Atmospheric Moisture Transport to the Arctic: Assessment of Reanalyses and Analysis of Transport Components

The atmospheric water cycle of the Arctic is evaluated via seven global reanalyses and in radiosonde observations covering the 1979–2013 period. In the regional moisture budget, evaporation and precipitation are the least consistent terms among different datasets. Despite the assimilation of radiosoundings, the reanalyses present a tendency to overestimate the moisture transport. Aside from this overestimation, the reanalyses exhibit a remarkable agreement with the radiosondes in terms of spatial and temporal patterns. The northern North Atlantic, subpolar North Pacific, and Labrador Sea stand out as the main gateways for moisture to the Arctic in all reanalyses. Because these regions correspond to the end of the storm tracks, the link between moisture transports and extratropical cyclones is further investigated by decomposing the moisture fluxes in the mean flow and transient eddy parts. In all reanalyses, the former term tends to cancel out when averaged over a latitude circle, leaving the latter to provide the bulk of the midlatitude moisture imports (89%–94% at 70°N). Although the Arctic warms faster than the rest of the world, the impact of these changes on its water cycle remains ambiguous. In most datasets, evaporation, precipitation, and precipitable water increase in line with what is expected from a warming signal. At the same time, the moisture transports have decreased in all the reanalyses but not in the radiosonde observations, though none of these trends is statistically significant. The fluxes do not scale with the Clausius–Clapeyron relation because the increasing humidity is not correlated with the meridional wind, particularly near the surface.

A low-order coupled chemistry meteorology model for testing online and offline data assimilation schemes: L95-GRS (v1.0)

Bocquet and Sakov (2013) introduced a low-order model based on the coupling of the chaotic Lorenz-95 (L95) model, which simulates winds along a mid-latitude circle, with the transport of a tracer species advected by this zonal wind field. This model, named L95-T, can serve as a playground for testing data assimilation schemes with an online model. Here, the tracer part of the model is extended to a reduced photochemistry module. This coupled chemistry meteorology model (CCMM), the L95-GRS (generic reaction set) model, mimics continental and transcontinental transport and the photochemistry of ozone, volatile organic compounds and nitrogen oxides. Its numerical implementation is described. The model is shown to reproduce the major physical and chemical processes being considered. L95-T and L95-GRS are specifically designed and useful for testing advanced data assimilation schemes, such as the iterative ensemble Kalman smoother (IEnKS), which combines the best of ensemble and variational methods. These models provide useful insights prior to the implementation of data assimilation methods into larger models. We illustrate their use with data assimilation schemes on preliminary yet instructive numerical experiments. In particular, online and offline data assimilation strategies can be conveniently tested and discussed with this low-order CCMM. The impact of observed chemical species concentrations on the wind field estimate can be quantitatively assessed. The impacts of the wind chaotic dynamics and of the chemical species non-chaotic but highly nonlinear dynamics on the data assimilation strategies are illustrated.

A low-order coupled chemistry meteorology model for testing online and offline data assimilation schemes

Bocquet and Sakov (2013) have introduced a low-order model based on the coupling of the chaotic Lorenz-95 model which simulates winds along a mid-latitude circle, with the transport of a tracer species advected by this zonal wind field. This model, named L95-T, can serve as a playground for testing data assimilation schemes with an online model. Here, the tracer part of the model is extended to a reduced photochemistry module. This coupled chemistry meteorology model (CCMM), the L95-GRS model, mimics continental and transcontinental transport and the photochemistry of ozone, volatile organic compounds and nitrogen oxides. Its numerical implementation is described. The model is shown to reproduce the major physical and chemical processes being considered. L95-T and L95-GRS are specifically designed and useful for testing advanced data assimilation schemes, such as the iterative ensemble Kalman smoother (IEnKS) which combines the best of ensemble and variational methods. These models provide useful insights prior to the implementation of data assimilation methods on larger models. We illustrate their use with data assimilation schemes on preliminary, yet instructive numerical experiments. In particular, online and offline data assimilation strategies can be conveniently tested and discussed with this low-order CCMM. The impact of observed chemical species concentrations on the wind field can be quantitatively estimated. The impacts of the wind chaotic dynamics and of the chemical species non-chaotic but highly nonlinear dynamics on the data assimilation strategies are illustrated.

The quasi 16-day wave in mesospheric water vapor during boreal winter 2011/2012

This study investigates the characteristics of the quasi 16-day wave in the mesosphere during boreal winter 2011/2012 using observations of water vapor from ground-based microwave radiometers and satellite data. The ground-based microwave radiometers are located in Seoul (South Korea, 37° N), Bern (Switzerland, 47° N) and Sodankylä (Finland, 67° N). The quasi 16-day wave is observed in the mesosphere at all three locations, while the dominant period increases with latitude from 15 days at Seoul to 20 days at Sodankylä. The observed evolution of the quasi 16-day wave confirms that the wave activity is strongly decreased during a sudden stratospheric warming that occurred in mid-January 2012. Using satellite data from the Microwave Limb Sounder on the Aura satellite, we examine the zonal characteristics of the quasi 16-day wave and conclude that the observed waves above the midlatitudinal stations Seoul and Bern are eastward-propagating s = −1 planetary waves with periods of 15 to 16 days, while the observed oscillation above the polar station Sodankylä is a standing wave with a period of approximately 20 days. The strongest relative wave amplitudes in water vapor during the investigated time period are approximately 15%. The wave activity varies strongly along a latitude circle. The activity of the quasi 16-day wave in mesospheric water vapor during boreal winter 2011/2012 is strongest over northern Europe, the North Atlantic Ocean and northwestern Canada. The region of highest wave activity seems to be related to the position of the polar vortex. We conclude that the classic approach to characterize planetary waves zonally averaged along a latitude circle is not sufficient to explain the local observations because of the strong longitudinal dependence of the wave activity.

Envelope to a Two-Parameter Family of Point Vectors Method for Surface Swept by Form Wheel

The form wheel generating curve is always a composited curve which is composed of lines and arcs, and the calculation of the envelope surface is more complex than conventional rotary cutter. Therefore, a method of envelope to a two-parametric family of point vectors is proposed, which defines the point and its normal vector on the wheel generating curve as a point vector. Each point vector corresponds to a virtual ball uniquely. The rotation of point vector forms latitude circle and the valid envelopment movement forms sectional circle. The intersection of latitude circle and sectional circle is grazing point; The analytical calculation model is established, in which the geometry parameters of virtual ball can be directly calculated from point vector; An example is presented to demonstrate the effectiveness of the method which can be used for form grinding geometry simulation, accuracy evaluation and optimization of the wheel path.

The quasi 16-day wave in mesospheric water vapor during boreal winter 2011/2012

This study investigates the characteristics of the quasi 16-day wave in the mesosphere during boreal winter 2011/2012 using observations of water vapor from ground-based microwave radiometers and satellite data. The ground-based microwave radiometers are located in Seoul (South Korea, 37° N), Bern (Switzerland, 47° N) and Sodankylä (Finland, 67° N). The quasi 16-day wave is observed in the mesosphere at all three locations, while the dominant period increases with latitude from 15 days at Seoul to 20 days at Sodankylä. The observed evolution of the quasi 16-day wave confirms that the wave activity is strongly decreased during a sudden stratospheric warming that occurred in mid-January 2012. Using satellite data from the Microwave Limb Sounder on the Aura satellite, we examine the zonal characteristics of the quasi 16-day wave and conclude that the observed waves above the mid-latitudinal stations Seoul and Bern are eastward-propagating s=−1 planetary waves with periods of 15 to 16 days, while the observed oscillation above the polar station Sodankylä is a standing oscillation with a period of approximately 20 days. The strongest relative wave amplitudes in water vapor during the investigated time period are approximately 15%. The wave activity varies strongly along a latitude circle. The activity of the quasi 16-day wave in mesospheric water vapor during boreal winter 2011/2012 is strongest over Northern Europe, the North Atlantic ocean and North-West Canada. The region of highest wave activity seems to be related to the position of the polar vortex. We conclude that the classic approach to characterize planetary waves zonally averaged along a latitude circle is not sufficient to explain the local observations because of the strong longitudinal dependence of the wave activity.

Surface Parameterization for the Shape Analysis of Tube-Like Objects

In bio-medicine and other fields, shape analysis is very important for diagnosis of diseases and prediction of shape variation. This paper focuses on the surface parameterization of tube-like 3D objects to obtain and analyze shape information from a sample shape, including its size and the shape variation between different samples. It can well represent the global and local shape information for statistical analysis and for the construction of Medial Shape Model. Firstly, we extract the axis curve of the object by a heat conduction model. Secondly, we obtain the latitude circles by using the normal planes to cross the surface. Then we get the final parameterized surface with quad-dominant meshes by registering the points of single latitude circle and between different circles through coordinate transformation and alignment. Subsequently, we apply the approach to parameterization of a rib bone.

A Comparison Study of Three Polar Grids

The circumference of a latitude circle decreases toward the Poles, making it difficult to present meteorological field variables on equally spaced grids with respect to latitude and longitude because of data aggregation. To identify the best method for displaying data at the Poles, three different grids are compared that have all been designed to reduce data aggregation: the reduced latitude–longitude (RL) grid, the National Snow and Ice Data Center Equal-Area Special Sensor Microwave Imager (SSM/I) Earth (EA) grid, and the National Meteorological Center octagonal (OG) grid. The merits and disadvantages of these grids are compared in terms of depictions of the Arctic summer circulation with wind vectors, streamfunction, and velocity potential at 400 hPa where maximum westerlies are located. Using geostrophy, the 400-hPa streamfunction at high latitudes can be formed from geopotential height. In comparison with this geostrophic streamfunction, the streamfunction generated from vorticity on the OG grid shows a negligible error (∼0.5%). The error becomes larger using vorticity on the EA (∼15%) and RL (∼30%) grids. During the northern summer, the Arctic circulation at 400 hPa is characterized by three troughs. The streamfunction and velocity potential of these three troughs are spatially in quadrature with divergent (convergent) centers located ahead of (behind) these troughs. These circulation features are best depicted by the streamfunction and velocity potential generated on the OG grid. It is demonstrated by these findings that the National Meteorological Center octagonal grid is the most ideal among the three grids used for the polar regions. However, this assessment is constrained by the hemispheric perspective of meteorological field variables, because these variables depicted on the octagonal grid at higher latitudes need to be merged with those on the equal-latitude-longitude grid at lower latitudes.

Empirical Wave Propagation

The purpose of this chapter is to demonstrate that, given a long data set of global extent, one can design a simple forecast method called Empirical Wave Propagation (EWP), which has modest forecast skill and allows us to explore aspects of atmospheric dynamics empirically, most notably aspects that help to explain mechanisms of teleconnection. The highlight of this chapter are dispersion experiments where we ask the question what happens to an isolated source at t = 0? Even though Nature has never done such an experiment, we will address this question empirically. In case the reader does not need/want to know the technical details of deriving wavespeeds he/she can skip to page 22 (EWP diagnostics sct 3.2) of this chapter. We will also discuss the skill of one-day EWP forecasts, in comparison to skill controls like “persistence”, as a function of season, hemisphere, level and variable. While short-range (1 day) forecasts are certainly not the topic of this book, we note that the short-term wave propagation features described here do nourish and maintain the teleconnection patterns thought to be important for longer range forecasts. EWP uses either zonal harmonic waves (sin/cos pairs) along each latitude circle separately, or global domain spherical harmonics (see Parkinson and Washington (1986) for the basics on spherical harmonics). The orthogonal functions used here are thus analytical. The atmosphere is to first order rotation-symmetric and obviously periodic in the east–west direction, which makes the zonal Fourier transform a natural. Moreover, many weather systems, wave-like in the upper levels, are seen to move from west to east (east to west) in the mid-latitudes (tropics), so a decomposition in sin/ cos functions should inform us about phase propagation and energy dispersion on the sphere. For any initial time we decompose the state of the atmosphere into harmonic waves. If we knew the wave speed, and made an assumption about the future amplitude, we could make forecasts by analytical means. But how do we know the phase speed? One way to proceed, with data alone, is to calculate from a large data set the climatological speeds of anomaly waves. This is where the empirical aspects come in.