| Cirrus clouds play an important role in the balance of ground-atmosphere radiation and the regulation of water vapor distribution in the upper troposphere.The relative humidity over ice controls the formation and further evolution of cirrus clouds.A large number of observations show that the distribution range of relative humidity in cirrus clouds is very wide(50% to 150%),and neither the field observation data nor the classical theory of cloud microphysics can better explain this phenomenon.In this study,under the assumption of adiabatic closure,based on the observation data and the cloud parcel model,ensemble simulations were carried out to explore the possible physical mechanism of the wide range disturbance of relative humidity in cirrus and deep convective cloud anvil caused by fluctuations.The data used in this study are from the in situ observation data in the MACPEX and TC4 experiments.In the process of using the data,the ice crystal data is processed,and the vertical disturbance time series used to drive the parcel model is constructed based on the observation data,physical background knowledge and artificial assumptions,and the ideal test of the parcel model is carried out.The test results show that the vertical velocity affects the relative humidity by affecting the temperature on the one hand,and the relative humidity by affecting the nucleation of ice crystals and the concentration of ice crystals generated by nucleation on the other hand.The influence of ice crystal deposition/sublimation process on relative humidity is mainly controlled by the average radius of ice crystal,number concentration and deposition coefficient.Further carry out collective simulation tests to study the contribution of vertical disturbance and ice crystal deposition/sublimation process to relative humidity disturbance under adiabatic closure conditions.The simulation results show that the standard test nearly reproduces the statistical characteristics of various physical quantities in cirrus clouds caused by the mid-latitude fluctuation of MACPEX,indicating that the vertical disturbance and the ice crystal deposition/sublimation process are the two main physical mechanisms to control the relative humidity disturbance,which can well explain the wide range disturbance of relative humidity in cirrus clouds caused by the fluctuation.Among them,the vertical disturbance is an external force that changes the relative humidity.The amplitude and frequency of the vertical disturbance determine the change of the relative humidity during the evolution of cirrus clouds.The ice crystal deposition/sublimation process plays an important role in suppressing the disturbance of relative humidity.The inhibition effect increases with the increase of deposition coefficient.In addition,the reasonable value of deposition coefficient can be deduced by comparing the simulation results with the observed statistical characteristics.On the basis of the existing conclusions,a set of simulation tests were carried out to analyze the effects of the vertical rising speed and the nucleation scheme on the relative humidity distribution in the deep convective cloud anvil.The simulation results show that the standard test can simulate the characteristics that the occurrence probability of relative humidity is greater than 100%,but the distribution range and the simulation of the peak value still have deviations.The vertical rising speed is the main mechanism to increase the probability of the occurrence of relative humidity when it is supersaturation.Without considering the homogeneous nucleation process,the distribution of relative humidity can be more dispersed.The vertical rising speed and nucleation process can also affect the distribution of relative humidity to a certain extent,but there are still other mechanisms that have not been found. |
