This study aims to validate and improve the universal evaporation duct (UED) model through a further analysis of the stability function (ψ). A large number of hydrometeorological observations obtained from a tower platform near Xisha Island of the South China Sea are employed, together with the latest variations in ψ function. Applicability of different ψ functions for specific sea areas and stratification conditions is investigated based on three objective criteria. The results show that, under unstable conditions, ψ function of Fairall et al. (1996) (i.e., Fairall96, similar for abbreviations of other function names) in general offers the best performance. However, strictly speaking, this holds true only for the stability (represented by bulk Richardson number RiB) range ?2.6 ? RiB < ?0.1; when conditions become weakly unstable (?0.1 ? RiB < ?0.01), Fairall96 offers the second best performance after Hu and Zhang (1992) (HYQ92). Conversely, for near-neutral but slightly unstable conditions (?0.01 ? RiB < 0.0), the effects of Edson04, Fairall03, Grachev00, and Fairall96 are similar, with Edson04 being the best function but offering only a weak advantage. Under stable conditions, HYQ92 is the optimal and offers a pronounced advantage, followed by the newly introduced SHEBA07 (by Grachev et al., 2007) function. Accordingly, the most favorable functions, i.e., Fairall96 and HYQ92, are incorporated into the UED model to obtain an improved version of the model. With the new functions, the mean root-mean-square (rms) errors of the modified refractivity (M), 0–5-m M slope, 5–40-m M slope, and the rms errors of evaporation duct height (EDH) are reduced by 21.65%, 9.12%, 38.79%, and 59.06%, respectively, compared to the classical Naval Postgraduate School model.
选取2014年7月31日安徽滁州一次飑线过程,使用地基雨滴谱仪资料分析此次过程的雨滴谱特征。根据雷达回波和地面降水强度将这次降水过程划分为对流降水、过渡性降水和层云降水,并以10 mm·h-1为临界值将对流降水进一步划分为对流前沿降水、对流中心降水、对流后沿降水。结果表明:对流中心降水、过渡性降水、层云降水的质量加权直径均比较稳定,平均值分别为1.8 mm, 1.0 mm, 1.7 mm。对流降水的标准化截距相比层云降水更大。对流中心降水各粒径段雨滴数浓度均较高;层云降水小雨滴浓度较低,且有少量大雨滴;过渡性降水由小雨滴组成。当雨水含量相同时,层云降水的质量加权直径相比对流降水更大。当雨强相同时,层云降水的反射率因子相比对流中心降水更大。更为精细的降水类型划分可有效改善Z-I关系。 相似文献
Frozen orbits are always important foci of orbit design because of their valuable characteristics that their eccentricity
and argument of pericentre remain constant on average. This study investigates quasi-circular frozen orbits and examines their
basic nature analytically using two different methods. First, an analytical method based on Lagrangian formulations is applied
to obtain constraint conditions for Martian frozen orbits. Second, Lie transforms are employed to locate these orbits accurately,
and draw the contours of the Hamiltonian to show evolutions of the equilibria. Both methods are verified by numerical integrations
in an 80 × 80 Mars gravity field. The simulations demonstrate that these two analytical methods can provide accurate enough
results. By comparison, the two methods are found well consistent with each other, and both discover four families of Martian
frozen orbits: three families with small eccentricities and one family near the critical inclination. The results also show
some valuable conclusions: for the majority of Martian frozen orbits, argument of pericentre is kept at 270° because J3 has the same sign as J2; while for a minority of ones with low altitude and low inclination, argument of pericentre can be kept at 90° because of
the effect of the higher degree odd zonals; for the critical inclination cases, argument of pericentre can also be kept at
90°. It is worthwhile to note that there exist some special frozen orbits with extremely small eccentricity, which could provide
much convenience for reconnaissance. Finally, the stability of Martian frozen orbits is estimated based on the trace of the
monodromy matrix. The analytical investigations can provide good initial conditions for numerical correction methods in the
more complex models. 相似文献
In the current study, the existence of periodic orbits around a fixed homogeneous cube is investigated, and the results have
powerful implications for examining periodic orbits around non-spherical celestial bodies. In the two different types of symmetry
planes of the fixed cube, periodic orbits are obtained using the method of the Poincaré surface of section. While in general
positions, periodic orbits are found by the homotopy method. The results show that periodic orbits exist extensively in symmetry
planes of the fixed cube, and also exist near asymmetry planes that contain the regular Hex cross section. The stability of
these periodic orbits is determined on the basis of the eigenvalues of the monodromy matrix. This paper proves that the homotopy
method is effective to find periodic orbits in the gravity field of the cube, which provides a new thought of searching for
periodic orbits around non-spherical celestial bodies. The investigation of orbits around the cube could be considered as
the first step of the complicated cases, and helps to understand the dynamics of orbits around bodies with complicated shapes.
The work is an extension of the previous research work about the dynamics of orbits around some simple shaped bodies, including
a straight segment, a circular ring, an annulus disk, and simple planar plates. 相似文献
