Pei-Ning Feng

Several configurations of the Canadian Precipitation Analysis system (CaPA) currently produce precipitation analyses at Environment and Climate Change Canada (ECCC). To improve CaPA’s performance during the winter season, the impact of assimilating the IMERG V06 product (IMERG: Integrated Multi-satellitE Retrievals for GPM—Global Precipitation Measurement mission) into CaPA is examined in this study. Tests are conducted with CaPA’s 10 km deterministic version, evaluated over Canada and the northern part of the United States (USA). Maps from a case study show that IMERG plays a contradictory role in the production of CaPA’s precipitation analyses for a synoptic-scale winter storm over North America’s eastern coast. While its contribution appears to be physically correct over southern portions of the meteorological system, and early in its intensification phase, IMERG displays unrealistic spatial structures over land later in the system’s life cycle when it is located over northern (colder) areas. Objective evaluation of CaPA’s analyses when IMERG is assimilated without any restrictions shows an overall decrease in precipitation, which has a mixed effect (positive and negative) on the bias indicators. But IMERG’s influence on the Equitable Threat Score (ETS), a measure of CaPA’s analyses accuracy, is clearly negative. Using IMERG’s quality index (QI) to filter out areas where it is less accurate improves CaPA’s objective evaluation, leading to better ETS versus the control experiment in which no IMERG data are assimilated. Several diagnostics provide insight into the nature of IMERG’s contribution to CaPA. For the most successful configuration, with a QI threshold of 0.3, IMERG’s impact is mostly found in the warmer parts of the domain, i.e., in northern US states and in British Columbia. Spatial means of the temporal sums of absolute differences between CaPA’s analyses with and without IMERG indicate that this product also contributes meaningfully over land areas covered by snow, and areas where air temperature is below −2 °C (where precipitation is assumed to be in solid phase).

The Canadian Precipitation Analysis System (CaPA) is an operational system that uses a combination of weather gauge and ground-based radar measurements together with short-term forecasts from a numerical weather model to provide near-real-time estimates of 6- and 24-h precipitation amounts. During the winter season, many gauge measurements are rejected by the CaPA quality control process because of the wind-induced undercatch for solid precipitation. The goal of this study is to improve the precipitation estimates over central Canada during the winter seasons from 2019 to 2022. Two approaches were tested. First, the quality control procedure in CaPA has been relaxed to increase the number of surface observations assimilated. Second, the automatic solid precipitation measurements were adjusted using a universal transfer function to compensate for the undercatch problem. Although increasing the wind speed threshold resulted in lower amounts and worse biases in frequency, the overall precipitation estimates are improved as the equitable threat score is improved because of a substantial decrease in the false alarm ratio, which compensates the degradation of the probability of detection. The increase of solid precipitation amounts using a transfer function improves the biases in both frequency and amounts and the probability of detection for all precipitation thresholds. However, the false alarm ratio deteriorates for large thresholds. The statistics vary from year to year, but an overall improvement is demonstrated by increasing the number of stations and adjusting the solid precipitation amounts for wind speed undercatch.

It was found in previous observational studies that the quasi-biennial oscillation (QBO) can modulate the teleconnection over the Atlantic basin related to the Madden–Julian Oscillation (MJO). In this study, we assess the modulation of the MJO-related teleconnection by the QBO in the operational models that participated in the subseasonal-to-seasonal prediction (S2S) project of the World Climate Research Programme/World Weather Research Programme. The enhancement of the positive North Atlantic Oscillation (NAO) after the occurrence of MJO phase 3, which corresponds to enhanced convection in the equatorial Indian Ocean and reduced convection in the tropical western Pacific, under westerly QBO (WQBO) conditions is seen to be captured by most S2S models but, not unexpectedly, to different degrees. In contrast, the enhancement of the NAO after the occurrence of MJO phase 7, when tropical convection anomalies have the opposite signs compared with MJO phase 3, under WQBO conditions is not reproduced in most S2S models. Under easterly QBO (EQBO) conditions, however, some S2S models can reproduce a significant negative NAO after the occurrence of MJO phase 7 but not a positive NAO after the occurrence of MJO phase 3. The results indicate that although the S2S models are able to predict a reasonable MJO up to around three weeks, representing the impact of the QBO on the extratropical teleconnection of the MJO remains challenging.

This study identifies the evolution of tropical vertical cloud regimes (CRs) and their associated heating structures on the intraseasonal time scales. Using the cloud classification retrievals of CloudSat during boreal winter between 2006 and 2017, the CR index is defined as the leading pair of the combined multivariate empirical orthogonal functions of the daily mean frequency of deep, high, and low clouds over the tropical Indian Ocean, Maritime Continents, and the Western Pacific. The principal components of the CR index exhibit robust temporal variance in the 30 to 80 day intraseasonal band. Based on the propagation stages of the CRs, the coherent vertical structures of cloud composition and large-scale moisture and vertical motion exhibit a westward-tilted structure. The associated Q1-QR diabatic heating and cloud radiative forcing are consistent with the key characteristics of the Madden Julian Oscillation (MJO) documented in the previous studies. Lastly, an MJO case study showcases that the presented approach characteristically captures the propagation of moisture, cloud vertical structure, and precipitation activity across spatial and temporal scales. The current results suggest that the CR index can potentially serve as an evaluation metric to cloud-associated processes in the simulated tropical intraseasonal variability in global climate models.

Previous studies have provided observational evidence for a correlation between certain phases of the Madden‐Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO), at a lag of about 10 days. Such a relation implies that the tropical convection of the MJO may provide sources of skill for subseasonal predictions in the extratropical regions. In this study, we show that the MJO‐NAO connection is influenced by the phase of the quasi‐biennial oscillation (QBO). During the westerly phase of QBO (WQBO), a stronger and longer lasting MJO‐NAO teleconnection is observed. About 10 days after the MJO phase 3 (7), which corresponds to enhanced (suppressed) diabatic heating anomaly in the tropical Indian Ocean and reduced (enhanced) convection in the western Pacific, a positive (negative) NAO tends to occur. On the other hand, under the easterly phase of QBO (EQBO), the MJO‐NAO teleconnection is also observed, but weaker and with less statistical significance. The QBO possibly influences the MJO‐related teleconnection by modulating the extratropical basic state. During WQBO years, there is subtropical anomalous westerly wind in the North Pacific, and as a result, the enhanced subtropical westerly jet provides a favorable environment for the MJO‐induced extratropical Rossby wave to propagate. The anomalous high‐latitude North Atlantic westerly is also favorable for troposphere‐stratosphere coupling and for the NAO to develop.