A Comparison of Convective and Stratiform Precipitation Microphysics of the Record-Breaking Typhoon In-Fa (2021)

In July 2021, Typhoon In-Fa attacked eastern China and broke many records for extreme precipitation over the last century. Such an unrivaled impact results from In-Fa’s slow moving speed and long residence time due to atmospheric circulations. With the supports of 66 networked surface disdrometers over eastern China and collaborative observations from the advanced GPM satellite, we are able to reveal the unique precipitation microphysical properties of the record-breaking Typhoon In-Fa (2021). After separating the typhoon precipitation into convective and stratiform types and comparing the drop size distribution (DSD) properties of Typhoon In-Fa with other typhoons from different climate regimes, it is found that typhoon precipitation shows significant internal differences as well as regional differences in terms of DSD-related parameters, such as mass-weighted mean diameter (Dm), normalized intercept parameter (Nw), radar reflectivity (Z), rain rate (R), and intercept, shape, and slope parameters (N0, µ, Λ). Comparing different rain types inside Typhoon In-Fa, convective rain (Nw ranging from 3.80 to 3.96 mm−1 m−3) shows higher raindrop concentration than stratiform rain (Nw ranging from 3.40 to 3.50 mm−1 m−3) due to more graupels melting into liquid water while falling. Large raindrops occupy most of the region below the melting layer in convective rain due to a dominant coalescence process of small raindrops (featured by larger ZKu, Dm, and smaller N0, µ, Λ), while small raindrops account for a considerable proportion in stratiform rain, reflecting a significant collisional breakup process of large raindrops (featured by smaller ZKu, Dm, and larger N0, µ, Λ). Compared with other typhoons in Hainan and Taiwan, the convective precipitation of Typhoon In-Fa shows a larger (smaller) raindrop concentration than that of Taiwan (Hainan), while smaller raindrop diameter than both Hainan and Taiwan. Moreover, the typhoon convective precipitation measured in In-Fa is more maritime-like than precipitation in Taiwan. Based on a great number of surface disdrometer observational data, the GPM precipitation products were further validated for both rain types, and a series of native quantitative precipitation estimation relations, such as Z–R and R–Dm relations were derived to improve the typhoon rainfall retrieval for both ground-based radar and spaceborne radar.

Global Mesoscale Convective System Latent Heating Characteristics from GPM Retrievals and an MCS Tracking Dataset

The distribution of latent heating released by Mesoscale Convective Systems (MCSs) plays a crucial role in global energy and water cycles. To investigate the characteristics of MCS latent heating, five years (2014-2019) of Global Precipitation Measurement (GPM) Ku-band Precipitation Radar observations and latent heating retrievals are combined with a newly developed global high-resolution (~10 km, hourly) MCS tracking dataset. The results suggest that mid-latitude MCSs are shallower and have a lower maximum precipitation rate than tropical MCSs. However, MCSs occurring in the mid-latitudes have larger precipitation areas and higher stratiform rain volume fraction, in agreement with previous studies. With substantial spatial and seasonal variability, MCS latent heating profiles are top-heavier in the middle and high latitudes than those in the tropics. Larger magnitudes of latent heating in the stratiform regions are found over the ocean than over land, which is the case for both the tropics and mid-latitudes. The larger magnitude is related to a larger precipitating area/volume rather than a higher storm height or more intense convective core typically associated with land systems. A majority of mid-latitude MCSs have a relatively high (> 70%) stratiform fraction while this is not the case for tropical MCSs, suggesting that mid-latitude MCSs tend to produce more stratiform rain while tropical MCSs are more convective. Importantly, the results of this study indicate that storm intensity, latent heating, and rainfall are different metrics of MCSs that can provide multiple constraints to inform development of convection parameterizations in global models.

Statistical Characteristics of Raindrop Size Distribution during Rainy Seasons in Northwest China

Raindrop size distribution (DSD) is of great significance for understanding the microphysical process of rainfall and the quantitative precipitation estimation (QPE). However, in the past, there was a lack of relevant research on Xinjiang in the arid region of northwest China. In this study, the rainy season data collected by the disdrometer in the Yining area of Xinjiang were used for more than two years, and the characteristics of DSDs for all samples, for two rain types (convective and stratiform), and for six different rain rates were studied. The results showed that nearly 70% of the total samples had a rainfall rate of less than 1 mm·h−1, the convective rain was neither continental nor maritime, and there was a clear boundary between convective rain and stratiform rain in terms of the scattergram of the standardized intercept parameter ( log 10 N w ) versus the mass-weighted average diameter ( D m ). When the raindrop diameter was less than 0.7 mm, DSDs of the two rainfalls basically coincided, while when the raindrop diameter was greater than 0.7 mm, DSDs of convective rainfall were located above the stratiform rain. As the rainfall rate increased, D m increased, while log 10 N w first increased and then decreased. In addition, we deduced the Z − R (radar reflectivity-rain rate) relationship and μ − Λ relationship (shape parameter-slope parameter of the gamma DSDs) suitable for the Yining area. These conclusions are conducive to strengthening the understanding of rainfall microphysical processes in arid regions and improving the ability of QPE in arid regions.

Linking rain into ice microphysics across the melting layer in stratiform rain: a closure study

This study investigates the link between rain and ice microphysics across the melting layer in stratiform rain systems using measurements from vertically pointing multi-frequency Doppler radars. A novel methodology to examine the variability of the precipitation rate and the mass-weighted melted diameter (Dm) across the melting region is proposed and applied to a 6 h long case study, observed during the TRIPEx-pol field campaign at the Jülich Observatory for Cloud Evolution Core Facility and covering a gamut of ice microphysical processes. The methodology is based on an optimal estimation (OE) retrieval of particle size distributions (PSDs) and dynamics (turbulence and vertical motions) from observed multi-frequency radar Doppler spectra applied both above and below the melting layer. First, the retrieval is applied in the rain region; based on a one-to-one conversion of raindrops into snowflakes, the retrieved drop size distributions (DSDs) are propagated upward to provide the mass-flux-preserving PSDs of snow. These ice PSDs are used to simulate radar reflectivities above the melting layer for different snow models and they are evaluated for a consistency with the actual radar measurements. Second, the OE snow retrieval where Doppler spectra are simulated based on different snow models, which consistently compute fall speeds and electromagnetic properties, is performed. The results corresponding to the best-matching models are then used to estimate snow fluxes and Dm, which are directly compared to the corresponding rain quantities. For the case study, the total accumulation of rain (2.30 mm) and the melted equivalent accumulation of snow (1.93 mm) show a 19 % difference. The analysis suggests that the mass flux through the melting zone is well preserved except the periods of intense riming where the precipitation rates were higher in rain than in the ice above. This is potentially due to additional condensation within the melting zone in correspondence to high relative humidity and collision and coalescence with the cloud droplets whose occurrence is ubiquitous with riming. It is shown that the mean mass-weighted diameter of ice is strongly related to the characteristic size of the underlying rain except the period of extreme aggregation where breakup of melting snowflakes significantly reduces Dm. The proposed methodology can be applied to long-term observations to advance our knowledge of the processes occurring across the melting region; this can then be used to improve assumptions underpinning spaceborne radar precipitation retrievals.

Distinguishing between Warm and Stratiform Rain Using Polarimetric Radar Measurements

Modeled statistical differential reflectivity–reflectivity (i.e., ZDR–Ze) correspondences for no bright-band warm rain and stratiform bright-band rain are evaluated using measurements from an operational polarimetric weather radar and independent information about rain types from a vertically pointing profiler. It is shown that these relations generally fit observational data satisfactorily. Due to a relative abundance of smaller drops, ZDR values for warm rain are, on average, smaller than those for stratiform rain of the same reflectivity by a factor of about two (in the logarithmic scale). A ZDR–Ze relation, representing a mean of such relations for warm and stratiform rains, can be utilized to distinguish between warm and stratiform rain types using polarimetric radar measurements. When a mean offset of observational ZDR data is accounted for and reflectivities are greater than 16 dBZ, about 70% of stratiform rains and approximately similar amounts of warm rains are classified correctly using the mean ZDR–Ze relation when applied to averaged data. Since rain rate estimators for warm rain are quite different from other common rain types, identifying and treating warm rain as a separate precipitation category can lead to better quantitative precipitation estimations.

Weather Types Affect Rain Microstructure: Implications for Estimating Rain Rate

Quantitative precipitation estimation (QPE) through remote sensing has to take rain microstructure into consideration, because it influences the relationship between radar reflectivity Z and rain intensity R. For this reason, separate equations are used to estimate rain intensity of convective and stratiform rain types. Here, we investigate whether incorporating synoptic scale meteorology could yield further QPE improvements. Depending on large-scale weather types, variability in cloud condensation nuclei and the humidity content may lead to variation in rain microstructure. In a case study for Bavaria, we measured rain microstructure at ten locations with laser-based disdrometers, covering a combined 18,600 h of rain in a period of 36 months. Rain was classified on a temporal scale of one minute into convective and stratiform based on a machine learning model. Large-scale wind direction classes were on a daily scale to represent the synoptic weather types. Significant variations in rain microstructure parameters were evident not only for rain types, but also for wind direction classes. The main contrast was observed between westerly and easterly circulations, with the latter characterized by smaller average size of drops and a higher average concentration. This led to substantial variation in the parameters of the radar rain intensity retrieval equation Z–R. The effect of wind direction on Z–R parameters was more pronounced for stratiform than convective rain types. We conclude that building separate Z–R retrieval equations for regional wind direction classes should improve radar-based QPE, especially for stratiform rain events.

Linking rain into ice microphysics across the melting layer in stratiform rain: a closure study

This study investigates the link between rain and ice microphysics across the melting layer in stratiform rain systems using measurements from vertically pointing multi-frequency Doppler radars. A novel methodology to examine the variability of the precipitation rate and the mass-weighted melted diameter (Dm)across the melting region is proposed and applied to a 6 h-long case study, observed during the TRIPEx-pol field campaign at the Julich Observatory for Cloud Evolution Core Facility and covering a gamut of ice microphysical processes. The methodology is based on an optimal estimation (OE) retrieval of particle size distributions (PSD) and dynamics (turbulence and vertical motions) from observed multi-frequency radar Doppler spectra applied both above and below the melting layer. The retrieval is first applied in the rain region; based on a one-to-one conversion of raindrops into snowflakes, the retrieved Drop Size Distributions (DSD) are propagated upward to provide a first guess for the snow PSDs. These ice PSDs are then used to constrain the OE snow retrieval where Doppler spectra are simulated based on different snow models, which consistently compute fall-speeds and electromagnetic properties. The results corresponding to the best matching models are then used to compute snow fluxes and Dm, which can be directly compared to the corresponding rain quantities. For the case study, the total accumulation of rain (2.65 mm) and the melted equivalent accumulation of snow (2.60 mm) show only a 2 % difference. The analysis suggests that the mass flux through the melting zone is well preserved except the periods of intense aggregation and intense riming where the precipitation rates were respectively larger and lower in ice than in the rain below. Moreover, it is shown that, the mean mass weighted diameter of ice is strongly related to the characteristic size of the underlying rain. With a simple scaling, Dmice = 1.21Dmrain, the characteristic size of snow can be predicted with a root-mean-square-error of 0.12 mm. This formula leads to slight underestimation of the ice size during aggregation, potentially due to the breakup of melting snowflakes, and to overestimation during riming where the additional particle growth within the melting layer cannot be unambiguously attributed to one process. The proposed methodology can be applied to long-term observations to advance our knowledge of the processes occurring across the melting region; this can then be used to improve assumptions underpinning space-borne radar precipitation retrievals.

Characterization of Ka-band Radar Observations for Different Rain Types over Akure, Nigeria

Radar is a unique tool that can measure precipitation parameters over a large aerial coverage. Its application spans over study of climate change and radiowave propagation. Inter-relation between the rain parameters can also be derived with the height of radar especially on vertical profiling or aloft ground level. Hence effect of precipitation parameters can be assessed along the satellite propagation path with the help of space-borne radar. Satellite communication links operating at frequencies above 10 GHz are usually affected by hydrometeors especially rainfall. These effects are expected to be quite severe in the tropical region like Akure due to the nature of precipitation which is mainly convective and stratiform rain type. Therefore, information on vertical rain structure is important for precise quantitative estimation of precipitation. Thus, the focus of this work is to characterize the vertical profile of rain structures using vertically-pointing Ka-band Micro Rain Radar (MRR) at Akure, Nigeria. This has been achieved by using 2-year (2013 and 2014) data of rain parameters namely: rain rate, reflectivity, liquid water content and fall velocity obtained from MRR to determine the bright band heights under different rain types and its implications on satellite and radio waves propagation in this region. Rain rate in this region has been categorized into four groups namely: 0.02- 0.2 mm/h, 0.2- 2 mm/h, 2-40 mm/h, and 40 - 200 mm/h. The very low rain rate group is related to the stratiform rain types whereas highest rain rate groups are for the convective rain type. Study shows that parameters that are much associated with rain attained peak value at different height depending on the period of the year. The vertical profile of Z shows peak around 3 to 4.2 km height. The peak region is associated with the bright band height and contribution to the melting layer. This study revealed that the occurrence of bright band heights varies with rain types. The overall results will be useful for determining rain height needed for the prediction of rain attenuation in this region.

Variations in Precipitating Convective Feature Populations with ITCZ Width in the Pacific Ocean

Many recent studies have aimed to better understand changes in the characteristics of the intertropical convergence zone (ITCZ), including ITCZ location, width, and precipitation intensity. However, very few studies have looked at the relationship between characteristics of convection within the ITCZ and ITCZ width. The present work uses information from an ITCZ identification database and Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database to quantify variations in convective characteristics across the ITCZ in the Pacific Ocean. Data are partitioned into wide and narrow ITCZ regimes to quantify differences in convection between different ITCZ regimes. Under the wide regime, convection deeper than 5 km, with areas greater than 100 km2, or stratiform rain fractions greater than 0.5 is, on average, 24%, 23%, and 12% more frequent, respectively. In the narrow regime, the signal is reversed, with average increases in the frequency of convection with heights below 5 km, areas less than 100 km2, or stratiform rain fractions less than 0.5 of 15%, 4%, and 6%, respectively. Positive and negative anomalies in columnar water vapor (CWV) and sea surface temperature (SST) across the ITCZ are observed in the wide and narrow regimes, respectively. There is also a strong positive correlation between an El Niño–Southern Oscillation (ENSO) index and ITCZ width anomalies, with wide (narrow) ITCZs occurring during warm (cold) phases of ENSO. This implies that the strengthening and weakening of the Walker circulation associated with ENSO may play a role in modulating the convective populations that contribute to the Pacific ITCZ width variations.