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1. Improving Polarimetric Radar-Based Drop Size Distribution Retrieval and Rain Estimation Using a Deep Neural Network

   https://doi.org/https://doi.org/10.1175/JHM-D-22-0166.1  

   Ho, Junho; Zhang, Guifu; Bukovcic, Petar; Parsons, David B.; Xu, Feng; Gao, Jidong; Carlin, Jacob T.; Snyder, Jeffrey C. (November 2023, Journal of Hydrometeorology)

   Abstract Raindrop size distributions (DSD) and rain rate have been estimated from polarimetric radar data using different approaches with the accuracy depending on the errors both in the radar measurements and the estimation methods. Herein, a deep neural network (DNN) technique was utilized to improve the estimation of the DSD and rain rate by mitigating these errors. The performance of this approach was evaluated using measurements from a two-dimensional video disdrometer (2DVD) at the Kessler Atmospheric and Ecological Field Station in Oklahoma as ground truth with the results compared against conventional estimation methods for the period 2006–17. Physical parameters (mass-/volume-weighted diameter and liquid water content), rain rate, and polarimetric radar variables (including radar reflectivity and differential reflectivity) were obtained from the DSD data. Three methods—physics-based inversion, empirical formula, and DNN—were applied to two different temporal domains (instantaneous and rain-event average) with three diverse error assumptions (fitting, measurement, and model errors). The DSD retrievals and rain estimates from 18 cases were evaluated by calculating the bias and root-mean-squared error (RMSE). DNN produced the best performance for most cases, with up to a 5% reduction in RMSE when model errors existed. DSD and rain estimated from a nearby polarimetric radar using the empirical and DNN methods were well correlated with the disdrometer observations; the rain-rate estimate bias of the DNN was significantly reduced (3.3% in DNN vs 50.1% in empirical). These results suggest that DNN has advantages over the physics-based and empirical methods in retrieving rain microphysics from radar observations. 

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2. Raindrop Size Spectrum in Deep Convective Regions of the Americas

   https://doi.org/10.3390/atmos12080979  

   Rivelli Zea, Lina; Nesbitt, Stephen W.; Ladino, Alfonso; Hardin, Joseph C.; Varble, Adam (August 2021, Atmosphere)

   This study compared drop size distribution (DSD) measurements on the surface, the corresponding properties, and the precipitation modes among three deep convective regions within the Americas. The measurement compilation corresponded to two sites in the midlatitudes: the U.S. Southern Great Plains and Córdoba Province in subtropical South America, as well as to one site in the tropics: Manacapuru in central Amazonia; these are all areas where intense rain-producing systems contribute to the majority of rainfall in the Americas’ largest river basins. This compilation included two types of disdrometers (Parsivel and 2D-Video Disdrometer) that were used at the midlatitude sites and one type of disdrometer (Parsivel) that was deployed at the tropical site. The distributions of physical parameters (such as rain rate R, mass-weighted mean diameter Dm, and normalized droplet concentration Nw) for the raindrop spectra without rainfall mode classification seemed similar, except for the much broader Nw distributions in Córdoba. The raindrop spectra were then classified into a light precipitation mode and a precipitation mode by using a cutoff at 0.5 mm h−1 based on previous studies that characterized the full drop size spectra. These segregated rain modes are potentially unique relative to previously studied terrain-influenced sites. In the light precipitation and precipitation modes, the dominant higher frequency observed in a broad distribution of Nw in both types of disdrometers and the identification of shallow light precipitation in vertically pointing cloud radar data represent unique characteristics of the Córdoba site relative to the others. As a result, the co-variability between the physical parameters of the DSD indicates that the precipitation observed in Córdoba may confound existing methods of determining the rain type by using the drop size distribution. 

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3. Evolution of Surface Precipitation Accumulations Upstream of the Olympic Mountains using Observations and Simulations: An OLYMPEX Case Study

   https://doi.org/10.5194/egusphere-egu24-20124  

   Hence, Deanna; James, Scott (April 2024, EGU General Assembly 2024)

   Analysis of surface precipitation accumulations upstream, near-shore, and adjacent to the Olympic mountains from the 17 December 2015 case during OLYMPEX using Weather Research and Forecasting (WRF) simulations, the NPOL dual-polarization radar, and high-resolution soundings investigates the role of low-level blocking on upstream precipitation enhancement. Past work shows that frontal systems often slow while approaching complex terrain if the Froude number is sufficiently low. Low-level blocking of stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding-derived vertical stability profiles indicate high levels of low-level stability and significant vertical wind shear, which showed little change while a warm front propagated northeastward and stalled as the stable air mass likely dammed against the terrain. Radial velocity from the NPOL radar and simulated wind fields indicate strong down-valley flow coupled with a frontal jet also contributed to long-lasting Kelvin-Helmholtz (KH) waves extending offshore.Using WRF simulations along with OLYMPEX observations, we examined the evolution of precipitation upstream of complex terrain by breaking down the distribution of pre-frontal and frontal precipitation accumulations as the warm front approached the Olympic Peninsula. Through dividing the event into regions upstream of NPOL and into timeframes relative to landfall, results indicate pre-warm frontal precipitation accumulations decrease with distance upstream of the coast with the highest accumulations present over the terrain. As the front's translation speed slowed and eventually stalled, the warm frontal period accumulations are highest far upstream of the coast and over the terrain, with lesser accumulations in the middle region. These results indicate that upstream precipitation enhancement upstream is an indirect effect of the terrain influencing the frontal shape and propagation, resulting in enhanced frontal precipitation accumulations. 

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4. Analyzing Surface Precipitation Accumulations Upstream of the Olympic Mountains using High-Resolution Simulations and Observations: An OLYMPEX Case Study

   James, S; Hence, D A (June 2022, 20th Conference on Mountain Meteorology)

   Analysis of surface precipitation accumulations upstream, near-shore, and adjacent to the Olympic mountains from the 17 December 2015 case during OLYMPEX using high-resolution Weather Research and Forecasting (WRF) simulations, and observations from the NPOL dual-polarization radar show that strong down-valley flow coupled with a frontal jet led to the formation of long-lasting Kelvin-Helmholtz (KH) waves. High-resolution soundings investigate the role of low-level blocking on upstream precipitation enhancement. Past work shows that frontal systems often slow while approaching complex terrain if the Froude number is sufficiently low. Low-level blocking by stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding profiles from OLYMPEX indicate little change in surface thermodynamic characteristics as the front propagated northeastward and stalled, a possible indication of blocking by the terrain. Initial findings from sounding-derived vertical stability parameters show high levels of low-level stability (~7.5 k km-1) and high vertical wind shear (dU/dz = 0.025 s-1). Precipitation accumulations from WRF simulations and OLYMPEX observations divide the event into five regions 100 km, 80 km, 40 km, 15 km, and 0 km upstream of NPOL and into timeframes relative to landfall to analyze the distribution of precipitation enhancement associated with a warm front. Preliminary results indicate that 40 km upstream of the coast saw nearly double 5-min precipitation accumulations compared to 15 and 0 km even though radar indicates that precipitation was stratiform across the region. We will isolate the impact of the KH waves and frontal dynamics on precipitation enhancement by analyzing each region’s respective model output precipitation accumulation distributions. Further analysis of the frontal characteristics, thermodynamic environment, and stability parameters will assist with determining if blocking had a role in a potential upstream shift in accumulations. 

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5. Analyzing Warm-Frontal Precipitation Distributions and Accumulations Upstream of the Olympic Mountains using Observations and High-Resolution Simulations: An OLYMPEX Case Study

   James, S; Hence, D A (January 2023, Third Symposium on Mesoscale Processes, 103rd American Meteorological Society Annual Meeting)

   Analysis of surface precipitation accumulations upstream, near-shore, and adjacent to the Olympic mountains from the 17 December 2015 case during OLYMPEX using high-resolution Weather Research and Forecasting (WRF) simulations, and observations from the NPOL dual-polarization radar show that strong down-valley flow coupled with a frontal jet led to the formation of long-lasting Kelvin-Helmholtz (KH) waves. High-resolution soundings investigate the role of low-level blocking on upstream precipitation enhancement. Past work shows that frontal systems often slow while approaching complex terrain if the Froude number is sufficiently low. Low-level blocking by stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding profiles from OLYMPEX indicate little change in surface thermodynamic characteristics as the front propagated northeastward and stalled, a possible indication of blocking by the terrain. Initial findings from sounding-derived vertical stability parameters show high levels of low-level stability (~7.5 k km ) and significant vertical wind shear (dU/dz = 0.025 s ). As a result, the Froude number was sufficiently low (Fr < 1) and the front was well within the Rossby radius of deformation for most of the case. Precipitation accumulations from WRF simulations and OLYMPEX observations divide the event into five regions 100 km, 80 km, 40 km, 15 km, and 0 km upstream of NPOL and into timeframes relative to the fronts position to analyze the distribution of precipitation enhancement associated with a warm front. Results indicate that 40 km upstream of the coast saw nearly double 5-min precipitation accumulations compared to 15 and 0 km even though radar indicates that precipitation was stratiform across the region. We assess that the precipitation enhancement upstream is not a direct result of orography, but more likely that the terrain influences the frontal shape and propagation which then results in enhanced near- front precipitation accumulations. 

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