NONHYDROSTATIC NUMERICAL SIMULATION STUDY ON THE DEVELOPING STRUCTURE AND MECHANISM OF THE
HEAVY RAINFALL MESOSCALE SYSTEM
Feng Wuhu
Department of Atmospheric Science, Lanzhou University, Lanzhou, 730000, P.R. of China
In this dissertation, the detailed investigation is conducted of the developing structure and mechanism of the heavy rainfall mesoscale system occurred in China by using observational analysis, numerical simulation and thermodynamic diagnosis. Mainly concerned four aspects of the following study:
1.The study on the developing structure and mechanism of the meso-a scale heavy rainfall system. Here an extraordinary heavy rainfall event("96.8") occurred in Henan, Hebei and Shanxi Provinces in China, resulting in severe flood catastrophe is chosen. The event is influenced by Typhoon, which landed in August 1,1996 and diminished into low pressure. This is one of typical meso-a scale system. Observational analyses indicate that the stable gross col field and the interaction between a northward moving typhoon-low pressure and its east lateral pacific subtropical high were the large-scale and mesoscale circulation conditions of the occurred "96.8" heavy rainfall; The mesoscale typhoon-low pressure and its specific dynamical and thermodynamical structure was directly related to this heavy rainfall event. We used the nonhydrostatic version of mesoscale numerical model MM5 to conduct investigation of numerical simulation for this case. Based on analysis of the simulated results, we found that the full physics process simulation of nonhydrostatic version MM5 was basically possessed of a capability to reproduce for the genesis, development and evolution of the large-scale and meso-a scale weather systems. The simulative results using a two-way interactive nesting procedure revealed that the typhoon-low pressure possessed an intensive coupled mechanism between the dynamical and thermodynamical fields, namely, the developing typhoon-low was possessed of a structure of the column of cyclonic vorticity with warm center and high humidity, within the lower levels of the vorticity column were associated with the moist convective instability and negative moist potential vorticity structure; the intensive ascending motion and the intense convergence (divergence) on the lower (upper) levels as well as the development of the convective cloud cluster were intercoupling; the intense south wind jet stream below 500 hPa accompanied by the typhoon-low was not only the interaccompany and intercoupling condition of the development and maintenance of the typhoon-low and convective cloud cluster, but also was the moisture source and heat energy transport belt of the "96.8" extraordinary heavy rainfall. The simulative results of precipitation show that the rain-band and rainfall distribution of precipitation with that of the observation were basically consistent in spite of some rainfall centers less or larger than that of observation for coarse or fine mesh, respectively. It is worth while that the heavy rain-band was in close relationship with the intensive ascending motion and the typographic forcing.
2. The study on the developing structure and mechanism of the meso-b scale heavy rainfall system. Here an abrupt extraordinary heavy rainfall event("98.7") occurred in the eastern and southwest parts of Hubei Province, which is seldom in Changjiang River basin is chosen. The event is induced by meso-b scale low vortex, which is one of typical system during Meiyu front. Precipitation analysis indicates that abrupt and insistence heavy rainfall is the main reason for the two-precipitation center between Wuhan and Huangshi. Analyses indicated this event was in close relationship with the short wave trough on 500hPa and low vortex with shear line on 700hPa as well as with MbCS of successive generating and intensive developing along the shear line. This event was conducted numerical simulation of full physics using nonhydrostatic version MM5(V2.12) with two-way interactive and double nested grid. The simulated results using two-way interactive fine grid revealed that the extraordinary heavy rainfall in Wuhan periphery was in direct relationship with the genesis and intensive development of a meso-b scale low vortex on 700hPa; this low vortex was possessed of obvious dynamic-thermodynamic structural feature: the extreme intensive ascendant motion was intercoupling with saturated air column; the surpassing intensive divergence column and intensive vorticity column were coupling developed; the moist static instability and the moist symmetric instability was coexist; the structure of the microphysic fields of the cluster developing in the deep convective moist air column is typical. The influence of the large-scale process to the mesoscale systems is important for the persistent heavy rainfall. The simulated precipitation is correspondence with high value of 700hPa equivalent temperature and Low Level Jet, the center of heavy rainfall occured in the northward of the center of strong low level jet, and the distribution of the simulated precipitation is almost the same as the low level potential vorticity, which indicates the development of mesoscale system is related with low level high potential vorticity value.
3. The study on the dynamics and thermodynamics of the meso-b scale heavy rainfall system. The mesoscale heat and moisture budgets were diagnosed using the output data of MM5. It was found as follow: apparent heat sources Q1 and apparent moisture sink Q2 were basically correspondent with regions of heavy rain when intensive convective heavy rainfall occurred, that is to say, the high value regions of the total latent heat plus surface fluxes were consitent with the regions of "98.7" heavy rainfall for corresponding time level; Q1 increased with height while the maximum heating level located at 486.1hPa(s=0.54); the deep and thick heating layer in the middle troposphere was the mainly thermodynamic mechanism for the both cumulus convective active and the persistent genesis and development of severe rainstorm; the relative cool layer in the upper troposphere provided a favorable thermodynamic unstable condition for intensive cumulus convection over the heavy rain areas. The condensation latent heating of cumulus convective in the lower and middle troposphere was not only heating the middle troposphere atmosphere, but also heating the upper environment atmosphere through transporting heat to up; the double-peak structure of Q2 at the early stage of heavy rainfall was relevant to drying associated with convective condensation of both stratocumulus in the lower levels and cumulus in the middle levels; the peak value of Q2 in the middle level was basically corresponding with that of Q1; the deep and thick drying layer was consistent with the deep and thick condensation heating layer of Q1.It showed that it was reasonable to diagnose Q1 and Q2 using successfully stimulated output data with high resolution. We can provide reliable physical proof for improving heating and moisturize profiles in the cumulus convective parameterization through diagnosing Q1 and Q2 of the rainstorm event.
The dynamic diagnoses of the development of the mesoscale vortex were also conducted by using the high spatial simulated output data(20km ). The analyses of vorticity indicate that one of the mainly physical mechanism of the persistent development of the low vortex along the shear line was the superposition and coupling of positive vorticity centers over the upper and lower of the regions of Wuhan periphery;Diagnoses of the total vorticity sources revealed that there was an almost vertical volume of high positive vorticity sources value generating and maintaining from low to upper level over Wuhan periphery during the intensive genesis and development of the abrupt heavy rainfall. The vertical structure and evolution of the positive vorticity change-rate centers were basically consistent with that of the positive vorticity centers of the formation and development of the vorticitiy. That is to say, total vorticity sources played important role in the genesis and development of the mesoscale low vortex, which was also a key dynamic mechanic of the persistent of the mesoscale system of the heavy rainfall. In the factors of contribution to the vortictiy change-rate, we can see the divergence term is very important below 650hPa, but from 650hPa to 200hPa, vertical vorticity advection term is bigger than divergence term, the horizontal advection is also positive. The divergence term was negative between 450hPa-250hPa,but the twisting term was negative through all the troposphere. Near the surface level, there was almost no any contribution to the vortictiy change-rate of the factors of vertical vorticity and horizontal advection. The time-mean and perturbation of vorticity change-rate played important roles in the genesis of the low vortex at the beginning stage;During the intensive development of the rainfall, the relative numeric of the nonlinear interactive vorticity change-rate is the most component, following which was the perturbation vorticity change-rate;When the nonlinear interactive vorticity change-rate diminished, the low vortex went eastward and diminished, although the time-mean and perturbation of vorticity change-rate were still positive.
The frontogenetical function, which defined as a rate of change of the horizontal gradient of a physical property with time, is also computed. The results showed the existence of frontgenises nearby the norhtward of Wuhan-Huangshi was important dynamics function for the maintenance and development of Meiyu front. Above the surface, the diabatic term is very crucial for the frontgenises and the deformation and convergence of wind field is important too.
The diagnoses of Moist Potential vorticity(MPV) show that the low level negative MPV was correspondence with the rainfall belt, which indicates the moist symmetric instability was one of the possible mechanism of the convective along the Meiyu front precipitation belt. When the low level pressure was moist static instability, once there was vertical motion induced by moist symmetrics instability, the heavy rain would be easily happened.
4. The study on the four dimension structure of the meso-b scale heavy rainfall system and Quantitative Precipitation Forecast(QPF).The Version of nonhydrostatic numerical model MM5.V3 with three nested domains run in two-way interactive mode of full physics is used to simulate "98.7" heavy rainfall. It shows that the simulation results used by three nested domains is better than two nested domains, especially Domain three can better reveal the four dimension thermodynamics and dynamics structure of the development of meso-b scale system. We form the concept model according to the result. Developing high spatial and temporal simulation technique of multi nested domain will not only be benefit to understand the multi-scale interaction and the four dimension thermodynamics and dynamics structure of the development of meso-b scale system but also improve the Quantitative Precipitation Forecast.
Key words: Nonhydrostatic numerical simulation Dynamics and thermodynamics diagnosis
Structure and mechanism of the Mesoscale heavy rainfall system
Four dimension structure of Meso-b scale system
Quantitative Precipitation Forecast