The primary goal of this study is to evaluate the performance of the Explicit One-dimensional (1D) Time-dependent Tilting Cloud Model (ETTM), which will be potentially used in a cumulus parameterization scheme. The Weather Research and Forecasting (WRF) model was used in a cloud-resolving mode to study 3D cloud characteristics under two sheared environments, one from the Rain in Cumulus over the Ocean (RICO) field experiment and the other from the International H2O Project (IHOP). Then, WRF 3D simulation results were used to evaluate ETTM performance. WRF simulations were performed with different radii from 1 km to 10 km of thermal bubbles for initiation. The three-dimensional cloud features were quite different between RICO and IHOP due to their environments, which were sub-tropical maritime sounding and mid-latitude continental sounding, respectively. ETTM 1D cloud simulations, corresponding to each of the WRF simulations, were conducted. The simulated 1D clouds were too weak when the original thermal bubbles were similar to those used in the 3D cloud simulations (i.e., no additional moisture within the thermal bubbles). The sensitivity of model results to relative humidity was tested by imposing a lower bound of 88% (ER88) and 95% (ER95) humidity to the thermal bubble in ETTM simulations. When compared with the original simulations, 1D results from ER88 and ER95 showed clear improvements, but they were still underestimated relative to 3D clouds, and the results from IHOP were slightly worse than those from RICO. Sensitivity tests with a zero-degree cloud tilting angle and with a different radius of the downdraft were also examined. Results show that the downdraft due to the tilting of the cloud slightly improved ETTM’s performance in terms of the heat and moisture fluxes, while the influence of using different downdraft sizes on 1D simulation results is not clear.
A new automated 222Rn measuring system suitable for a wide range of atmospheric observations has been developed. This system comprises a 222Rn analyzer and an air-sampling unit, along with a pump, drying equipment, and a filter. The operation of this 222Rn analyzer is based on an electrostatic collection method involving the use of a PIN photodiode to separately detect a particles emitted from 218Po and 214Po released from the decay of 222Rn in a hemispheric air-sample chamber. The response of the 222Rn analyzer with respect to the applied voltage, chamber volume size, and sample flow rate is investigated to determine the appropriate measuring conditions. With this instrument, the detection limits have improved to 0.16-0.20 Bq m-3 for 1 h. The new 222Rn analyzer shows several technical advancements over the previous electrostatic collection methods, such as an improved detection limit, higher sensitivity, higher time-resolution analysis, and longer life. During the preliminary observational test, the 222Rn measuring system was able to detect the diurnal cycle in 222Rn resulting from the mixing of rapid vertical air in the surface boundary layer. At the Minamitorishima station located far from the continent, very small 222Rn peaks were clearly detected when long-range transport of polluted air masses from the Asian continent was observed at the station. We demonstrate that our compact 222Rn measuring system has the potential to be widely used for high time-resolution measurements of low-level 222Rn.
The mechanism of supercell tornadogenesis and its vorticity budget are investigated by means of a high-resolution (horizontally uniform grid size of 70 m) numerical simulation of the Del City storm, which occurred in Oklahoma, USA, on May 20, 1977. After 50 min of the simulation, a meso-low, which is generated by nonlinear interaction between the storm updraft and vertical wind shear associated with both environmental and storm-induced horizontal flow, develops at around 1.8 km above ground level (AGL). The meso-low acts to strengthen the underlying updraft, and generates a low-level mesocyclone via the tilting of horizontal vorticity associated with the environmental wind and that generated by baroclinic processes. In turn, a pressure depression associated with this low-level mesocyclone generates an updraft exceeding 43 m s1 at 1.5 km AGL. In addition, small-scale vortices (pretornadic vortices) develop along a gust front. When the low-level updraft strengthened, one of these pretornadic vortices located immediately beneath the updraft shows a rapid growth into a major tornado. As the tornado develops, a downward pressure gradient force associated with an intense rotation of the tornado strengthens a tornado-scale downdraft on its north side. The developed downdraft compresses the vertical vortex of the tornado, eventually resulting in its dissipation. We also performed a vorticity budget analysis along a typical air-parcel trajectory. Air parcels in the mature tornado vortex originate mainly from the northwest in a layer between 10 and 500 m AGL. Vertical vorticity within the mature tornado is initially produced during descent via tilting of the horizontal vorticity, which is enhanced by stretching of the vortex tube.
A numerical simulation of the strong southeasterly (SE) “Kiyokawa-dashi” wind in Yamagata, Japan on August 30, 2004, is examined and compared to the Coherent Doppler Lidar (CDL) observation. Three-dimensional numerical simulations were performed using a non-hydrostatic meso-scale model developed by the Japan Meteorological Agency. The sensitivity of the numerical simulation was examined with respect to the resolution of the horizontal and vertical grid, surface roughness, and ground surface temperature. For the case of a 1-km grid with 85 vertical layers including realistic surface settings, the observed characteristics of Kiyokawa-dashi were well reproduced: the strong SE wind (10 m s-1) was extremely low, about 0.1-0.5 km AGL, and the maximum wind speed over 12 m s-1 was observed at 0.1-0.2 km, AGL, under the low-level stable layer. Strong winds appeared at the foot of the final lee slope facing the Shonai Plain and 1 km south of the valley; this result was in good agreement with the CDL observation. On the basis of the stream line analysis, Kiyokawa-dashi was strongly affected by the upper SE wind, i.e., upper air on the eastern side of the mountains blew down to the middle and lower layer on the western side. The strongest wind field located 1 km south of the valley was affected by the mountain at the southwestern end of the valley. It was observed from the sensitivity experiments that the height of the jet-like flow was found to be closely related to the low-level stable layer, and the critical layer was not very important in reinforcing the low-level SE wind. This case of Kiyokawa-dashi was explained on the basis of the hydraulic theory.
Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropical North and South Pacific associated with the ENSO warm (cold) events are examined using reanalysis and in-situ observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over the subtropical North Pacific in the February-March period and over the subtropical South Pacific after April, and vice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, the negative air humidity anomaly and anomalous downward motion appear at the same location in either the Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending branch of local Hadley circulation is observed during the January-February period in the Northern hemisphere and after March in the Southern hemisphere, which coincides with the seasonal transition of the climatological local Hadley circulation from the Northern to Southern hemisphere. Our linear decomposition analysis of surface heat flux anomalies indicates that the negative air humidity anomaly, as well as anomalies in wind speed, contributes to the formation of the subtropical negative SSTAs through the enhanced latent heat flux induced by the anomalous air-sea humidity difference. These results suggest that the anomalous downward motion associated with the changes in local Hadley circulation can induce the subtropical negative SSTAs through the surface humidity variability. A possible mechanism for the subtropical air-sea interaction associated with the local Hadley circulation is discussed.
By analyzing the long-term (1950-2007) variability of diurnal temperature range [DTR; daytime maximum (Tmax) minus nighttime minimum (Tmin)] at 21 stations in Taiwan, this study applies univariate analysis, a trend-free prewhitening procedure combined with a modified Mann-Kendall test, and EOF-based multivariate trend analysis (TEOFA) to the ranked station DTR, Tmax, and Tmin. To reveal the large-scale associations with the local TEOFA results, this study also uses global gridded Tmax,Tmin, nighttime marine air temperature, and sea level pressure (SLP) datasets archived at the U.K. Meteorological Office Hadley Centre. On the basis of the signs and relative magnitudes between the annually-mean Tmax and Tmin trends, the stations can be classified into three types through univariate analysis. With a common increasing Tmin, type-A (type-B) stations have an increasing Tmin (Tmax) faster than Tmax (Tmin) whereas type-C stations have a decreasing Tmax. Both type-A and type-C (type-B) thus show(s) a decreasing (an increasing) DTR. For most stations, the increasing Tmin is the largest in December-February. In contrast, all type-B stations have an increasing Tmax peaking in June-August. Noticeably, six of nine type-B stations, either in the remote islands or in the seaports, are particularly influenced by the ocean. Three DTR trend modes stand out of TEOFA. The first mode, TEOF1, captures increasing DTR trends at four type-B stations and decreasing DTR trends elsewhere. The associated increasing Tmin (Tmax) trend is consistent with the increasing (decreasing) clouds during nighttime (daytime) and is well-correlated with the large-scale patterns, suggesting that it is part of the global warming scenario. Nevertheless, TEOF1 also captures a decreasing Tmax trend in Taiwan’s highly developed western plains (i.e., type C) and eastern China where an increasing SLP pattern is observed, implying the anthropogenic forcings on DTR. TEOF2 (TEOF3) depicts the decadal-to-interdecadal DTR variability in central (northern) Taiwan. Evolution of TEOF2 shows smaller (larger) amplitude before 1970s (after mid-1980s). The associated large-scale patterns suggest that TEOF2 (TEOF3) captures the relationship between an intensified (weakened) East Asian winter monsoon and La Niña (El Niño)-like condition in eastern (central) equatorial Pacific. Embedded in a Pacific-Japan-like teleconnection pattern, the southwestward intrusion of Pacific subtropical anticyclone in June-August that signifies the weakened southwesterly monsoon is also depicted by TEOF2.
On the basis of buoyancy-vorticity (BV) formulation of Harnik et al. (2008), the initial value problem of vertically propagating gravity waves is analytically solved in a zonal-vertical two-dimensional system. The analytical solutions provide an example of the visualization of BV thinking. Further, the analytical solutions enable a qualitative understanding of the growth of gravity waves in a vertically sheared zonal flow (so-called shear instability of gravity waves) by BV thinking. To this end, the basic buoyancy (i.e., basic potential temperature) is assumed to be piecewise uniform in the vertical direction, and the Green function method is employed. The obtained analytical solutions show the following. In a vertically uniform basic zonal flow, the gravity wave, which is initially at the lowest level, propagates vertically upwards, gets reflected from the highest level back to the lowest level and again from the lowest level to the highest level, and so on. In a vertically sheared basic zonal flow, the behavior of the gravity waves depends on the horizontal wave number. This is caused by the dependence of horizontal propagation velocity on the horizontal wave number. Here, horizontal propagation is defined relative to the fluid. If the horizontal propagation and advection by the basic zonal flow are successfully balanced so that the lower and upper phase velocities are nearly equal, then the gravity wave propagates vertically, and the upper and lower disturbances are phase-locked to each other; this results in an effective interaction between them and in growth as an exponential function of time. On the other hand, if the horizontal propagation and advection by the basic zonal flow are out of balance so that the lower and upper phase velocities are different from each other, then the gravity wave hardly propagates vertically, and the upper and lower disturbances horizontally flow away from each other resulting in an absence of interaction between them and in the oscillation (i.e., no growth). At the marginal points between oscillation and growth, the gravity wave grows as a linear function of time. The behavior of analytical solutions can be qualitatively explained by the BV thinking of Harnik et al. (2008).