Takeshi Horinouchi

Convectively generated mesoscale gravity waves simulated throughout the middle atmosphere

[1]xa0A three-dimensional simulation was conducted with a cloud-resolving model to investigate mesoscale gravity waves generated by cumulus convection that propagate to the mesospheric and lower thermospheric (MLT) region. It is found that both individual convective turrets and mesoscale convective systems excite gravity waves, resulting in a broad scale distribution. Waves excited by the former have conically shaped phase surfaces as previously reported and are conspicuous up to around the stratopause. Waves excited by the latter dominate in the MLT region. Frequent wave breaking is found above 85 km, where convective instability is found typically in the initial stage, leading to unstable shear with billows resembling the ripple-type structures observed frequently in airglow imaging.

Turbulence at the tropopause due to breaking Kelvin waves observed by the Equatorial Atmosphere Radar

[1]xa0The Equatorial Atmosphere Radar (EAR) installed in Sumatra Island, Indonesia, observed significant enhancement of turbulence in the tropopause region, 15–17 km, intermittently for ∼5 days in November 2001. The turbulence intensity was estimated with the spectral width of the radar echo power spectrum, and the turbulence during the period was a factor of up to ∼5 larger in kinetic energy than that in other periods. Further analyses confirm that the enhanced turbulence was convectively generated in the breaking phase of an equatorial Kelvin wave. Between July and December 2001, we observed at least three more prominent cases of the turbulence generation by breaking Kelvin waves in the tropopause region.

Kelvin-Helmholtz instability around the tropical tropopause observed with the Equatorial Atmosphere Radar

[1]xa0In November 2001, the Equatorial Atmosphere Radar (0.20°S, 100.32°E) observed a continuous strong eastward wind shear (10–50 m s−1 km−1), westward wind (2–27 m s−1), and the radar echo layer tilted downward to the west in the region 0–1 km above the tropopause. During the same period, the Richardson number calculated with hourly-averaged horizontal wind and radiosonde temperature data was almost continuously <0.5 and sometimes <0.25, which seems to indicate that the Kelvin-Helmholtz instability (KHI) frequently occurs in that region. The existence of the tilted radar echo layer can be explained by KHI billows. A spurious updraft caused by the KHI-induced tilted echo layer and by the strong westward wind was also observed in the region.

Modulation of the midlatitude ionospheric E region by atmospheric gravity waves through polarization electric field

[1]xa0We have studied coupling between the neutral atmosphere and the ionospheric E region in the midlatitude by combining two numerical simulation models. Atmospheric gravity waves generated in the troposphere propagate through the stratosphere and the mesosphere and can reach the lower thermosphere. When a zonal wind shear that can accumulate a sporadic-E (Es) layer (eastward below and westward above) is applied, the accumulated Es layer is strongly modulated by the gravity waves, and polarization electric field due to the modulated Es layer produces wave-like patterns of plasma density in the upper E region. Since the eastward wind below the shear node filters out gravity waves with eastward phase velocity, those with westward phase velocity are dominant where the Es layer is accumulated. Because of the angle between phase front of gravity waves and the geomagnetic field line, gravity waves with southward phase velocity is more effective to generate polarization electric field than those with northward phase velocity. Since gravity waves with southwestward phase velocity have phase fronts aligned from northwest to southeast, polarization electric field also has the similar structure. This mechanism can explain the spatial structure of quasi-periodic (QP) echoes associated with plasma irregularities in the midlatitude E region which often shows northwest-southeast alignment.

Sea-Breeze Circulation over Jakarta, Indonesia: A Climatology Based on Boundary Layer Radar Observations

Abstract Characteristics of sea-breeze circulation over the tropical site of Jakarta, Indonesia, have been documented based on analyses of satellite images and data from long-term L-band boundary layer radar measurements carried out at Serpong (6.4°S, 106.7°E). Inspection of satellite imagery reveals that a sea-breeze front develops well along the northern coastal plain of West Java and propagates inland until its structure is deformed over complex topography. It is found that the sea-breeze signal detected by the boundary layer radar is most well defined during the dry season months of July–October. In all of these months, radar observations indicate a late afternoon intensification of sea-breeze flow in the 0.5–0.8-km height range between 1700 and 1800 LT, which is not elucidated upon by surface measurements. The effect of weather conditions on the sea-breeze pattern is investigated by using a cloudiness index derived from data of incoming solar radiation. The results show that sea-breeze intrusion over...

High time resolution determination of the tropical tropopause by the Equatorial Atmosphere Radar

[1]xa0The Equatorial Atmosphere Radar (EAR) is located at the equator (0.20°S, 100.32°E) in West Sumatra, Indonesia. The capability of the EAR to observe short-term variations of the tropopause altitude is shown. Near the tropopause, the EAR receives strong echoes caused by the rapid increase of hydrostatic stability with altitude. By applying this characteristic, the radar tropopause (RT) was determined. The RT agreed well with both the lapse-rate tropopause (LRT) and the cold-point tropopause (CPT) in altitude with a time resolution of 3 hours. The power spectrum of the RT altitude from 1 July to 18 December, 2001 showed a clear 1-day peak. The amplitude of its diurnal cycle as revealed in composites is as small as ∼0.1 km. Nevertheless, the RT was able to reproduce the diurnal cycle in the LRT and the CPT well, which indicates that the EAR can observe short-term variations of the tropical tropopause.

Vertical wind observation in the tropical upper troposphere by VHF wind profiler: A case study

[1]xa0Features of upper tropospheric vertical wind (W) over Sumatra, Indonesia, are presented using data observed by a VHF wind profiler installed at West Sumatra (0.2°S, 100.32°E). During 5–9 May 2004, W from the middle to upper troposphere (8–14 km) changed in accordance with the cumulus activity over Sumatra. During 5–6 May, 3-hourly averaged W continuously showed upward motions up to 0.09 m s−1. The upward motions were observed in the vicinity of deep convective events, which were continuously seen over Sumatra within a synoptic-scale convectively active envelope. After 7 May, when cumulus activity was suppressed over Sumatra, 3-hourly averaged upward motions of greater than 0.05 m s−1 almost disappeared. During 5–6 May, downward motions up to ∼0.11 m s−1 were observed above 14 km, while upward motions were observed below 14 km. Estimation of W by the European Centre for Medium-Range Weather Forecasts operational analysis have revealed that a major part of observed downward motions above 14 km is explained by the leeward (southwestward) wind and leeward downward tilt of isentropes that existed over western Sumatra. The observed downward motions above 14 km during 5–6 May suggest that downward motions caused by leeward downward tilt of isentropes can be produced in the vicinity of the convectively active region, and leeward downward tilt of isentropes can suppress an upward transport of air mass into the tropical tropopause layer (TTL) by producing downward motions in the TTL.

Mesoscale Variability of Tropical Precipitation: Validation of Satellite Estimates of Wave Forcing Using TOGA COARE Radar Data

Abstract Satellite-derived brightness temperature has been used to estimate tropical precipitation. Ricciardulli and Garcia applied it to quantify forcing of atmospheric waves that are excited by tropical cumulus convection and propagate into the middle atmosphere. Because of the broad coverage of the satellite data, this method provides exclusively dense information on wave forcing and is especially valuable for middle-atmosphere modeling. However, the validity of the method has not been investigated, which is done in this study using radar-derived precipitation during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) field experiment. The method is shown to overestimate the variance of precipitation so that the wave forcing derived with it is too strong. The overestimation is most severe at coarse resolution, reaching nearly an order of magnitude at a grid scale of 2°, which is comparable to typical resolutions of current global climate models. Although t...

A numerical study of upward‐propagating gravity waves in two different MJO phases

[1] Convectively generated atmospheric gravity waves that propagate into the equatorial stratosphere were investigated using a cloud resolving model. Numerical simulations were conducted to study wave generation during convectively inactive and active phases in a cycle of the Madden-Julian oscillation, which occurred during an observational campaign conducted in Sumatra. Greater convective activity during the active phase caused stronger disturbances in the troposphere at relatively low phase speeds less than about 20m/s than the inactive phase. However, the contrast was weaker at higher phase speeds. Furthermore, the vertical structure of convective forcing in the inactive phase was more suitable to excite waves with wavelength longer than twice the depth of convection. Owing to the combination of these factors, upward gravity wave propagation was significantly enhanced during the inactive phase. This study demonstrates the necessity to study convection spectrally to investigate possible links between intraseasonal variabilities in the troposphere and the middle atmosphere.

An intense gravity wave near the mesopause region observed by a Fabry‐Perot interferometer and an airglow imager

[1]xa0We report an intense gravity wave event observed at Shigaraki, Japan, at 1500–2000 UT (0000–0500 LT) on 22 December 2001 using a Fabry-Perot interferometer, which measures the Doppler shift of nocturnal airglow emission at a wavelength of 557.7 nm (emission altitude: 90–100 km). The wave had a period of 1.5 hours and a peak-to-peak amplitude of 68 m/s in the horizontal wind velocity. The hodograph of the east-west and north-south wind oscillations showed a polarization direction of the wave of ENE-WSW. An all-sky airglow imager at Shigaraki also recorded a similar southwestward moving wave with a period of ∼1.5 hours and a velocity of 146 m/s in the 557.7-nm airglow images. The horizontal wavelength was estimated to be 790 km from the images. These facts indicate that the wave was generated in the northeast of Japan, where a well-developed low-pressure cell (968 hPa) and a distortion of the jet stream were observed in the troposphere. However, the large horizontal wavelength, the fast phase velocity, and the possible source location apart from the exit of the jet streak are different from those predicted from previous numerical simulations.