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Study on cloud microphysics

@We have been investigating the inner structures and precipitation mechanisms for various types of clouds using hydrometeor videosonde (HYVIS), instrumented aircraft and Doppler radar, etc.

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Snow clouds over the Sea of Japan

@Shallow snow bands were examined over the Sea of Japan with an instrumented aircraft and dual-Doppler radars. During the aircraft observation period, two types of snow bands occurred. One is the snow bands with their orientation deviating from the mean wind direction by 30 - 40 degrees to the right (primary mode). The other type of snow bands oriented parallel to the mean wind direction (secondary mode). Both types of snow bands consisted of convective clouds with top height of 2.5 km and horizontal dimension of 5-10 km, and loosely organized in a line.

@Aircraft observations were made in the vertical cross sections that passed the center of convective cells and were perpendicular to the band orientation simultaneously with dual-Doppler radar observation. For the snow bands of primary mode, structure of system relative wind in the vertical cross section was symmetrical except for the air flow from the southwest at lower levels. By comparison, for the secondary mode, the air flow structure was symmetrical at all levels.

@For both types of snow bands, maximum updraft velocity in convective cells was 3-4 m/s and existed at around the 2 km level. Above this level, updraft velocity weakened and horizontal divergent flow became prominent.

@In updraft cores, total number concentrations of ice crystals and precipitation particles were low. But many large graupel particles were found. This is consistent with the results of dual-Doppler radar observations that updraft cores were almost collocated with the high reflectivity regions. In updraft core near cloud base levels, many graupel particles with their fall velocities smaller than updraft velocities were found. This fact suggests that recirculation of precipitation particles played an important role in producing large graupel particles (5-6 mm) in shallow convective snow clouds.

VW component of system relative wind (a), cloud water content (b), ice crystal concentration (c), and precipitation particle concentration (d) in the vertical cross section perpendicular to the band orientation.
2D-P images in the center and on both sides of the updraft core at three different levels.

Snow clouds Associated with Japan Sea Polar Air Mass Convergence Zone (JPCZ)

@The organized snow band associated with the JPCZ, which extended southeastward from the root of the Korean Peninsula to the San-in and Hokuriku districts over the Japan Sea on 14 January, 2001, was investigated mainly with an instrumented aircraft (Gulfstream-II). The conceptual model of the snow bands associated with the JPCZ, which is derived on a basis of in-situ measurements, cloud radar observations, and dropsonde sounding from Citation-V and radiosonde sounding from observation vessels and aerological stations, is shown in below figures.

@The JPCZ formed between WNW flow with warmer temperatures and NNW flow with colder temperatures. Such a confluent airflow structure was prominent below the height of 1 km. On the southwestern edge of the JPCZ, warmer airstream from WNW induced intense updraft and formed relatively deep (~4 km) convective clouds. Longitudinal mode cloud streets with top heights of ~3 km were observed to the SW side of the JPCZ. To the far NE side of the JPCZ, shallow (~2 km) convective clouds were confined to below an intense temperature inversion. A strong SW wind at 2.5~3.0 km blew parts of cloud mass off the deep convective clouds and formed transverse mode, anvil-like clouds over the shallow convective clouds.

@The maximum cloud water content in the convective clouds was ~1 gm-3 and most of precipitation particles were graupel and/or densely rimed snow particles, suggesting that the main mechanisms for precipitation growth was the accretion of supercooled cloud droplets by snow particles.

G-II flight track of southwest bound leg at 5.6 km, 1-min averaged wind barbs, CAPPI at 2 km level of radar reflectivity at 1445, vertical cross-section of reflectivity measured with the w-band cloud radar, and photos of clouds taken from G-II at positions 1, 2 and 3.

Meso-Low over the Sea of Japan

@The organized cloud system associated with the polar low, which formed between Noto Peninsula and Sado Island over the Japan Sea, on 27 January 2001, were investigated mainly with an instrumented aircraft (Gulfstream-?).

@Mesoscale structures of the polar low, which is derived on a basis of in-site measurement, cloud radar observation and dropsonde sounding from G-II as well as ground-based radar observations, rawinsonde sounding from additional aerological stations and observation ships and Citation-V's dropsonde sounding, are shown below.

@The cloud system had the horizontal scale of 100`200 km and vertical scale of 3`4 km. Characteristic wind fields were detected below the height of 1.5 km. Remarkable cyclonic wind patterns were found at the lowest level (0.3 km). The warm core was most remarkable at 850 hPa level and ƒÆe in the warm core was higher by 2`3 degrees than its surroundings. Neither the cyclonic circulation nor the warm core were found at 700 hPa level.

@Most of excess vapor produced by the polar low circulation was consumed through the deposition of ice crystals seeded from upper clouds associated with the synoptic-scale low passed by along the Pacific coast of the Japan Islands. As a result, cloud droplet regions were spatially and temporally limited and their water contents were 0.1`0.2 gm-3 at most. The primary growth mechanism of precipitation particles was the depositional growth of ice and snow particles and accretional growth of snow particles was the secondary mechanism in this cloud system.

Horizontal distribution of equivalent potential temperature and horizontal winds at 0.3, 1.5 and 3.5 km level.

Orographic Snow Clouds

@The thermodynamic, kinematic and microphysical structures of orographic snow clouds were investigated over the Echigo Mountains with peak height of 2 km, in central Japan with an instrumented aircraft (B200), hydrometeor videosonde, microwave radiometer, X-band Doppler radar, Ka-band Doppler for the last six years.

@Most of snow clouds had their top heights of 3 to 4 km and were confined to below a strong temperature inversion. Airflow structures are rather smooth with maximum updraft of 1-2 m/s over the windward slope while they are rough with strong downdraft and updraft of 5 m/s or more over the leeward slope. Horizontal winds are accelerated and veered clockwise passing over the crest line.

@With larger Froude numbers, gentle updraft and supercooled cloud water regions exist over rather narrow region of windward slope and crest line. However, they extend windward up to 20 km from crest line with decreasing Froude number.

@Ice and snow particle concentrations gradually increase over the windward slope, suggesting that ice nucleation occurs there due to a gradual decrease of cloud top temperature. Higher concentrations are leeward slope. Ice crystals initiated over the windward slope grew rapidly by vapor deposition and accretion during their descent through supercooled cloud droplet regions. An increase in spillover ratio with large Froude numbers is caused by high wind speeds which advect precipitation particles far downwind and by supercooled cloud water region closer to the crest line where precipitation particles falling down to leeward slope are allowed to spend much more time and grow farther.

Flight track of an instrumented aircraft and vertical velocity measured on flight track (upper left) and vertical cross section of vertical velocity (upper right), equivalent potential temperature (lower left) and cloud water content (lower right).

Convective clouds associated with Baiu front

@Precipitation bands associated with Baiu front were investigated with an instrumented aircraft (Gulfstream-II) on 22 June 2002. In-situ measurements of airflow, thermodynamic and microphysical fields in/around the cloud system were made as well as measurements of reflectivity and Doppler velocity with a w-band cloud radar and GPS dropsonde sounding.

@Baiu front formed between warm and humid westerly flow with CAPE values of 2000 to 3000 and cool northerly flow with CAPE values of nearly 0. Such a confluent airflow structure was prominent below the height of 1.5 km. In the center of Baiu frontal zone, the warm and humid airflow from west-southwest induced intense updraft and formed deep (~14 km) convective clouds with top temperature of ~-60C. The strong updraft blew a large amount of hydrometers upward to cloud top. Ice crystal concentration and ice water content near the top were 1000L-1 and 0.3 gm-3, respectively. The northwesterly blew off these ice crystals downwind and formed anvil clouds. At middle layers above freezing level, snow particles with ice water contents of 0.5 gm-3 and supercooled cloud water of 0.3 gm-3 coexisted in convective regions. Air on the both sides, especially northern side, of convective regions was dry, and confined clouds below 6 km. Below freezing level (5km), cloud water contents were mostly less than 0.5 gm-3 except for high cloud water contents of ~1 gm-3 in strong updraft regions. Above freezing level, the precipitation formation mechanisms were primarily depositional and econdarily accretional growth of snow particles. Below freezing level, rain drops produced by melting of snow particles grew through the collection of cloud droplets.

Infrared satellite imagery and flight track (box ). Arrows indicate release points of GPS dropsondes
Horizontal distributions of u,v and w components of wind obtained from the instrumented aircraft at 12.6, 7.7, 3.5, 0.5 km along 130E
Horizontal distributions of air temperature, dewpoint temperature and equivalent potential temperature at 12.6, 7.7, 3.5, 0.5 km along 130E.
Horizontal distributions of cloud water content(clwc2), vertical velocity(hw_corr/10), 2DC concentration(concic_log) and 2DP concentration(concip_log) at 12.6, 7.7, 3.5, 0.5 km along 130E.

Stratiform Precipitating Clouds Associated with Warm Front

@A thin upglide cloud (400 km north of the surface warm front) and the deeper upglide cloud (200 km north of the warm front) associated with the same warm frontal system were investigated. The microphysical structure and precipitation mechanisms associated with these clouds were studied using data collected with a hydrometeor videosonde (HYVIS) and a Doppler radar as well as routine rawinsonde and surface measurements operated by Japan Meteorological Agency.

@The HYVIS observations showed the following microphysical structures which were common to both the clouds. Columnar type crystals predominated throughout the whole cloud layer, and some capped columns and hexagonal plates were also observed below the -17?C level. These crystals were hardly rimed nor aggregated. Only in the melting layer of the deep upglide cloud, wet aggregates of these crystals were found. No supercooled cloud droplets were found in the clouds as expected from the lack of rime on the crystals. However, a low concentration of drizzle drops coexisted with snow crystals in the shallow layer (2 km deep) just above the 0?C level. These results demonstrate that the dominant mechanism of precipitation formation in these clouds was the depositional growth of snow crystals above the warm-frontal surfaces.

@In the thinner cloud, snow crystals rapidly evaporated in a dry layer (R.H.~30%) just below the warm-frontal surface before they melted.

@In the deep upglide cloud, the increase of precipitation rate due to the collection of cloud droplets was less than 10 % of precipitation rate at the surface. Between the 0?C and -10?C levels, low concentrations of drizzle drops were observed and the air was almost water-saturated. In this region, snow crystals rapidly grew by vapor deposition and formed 80 % of the total precipitation mass reaching the ground. Such a humid condition was produced by strong updrafts (with the maximum value of ~30 cm/s) associated with strong southerly flow over the frontal surface.

Schematic illustration of microphysical and dynamical structure associated with warm-frontal clouds, based on the HYVIS observations of 0530 and 1044, single Doppler radar data and routine rawinsonde observations.

Stratocumuli and Shallow Cumuli in Warm Season

@Shallow warm clouds with top heights of 2 ~ 3 km were investigated with an instrumented aircraft (B200T) and several ground-based remote sensors in western Japan. Shallow warm clouds showed a wide range of ability for producing drizzle and rain drops.

@Even in southerly wind (supposed to be in maritime air mass), typical number concentrations of cloud droplets are 400 ~ 600 droplets/cc and not purely maritime, but polluted to some extent.

@Typical microphysical structures of clouds that hardly precipitate were 1) cloud droplet number concentrations gradually decrease with height from 600 to 400 droplets/cc, 2) droplet sizes are confined to smaller than 30 microns. On the other hand, microphysical structures of clouds that efficiently produced precipitation were 1) cloud droplet concentrations are rather high 500 ~ 600 droplets/cc at lower levels, but very low (several tens droplets/cc) at upper and middle levels. Drizzles initiated through collision-coalescence among large droplets grew by accretion of small cloud droplets in lower levels of clouds and produced radar-detectable precipitation.

@Back trajectory analysis showed that the air at lower levels of clouds came from over the sea near Japan Islands whereas the air at middle and upper levels came from over the sea far away from Japan Islands. The very low concentrations of CCN at middle and upper levels may be also attributed to long life of the cloud system.

Vertical profiles of cloud droplets (FSSP; red), drizzle drops (CIP; green), rain drops (2DC; blue) and cloud water contents (Black line; KLWC-5) for the June 4 (left) and June 22 (right) cases

Stratocumuli and Shallow Cumuli in Cold Season

@Under construction...

Cirrus Clouds

@This study reports on the concentration of ice crystals measured in midlatitude cirrus clouds by a balloonborne hydrometeor videosonde (HYVIS), which has the advantage of more reliably measuring small ice crystals in the size range of 10-100 ƒÊm. The cirrus clouds were generally associated with warm or stationary fronts of synoptic-scale lows. The microphysical data set consisted of 37 launches from Tsukuba, Japan. Based on the comparison with concurrent data by other airborne instruments in the laboratory, the ice crystal concentrations can be measured by the HYVIS with uncertainty of a factor 2 to 3, although significant uncertainties are still included in the size range of 10-30 ƒÊm. The reliability of the measured concentrations is supported by the observed size spectra of the data set in this study and the simulated total concentrations of ice particles with a parcel model.

@Vertical profiles of cirrus cloud ice size distributions were obtained for clouds with top temperatures ranging from -33‹ to -72‹C, and base temperatures from -3‹ to -49‹C. Ice crystal concentrations varied approximately from the order of 10-1 to 102 L-1. Median ice crystal concentrations were typically several tens per liter regardless of temperature or their vertical location. While the concentrations were sometimes highest near cloud top, some clouds had their maximum concentration near the cloud base. As ice particles near the cloud base were usually in sublimation zones, it is suggested that crystal breakup through the sublimation process enhanced the concentrations in some cases. There was a large difference between the measured concentrations and the simulated ones in earlier modeling studies of cirrus cloud formation that treated ice crystal generation process through homogeneous ice nucleation, although the measured ones are probably affected by other physical processes such as secondary ice formation and gravitational sedimentation and turbulent mixing of ice particles after the initial cloud formation. Furthermore, a strong temperature-dependency expected from heterogeneous ice nucleation formulas at relatively warm temperatures (> -25‹C) was not found over all temperature ranges. In comparison with recent modeling studies involving heterogeneous ice nucleation at temperatures below -40‹C, some implications for ice nucleation mechanisms in cirrus clouds are briefly discussed.

Examples of HYVIS images taken with a close-up camera (top) and a microscopic camera (bottom).
Vertical distributions of ice crystal concentration observed in this study as a function of normalized height (NormH), sorted by cloud top temperature; an asterisk indicates each measurement of the HYVIS. In each panel, the dotted line, thin dashed line, and solid line indicate 10th, 50th, and 90th percentiles of concentrations of particles larger than 10 ƒÊm, respectively, whereas the thick dashed line indicates the 50th percentile of concentrations of particles larger than 30 ƒÊm.