We estimated monthly atmospheric turbidity from hourly observation data of global solar radiation under cloudless skies at 67 stations all over Japan. Using an empirical formula for calculating global solar radiation, we determined atmospheric turbidity as the best-fit parameter. The estimated turbidity is consistent with the observation by the Japan Meteorological Agency. Sub-nationwide increases in atmospheric turbidity, caused by forest fires or Asian dust events, are detected in time series. Some temporal changes in atmospheric turbidity are also found in a small area. Such local phenomena are hardly detectable by sparse distribution of the stations for direct solar radiation observation. The accuracy of the global solar radiation estimation can be improved by considering the seasonal variation of the turbidity. Since Ångström’s turbidity coefficient is directly linked with the volumetric aerosol content, the present method is useful for monitoring the spatial distribution of aerosols. In addition, the present method using the densely distributed observation network of global solar radiation has an advantage in compensating for the sparseness of the current atmospheric turbidity observation in Japan.
Characteristics of polarimetric radar variables in three rainfall types in a Baiu front event over the East China Sea observed on 1 June 2004 were studied and compared using a C-band polarimetric radar, the CRL Okinawa bistatic polarimetric radar (COBRA). The selected rainfalls are common types in the Baiu season in this area: (1) stratiform type (ST), (2) isolated convective type (ICT) and (3) embedded convective type (ECT). ST was characterized by an obvious bright band in the field of radar reflectivity (Zhh). ICT and ECT had almost the same 30-dBZ echo-top height of about 5.5 km, and their strong echo region (Zhh > 40 dBZ) did not exceed the 0°C level (4.4 km altitude) even in their mature stages. Around the 0°C level, overall decrease in correlation coefficient between horizontal and vertical polarization signals (ρhv) and increase in differential reflectivity (Zdr) were observed in ST and ECT, which indicated the presence of a layer of mixed-phase precipitation. By contrast, significant decrease in ρhv and increase in Zdr were not found in ICT. At lower levels, Zdr ranged from 0 to 1.5 dB and most of ρhv were higher than 0.98 in ST and ECT. The values of Zdr and ρhv had wider variations in ICT. The characteristics of the vertical profiles of Zdr and ρhv in ECT are consequently more similar to those in ST rather than to those in ICT, although their echo-top heights of 30 dBZ and maximum Zhh near the surface were almost equal.
The land-ocean contrasts of near-surface temperature and precipitation averaged over tropics (40°S–40°N) and extra tropics in the Northern Hemisphere (40°N-70°N) were studied using the global version of the Experimental Climate Prediction Center (ECPC) global to regional forecast system (G-RSM) under current and projected sea surface temperatures (SSTs) with four different stratiform cloud parameterizations. As common features of the simulations, we found that there is a large land-ocean contrast in surface downward longwave radiation flux (LWd), downward shortwave flux (SWd), upward shortwave flux (SWu), latent heat flux (LHF) and sensible heat flux (SHF). Since smaller LWd is compensated by larger SWd, and larger SWu is compensated by smaller LHF, there is a tendency toward the reduction of the land-ocean contrast of the total surface fluxes. There were significant differences in near-surface air temperature and precipitation among the four simulations. The main cause is due to clear-sky LWd. The second largest term is the SWd, which works in the opposite direction of the difference in LWd. The difference in the near-surface temperature among the schemes is mostly determined by the sum of the LWd and SWd. The variability of precipitation among the four cloud schemes is large everywhere except in tropical oceans. In general, the difference in evaporation explains the precipitation difference among the schemes everywhere except tropical oceans where moisture convergence is negative. When the simulations are performed with warmer SST, the differences among the cloud schemes are very similar to those in present-day simulations. However, the magnitude of the response to warm SST depends on the cloud scheme. Those sensitivities are connected to that of the effect of cloudiness on SWd and surface albedo.
An improved Mellor–Yamada (MY) turbulence closure model (MYNN model: Mellor–Yamada– Nakanishi–Niino model) that we have developed is summarized and its performance is demonstrated against a large-eddy simulation (LES) of a convective boundary layer. Unlike the original MY model, the MYNN model considers effects of buoyancy on pressure covariances and effects of stability on the turbulent length scale, with model constants determined from a LES database. One-dimensional simulations of Day 33 of the Wangara field experiment, which was conducted in a flat area of southeastern Australia in 1967, are made by the MY and MYNN models and the results are compared with horizontal-average statistics obtained from a three-dimensional LES. The MYNN model improves several weak points of the MY model such as an insufficient growth of the convective boundary layer, and underestimates of the turbulent kinetic energy and the turbulent length scale; it reproduces fairly well the results of the LES including the vertical distributions of the mean and turbulent quantities. The improved performance of the MYNN model relies mainly on the new formulation of the turbulent length scale that realistically increases with decreasing stability, and partly on the parameterization of the pressure covariances and the expression for stability functions for third-order turbulent fluxes.
Using a Martian general circulation model, we investigated the changes in the meridional circulation during planet-encircling dust storms on Mars that produce strong temperature vertical inversions in the middle atmosphere over winter polar regions. It is shown that vigorous poleward and downward transport, and, consequently, the adiabatic heating are caused by dissipating thermal tides, planetary and resolved small-scale gravity waves and eddies in almost equal degree. The increase of tidal forcing is mainly due to a stronger excitation in the summer hemisphere. Contribution of the stationary planetary wave (SPW) with the zonal wavenumber s = 1 increases during dust storms due to intensified generation in the lower atmosphere as well as due to more favorable vertical propagation. SPW (s = 2) varies less with the dust load, dissipates lower, and contributes to the warming only below ∼ 0.1 mb. Transient planetary wave (s = 1, period ∼ 5 sols) with a barotropic/baroclinic vertical structure provides up to 1/3 of the forcing by SPW (s = 1). For the first time, we demonstrated a significance of small-scale gravity waves and eddies in maintaining the meridional circulation in Martian middle atmosphere, at least in high winter latitudes during dust storms.