Abstract

Simple theoretical models of evaporation and sensitive high-resolution Doppler radars are used to study the precipitation and velocity structures resulting from evaporation at the base of layers of ice particles.

The models show that most ice particles will evaporate completely within a 2 km depth of fall, with the depth being dependent primarily upon particle size and relative humidity. Vertical gradients of reflectivity factor of 20–30 dBZ per 750 m are predicted for relative humidities <75% for a non-rimed collection of particles with a distribution of sizes. The evaporative cooling produces a destabilized layer, the depth and intensity of which are most dependent upon the relative humidity and precipitation characteristics. A dynamical model shows that downdrafts of at least 6 m s⁻¹ penetrating to a depth of 2 km can be produced by evaporation. The intensity and penetration depth of the downdrafts depend primarily on the ambient lapse rate of temperature.

The magnitudes of vertical gradients of reflectivity factor predicted by the models were seen in radar observations. On one occasion the base of the precipitation layer lowered with time at 200 m h⁻¹, in excellent agreement with the calculations. Updrafts and downdrafts of 1-3 m s⁻¹ were observed in the region of intense vertical gradients of reflectivity by a vertically pointing Doppler radar. These motions perturbed the precipitation field such that the downdrafts were in downward extending appendages called “stalactites” and updrafts in the holes between.

Doppler radar observations are presented of stalactites and convective motion fields which were associated with a uniformly generated precipitation layer and with trails from cellular generators at cloud top. The motions associated with the trails appeared to be better organized and to have greater vertical extent than those associated with the uniformly generated layer. In the latter case, the largest scales varied from 500 m to 1.5 km with several preferred scales at smaller wavelengths which suggested a tendency toward a breakdown of the stalactite associated motion fields. In the former case the scales were more uniform at 600–900 m.

An evolution of a stalactite layer was observed in the case of the trails. Initially, when the trails entered the top of the dry layer with a small angle of incidence, the stalactites were simply extensions of the trails and the updrafts and downdrafts appeared to follow the stalactites. The vertical extent of these perturbations at this point was 1.5-2 km. The shear at the top of the layer increased with time and the trails became more horizontal. Precipitation tended to be carried out of the trails by downdrafts resulting from locally enhanced chilling by evaporation. This resulted in more vertical stalactites and more vertical updrafts and downdrafts. When the trials became horizontal, they were much more disuse and the whole layer was destabilized. However, the destabilization was not sufficient to result in significant convection. The increase in shear was due to a decrease in the mean horizontal wind at the top of the dry layer which in turn was deduced to be the result of a transfer of energy to the perturbations, probably making them more intense. It appears that this interaction between the perturbations and the mean flow resulted in more intense but shorter lived stalactite associated motion fields than otherwise would have occurred.

Abstract

A kinematic mesocyclone model is developed to better approximate mesocyclone flows observed by single-Doppler radar. The model is described by two general flow regimes: an inner core region where velocity varies directly with radius from the center of the flow and an outer flow region where velocity varies inversely with radius. The new model differs from the traditional circular mesocyclone model in that the shape of the inner flow is described by an ellipse of specified eccentricity, and the vorticity and divergence structures of the inner flow region are nonuniform and described by simple functions. The effects of flow shape, vorticity and divergence structures, radar viewing angle, and radar resolution on the flow appearance and data interpretation are examined.

One traditional measure of mesocyclone intensity is the shear measured between the relative peaks of incoming and outgoing Doppler velocity. In noncircular flows or flows where the vorticity structure is not uniform, shear is found to be an unreliable measure of mesocyclone intensity. A correction for shear is possible if the flow shape, internal structure, and orientation to the radar are known. Techniques to assess these characteristics from single-Doppler data are presented.

The elliptical mesocyclone model is compared with observations of the 20 May 1977 Del City, Oklahoma, mesocyclone from two Doppler radars. From characteristics of the flow estimated from single-Doppler data, a simulation of the mesocyclone is produced that closely approximates the observed single-Doppler fields. The associated model fields of vorticity and divergence are comparable in structure and magnitude to the fields determined from dual-Doppler analysis.

Abstract

The potential for single-Doppler radar determination of wind field characteristics in cyclonic flow is examined. The influence of the four independent first-order derivatives of a wind field, namely curvature, diffluence, downwind shear, and crosswind shear, upon the Doppler radial velocities is studied. Simple models of wind fields containing each of the derivatives defined in natural coordinates are presented. When only one derivative is present at a time, it has been found that there are unique signatures for diffluence and downwind shear and qualitatively similar signatures for curvature and crosswind shear. With a model incorporating all four derivatives, techniques are developed for the recovery of these derivatives. A method is also presented that corrects the mean speed estimate. It is concluded that in most cases the recovery of the downwind shear, diffluence, the sum of curvature and crosswind shear, and mean wind is possible to within 5 percent of the true values.

Application of these techniques to radar data collected from Hurricane Gloria is discussed. A storm strength indicator based on shearing deformation and distance of cyclone center yielded signs of the declining trend of the storm an hour or two before this trend manifested itself significantly in the wind speed as estimated by the Doppler radar, therefore suggesting potential as a forecast tool.

Abstract

—J. BLUNDEN, T. BOYER, AND E. BARTOW-GILLIES

Earth’s global climate system is vast, complex, and intricately interrelated. Many areas are influenced by global-scale phenomena, including the “triple dip” La Niña conditions that prevailed in the eastern Pacific Ocean nearly continuously from mid-2020 through all of 2022; by regional phenomena such as the positive winter and summer North Atlantic Oscillation that impacted weather in parts the Northern Hemisphere and the negative Indian Ocean dipole that impacted weather in parts of the Southern Hemisphere; and by more localized systems such as high-pressure heat domes that caused extreme heat in different areas of the world. Underlying all these natural short-term variabilities are long-term climate trends due to continuous increases since the beginning of the Industrial Revolution in the atmospheric concentrations of Earth’s major greenhouse gases.

In 2022, the annual global average carbon dioxide concentration in the atmosphere rose to 417.1±0.1 ppm, which is 50% greater than the pre-industrial level. Global mean tropospheric methane abundance was 165% higher than its pre-industrial level, and nitrous oxide was 24% higher. All three gases set new record-high atmospheric concentration levels in 2022.

Sea-surface temperature patterns in the tropical Pacific characteristic of La Niña and attendant atmospheric patterns tend to mitigate atmospheric heat gain at the global scale, but the annual global surface temperature across land and oceans was still among the six highest in records dating as far back as the mid-1800s. It was the warmest La Niña year on record. Many areas observed record or near-record heat. Europe as a whole observed its second-warmest year on record, with sixteen individual countries observing record warmth at the national scale. Records were shattered across the continent during the summer months as heatwaves plagued the region. On 18 July, 104 stations in France broke their all-time records. One day later, England recorded a temperature of 40°C for the first time ever. China experienced its second-warmest year and warmest summer on record. In the Southern Hemisphere, the average temperature across New Zealand reached a record high for the second year in a row. While Australia’s annual temperature was slightly below the 1991–2020 average, Onslow Airport in Western Australia reached 50.7°C on 13 January, equaling Australia's highest temperature on record.

While fewer in number and locations than record-high temperatures, record cold was also observed during the year. Southern Africa had its coldest August on record, with minimum temperatures as much as 5°C below normal over Angola, western Zambia, and northern Namibia. Cold outbreaks in the first half of December led to many record-low daily minimum temperature records in eastern Australia.

The effects of rising temperatures and extreme heat were apparent across the Northern Hemisphere, where snow-cover extent by June 2022 was the third smallest in the 56-year record, and the seasonal duration of lake ice cover was the fourth shortest since 1980. More frequent and intense heatwaves contributed to the second-greatest average mass balance loss for Alpine glaciers around the world since the start of the record in 1970. Glaciers in the Swiss Alps lost a record 6% of their volume. In South America, the combination of drought and heat left many central Andean glaciers snow free by mid-summer in early 2022; glacial ice has a much lower albedo than snow, leading to accelerated heating of the glacier. Across the global cryosphere, permafrost temperatures continued to reach record highs at many high-latitude and mountain locations.

In the high northern latitudes, the annual surface-air temperature across the Arctic was the fifth highest in the 123-year record. The seasonal Arctic minimum sea-ice extent, typically reached in September, was the 11th-smallest in the 43-year record; however, the amount of multiyear ice—ice that survives at least one summer melt season—remaining in the Arctic continued to decline. Since 2012, the Arctic has been nearly devoid of ice more than four years old.

In Antarctica, an unusually large amount of snow and ice fell over the continent in 2022 due to several landfalling atmospheric rivers, which contributed to the highest annual surface mass balance, 15% to 16% above the 1991–2020 normal, since the start of two reanalyses records dating to 1980. It was the second-warmest year on record for all five of the long-term staffed weather stations on the Antarctic Peninsula. In East Antarctica, a heatwave event led to a new all-time record-high temperature of −9.4°C—44°C above the March average—on 18 March at Dome C. This was followed by the collapse of the critically unstable Conger Ice Shelf. More than 100 daily low sea-ice extent and sea-ice area records were set in 2022, including two new all-time annual record lows in net sea-ice extent and area in February.

Across the world’s oceans, global mean sea level was record high for the 11th consecutive year, reaching 101.2 mm above the 1993 average when satellite altimetry measurements began, an increase of 3.3±0.7 over 2021. Globally-averaged ocean heat content was also record high in 2022, while the global sea-surface temperature was the sixth highest on record, equal with 2018. Approximately 58% of the ocean surface experienced at least one marine heatwave in 2022. In the Bay of Plenty, New Zealand’s longest continuous marine heatwave was recorded.

A total of 85 named tropical storms were observed during the Northern and Southern Hemisphere storm seasons, close to the 1991–2020 average of 87. There were three Category 5 tropical cyclones across the globe—two in the western North Pacific and one in the North Atlantic. This was the fewest Category 5 storms globally since 2017. Globally, the accumulated cyclone energy was the lowest since reliable records began in 1981. Regardless, some storms caused massive damage. In the North Atlantic, Hurricane Fiona became the most intense and most destructive tropical or post-tropical cyclone in Atlantic Canada’s history, while major Hurricane Ian killed more than 100 people and became the third costliest disaster in the United States, causing damage estimated at $113 billion U.S. dollars. In the South Indian Ocean, Tropical Cyclone Batsirai dropped 2044 mm of rain at Commerson Crater in Réunion. The storm also impacted Madagascar, where 121 fatalities were reported.

As is typical, some areas around the world were notably dry in 2022 and some were notably wet. In August, record high areas of land across the globe (6.2%) were experiencing extreme drought. Overall, 29% of land experienced moderate or worse categories of drought during the year. The largest drought footprint in the contiguous United States since 2012 (63%) was observed in late October. The record-breaking megadrought of central Chile continued in its 13th consecutive year, and 80-year record-low river levels in northern Argentina and Paraguay disrupted fluvial transport. In China, the Yangtze River reached record-low values. Much of equatorial eastern Africa had five consecutive below-normal rainy seasons by the end of 2022, with some areas receiving record-low precipitation totals for the year. This ongoing 2.5-year drought is the most extensive and persistent drought event in decades, and led to crop failure, millions of livestock deaths, water scarcity, and inflated prices for staple food items.

In South Asia, Pakistan received around three times its normal volume of monsoon precipitation in August, with some regions receiving up to eight times their expected monthly totals. Resulting floods affected over 30 million people, caused over 1700 fatalities, led to major crop and property losses, and was recorded as one of the world’s costliest natural disasters of all time. Near Rio de Janeiro, Brazil, Petrópolis received 530 mm in 24 hours on 15 February, about 2.5 times the monthly February average, leading to the worst disaster in the city since 1931 with over 230 fatalities.

On 14–15 January, the Hunga Tonga-Hunga Ha'apai submarine volcano in the South Pacific erupted multiple times. The injection of water into the atmosphere was unprecedented in both magnitude—far exceeding any previous values in the 17-year satellite record—and altitude as it penetrated into the mesosphere. The amount of water injected into the stratosphere is estimated to be 146±5 Terragrams, or ∼10% of the total amount in the stratosphere. It may take several years for the water plume to dissipate, and it is currently unknown whether this eruption will have any long-term climate effect.