Medium Range Weather Forecast (ECMWF) analyses. The focus is on three lows that developed on the monsoon shear line, two of which eventually became tropical cyclones. One of the disturbances intensified as it made a loop over land. Interpretations of the formation are given in terms of vorticity dynamics. The roll up of low-level absolute vorticity associated with the shear line is a prominent feature of the early development of the lows, a mechanism akin to that believed to operate in the formation of hurricanes from Atlantic easterly waves. A notable feature of the two lows that eventually became tropical cyclones was the occurrence of persistent bouts of deep convection near the circulation centre. The development of the low that didn't achieve tropical cyclone status was apparently thwarted by strong vertical shear and drying of the middle troposphere.

An analysis of numerical simulations of tropical low intensification over land is presented. The simulations are carried out using the MM5 mesoscale model with initial and boundary conditions provided by ECMWF analyses. Seven simulations are discussed: a control simulation, five sensitivity simulations in which the initial moisture availability is varied, and one simulation in which the coupling between moisture availability is suppressed. Changing the initial moisture availability adds a stochastic element to the development of deep convection. The results are interpreted in terms of the classical axisymmetric paradigm for tropical cyclone intensification with recent modifications. Spin‐up over land is favoured by the development of deep convection near the centre of the low circulation. As for tropical cyclones over sea, this convection leads to an overturning circulation that draws absolute angular momentum surfaces inwards in the lower troposphere leading to spin‐up of the tangential winds above the boundary layer. The intensification takes place within a moist monsoonal environment, which appears to be sufficient to support sporadic deep convection. A moisture budget for two mesoscale columns of air encompassing the storm shows that the horizontal import of moisture is roughly equal to the moisture lost by precipitation. Overall, surface moisture fluxes make a small quantitative contribution to the budget although, near the circulation centre, these fluxes appear to play an important role in generating local conditional instability. Suppressing the effect of rainfall on the moisture availability has little effect on the evolution of the low, presumably because, at any one time, deep convection is not sufficiently widespread.

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Abstract. Doppler wind lidar (DWL) has been shown to obtain fairly accurate wind speeds in normal wind conditions. However, the evaluation of DWL winds under typhoon conditions is less common. This study evaluated the accuracy of wind data measured by two types of DWLs (WindPrint S4000 and WindCube V2), and investigated the impact of factors (e.g., precipitation and humidity) on the DWL-observed wind speed and direction. Data were collected from joint observations in Baoshan, Zhoushan and Taizhou (China) by the Shanghai Typhoon Institute during the passage of Super Typhoon Lekima in 2019. The DWL observations were compared with measured data from balloon-borne radiosonde released at the same location. The results showed that the 1-min average wind speed and direction of WindPrint S4000 were more consistent with the instantaneous observation data of the sounding balloon than those of WindCube V2. The applicability of DWL was poor when the precipitation intensity was larger than 50 mm · h−1. The DWL wind speed bias significantly increased when the relative humidity exceeded 85 %. When the drift distance of the sounding balloon (ldrift) was less than 1 km, the DWL wind speed bias decreased with an increase of ldrift, whereas it increased with an increase of ldrift when the drift exceeded 1.5 km. Within a radius of 700 km, the root mean square of wind speeds between DWL and sounding balloon measurements showed a trend of increasing as the distance from the typhoon center decreased.

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