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Clouds and Precipitation in Subtropical Highs and Overshooting Convection in Tropics and Subtropics

Author: FengSha
Tutor: FuYunFei; Qingnong Xiao
School: University of Science and Technology of China
Course: Space Physics
Keywords: subtropics subtropical high(s) precipitation cloud overshootingconvection tropopause layer climatological characteristics trend Super Typhoon Jangmi ISCCP GPCP COSMIC TRMM WRF-ARW WRF3DVAR
CLC: P433
Type: PhD thesis
Year: 2012
Downloads: 88
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Abstract


Subtropical highs is one of the most important weather and climate system. The research about the relationship of its activity and the peripheral cloud and precipitation is increasing in recent decades. However, there are still gaps in the climatological characteristics of cloud and precipitation in the subtropical highs due to the limited coverage of observations.With the increasing application of meteorological satellites, overshooting convection has been observed and realized. What is more, the knowledge of the structure of overshooting convection is greatly benefited from the application of active microwave radar aboard satellites. Nevertheless, its detailed structure is still unknown for the limited swath of radar detection. Furthermore, the definition of overshooting convection is on the basis of where the tropopause is, which is still undefined well.Using multiple observation, reanalysis, and model datasets, the climatologic characteristics of the cloud and precipitation in the subtropical highs and overshooting convection in the tropics and subtropics were fully analyzed. As a case study, Super Typhoon Jangmi was simulated to present the detailed structure of overshooting convection at synoptic scale. Meanwhile, the climatological characteristics and trend in the tropopause layer were discussed, and a new definition of tropopause layer was presented to locate the overshooting convection. The preliminary results are presented as follows.(1) Climatological characteristics of precipitation in subtropical highsThe results indicate that shallow precipitation occurs in subtropical highs in all seasons, which is inconsistent with the common knowledge. In summer there is40-80%of the precipitation frequency in subtropical highs against less than40%in other seasons. The mean rain rate in subtropical highs is about1-2mm d-1in winter and4mm d"1in summer, which contributes about30-90%of the precipitation amount to the local precipitation. While in summer such contribution is about50-90%basically contrary to less than40%in other seasons.Comparison of the conditional probability (intensity) of precipitation under subtropical high conditions with that under non-subtropical high conditions suggests that the presence of subtropical high has a limited impact on local precipitation, which mainly weakens the intensity of precipitation. In the west Pacific subtropical high and the north Atlantic subtropical high, precipitation was only30%lower under subtropical high than under non-subtropical high, but the precipitation intensity under subtropical high was50%less than that under non-subtropical high.(2) Climatological characteristics of cloud in subtropical highsThe results reveal that the low and thin cloud occupies subtropical highs. The average amount of total clouds exceeds30with abundant low clouds which contribute over60%within the Pacific subtropical high and over40%within the Atlantic subtropical high, impacted by a circulation pattern around150°-180°E and850hPa suppressing both of the upward development of the cloud tops and the water vapor divergences near the surface.Furthermore, clouds present great geographical incoherence within subtropical highs. In the east, the amount of middle and low clouds increases to peak in the early morning and decreases to a trough in the afternoon, while the amount of high clouds remains stable throughout the day. Conversely, in the west, the diurnal amplitude of low and middle clouds is less than three, while high clouds dramatically reach the maximum in the afternoon and drop to the minimum in the evening. Among the nine cloud categories, stratocumulus clouds with greater optical thickness account for the most in subtropical highs, no matter their occurrence or amount, causing more shortwave cloud radiative forcing to cool the local atmosphere and surface as a consequence.(3) Climatological characteristics of tropopause layer in subtropicsThe results exhibit that the percentage of multiple tropopause in subtropical region is42.46%. The tropopause layer has been thickening in recent decades for the entire globe, especially in the northern subtropics and northern poles with marked positive linear trends of0.28km/decade and0.39km/decade, respectively. Accompanied by overall cooling in the global tropopause layer, remarkable rising trends are observed in the top of it for all of the five latitude bands, but not for all regions in the bottom, which suggests that the tropopause layer temperature primarily couple with the height of the top of the tropopause layer.Comparison of the tropopause over the Tibetan Plateau to that over the same latitudinal plain area show that as an outstanding heat source in boreal summer, the Tibetan Plateau thermally pushes the lapse rate tropopause upwards by-2km compared with the plain, while the lapse rate tropopause drops below that over the plain in boreal winter. As the "roof of the world," the elevated topography of the Tibetan Plateau dynamically lifts the cold point tropopause to a higher altitude without significantly seasonal variations. Along the latitude of32.5°N, the cold point tropopause is located at-18km over the main body of the Tibetan Plateau and drops to~15km over the plain.(4) Characteristics of overshooting convection in subtropical regionsResults indicate that the overshooting convective precipitation is mainly distributed over Intertropical Convergence Zone, South Pacific Convergence Zone, Asia Monsoon Region, southward of20°N in Africa and America. Its distribution shows remarkable regionality and seasonal variety. Furthermore, the overshooting convective precipitation more frequently occurs over land than over ocean, although its frequency is below2%. The conditional rain rate of overshooting convection is about10mm h-1, which is much stronger than local precipitation. Over land, the conditional rain rate of overshooting convection is with a relative narrow range of5-20mm h-1, compared to that over ocean.Using WRF3-dimensional variational system that assimilated the precipitable water vapor from AMSR-E and wind fields from QuikSCAT, Super Typhoon Jangmi was simulated to explore the overshooting convection for typhoon systems. During the entire period of simulation, there were overshooting convections in the whole system. Meanwhile, there were also updrafts above the cloud top, which might bring water vapor and chemical traces to penetrate into the tropopause layer. The ice and water vapor amount reach606t s-1and67t s-1at the first lapse rate tropopause level, respectively, where their variations are basically opposite.

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