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We investigate transport pathways of water vapor associated with landfalling atmospheric river (AR) events that result in precipitation along the West Coast of the U.S. for winters of 1997-2010. The water vapor transport pathways are determined by computing back trajectories with a trajectory model using the Modern Era Retrospective analysis for Research and Applications reanalysis data set. The majority of AR events (86%) over the West Coast of the U.S. are grouped into three trajectory types, and two of them are closely associated with the AR events. We designate the first type as Ascending near landfall and of Tropical Origin (AT), the second type as Ascending near landfall and of Extratropical Origin (AE), and the third type as Descending or parallel near landfall and of Extratropical Origin (DE), which is accompanied but not directly associated with the AR events. The magnitude and spatial distribution of precipitation of a given AR event are found to be strongly determined by the type of trajectories. In general, AR events composed of both AT and AE trajectories have more frequent precipitation over a broad region of the western U.S. and AR events composed of both AT and DE trajectories have intense precipitation over the southwestern U.S. due to AT trajectories. AR events of AT-only trajectories have intense precipitation, especially over the northwestern U.S., but are less frequent compared to those of AT + AE trajectories. In addition, different patterns of trajectory types among AR events are closely linked to upper level potential vorticity (PV) anomalies; 66% of AR events are associated with anticyclonic Rossby wave breaking events.

In this study, the Atmospheric Infrared Sounder (AIRS) Observations for Model Intercomparison Projects (Obs4MIPs) V2.1 tropospheric air temperature, specific humidity, and relative humidity data are utilized to evaluate the global tropospheric temperature and humidity simulations in the fully coupled global climate models from the Coupled Model Intercomparison Project phases 3, 5, and 6 (CMIP3, CMIP5, and CMIP6), and possible simulation improvement in CMIP6 models in comparison to CMIP3 and CMIP5 models. Our analyses indicate that all three phases of CMIP models share similar tropospheric air temperature, specific humidity, and relative humidity biases in their multi‐model ensemble means relative to AIRS. Cold biases up to 4 K and positive relative humidity biases up to 20% are found in the free troposphere almost globally with maxima over the mid‐latitude storm tracks. Warm biases up to 2 K are seen over the Southern Ocean in the lower troposphere. Positive specific and relative humidity biases exist over the off‐equatorial oceans while negative specific and relative humidity biases are seen near the equator in the tropical free troposphere, which are related to the double‐intertropical convergence zone bias in the models. Both the air temperature and specific humidity biases are important to the relative humidity biases except in the tropical free troposphere where the specific humidity biases dominate. The tropospheric air temperature, specific humidity, and relative humidity biases are reduced from CMIP3 to CMIP5 and to CMIP6 at almost all pressure levels except at 300 hPa for specific humidity and in the boundary layer for relative humidity. Plain Language Summary: In this study, we use the Atmospheric Infrared Sounder (AIRS) data to assess how well the fully coupled global climate models simulate temperature and humidity in the troposphere. We focus on the climate models from the recent three phases of Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6) and investigate if there is any bias reduction in the recent CMIP6 models compared to the earlier CMIP3 and CMIP5 models. To conduct our evaluation, we compare the CMIP model ensemble averages with the AIRS data and find that all three phases of CMIP models exhibit similar biases in tropospheric temperature and humidity. For example, cold biases up to 4 K and positive relative humidity biases up to 20% are seen in the free troposphere over most regions, with the maximum biases over the midlatitudes. Positive specific and relative humidity biases are found over the off‐equatorial oceans while negative specific and relative humidity biases are seen near the equator in the tropical free troposphere. We also note a possible temperature and humidity bias reduction from the CMIP3 models to the CMIP5 models and then to the CMIP6 models at most pressure levels. Key Points: CMIP Phase 3, 5 and 6 climate models share similar tropospheric air temperature, specific and relative humidity biases relative to Atmospheric Infrared SounderCold biases and positive relative humidity biases are prevalent in free troposphere almost globally with maxima over the mid‐latitude oceansDouble‐intertropical convergence zone bias‐related specific and relative humidity biases are evident in tropical free troposphere: positive (negative) off (near) the equator [ABSTRACT FROM AUTHOR]