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Robert S. Schrom
,
S. Joseph Munchak
, and
Ian S. Adams

Abstract

The scattering properties of aggregates are studied herein. Early aggregates (< 7 monomers) of branched planar crystals and mature aggregates (up to 100 monomers) of columns are randomly generated with varying assumptions about the monomer attachment processes and the orientation behavior during collection. The resulting physical properties of the aggregates correspond well with prior in situ and retrieved sizes and shapes. Assumed azimuthally uniform orientations during collection and monomer pivoting upon attachment resulted in flatter and denser aggregates. The column aggregates had lower density and more spherical shapes than the branched planar crystal aggregates. The scattering properties were calculated using the Discrete Dipole Approximation for a set of orientation angles and transformed to spectral coefficients representing modes of orientation angle variability. The zeroth- and second-order coefficients dominate this variability, with the zeroth-order coefficients representing the scattering properties for randomly oriented particles. The second order coefficients for backscatter showed differences between horizontal and vertical polarization increasing with density, and these coefficients for specific differential phase increase with both mass and density. Similarly, coefficients for the copolar covariance decreased with density. Rapid changes in the contributions to the radar moments from the second-order coefficients from low to moderate density were observed, likely due to the increasing presence of horizontally aligned monomers in the aggregate structure. Differences in how differential reflectivity and correlation coefficient evolve with the orientation distribution parameters, suggest that these measurements, along with specific differential phase and reflectivity, provide complimentary information about aggregate sizes, shapes, and orientation distributions.

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Wei Chen

Abstract

El Niño events are likely to be followed by significant positive sea surface temperature anomalies (SSTA) over the north tropical Atlantic (NTA). We find that this is not always true for the El Niño with moderate intensity. Nearly half of the moderate El Niño cases are followed by the strong NTA SSTA but others are followed by the weak NTA SSTA, indicating the uncertainty in the connection between the moderate El Niño and the NTA. The differences in the El Niño-related NTA SSTA are due to the different extratropical teleconnection between El Niño and the NTA, manifesting as a Pacific-North American (PNA) pattern.

Further analysis suggests that the deepened Aleutian Low (AL) induces the negative north Pacific SSTA which is associated with a wave train propagating eastward and in turn modulates the El Niño-generated PNA pattern. Therefore, El Niño that are accompanied by the strong AL induces a strong PNA pattern, and in turn leads to the strong NTA SSTA. The strength of the AL coinciding with El Niño plays a crucial role in modulating the connection between the moderate El Niño and the NTA SSTA. These observational evidences about the uncertainty in the El Niño-NTA connection and the role of the AL in modulating this connection are further supported by the model simulations that participated in the Coupled Model Intercomparison Project (CMIP) 6. These results indicates that the combined effects of El Niño and the AL must be considered in order to fully understand the NTA SSTA variability.

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Iaroslav Verevkin
and
Ian Folkins

Abstract

During the summer, rainfall over the Rocky Mountains peaks in the afternoon. The diurnal time of maximum rain becomes progressively delayed in going eastward from the Rocky Mountains, such that over much of the Great Plains, maximum rain occurs near 3 am local time. We use Rapid Refresh (RAP) reanalysis data from July and August of 2009-2019 to show that there is continuous spatial variation in the mean structure of the high rain events that occur along this anomalous diurnally propagating rainfall feature. High rain events that occur further from the Rockies are more likely to be associated with a larger warm anomaly to south of the rain event center, a smaller cold anomaly, a larger negative surface pressure anomaly, and increased low level southerly meridional wind. We also use the Integrated Multi-satellitE Retrievals for GPM (IMERG) rainfall dataset to show that, on hourly timescales, afternoon rainfall that occurs over the Rocky Mountains propagates eastward onto the Great Plains, but is rapidly attenuated and becomes negligible east of 100 W. This eastward rainfall propagation appears to be mediated in part by the formation of a low level cold anomaly and increased surface pressure over the Rocky Mountains, a reversal of the upslope wind, and increased low level zonal mass convergence over the adjacent Near Plains.

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Xuyan Fan
,
Yueqing Li
, and
Juan Li

Abstract

This study demonstrates the synoptic-scale relationship between atmospheric heat source over the Tibetan Plateau (TP<Q1>) and summer rainfall in eastern China. The results show that the summer TP<Q1> has three key areas-eastern, western, northeastern TP (TPE, TPW, TPNE) and 3-8-day dominant oscillation periods on the synoptic time scale for each subregion. Analogously, on synoptic-scale, the downstream rainfall of the TP in China have six sensitive regions (zone-I to zon-VI) each with 3-8-day dominant periods. The TPE<Q1>, TPW<Q1> and TPNE<Q1> lead rainfall 4 days with a southeastward propagation pathway, 5 days with both southeasteward and northeasteward propagation routes, 4 days with an easteward way, respectively. Mechanistically, when TP<Q1> lies in the positive to negative transition phase, the rainfall moves off the TP and reaches the maximum in its most inactive phase; the jet cores embedded in the Subtropical Westerly Jet band and the trough from the region of the TP<Q1> lead the eastward movement of the rainfall by the propagation of the divergence center and generalized moist potential vorticity. Besides, the southwest vortex and northwest vortex generate at the southeastern and northeastern flank of the TP are the main cause for the southeastward and northesastward or eastward movement of the precipitation. Further internal atmospheric dynamic structures indicate that the horizontal moisture advection and generalized moist potential vorticity advection both act on the entire propagation processes of the synoptic-scale signals for each TP<Q1> key area. In conclusion, the study have obvious theoretical significance and application value for the development of plateau synoptics.

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Yanyi He
,
Kun Yang
,
Yanghang Ren
,
Mijun Zou
,
Xu Yuan
, and
Wenjun Tang

The 2021 low solar radiation over southeastern Tibetan Plateau was mainly caused by abnormally strong southerlies and was further enhanced by anthropogenic aerosols and GHGs-induced warming, and consequently reduced vegetation growth.

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Vannia Aliaga-Nestares
,
Gustavo De La Cruz
, and
Ken Takahashi

Abstract

Multiple linear regression models were developed for 1-3-day lead forecasts of maximum and minimum temperature for two locations in the city of Lima, in the central coast of Peru (12°S), and contrasted with the operational forecasts issued by the National Meteorological and Hydrological Service - SENAMHI and the output of a regional numerical atmospheric model. We developed empirical models, fitted to data from the 2000-2013 period, and verified their skill for the 2014-2019 period. Since El Niño produces a strong low-frequency signal, the models focus on the high-frequency weather and subseasonal variability (60-day cutoff).

The empirical models outperformed the operational forecasts and the numerical model. For instance, the high-frequency annual correlation coefficient and root mean square error (RMSE) for the 1-day lead forecasts were 0.37-0.53 and 0.74-1.76°C for the empirical model, respectively, but around −0.05-0.24 and 0.88-4.21°C in the operational case. Only three predictors were considered for the models, including persistence and large-scale atmospheric indices. Contrary to our belief, the model skill was lowest for the austral winter (June-August), when the extratropical influence is largest, suggesting an enhanced role of local effects. Including local specific humidity as a predictor for minimum temperature at the inland location substantially increased the skill and reduced its seasonality.

There were cases in which both the operational and empirical forecast had similar strong errors and we suggest mesoscale circulations, such as the Low-Level Cyclonic Vortex over the ocean, as the culprit. Incorporating such information could be valuable for improving the forecasts.

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Vishal Batchu
,
Grey Nearing
, and
Varun Gulshan

Abstract

We develop a deep learning based convolutional-regression model that estimates the volumetric soil moisture content in the top ~5 cm of soil. Input predictors include Sentinel-1 (active radar), and Sentinel-2 (multispectral imagery) as well as geophysical variables from SoilGrids and modelled soil moisture fields from SMAP-USDA and GLDAS. The model was trained and evaluated on data from ~1000 in-situ sensors globally over the period 2015 - 2021 and obtained an average per-sensor correlation of 0.707 and ubRMSE of 0.055 m3/m3, and can be used to produce a soil moisture map at a nominal 320m resolution. These results are benchmarked against 14 other soil moisture evaluation research works at different locations, and an ablation study was used to identify important predictors.

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Ali Tokay
,
Charles N. Helms
,
Kwonil Kim
,
Patrick N. Gatlin
, and
David B. Wolff

Abstract

Improving estimation of snow water equivalent rate (SWER) from radar reflectivity (Ze), known as a SWER(Ze) relationship, is a priority for NASA’s Global Precipitation Measurement (GPM) mission ground validation program as it is needed to comprehensively validate spaceborne precipitation retrievals. This study investigates the performance of eight operational and four research based SWER(Ze) relationships utilizing Precipitation Imaging Probe (PIP) observations from the International Collaborative Experiment – Pyeongchang Olympics and Paraolympics (ICE-POP 2018) field campaign. During ICE-POP 2018, there were 10 snow events that are classified by synoptic conditions as either cold low or warm low and a SWER(Ze) relationship is derived for each event. Additionally, a SWER(Ze) relationship is derived for each synoptic classification by merging all events within each class. Two new types of SWER(Ze) relationships are derived from PIP measurements of bulk density and habit classification. These two physically-based SWER(Ze) relationships provided superior estimates of SWER when compared to the operational, event-specific, and synoptic SWER(Ze) relationships. For estimates of the event snow water equivalent total, the event-specific, synoptic, and best-performing operational SWER(Ze) relationships outperformed the physically-based SWER(Ze) relationship, although the physically-based relationships still performed well. This study recommends using the density or habit based SWER(Ze) relationships for microphysical studies, whereas, the other SWER(Ze) relationships are better suited towards hydrologic application.

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