Intercomparison of middle atmospheric meteorological analyses for the Northern Hemisphere winter 2009–2010

Detailed meteorological analyses based on observations extending through the middle atmosphere (∼ 15 to 100 km altitude) can provide key information to whole atmosphere modeling systems regarding the physical mechanisms linking day-to-day changes in ionospheric electron density to meteorological variability near the Earth's surface. However, the extent to which independent middle atmosphere analyses differ in their representation of wave-induced coupling to the ionosphere is unclear. To begin to address this issue, we present the first intercomparison among four such analyses, JAGUAR-DAS, MERRA-2, NAVGEM-HA, and WACCMX+DART, focusing on the Northern Hemisphere (NH) 2009–2010 winter, which includes a major sudden stratospheric warming (SSW). This intercomparison examines the altitude, latitude, and time dependences of zonal mean zonal winds and temperatures among these four analyses over the 1 December 2009 to 31 March 2010 period, as well as latitude and altitude dependences of monthly mean amplitudes of the diurnal and semidiurnal migrating solar tides, the eastward-propagating diurnal zonal wave number 3 nonmigrating tide, and traveling planetary waves associated with the quasi-5 d and quasi-2 d Rossby modes. Our results show generally good agreement among the four analyses up to the stratopause (∼ 50 km altitude). Large discrepancies begin to emerge in the mesosphere and lower thermosphere owing to (1) differences in the types of satellite data assimilated by each system and (2) differences in the details of the global atmospheric models used by each analysis system. The results of this intercomparison provide initial estimates of uncertainty in analyses commonly used to constrain middle atmospheric meteorological variability in whole atmosphere model simulations.

Variations of Arctic winter ozone from the LIMS Level 3 dataset

The Nimbus 7 limb infrared monitor of the stratosphere (LIMS) instrument operated from October 25, 1978, through May 28, 1979. Its Version 6 (V6) profiles and their Level 3 or zonal Fourier coefficient products have been characterized and archived in 2008 and in 2011, respectively. This paper focuses on the value and use of daily ozone maps from Level 3, based on a gridding of its zonal coefficients. We present maps of V6 ozone on pressure surfaces and compare them with several rocket-borne chemiluminescent ozone measurements that extend into the lower mesosphere. Daily, synoptic maps of V6 ozone and temperature illustrate that they are an important aid in interpreting satellite limb-infrared emission versus local measurements, especially when they occur during dynamically active periods of northern hemisphere winter. We then show a sequence of V6 maps of upper stratospheric ozone, spanning the minor stratospheric warmings of late January and early February 1979. The map sequence of V6 geopotential height reveals how ozone was changing in the vortex and at the centers of adjacent anticyclones. We also report on zonal variations of the tertiary ozone maximum of the upper mesosphere and its associated temperature fields during winter. These several examples provide a guide to researchers for further exploratory analyses of middle atmosphere ozone from LIMS.

Orographic resolution driving the improvements associated with horizontal resolution increase in the Northern Hemisphere winter mid-latitudes

In recent years much attention has been devoted to the investigation of the impact of increasing the horizontal resolution of global climate models. In the present work, a set of atmosphere-only idealized sensitivity simulations with EC-Earth3 have been designed to disentangle the relative roles of increasing the resolution of the resolved orography and of the atmospheric grid. Focusing on the winter Northern Hemisphere, it is shown that if the grid is refined while keeping the resolved orography unchanged, model biases are reduced only in some specific occasions. Conversely, increasing the resolved (or mean) orography is found to clearly reduce several important systematic model errors, including synoptic transient eddies, the North Atlantic jet stream variability and atmospheric blocking frequency and duration. From an analysis of the radiation budget it is concluded that the large changes in radiative fluxes caused by the resolution increase – something commonly observed in climate models – have a relevant impact on the atmospheric circulation, partially offsetting the benefits obtained from the increase in orographic resolution. These findings point to the necessity of always tuning climate models to fully exploit the benefits of high horizontal resolution.

Stratospheric modulation of winter temperature trends in a warming climate

Variability in the circumpolar westerly winds of the Northern Hemisphere winter polar stratosphere-- the stratospheric polar vortex-- has a known downward influence on the extratropical surface climate on sub-seasonal timescales. On longer timescales, observed trends towards a weakening stratospheric polar vortex have been linked to cooling surface temperatures over Eurasia from 1990-2009. Here, we show that 10-40 year polar vortex weakening trends occur as often as strengthening trends in large-ensemble historical climate simulations, and that decadal variability in polar vortex trends is significantly linked to decadal variability in regional surface temperature trends across the Northern Hemisphere even as the climate warms. We find that 74% of ensemble members with cooling trends over Eurasia during an 1850-2099 climate simulation also exhibit a weakening polar vortex, while 70% of members with accelerated warming over Eurasia exhibit a strengthening polar vortex. Decadal variability in the polar vortex thus modulates extratropical anthropogenically-forced warming trends.

Current and Predicted Future Winter Warm Spells Would Affect Douglas Fir (Pseudotsuga menziesii (Mirb.), Franco) Seeds in the Early Stage of Germination More Than in the Late Stage of Germination

Most tree species in the temperate climatic zone (including Douglas fir) disperse seeds in autumn. Some of them must be exposed to cold (0–10 °C) and moist conditions (cold stratification) to overcome dormancy and trigger germination. In the Northern Hemisphere, winter warm spells occur more frequently and last longer than in recent decades from eastern Canada to Europe. Our main research objective was to investigate the influence of current (1 or 3 days at day/night temperatures: 15 °C/10 °C) and future predicted (5 days at day/night temperatures: 25 °C/15 °C) winter warm spells on dormancy breaking and germination traits (germination energy—GE; germination capacity—GC; final germination capacity—FGC) of Douglas fir seeds from four old-growth stands in northern Poland. For this purpose, we interrupted cold stratification of seeds at different time points, i.e., after 3 weeks; 6 weeks; 9 or 3 weeks and 6 weeks; 3 and 9 weeks; 6 and 9 weeks; 3 and 6 weeks and 9 weeks. We found that for GE and GC, all main effects (populations—P; days of warm spell—D; stratification duration—W) and interactions were significant (except interaction P×D). FGC was significantly affected by the effects P and D and interactions of D × W and P × D × W. In addition, we found that the predicted warm spells negatively affected the early germination stage (GE and GC) of Douglas fir, but both current and future predicted winter conditions will not negatively affect the late germination stage (FGC).

A Climatology of Atmospheric Rivers in New Zealand

The occurrence of extreme precipitation events in New Zealand regularly results in devastating impacts to the local society and environment. An automated atmospheric river (AR) detection technique (ARDT) is applied to construct a climatology (1979–2019) of extreme midlatitude moisture fluxes conducive to extreme precipitation. A distinct seasonality exists in AR occurrence aligning with seasonal variations in the midlatitude jet streams. The formation of the Southern Hemisphere winter split jet enables AR occurrence to persist through all seasons in northern regions of New Zealand, while southern regions of the country exhibit a substantial (50%) reduction in AR occurrence as the polar jet shifts southward during the cold season. ARs making landfall on the western coast of New Zealand (90% of all events) are characterized by a dominant northwesterly moisture flux associated with a distinct dipole pressure anomaly, with low pressure to the southwest and high pressure to the northeast of New Zealand. Precipitation totals during AR events increase with AR rank (five-point scale) throughout the country, with the most substantial increase on the windward side of the Southern Alps (South Island). The largest events (rank 5 ARs) produce 3-day precipitation totals exceeding 1000 mm. ARs account for up to 78% of total precipitation and up to 94% of extreme precipitation on the west coast of the South Island. Assessment of the multiscale atmospheric processes associated with AR events governing extreme precipitation in the Southern Alps of New Zealand should remain a priority given their hydrological significance and impact on people and infrastructure.

EFSO at different geographical locations verified with observing-system experiments

An ensemble-based forecast sensitivity to observations (EFSO) diagnosis has been implemented in an atmospheric general circulation model–ensemble Kalman filter data assimilation system to estimate the impacts of specific observations from the quasi-operational global observing system on weekly short-range forecasts. It was examined whether EFSO reasonably approximates the impacts of a subset of observations from specific geographical locations for 6-hour forecasts, and how long the 6-hour observation impacts can be retained during the 7-day forecast period. The reference for these forecasts was obtained from 12 data denial experiments in each of which a subset of three radiosonde observations launched from a geographical location was excluded. The 12 locations were selected from three latitudinal bands comprising (i) four Arctic regions, (ii) four midlatitude regions in the Northern Hemisphere, and (iii) four tropical regions during the Northern Hemisphere winter of 2015/16. The estimated winter-averaged EFSO-derived observation impacts well corresponded to the 6-hour observation impacts obtained by the data denials and EFSO could reasonably estimate the observation impacts by the data denials on short-range (6-hour to 2-day) forecasts. Furthermore, during the medium-range (4-day to 7-day) forecasts, it was found that the Arctic observations tend to seed the broadest impacts and their short-range observation impacts could be projected to beneficial impacts in Arctic and midlatitude North American areas. The midlatitude area located just downstream of dynamical propagation from the Arctic toward the midlatitudes. Results obtained by repeated Arctic data-denial experiments were found to be generally common to those from the non-repeated experiments.

Revisiting the Role of Mountains in the Northern Hemisphere Winter Atmospheric Circulation

The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, WACCM6. A suite of experiments explores the roles of both resolved orography, and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extra-tropical tropospheric and stratospheric zonal mean zonal wind, , by up to 80%; this is substantially greater than previous estimates. Ultimately parameterized orography accounts for 60-80% of this reduction; however, away from the surface most of the forcing of by parameterized orography is accomplished by resolved planetary waves. We propose that a catalytic wave-mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land-sea contrast contribute approximately equally to the strength of the mid-latitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface, and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in which is prescribed. We show that orography has less impact on stationary waves, and therefore on , on a backwards rotating Earth. Lastly, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths.

The influence of the enhancement of the convective activities of tropical Pacific on the trend of stratospheric sudden warmings in the period of 1948-2003

The exploration of the trend of stratospheric sudden warming (SSW) in the Northern Hemisphere is conducive to predict SSWs in the future. Utilizing the National Centre for Environmental Prediction (NCEP) (1948–2017) and Japanese 55-year reanalysis data (JRA55) (1958–2017), we investigated the duration and strength of SSWs in the Northern Hemisphere winter (December-February). We found the duration of SSWs has an increasing trend and the strength of SSWs tends to strengthen from 1948 to 2003. However, after 2003, these trends did not continue. We also utilize the observed cloudiness from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) to examine the convective activities in the tropical Pacific and found that the convective activities in the tropical central Pacific are enhanced during the period of 1948–2003, and the trend of the enhancement of the convective activities ceases after 2003. The circulation anomalies caused by the enhanced convective activities propagate to the troposphere at high latitudes through wave trains. The anomalies of circulation and the climatic circulation at high latitudes interfere with each other and superimpose, which has a significant impact on planetary wave 1 (PW1). As a result, the PW1 in the troposphere also showed an increasing trend from 1948 to 2003 and a decreasing trend after 2003. After the stratosphere filters out the planetary wave with a large wavenumber, PW1 accounts for more proportion of planetary waves, which causes the trend of SSW to change synchronously.