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P. Gates
and
H. Tong

Abstract

Using the method of Akaike's Information Criterion (AIC), we present a critical discussion on the determination of the order (i.e., “memory”) of an ergodic Markov chain with a finite number of states. We apply this method to sequences of wet and dry days observed at Manchester and Liverpool, England. We reexamine the Tel Aviv data and argue that a Markov chain of order not lower than 2 should be fitted, instead of the previously fitted first order. We further consider the use of AIC in investigating local stationarity. Finally, the sensitivity of the method when the sample size is reduced is briefly examined. The method proposed in this paper will enable practicing meteorologists to set up an automatic “Markov chain modeler”.

Full access
Tong Wan
,
Brenden H. Covert
,
Charles N. Kroll
, and
Craig R. Ferguson

Abstract

Portions of the northeastern United States (NE) have experienced drought every year since 2016. The U.S. Drought Monitor (USDM) has played an important role in drought characterization and management by providing weekly drought maps across the entire United States, including the NE. Unfortunately, the USDM lacks consistency between input variables leading to difficulties in defining boundaries between drought categories. This paper evaluates the National Water Model’s (NWM) ability to model streamflow and soil moisture, two important hydrological products that are frequently incorporated in drought indices. Using a 26-yr NWM retrospective simulation, comparisons were conducted between NWM output and observations of streamflow and soil moisture, as well as between drought categories derived from the NWM and observations and the USDM. Results indicate that NWM provides moderate predictions of streamflow at NE stations when comparing to historical observations, that NWM streamflow estimators are generally upwardly biased, and performance is worse at lower streamflow magnitudes. The NWM’s ability to predict soil moisture is worse than streamflow, with again a positive bias at most sites and strong variations in anomaly correlation across sites. When predicting drought categories, NWM streamflow is as strong a predictor of USDM drought categories as observed streamflow. Extending the NWM streamflow series using a maintenance of variance technique and only past records provides slight improvements over drought categories derived from the entire 26-yr retrospective simulation. Output from the NWM appears to have some skill in characterizing drought in the NE and provides a spatial resolution to improve the designation of drought boundaries.

Free access
Hoffman H. N. Cheung
,
Wen Zhou
,
Sai-ming Lee
, and
Hang-wai Tong

Abstract

During the past decade (2004/05–2013/14), the number of cold days in Hong Kong (N CD), as a proxy of the temperature of southern China, appeared to have increased from the historical minimum, in contrast to a remarkable decline in the entire postwar period. This is related to the recent apparent changes in the large-scale circulation upstream and downstream of the East Asian winter monsoon (EAWM) region: the increase in Ural blocking (UB) that enhances cold advection from the polar region and reinforces the Siberian high and the decrease in a western Pacific (WP)-like index that corresponds to increasing meridional gradient of geopotential height over the EAWM region. Overall, UB and WP account for 26.4% of the interannual (≤8 yr) variance and 83.7% of the decadal (>8 yr) variance of N CD for the period 1948/49–2013/14, indicating that further study could lead to improvement in the prediction of N CD.

Full access
Agnes H. N. Lim
,
James A. Jung
,
Sharon E. Nebuda
,
Jaime M. Daniels
,
Wayne Bresky
,
Mingjing Tong
, and
Vijay Tallapragada

Abstract

The assimilation of atmospheric motion vectors (AMVs) provides important wind information to conventional data-lacking oceanic regions, where tropical cyclones spend most of their lifetimes. Three new AMV types, shortwave infrared (SWIR), clear-air water vapor (CAWV), and visible (VIS), are produced hourly by NOAA/NESDIS and are assimilated in operational NWP systems. The new AMV data types are added to the hourly infrared (IR) and cloud-top water vapor (CTWV) AMV data in the 2016 operational version of the HWRF Model. In this study, we update existing quality control (QC) procedures and add new procedures specific to tropical cyclone assimilation. We assess the impact of the three new AMV types on tropical cyclone forecasts by conducting assimilation experiments for 25 Atlantic tropical cyclones from the 2015 and 2016 hurricane seasons. Forecasts are analyzed by considering all tropical cyclones as a group and classifying them into strong/weak storm vortices based on their initial model intensity. Metrics such as track error, intensity error, minimum central pressure error, and storm size are used to assess the data impact from the addition of the three new AMV types. Positive impact is obtained for these metrics, indicating that assimilating SWIR-, CAWV-, and VIS-type AMVs are beneficial for tropical cyclone forecasting. Given the results presented here, the new AMV types were accepted into NOAA/NCEP’s operational HWRF for the 2017 hurricane season.

Full access
Marco Y.-T. Leung
,
Wen Zhou
,
Dongxiao Wang
,
P. W. Chan
,
S. M. Lee
, and
H. W. Tong

Abstract

In this study, remote influence originating from the tropical western Indian Ocean on June precipitation in South China and the Indochina Peninsula is documented. Based on numerical simulation and statistical analysis, it is noted that the warm anomaly in the tropical western Indian Ocean can induce a weaker-than-normal Walker circulation across the tropical Indian Ocean and western Pacific Ocean. This further leads to a northeast–southwest-oriented western North Pacific subtropical high and a weaker-than-normal monsoon trough in the South China Sea. In addition, the weak monsoon trough is concurrent with an anomalous rising motion in South China and a sinking motion in the Indochina Peninsula. This enhances precipitation in South China and suppresses precipitation in the Indochina Peninsula on an interannual time scale. On the other hand, the warming trend in the tropical western Indian Ocean also supports the long-term trends of precipitation in the two regions.

Open access
Janet Barlow
,
Martin Best
,
Sylvia I. Bohnenstengel
,
Peter Clark
,
Sue Grimmond
,
Humphrey Lean
,
Andreas Christen
,
Stefan Emeis
,
Martial Haeffelin
,
Ian N. Harman
,
Aude Lemonsu
,
Alberto Martilli
,
Eric Pardyjak
,
Mathias W Rotach
,
Susan Ballard
,
Ian Boutle
,
Andy Brown
,
Xiaoming Cai
,
Matteo Carpentieri
,
Omduth Coceal
,
Ben Crawford
,
Silvana Di Sabatino
,
Junxia Dou
,
Daniel R. Drew
,
John M. Edwards
,
Joachim Fallmann
,
Krzysztof Fortuniak
,
Jemma Gornall
,
Tobias Gronemeier
,
Christos H. Halios
,
Denise Hertwig
,
Kohin Hirano
,
Albert A. M. Holtslag
,
Zhiwen Luo
,
Gerald Mills
,
Makoto Nakayoshi
,
Kathy Pain
,
K. Heinke Schlünzen
,
Stefan Smith
,
Lionel Soulhac
,
Gert-Jan Steeneveld
,
Ting Sun
,
Natalie E Theeuwes
,
David Thomson
,
James A. Voogt
,
Helen C. Ward
,
Zheng-Tong Xie
, and
Jian Zhong
Open access