大气热源对高原低涡不同发展阶段的影响——2013年7月个例分析

高原低涡是夏季青藏高原(简称高原)及其下游地区的主要降水系统。本文利用ERA5逐小时再分析资料、FY-2E卫星云顶亮温逐小时数据和TRMM 3 h降水资料,对2013年7月19~21日活动于高原的一次低涡过程进行了诊断分析。此低涡在高原期间的活动时间长达56 h,将其分为初生、发展及移出高原前三个阶段,着重分析了高原大气热源在低涡不同阶段的关键作用和机理。结果表明:此低涡在发展过程中表现为阶段性增强的特征,位势涡度倾向方程诊断发现非绝热加热的垂直梯度是造成低涡发展增强的主要因素,即非绝热加热极值所在高度的下方和上方分别有正的和负的位涡制造,从而加强了低层的气旋和高层的反气旋。进一步分析可知大气热源在低涡发展过程中也表现出阶段性增强的特征,最大值出现在正午时段,且在低涡移出高原前阶段最强。低涡的生成与地面暖中心有关,这归因于地表感热加热的作用;而低涡的后续发展则主要依赖于凝结潜热加热,加热高度位于对流层中层,这主要是由垂直运动将低层的水汽集中到中层,产生水汽凝结所致。

A Synthetic Study of the Position Difference of the Southern Branch Trough of the Qinghai-Tibet Plateau Based on Objective Identification

The southern branch trough (SBT) mainly appears in the winter half year (November to May of the following year), using the 4 times daily NCEP/NCAR re-analysis data nearly 41 years (1979-2019) to analyze the differences of the SBT distribution of spatial location, frequency in winter and spring, then selects the “eastern type” and “western type” of the 10 most typical SBT, using simplified vertical vorticity tendency equation, using simplified vertical vorticity tendency equation to diagnosis of the SBT in power, heat, water vapor and wave energy in different positions. The results show that: 1) The location of the SBT is more eastward in winter, and more westward in spring. 2) The diagnosis results of the vorticity equation show that the vorticity of the southern branch of the “western type” is mainly contributed by advection term;the vorticity of the “eastern type” south branch is mainly contributed by the non-adiabatic heating term. 3) The SBT of the “eastern type” has more obvious vorticity advection than the southern branch of the “western type”, and the dynamic action is stronger. The “western type” SBT has stronger Q1, specific humidity advection and water vapor flux than the “eastern type” SBT, which is greatly affected by thermal action and water vapor. When the “eastern type” and “western type” SBT occur, the T-N wave activity flux appears obvious abnormal energy fluctuation propagation.

Impacts of the Diurnal Cycle of Solar Radiation on Spiral Rainbands

Based on idealized numerical simulations, the impacts of the diurnal cycle of solar radiation on the diurnal variation of outer rainbands in a tropical cyclone are examined. It is found that cold pools associated with precipitation-driven downdrafts are essential for the growth and propagation of spiral rainbands. The downdrafts result in surface outflows, which act as a lifting mechanism to trigger the convection cell along the leading edge of the cold pools. The diurnal cycle of solar radiation may modulate the diurnal behavior of the spiral rainbands. In the daytime, shortwave radiation will suppress the outer convection and thus weaken the cold pools. Meanwhile, the limited cold pool activity leads to a strong modification of the moisture field, which in turn inhibits further convection development.

Quantifying the Contribution of Track Changes to Interannual Variations of North Atlantic Intense Hurricanes

Previous studies have linked interannual variability of tropical cyclone(TC)intensity in the North Atlantic basin(NA)to Sahelian rainfall,vertical shear of the environmental flow,and relative sea surface temperature(SST).In this study,the contribution of TC track changes to the interannual variations of intense hurricane activity in the North Atlantic basin is evaluated through numerical experiments.It is found that that observed interannual variations of the frequency of intense hurricanes during the period 1958–2017 are dynamically consistent with changes in the large-scale ocean/atmosphere environment.Track changes can account for~50%of the interannual variability of intense hurricanes,while no significant difference is found for individual environmental parameters between active and inactive years.The only significant difference between active and inactive years is in the duration of TC intensification in the region east of 60°W.The duration increase is not due to the slow-down of TC translation.In active years,a southeastward shift of the formation location in the region east of 60°W causes TCs to take a westward prevailing track,which allows TCs to have a longer opportunity for intensification.On the other hand,most TCs in inactive years take a recurving track,leading to a shorter duration of intensification.This study suggests that the influence of track changes should be considered to understand the basin-wide intensity changes in the North Atlantic basin on the interannual time scale.

The Error Analysis for the Remote Sensing of Water Vapor Data by Ground Based GPS in Tengchong, Yunnan Province

Due to its special observation principle, GPS remote sensing atmospheric precipitation has the advantages of high time resolution and no weather conditions, and has been widely used in the research field of atmospheric precipitation. Using ground-based GPS precipitate water vapor data (GPS-PWV) and radiosonde-precipitate water vapor data (RS-PWV) that integrated by Radiosonde data, the error between GPS-PWV and RS-PWV in Tengchong is analyzed on its distribution of wet and dry seasons, also the difference between 00:00 UTC and 12:00 UTC. Results show that the RMSE of GPS-PWV and RS-PWV on both 00:00 UTC and 12:00 UTC are less than 5 mm, they correspond with each other well and their correlation coefficient is above 0.95, additionally, GPS-PWV value is stable than RS-PWV value. On the whole, the value of GPS-PWV is slightly larger than RS-PWV. And the mean absolute error between them has higher values, 4.5 mm in 2011 and 4.7 mm in 2012 from May to October (local rainy season) and lower values, 2.8 mm in 2011 and 3.1 mm in 2012 in November to April (local dry season). Besides, the mean absolute error in the morning seems has a difference with its component in the evening. Specifically, it is bigger on 12:00 UTC than on 00:00 UTC and the mean absolute errors on 12:00 UTC of two years are 27% and 11% larger than errors on 00:00 UTC respectively. The correlation of mean absolute error and surface vapor pressure, surface air temperature is examined in this study as well. We achieved that the correlation coefficient between mean absolute error and surface vapor pressure, surface air temperature equals 0.32, 0.37 separately. Diverse characters of mean absolute error under different precipitation conditions are also discussed. The outcome is that the mean absolute error has a higher value on rainy days and a lower value on clear days. However, during the precipitation periods, it appears that the mean absolute error and the rainfall situation don’t agree with each other well, it is likely to change randomly.