High-resolution numerical experiments of Typhoons

Some typhoons usually attack Japan and its surroundings and cause severe disaster. In particular, ten typhoons landed over the main lands of Japan in 2004. Accurate prediction of the associated intense wind and rainfall is very important for disaster prevention. We performed two simulation experiments of typhoons using the Cloud Resolving Storm Simulator (CReSS) on the Earth Simulator. One is the typhoon T0418 which brought a very intense wind and caused huge disaster. The other is the typhoon T0423 which brought a heavy rainfall and caused severe floods.

The main objectives of the simulation experiments of T0418 are to study the eye wall as well as spiral rainbands, and to examine structure of the strong wind associated with the typhoon around Okinawa Island. The simulation experiments of T0418 started from 0000 UTC, 5 September 2004. The Typhoon T0418 moved northwestward over the northwest Pacific Ocean and passed Okinawa Island on 5 September 2004. Its center passed Nago City around 0930 UTC, 5 September with the minimum sea level presser of 924.4 hPa. The experiment with a horizontal resolution of 1 km shows the detailed structure of the eye wall and the spiral rainbands (Fig.1). Individual cumulus clouds are resolved. They are present along the spiral rainband as well as the inside of the eye. The high resolution experiments provide detailed data of the cloud and precipitation systems associated with the typhoon.

The purpose of the simulation experiment of T0423 is to study process of the heavy rainfall. At the initial time of 1200 UTC, 19 October 2004, T0423 was located to the south-southwest of Amami Island. The most part of Typhoon was out of the calculation domain. The movement of T0423 and the rainfall were successfully simulated. In the simulation, a northward moisture flux was large in the east side of the typhoon center. When the large moisture flux reached to the Japan Islands, heavy rainfalls occurred along the Pacific Ocean side. The heavy rainfall moved with the movement of the typhoon from Kyushu to Shikoku. When the typhoon reached to the south of Shikoku, heavy rainfall began in the Kinki District as well as along the east coast of the Kii Peninsula around 04 UTC, 20 October (Fig.2). The distribution well corresponds to the radar observation. The heavy rainfall along the Pacific Ocean side moved eastward, while that in the Kinki District lasted until 09 UTC. After the typhoon moved to the east of the Kinki District, the northeasterly wind intensified significantly. Consequently, orographic rainfall was formed in the northern part of the Kinki District. As a result, the accumulated rainfall was large and the severe flood occurred.


Fig. 1: Rainfall intensity (mm hr-1; gray levels), and pressure (contour lines) obtained from the simulation experiment (1 km resolution) of T0418 at 08 UTC, 5 September 2004.


Fig. 2: Rainfall intensity (mm hr-1; gray levels), horizontal velocity at a height of 1458 m (arrows), and pressure (contour lines) obtained from the simulation experiment of T0423 at 04 UTC, 20 October 2004.

Aircraft observation of a water vapor front over the East China Sea

A water vapor front proposed by Moteki et al. (2004, JMSJ) using a cloud resolving model has meridional water vapor gradient in the lower troposphere over the East China Sea in the southern region of the Baiu front. In order to examine the existence of the water vapor front and its amounts of temperature and moisture gradient, we performed aircraft observations using the Gulfstream-II with temperature and humidity sensors and drop-sounding system.

We set a base camp of our observation at the Kagoshima Airport from June 23 to 27, 2004. We predicted the location and alignment of the Baiu front by using the daily forecasted results by the Cloud Resolving Storm Simulator (CReSS) and decide the flight time, flight area and points of the drop sounding observation. The aircraft observation has been performed two times during this period: on June 24 and 27, 2004. On June 27, we performed a successive temperature and humidity observation across the Baiu front from the northern area to the southern one at the height of 550 m over the East China Sea, and six drop-sounding observations both northern and southern regions of the Baiu front (Fig. 3).

Figure 4 shows a series of temperature and dew-point temperature at the height of flight level (550 m). There are two significant temperature variations between 30.7N and 31.0N, and around 31.5N. Both points have also variations of dew-point temperature. On the other hand, the dew-point temperature changes between 20 and 18 ℃ around 29.6N without no temperature variation. This shows that the southern region is moister than the northern one. This humidity variation without temperature variation is corresponding to the water vapor front located in the southern region of the Baiu front over the East China Sea.

In this aircraft observation, we confirm the existence not only the Baiu front with both temperature and humidity variations, but also the water vapor front defined by the humidity variation without the temperature variation. The southern region of the water vapor front is moister than the northern region of it. This is corresponding to a concept of the water vapor front proposed by Moteki et al. (2004, JMSJ).


Fig. 3: Flight path of the aircraft observation in the lower troposphere from 08:54 to 09:56 JST on June 27, 2005 is shown by a solid line. Drop sounding observations (star) are performed from 10:23 to 12:09 JST.


Fig. 4: Series of temperature (solid line) and dew-point temperature (dashed line) from 08:54 to 09:56 JST along the flight path in the lower troposphere. Horizontal axis is latitude. Black and gray triangles show the location of the water vapor front and the Baiu front, respectively.

Structure and maintenance process of a rainband over the East China Sea during the early Baiu season

During the early Baiu season in 2003 and 2004, field experiments on the boundary layer and precipitation systems were carried out around Okinawa Island in cooperation with Hydrospheric Atmospheric Research Center (HyARC), Nagoya University and Okinawa Subtropical Environment Remote-Sensing Center, National Institute of Information and Communications Technology (NICT). Three-dimensional wind fields within the precipitation systems were analyzed with the dual Doppler radar observation: using a C-band polarimetric Doppler radar (COBRA) of NICT and X-band Doppler radar of HyARC (Fig. 5).

A long lasting rainband of 25 May 2003 was observed during this observation period. The rainband was composed of precipitation cells aligned parallel to the band in the northwestern edge of the precipitation system. The system moved southeast passing over Okinawa Island. Vertical cross sections of reflectivity and updraft fields along the X-axis of Fig. 5 are shown in Fig. 6 for the time before, during and after the passage of the system over Okinawa Island. A stratiform region was ahead of the system movement and the convective region was in the rear edge of the system. The stratiform region did not change much before and after the passage. On the other hand, the height of the convective core of 3.5 km ASL before the passage decreased to the level below 2.0 km ASL after the passage at 0548 JST.

The development of the convective cells was constrained after the passage, when the convergence in the low altitude decreased because of the weakening of the northwesterly behind the Okinawa Island. A few hours after the passage, when northwesterly recovered the strength at the northwest edge of the precipitation system, the rainband re-strengthened. These observational results revealed that the maintenance mechanism of the rainband and the precipitation system is continuous formation of not so tall convective clouds on a weak convergence line in the moist environment of the low atmosphere (Fig. 7).


Fig. 5: Observation ranges of COBRA (Nago) and X-band Doppler radar (Katsuren) are shown by large and small circles, respectively. Analysis areas of three-dimensional wind fields are shown by dashed circles. Radar echo at 03 JST, 25 May 2003 is shown by shades. The analysis area is shown by a square.


Fig. 6: Time variations (at 05, 06 and 08 JST 25 May 2003) of vertical cross section of reflectivity (contour) and updraft fields (shades) along the X-axis of Fig. 5. Values are averaged along Y-axis in Fig. 5. The rainband passed over Okinawa Island at around 06 JST. Contours are every 3 dBZ from 15 dBZ. Updrafts are at the low altitudes in the northwestern edge of the precipitation system.


Fig. 7: Schematic illustration of the structure and maintenance mechanism of the rainband in the system formed over East China Sea. Low convective clouds developed along the weak convergence below 1 km ASL using the abundant water vapor of the southwesterly. Cloud droplets and raindrops formed rapidly using the moist air in the low altitude. A stratiform region extended to the downstream of the system movement and the downdraft in this stratiform region did not reach to the ground surface, tahen low altitude moist air reached to the northwestern edge (convective region) of the precipitation system. In this way, the rainband maintained its linear shape for long time.

Structure and mechanism of convective clouds and their clusters developed in a highly humid environment

Convective clouds are a basic element of precipitation phenomena in a humid environment. The structure and mechanism of clouds are important problems because they determine the morphology and the time variation of the convective systems. Previous studies focused on the effect of evaporative cooling which accelerates the downdraft, because the low-level outflow from the downdraft plays an important role to maintain convective clouds. In a highly humid environment, however, the evaporative cooling is ineffective. There have been few studies about convective cloud in such a highly humid environment. The present paper studies the structure and mechanism of convective clouds and their clusters using Doppler radar observation at Miyakojima Island during the early Baiu season and three-dimensional cloud-resolving model.

The convective system, which formed in the highly humid environment on 6 June 2003 was studied in detail. This convective system had a multiscale structure, which was consisted of three elements: convective cells, cell groups and cell-group lines. Cell groups consisted of two types of cells: long-lasting type and short-lived type. Cell groups consisted of the former type included only two long-lasting convective cells. On the other hand, the latter type were often observed during this precipitating event (Fig 8). The long-lasting convective cells did not form low-level outflow, but the convective cells maintained long time by re-development. It is considered that the cells re-developed by intensification of upward acceleration, which resulted from falling a large amount of precipitation loading and condensation heating by entrainment of almost saturated air.

This paper examined the mechanism of the re-development using the Cloud Resolving Storm Simulator (CReSS). Moreover, some sensitivity tests to examine the humidity of the environment. When the humidity was lower, the convective cell did not re-develop because the condensation heating was suppressed. Thus, it was revealed that the convective cells had lasted for long time by re-development due to unloading in the highly humid environment (Fig. 9).


Fig. 8: The lifetime and maximum reflectivity of convective cells, which were consisted two cell groups. Shades indicate existence of convective cells and length of shaded boxes indicate lifetime of convective cells. Maximum reflectivity of convective cells are shown by contrast.


Fig. 9: Conceptual model of the long-lasting convective cell developed in a highly humid environment.

Profiles of heating budgets in the developed MCSs during the Australian summer monsoon season

Latent heat release in mesoscale convective systems (MCSs) in the Tropics is a major driving force of atmospheric circulations, so it is necessary to exactly estimate the vertical profiles of heating in the MCSs. MCSs frequently develop over the northern Australia (NAU) during the the Australian summer monsoon season in relation to the passage of the Madden-Julian Oscillation (MJO). By using the Cloud Resolving Simulator (CReSS) with 1 km horizontal grid resolution and mesoscale model (MM5), we investigate the profiles of heating budgets of convective and stratiform precipitation regions within two MCSs.

The profiles of time-area averaged heating budgets are shown in Fig. 10. The large-scale environment in the west part of the MJO is characterized by abundant moisture transport by west-southwesterly, large-scale convergence, and large vertical wind shear. The maritime MCS develop in this phase. The heating effect by deposition to ice particles in the stratiform region is found to be larger than that of continental MCS in the middle level (Fig. 10b and 10d).

On the other hand, the large-scale environment in the east part of the MJO is characterized by a southeasterly intrusion of very dry air mass in the middle level, and large-scale divergence and large vertical wind shear in the low level. The continental MCS develop in this phase. The heating effects by deep condensation and riming of graupel at the middle level in the convective region are dominantly larger than that of maritime. The larger evaporation cooling of rainwater is also indicated in the convective region (Fig. 10a and 10c). The cooling effects by sublimation of ice hydrometer in the middle level and evaporation of rainwater in the low level in the stratiform region distinctly are much larger than those of maritime.

The relations among heating budgets in the MCSs, wind shear and moisture fields are revealed in contrasting phases of the MJO.


Fig. 10: Vertical profiles of area averaged Q1 budgets normalized by convective and stratiform rain rate (averaged for 3-10 hours). (a) convective area and (b) stratifom area of maritime MCS. (c) convective area and (d) stratiform area of continental MCS. Notations show vertical eddy heat flux convergence term (vehfc), condensation heating term (con), rain-evaporation cooling term (eva), melting/freezing term (mf), deposition/sublimation term (ds), respectively.

Estimation of water source into the Meiyu front

In order to explore the source of water supplied into the Meiyu front, we employ a ``Colored Moisture Analysis (CMA)'' developed by Yoshimura et al. (2004, JMSJ) for the water circulation around the Meiyu front in 1998. CMA is an analytic method that allows for the estimation of atmospheric moisture advection from specific source regions in a daily time scale. The CMA's water transport model includes atmospheric water balance equations and the well-mixed assumption, and uses external meteorological forcings: that is, precipitation, evaporation, and vertically integrated horizontal moisture fluxes, obtained from the GAME reanalysis. We use 16 regions as water sources that are closely related to the water circulation in the East Asian region (Fig. 11). When water evaporating from any specific regions is tagged as one of 16 kinds of water and is mixed into precipitable water. Thus, the total precipitable water at a specific point is recognized as a sum of 16 kinds of tagged water that shows the source (evaporating) region.

Figure 12 shows a daily time series of the water sources of the total precipitable water at the grid of 33.125N, 115.625E over mainland China. The Meiyu front marched northward from the Yangtze River Valley over this region from June 28 to July 3, 1998. Before passing the Meiyu front (June 23: days of year = 174), this region was located in the northern region far from the Meiyu front: this is dry region. The total precipitable water is about 20 to 40 kg m^{-2}, and water sources are almost found in the Central Eurasian Continent, northeastern China, and northwestern China in this period. After approaching the Meiyu front, water sources dramatically change from those in the earlier period. Instead of the remarkable decrease of the contribution of the northern part of the Eurasian Continent to the total precipitable water, water from the upstream regions of the Asian summer monsoon; that is, the southern Indian Ocean, the northern Indian Ocean, the Indochina Peninsula, and the South China Sea, increases over this region. For example, the sum of the contribution amount (rate) of the upstream regions of the Asian summer monsoon is 25.54 kg m-2 (41.99 %) on July 14, 1998. The contribution amount (rate) of the southeastern region of China to the total precipitable water over this region in this period is about 10 kg m-2 (over 15 %). Therefore, the land surface over southeastern China can be regarded as a major source of water into the Meiyu front.


Fig. 11: Map of 16 sections as the sources of water in this calculation. The names of these 16 sections are presented in the notation.


Fig. 12: Daily time series of water sources of the total precipitable water at the grid of 33.125N, 115.625E over mainland China. Every shades show the contribution of each source of water and corresponds to the 16 sections shown in Fig. 11.