude than that observed, although the 7-km version shows slight improvement. The fast-growth phases are induced by the fast-growing low-level convergence in the Indian Ocean and the strongly biased ITCZ in the west Pacific when the model undergoes a spinup. The simulated OLR has a substantial bias in the tropics. Possible solutions to the deficiencies are discussed.

scholarly journals Including Atmospheric Layers in Vegetation and Urban Offline Surface Schemes

2009 ◽  
Vol 48 (7) ◽  
pp. 1377-1397 ◽  
Author(s):  
Valéry Masson ◽  
Yann Seity
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Single Layer ◽  
Urban Canopy ◽  
Atmospheric Model ◽  
Surface Fluxes ◽  
Surface Boundary ◽  
Surface Cooling ◽  
Stable Conditions ◽  
Surface Scheme

Abstract A formulation to include prognostic atmospheric layers in offline surface schemes is derived from atmospheric equations. Whereas multilayer schemes developed previously need a complex coupling between atmospheric-model levels and surface-scheme levels, the coupling proposed here remains simple. This is possible because the atmospheric layers interacting with the surface scheme are independent of the atmospheric model that could be coupled above. The surface boundary layer (SBL; both inside and just above the canopy) is resolved prognostically, taking into account large-scale forcing, turbulence, and, if any, drag and canopy forces and surface fluxes. This formulation allows one to retrieve the logarithmic law in neutral conditions, and it has been validated when coupled to a 3D atmospheric model. Systematic comparisons with 2-m observations and 10-m wind have been made for 2 months. The SBL scheme is able to model the 2-m temperature accurately, as well as the 10-m wind, without any use of analytical interpolation. The largest improvement takes place during stable conditions (i.e., by night), during which analytical laws and interpolation methods are known to be less accurate, and in mountainous areas, in which nocturnal low-level flow is strongly influenced by surface cooling. The prognostic SBL scheme is shown to solve the nighttime physical disconnection problem between surface and atmosphere models. The inclusion of the SBL into the urban Town Energy Balance scheme is presented in a paper by Hamdi and Masson in which the ability of the method to simulate the profiles of both mean and turbulent quantities from above the building down to the road surface is shown using data from the Basel Urban Boundary Layer Experiment (BUBBLE). The proposed method will allow the inclusion of the detailed physics of the multilayer schemes (e.g., the interactions of the SBL flow with forest or urban canopy) into a single-layer scheme that is easily coupled with atmospheric models.

scholarly journals Physical Processes Controlling the Tide in the Tropical Lower Atmosphere Investigated Using a Comprehensive Numerical Model

2017 ◽  
Vol 74 (8) ◽  
pp. 2467-2487 ◽  
Author(s):  
T. Sakazaki ◽  
K. Hamilton
Keyword(s):  
Linear Theory ◽  
Large Scale ◽  
Regional Climate ◽  
Atmospheric Model ◽  
Solar Heating ◽  
Radiative Heating ◽  
Lower Atmosphere ◽  
Limited Area ◽  
Daily Variations ◽  
The Tropics

Abstract The lower-atmospheric circulation in the tropics is strongly influenced by large-scale daily variations referred to as atmospheric solar tides. Most earlier studies have used simplified linear theory to explain daily variations in the tropics. The present study employs a comprehensive limited-area atmospheric model and revisits some longstanding issues related to atmospheric tidal dynamics. The tides in the tropical lower atmosphere are realistically simulated in the control experiment with a near-global (75°S–75°N) version of the model. Sensitivity experiments with different aspects of the solar heating suppressed showed that the semidiurnal (S2) tide near the surface can be attributed roughly equally to stratospheric and tropospheric direct solar heating and that the diurnal (S1) tide is excited almost entirely by tropospheric direct solar heating as well as solar heating of Earth’s surface. Linear theory with forcing only by direct radiative heating predicts the phase of the S2 barometric oscillation should be ~0910 LT versus the ~0945 LT phase seen in low-latitude observations. The roles of (i) convective and latent heating and (ii) mechanical dissipation, in determining the S2 phase, are assessed in the model. It is found that the former effect delays the phase by ~25 min and the latter by ~5 min; these two effects together explain the observed phase. When the model is run in limited-area domains comparable to those used in typical regional climate studies the S2, but not S1, tide is found to be significantly weaker than observed, even using atmospheric reanalysis data to drive the lateral boundaries.

scholarly journals The Dissipation of Trapped Lee Waves. Part II: The Relative Importance of the Boundary Layer and the Stratosphere

2016 ◽  
Vol 73 (3) ◽  
pp. 943-955 ◽  
Author(s):  
Matthew O. G. Hills ◽  
Dale R. Durran ◽  
Peter N. Blossey
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Surface Friction ◽  
Static Stability ◽  
Relative Strength ◽  
Relative Importance ◽  
Trapped Waves ◽  
Wind Speeds ◽  
Lee Waves ◽  
The Waves

Abstract Decaying trapped waves exert a drag on the large-scale flow. The two most studied mechanisms for such decay are boundary layer dissipation and leakage into the stratosphere. If the waves dissipate in the boundary layer, they exert a drag near the surface, whereas, if they leak into the stratosphere, the drag is exerted at the level where the waves dissipate aloft. Although each of these decay mechanisms has been studied in isolation, their relative importance has not been previously assessed. Here, numerical simulations are conducted showing that the relative strength of these two mechanisms depends on the details of the environment supporting the waves. During actual trapped-wave events, the environment often includes elevated inversions and strong winds aloft. Such conditions tend to favor leakage into the stratosphere, although boundary layer dissipation becomes nonnegligible in cases with shorter resonant wavelengths and higher tropopause heights. In contrast, idealized two-layer profiles with constant wind speeds and high static stability beneath a less stable upper troposphere support lee waves that are much more susceptible to boundary dissipation and relatively unaffected by the presence of a stratosphere. One reason that trapped waves in the two-layer case do not leak much energy upward is that the resonant wavelength is greatly reduced in the presence of surface friction. This reduction in wavelength is well predicted by the linear inviscid equations if the basic-state profile is modified a posteriori to include the shallow ground-based shear layer generated by surface friction.

scholarly journals A new chemistry-climate tropospheric and stratospheric model MOCAGE-Climat: evaluation of the present-day climatology and sensitivity to surface processes

2007 ◽  
Vol 7 (4) ◽  
pp. 11295-11398 ◽  
Author(s):  
H. Teyssèdre ◽  
M. Michou ◽  
H. L. Clark ◽  
B. Josse ◽  
F. Karcher ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Dry Deposition ◽  
Large Scale ◽  
Transport Model ◽  
Chemical Species ◽  
Surface Processes ◽  
Systematic Bias ◽  
Deposition Velocities ◽  
On Line ◽  
The Tropics

Abstract. We present the chemistry-climate configuration of the Météo-France Chemistry and Transport Model, MOCAGE-Climat. MOCAGE-Climat is a state-of-the-art model that simulates the global distribution of ozone and its precursors (82 chemical species) both in the troposphere and the stratosphere, up to the mid-mesosphere (~70 km). Surface processes (emissions, dry deposition), convection, and scavenging are explicitly described in the model that has been driven by the ECMWF operational analyses of the period 2000–2005, on T21 and T42 horizontal grids and 60 hybrid vertical levels, with and without a procedure that reduces calculations in the boundary layer, and with on-line or climatological deposition velocities. Model outputs have been compared to available observations, both from satellites (TOMS, HALOE, SMR, SCIAMACHY, MOPITT) and in-situ instrument measurements (ozone sondes, MOZAIC and aircraft campaigns) at climatological timescales. The distribution of long-lived species is in fair agreement with observations in the stratosphere putting apart shortcomings linked to the large-scale circulation. The variability of the ozone column, both spatially and temporarily, is satisfactory. However, the too fast Brewer-Dobson circulation accumulates too much ozone in the lower to mid-stratosphere at the end of winter. Ozone in the UTLS region does not show any systematic bias. In the troposphere better agreement with ozone sonde measurements is obtained at mid and high latitudes than in the tropics and differences with observations are the lowest in summer. Simulations using a simplified boundary layer lead to ozone differences between the model and the observations up to the mid-troposphere. NOx in the lowest troposphere is in general overestimated, especially in the winter months over the northern hemisphere, which might result from a positive bias in OH. Dry deposition fluxes of O3 and nitrogen species are within the range of values reported by recent inter-comparison model exercises. The use of climatological deposition velocities versus deposition velocities calculated on-line had greatest impact on HNO3 and NO2 in the troposphere.

scholarly journals Thermodynamic constraint on the depth of the global tropospheric circulation

2017 ◽  
Vol 114 (31) ◽  
pp. 8181-8186 ◽  
Author(s):  
David W. J. Thompson ◽  
Sandrine Bony ◽  
Ying Li
Keyword(s):  
Water Vapor ◽  
Large Scale ◽  
Water Vapor Pressure ◽  
Static Stability ◽  
Radiative Cooling ◽  
Clear Sky ◽  
Tropospheric Circulation ◽  
Basic Physics ◽  
The Tropics ◽  
Tropopause Temperature

The troposphere is the region of the atmosphere characterized by low static stability, vigorous diabatic mixing, and widespread condensational heating in clouds. Previous research has argued that in the tropics, the upper bound on tropospheric mixing and clouds is constrained by the rapid decrease with height of the saturation water vapor pressure and hence radiative cooling by water vapor in clear-sky regions. Here the authors contend that the same basic physics play a key role in constraining the vertical structure of tropospheric mixing, tropopause temperature, and cloud-top temperature throughout the globe. It is argued that radiative cooling by water vapor plays an important role in governing the depth and amplitude of large-scale dynamics at extratropical latitudes.

scholarly journals Boundary Layer and Cloud Structure Controls on Tropical Low Cloud Cover Using A-Train Satellite Data and ECMWF Analyses

2011 ◽  
Vol 24 (1) ◽  
pp. 194-215 ◽  
Author(s):  
Terence L. Kubar ◽  
Duane E. Waliser ◽  
J-L. Li
Keyword(s):  
Boundary Layer ◽  
Cross Section ◽  
Large Scale ◽  
Layer Structure ◽  
Deep Convection ◽  
Static Stability ◽  
Boundary Layer Stability ◽  
Middle Point ◽  
Cloud Data ◽  
Low Cloud

Abstract The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), CloudSat radar, and the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud data on the A-Train constellation complemented with the European Centre for Medium-Range Forecasts (ECMWF) analyses are used to investigate the cloud and boundary layer structure across a 10° wide cross section starting at 5°S near the international date line and extending to 35°N near the California coast from March 2008 to February 2009. The mean large-scale inversion height and low-level cloud tops, which correspond very closely to each other, are very shallow (∼500 m) over cold SSTs and high static stability near California and deepen southwestward (to a maximum of ∼1.5–2.0 km) along the cross section as SSTs rise. Deep convection near the ITCZ occurs at a surface temperature close to 298 K. While the boundary layer relative humidity (RH) is nearly constant where a boundary layer is well defined, it drops sharply near cloud top in stratocumulus regions, corresponding with strong thermal inversions and water vapor decrease, such that the maximum (−∂RH/∂z) marks the boundary layer cloud top very well. The magnitude correlates well with low cloud frequency during March–May (MAM), June–August (JJA), and September–November (SON) (r 2 = 0.85, 0.88, and 0.86, respectively). Also, CALIPSO and MODIS isolated low cloud frequency generally agree quite well, but CloudSat senses only slightly more than one-third of the low clouds as observed by the other sensors, as many clouds are shallower than 1 km and thus cannot be discerned with CloudSat due to contamination from the strong signal from surface clutter. Mean tropospheric ω between 300 and 700 hPa is examined from the ECMWF Year of Tropical Convection (YOTC) analysis dataset, and during JJA and SON, strong rising motion in the middle troposphere is confined to a range of 2-m surface temperatures between 297 and 300 K, consistent with previous studies that show a narrow range of SSTs over which deep ascent occurs. During December–February (DJF), large-scale ascending motion extends to colder SSTs and high boundary layer stability. A slightly different boundary layer stability metric is derived, the difference of moist static energy (MSE) at the middle point of the inversion (or at 700 hPa if no inversion exists) and the surface, referred to as ΔMSE. The utility of ΔMSE is its prediction of isolated uniform low cloud frequency, with very high r 2 values of 0.93 and 0.88, respectively, for the MODIS and joint lidar plus radar product during JJA but significantly lower values during DJF (0.46 and 0.40), with much scatter. To quantify the importance of free tropospheric dynamics in modulating the ΔMSE–low cloud relationships, the frequency as a function of ΔMSE of rising motion profiles (ω < −0.05 Pa s−1) is added to the observed low cloud frequency for a maximum hypothetical low cloud frequency. Doing this greatly reduces the interseasonal differences and holds promise for using ΔMSE for parameterization schemes and examining low cloud feedbacks.

scholarly journals Extraction, classification and visualization of 3-dimensional clouds simulated by cloud-resolving atmospheric model

2017 ◽  
Vol 08 (04) ◽  
pp. 1750051 ◽  
Author(s):  
Daisuke Matsuoka
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Vertical Direction ◽  
General Circulation Models ◽  
Atmospheric Model ◽  
Global Scale ◽  
3 Dimensional ◽  
Cloud Ice ◽  
The Tropics ◽  
Atmospheric Phenomena

Cloud-resolving atmospheric general circulation models using large-scale supercomputers reproduce realistic behavior of 3-dimensional atmospheric field on a global scale. To understand the simulation result for scientists, conventional visualization methods based on 2-dimensional cloud classification are not enough for understanding individual clouds and their physical characteristics. In this study, we propose a new 3-dimensional extraction and classification method of simulated clouds based on their 3-dimensional shape and physical properties. Our proposed method extracts individual clouds by cloud water and cloud ice, and classifies them into six types by their altitude and upward flow. We applied the method to time-varying atmospheric simulation data, and attempted to visualize atmospheric phenomena on the tropics such as developing cumulonimbus and tropical cyclone. Two case studies clearly visualize the behavior of individual cloud type and clarify that some cloud types have a relationship with rainfall during active weather phenomena. The proposed method has the potential to analyze such phenomena that develop in the vertical direction as well as the horizontal direction.

scholarly journals On Thermally Forced Circulations over Heated Terrain

2013 ◽  
Vol 70 (6) ◽  
pp. 1690-1709 ◽  
Author(s):  
Daniel J. Kirshbaum
Keyword(s):  
Boundary Layer ◽  
Heat Source ◽  
Stable Boundary Layer ◽  
Large Scale ◽  
Numerical Models ◽  
Theoretical Models ◽  
Static Stability ◽  
Boussinesq System ◽  
Stable Boundary ◽  
Convection Initiation

Abstract A combination of analytical and numerical models is used to gain insight into the dynamics of thermally forced circulations over diurnally heated terrain. Solutions are obtained for two-layer flows (representing the boundary layer and the overlying free troposphere) over an isolated mountainlike heat source. A scaling based on the linearized Boussinesq system of equations is developed to quantify the strength of thermally forced updrafts and to identify three flow regimes, each with distinct dynamics and parameter sensitivities. This scaling closely matches corresponding numerical simulations in two of these regimes: the first characterized by a weakly stable boundary layer and significant background winds and the second by a strongly stable boundary layer. In the third regime, characterized by weak winds and weak boundary layer stability, this scaling is outperformed by a fundamentally different scaling based on thermodynamic heat engines. Within this regime, the inability of wind ventilation or static stability to diminish the buoyancy over the heat source leads to intense updrafts that are controlled by nonlinear dynamics. These nonlinearities create a positive feedback loop between the thermal forcing and vorticity that rapidly strengthens the circulation and contracts its central updraft into a narrow core. As the circulation intensifies under daytime heating, the warmest surface-based air is ventilated into the upper boundary layer, where it spreads laterally to occupy a broader area and, ultimately, restrain the circulation strength. The success demonstrated herein of simple theoretical models at predicting key aspects of thermally forced circulations offers hope for improved parameterization of related processes (e.g., convection initiation and aerosol venting) in large-scale models.

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