Atmospheric Latent Heat Research Articles

Impact of May atmospheric latent heating over the Southeast Asian low-latitude highlands on interannual variability in the Meiyu onset date

The Yangtze-Huai River Basin (YHRB) is an economically developed area and one of the most densely populated regions in China. Based on ERA5 reanalysis and Meiyu monitoring data, this study investigates the relationship between interannual variability in atmospheric latent heating (LH) over the Southeast Asian low-latitude highlands (SEALLH) during early summer and the Meiyu onset date (MOD) in the YHRB. The mean MOD is 9 June, with a quasi-6-year interannual oscillation period. A stronger early summer LH over the SEALLH coincides with earlier MOD; below 500 hPa, LH increases with height and drives an anomalous southwesterly airflow from the Bay of Bengal (BOB) to the SEALLH via β effects. Circulation over the western North Pacific (WNP) is dominated by an anomalous anticyclone, causing the WNP subtropical high (WNPSH) to extend westward and northward. Continuous positive LH anomalies over the SEALLH cause anomalous southwesterly winds to transport moist enthalpy air masses from low latitudes into the YHRB, resulting in abnormal convergence there. Favorable dynamic and thermal conditions drive enhanced precipitation over the YHRB, eventually leading to a MOD that is 13 days earlier than that during periods when early summer LH is relatively weak over the SEALLH. This result potentially supports our capacity to predict the Meiyu in the YHRB.

Role of Winter Soil Moisture in Subsequent Summer Thermal Anomalies on the Tibetan Plateau

Abstract The Tibetan Plateau (TP) exerts strong powerful thermal forcing, which plays a vital role in influencing weather–climate variations in Asia and even the Northern Hemisphere. However, the causes of thermal variation over the TP have not been fully revealed. Here, the role of winter soil moisture (SM) in subsequent summer thermal anomalies on the TP was investigated through multisource datasets for the period 1979–2014. Results indicate a significant positive relationship (r = 0.52) between winter SM and subsequent summer-mean surface air temperature (SAT). Further investigations show that more (less) winter SM results in abundant (deficient) atmospheric water vapor in subsequent summer owing to its persistence. Furthermore, Earth’s surface energy budget equation confirms that strengthened (weakened) surface downward longwave radiation caused by increased (decreased) water vapor is the dominant factor leading to SAT variations, even more significant for nocturnal SAT. The winter SM–atmospheric temperature positive relationship can extend from the surface to 200 hPa over the TP. In addition, the enhanced (weakened) atmospheric latent heat release associated with increased (decreased) water vapor content may be another important factor contributing to changes in atmospheric temperatures over the TP. Therefore, our results contribute to a better understanding of the effect of land–surface processes on thermal anomalies over the TP. Significance Statement Understanding the causes of thermal anomalies over the Tibetan Plateau (TP) is crucial for weather and climate variations, but the role of winter soil moisture (SM) in subsequent summer thermal effects is largely unknown. This study investigated the relationship between winter SM and thermal anomalies in subsequent summers on the TP. Results show that the above (below)-normal winter SM will cause warm (cold) atmospheric temperatures in the subsequent summer. The above (below)-normal winter SM anomalies bring increased (decreased) atmospheric water vapor in summer owing to its persistence, resulting in strengthened (weakened) downward longwave radiation and atmospheric latent heat release to heat (cool) the atmosphere.

Tropical Surface Temperature and Atmospheric Latent Heating: A Whole-Tropics Perspective Based on TRMM and ERA5 Datasets

Tropical surface temperature (TST) and its connection with atmospheric heating, including tropical latent heating (TLH), is essential to the interannual variability of tropical atmospheric circulation and global teleconnection. Utilizing seasonally averaged satellite-based TRMM precipitation data as a proxy of TLH and ERA5-based TST data from 1998 to 2018, we reveal some new features in terms of cross-hemispheric connection in the TLH and TST variability by decomposing them into equatorially symmetric and antisymmetric components. We find surprisingly that the spatial patterns of TLH projected upon the first principal components (PC1) of symmetric and antisymmetric TSTs over the whole-tropics, are very similar to each other, seemingly at odds with the classic Mastuno–Gill theory. The similarity in the projected TLH patterns is mainly because the PC1s of symmetric and antisymmetric TSTs co-vary temporally with a very high correlation. We use the spatial pattern of local correlation between symmetric and antisymmetric components, for both TST and TLH to depict geographic dependence of the symmetric–antisymmetric connection. We suggest that a whole-tropics perspective, which takes the different but connected nature of equatorially symmetric and antisymmetric modes across the whole-tropics into consideration, may well be useful in understanding and predicting tropical climate variability because clarifying the puzzle raised in this research from such a perspective about the consistency between the observation and the classic Mastuno–Gill theory is directly related to the fundamental dynamics of tropical systems, such as Walker circulation, monsoons, and their relationship with underlying land and sea conditions.

Atmospheric heat source/sink response to the snow depth over the Tibetan plateau in melting season: A modeling study

AbstractAtmospheric heat source/sink (AHS) of the Tibetan Plateau (TP) is strongly linked to the local snow depth. Based on the numerical simulations with Weather Research and Forecasting (WRF) model, connections between snow depth and the AHS across the TP in melting season (May–August) are investigated. Compared with the derived AHS from reanalysis data, spatial patterns and monthly evolution of the AHS are reasonably reproduced by the WRF model. The AHS can be significantly reduced (~80 W m−2) by snow over both the western and southeastern TP in early May, which is mainly ascribed to the weakened surface sensible heat (SH) and atmospheric latent heat. Obvious cooling effect of snow on the AHS is still found over the western TP even in mid‐summer. However, the cooling strength in late summer has declined to one‐fifth that of May and it is totally dominated by the reduced surface SH. As to the southeastern TP, snow is thin and scattered since June, resulting that the AHS response to snow depth declines significantly and even vanishes in mid‐summer. It is further clarified that there is no elevation‐dependent relationship between the AHS and snow over the TP, while the cooling amplitudes of the AHS varied with altitude are mainly determined by the corresponding snow depth.

To Bring an End to Global Warming -Make it Rain

Over the last 40 years, the average global temperature has risen by 1°C and the catastrophic storm risk has tripled, as the latent heating power of the atmosphere grew, driven by the 15% increase in the average global concentration of the primary greenhouse gas, water vapor. Global warming and the catastrophic storm risk only worsen as the average global concentration of water vapor continues to increase at 0.4% yr.-1driving the average global temperature up at 0.2°C per decade. As the latent heating power of the atmosphere rose, the annual number of catastrophic, weather-related events increased to over 750, by 2019, 525 above the 1980 baseline of 225 annual events. Since 1980, these weather-related catastrophic events have taken tens of thousands of lives, wiped out whole communities while wreaking4.6 trillion dollars in cumulative worldwide weather-related destruction, of which 2.4 trillion dollars is the result of global warming driven increasing atmospheric latent heating power, as shown by the close correlation of major weather-related events with the average global temperature record (correlation coefficient 0.84).

Topographic Controls on the Distribution of Summer Monsoon Precipitation over South Asia

We use a high-resolution (9 km) one-way nested regional climate modeling framework to systematically evaluate individual and aggregate roles of various components of topography in the precipitation distribution during South Asian Summer Monsoon (SASM). While it is not the arrival but the north-northwestward extent of summer monsoon precipitation that is regulated by the presence of elevated surfaces in South Asia, both the thermal and mechanical topographic forcing play a role. Topography not only provides mechanical uplifting and prevents dry air entrainment, it also helps regulate the strength of upper level anticyclone and tropical easterly jet, all of which are necessary to sustain a conducive environment for monsoon precipitation in the South Asian summer. In turn, precipitation feedbacks positively by accelerating the tropospheric diabatic warming through atmospheric latent heating and by maintaining the moisture supply through local recycling. Modifications in the topography, particularly the removal of the Tibetan Plateau, substantially modulates the moist flow in the lower troposphere and the anticyclonic circulation in the upper troposphere to an extent that the northern branch of SASM becomes substantially weaker. These changes give way to negative precipitation anomalies and eventually to an environment where the cycle of feedbacks that runs the process of monsoon progression over land becomes inefficient. The design of topographic modification experiments can influence precipitation response; therefore, we advise caution in the interpretation of these and previously reported results.

Assessing Global and Regional Effects of Reconstructed Land-Use and Land-Cover Change on Climate since 1950 Using a Coupled Land–Atmosphere–Ocean Model

AbstractLand-use and land-cover change (LULCC) is one of the most important forcings affecting climate in the past century. This study evaluates the global and regional LULCC impacts in 1950–2015 by employing an annually updated LULCC map in a coupled land–atmosphere–ocean model. The difference between LULCC and control experiments shows an overall land surface temperature (LST) increase by 0.48 K in the LULCC regions and a widespread LST decrease by 0.18 K outside the LULCC regions. A decomposed temperature metric (DTM) is applied to quantify the relative contribution of surface processes to temperature changes. Furthermore, while precipitation in the LULCC areas is reduced in agreement with declined evaporation, LULCC causes a southward displacement of the intertropical convergence zone (ITCZ) with a narrowing by 0.5°, leading to a tripole anomalous precipitation pattern over the warm pool. The DTM shows that the temperature response in LULCC regions results from the competing effect between increased albedo (cooling) and reduced evaporation (warming). The reduced evaporation indicates less atmospheric latent heat release in convective processes and thus a drier and cooler troposphere, resulting in a reduction in surface cooling outside the LULCC regions. The southward shift of the ITCZ implies a northward cross-equatorial energy transport anomaly in response to reduced latent/sensible heat of the atmosphere in the Northern Hemisphere, where LULCC is more intensive. Tropospheric cooling results in the equatorward shift of the upper-tropospheric westerly jet in both hemispheres, which, in turn, leads to an equatorward narrowing of the Hadley circulation and ITCZ.

Urbanization in an Underdeveloped City—Nanning, China and its Impact on a Heavy Rainfall Event in July

AbstractA rapid urbanization process is undergoing in China, exerting significant impacts on local weather and climate. However, rare study pays attention to the underdeveloped regions. An underdeveloped city of Nanning, in southwestern China, is experiencing considerable urbanization. The present urban area of Nanning is 2.8 times larger than that in 1980. Using the Weather Research and Forecasting model coupled with an urban canopy model, two experiments with different urban land use are designed to investigate impacts of Nanning urbanization on a rainstorm occurred in July 2009. Results show simulated precipitation can be significantly improved by the experiment with urbanization effects included. Urbanization leads to a warmer and drier surface, which is favored for the formation of near‐surface low pressure and wind convergence over the city. But the warm‐dry air near urban surface cannot lead to deep convective activity, resulting in urban‐enhanced precipitation is dominated by the large‐scale precipitation. Such enhanced large‐scale precipitation has a positive contribution to atmospheric latent heat release, inducing a heating source in middle troposphere over the urban area. Then the cyclonic circulation around Nanning is intensified according to thermal adaption theory. The enhanced cyclonic circulation is beneficial for stronger moisture convergence and warm‐moist southwesterly flow transports into Guangxi from tropical oceans. Vertical motion is amplified by the coastal orographic uplift effect, and wind convergence over the Nanning downstream is also enhanced. Therefore, the convective precipitation dominantly enhances over the coastal and city's downstream regions, which is different from the urban‐increased large‐scale precipitation in the Nanning City area.

Global Circulation of Latent Heat in the Earth’s Atmosphere According to Data from Satellite Radiothermovision

The methodical bases and some results of using satellite radiothermovision to study global atmospheric latent heat circulation according to the data of regular satellite radiothermal monitoring are described. This approach does not use an a priori model of circulation; it implements an objective procedure for the analysis of the dynamics of periodically measured fields of total precipitable water. The reconstructed directions and the values of the mean-zonal transport velocity, the average position of the thermal equator at ~5° N, and the positions of the axis of the intertropical convergence zone over individual oceans are very consistent with the known results of independent observations and numerical modeling. Some problematic aspects of the analysis procedure are discussed.

Impacts of Diabatic Heating on the Genesis and Development of an Inner Tibetan Plateau Vortex

AbstractA short‐lived (less than 2 days) inner Tibetan Plateau vortex (TPV) with heavy precipitation was simulated using the Weather Research and Forecasting model, and the sensitivity experiments with and without surface diabatic heating and atmospheric latent heating released by precipitation were conducted to investigate the roles of surface and atmospheric diabatic heating on the genesis and development of TPV. Results show that before the genesis of the TPV, there is an anomalous increase of surface heating flux; while a disturbance of potential vorticity (PV) at 500 hPa moves over this region, the PV is enhanced and induces the generation of TPV. The precipitation in turn reduces the surface heating flux. The release of condensation latent heating by precipitation during the development of TPV does not further contribute to its reinforcement. Instead, it is more likely a result of precipitation, tending to stabilize the atmosphere and conducive to the decay of TPV. The PV diagnoses further reveal that in the early phase, the strong diabatic heating in the lower levels associated with the increasing surface heating flux has a significantly positive contribution to the generation of the TPV. The precipitation results in the decrease of positive diabatic heating contribution at low levels. Inversely, the diabatic heating in the middle levels associated with the released latent heating of precipitation condensation tends to have the negative contribution to TPV, resulting in the decay of TPV. The study of this case can deepen our understanding on the mechanism of TPV and its precipitation.

Humid heat and climate change

Extreme heat events cause significant societal impacts, prompting much concern and research about possible changes to their frequency and intensity as the climate warms. However, to date, extremes in air temperature have been emphasised at the expense of ‘heat-humidity’ indices, measures which incorporate the effect of atmospheric latent heat content on heat stress and provide a more complete picture of the thermal environment for human thermoregulation. This progress report restores balance by reviewing recent developments in the understanding of how heat-humidity indices have changed, and may continue to, as the climate warms further. The literature indicates that a concurrent rise in temperature and absolute humidity has already increased the frequency of potentially deadly conditions, and has reduced labour potential worldwide. More serious consequences may result if mitigation efforts are unsuccessful. The energetic basis of a heat-humidity perspective has permitted researchers to identify, for example, that by the end of the century, substantial parts of the Earth’s surface may be too hot and humid for human thermoregulation. Such consequences are avoided for less pessimistic scenarios of climate warming, but the societal impacts may still be very severe, as densely-populated low-latitude environments emerge as particularly at risk when a humid heat perspective is adopted. Counter to air temperature, changes in mean heat-humidity indices are actually amongst the largest worldwide at lower latitudes, where only small increases in the mean may be required to substantially enhance the frequency of dangerous conditions. The report concludes by outlining areas requiring improved process understanding, and it highlights the urgent role for societal adaptation if the worst impacts from rising humid heat are to be avoided.

Sources of errors in the simulation of south Asian summer monsoon in the CMIP5 GCMs

Accurate simulation of the South Asian summer monsoon (SAM) is still an unresolved challenge. There has not been a benchmark effort to decipher the origin of undesired yet virtually invariable unsuccessfulness of general circulation models (GCMs) over this region. This study analyzes a large ensemble of CMIP5 GCMs to show that most of the simulation errors in the precipitation distribution and their driving mechanisms are systematic and of similar nature across the GCMs, with biases in meridional differential heating playing a critical role in determining the timing of monsoon onset over land, the magnitude of seasonal precipitation distribution and the trajectories of monsoon depressions. Errors in the pre-monsoon heat low over the lower latitudes and atmospheric latent heating over the slopes of Himalayas and Karakoram Range induce significant errors in the atmospheric circulations and meridional differential heating. Lack of timely precipitation further exacerbates such errors by limiting local moisture recycling and latent heating aloft from convection. Most of the summer monsoon errors and their sources are reproducible in the land–atmosphere configuration of a GCM when it is configured at horizontal grid spacing comparable to the CMIP5 GCMs. While an increase in resolution overcomes many modeling challenges, coarse resolution is not necessarily the primary driver in the exhibition of errors over South Asia. These results highlight the importance of previously less well known pre-monsoon mechanisms that critically influence the strength of SAM in the GCMs and highlight the importance of land–atmosphere interactions in the development and maintenance of SAM.

Two types of summertime heating over the Asian large-scale orography and excitation of potential-vorticity forcing I. Over Tibetan Plateau

Based on numerical simulation, this study explored the characteristics and interactions of surface sensible heating and atmospheric latent heating over the main part of the Tibetan Plateau, i.e., terrain at elevations >2 km in summer. The impacts of these two types of heating on local vertical motion and monsoonal meridional circulation were compared. Theoretical analysis and numerical experimentation demonstrated that by changing the configuration of the upper-tropospheric air temperature and circulation, the two types of heating could generate both minimum absolute vorticity and abnormal potential vorticity forcing near the tropopause, enhance the meridional circulation of the Asian summer monsoon, and produce an eastward- propagating Rossby wave train within the mid-latitude westerly flow. Consequently, the manifestations of these features were shown to influence the circulation of the Northern Hemisphere.

Tropical Gravity Wave Momentum Fluxes and Latent Heating Distributions

Recent satellite determinations of global distributions of absolute gravity wave (GW) momentum fluxes in the lower stratosphere show maxima over the summer subtropical continents and little evidence of GW momentum fluxes associated with the intertropical convergence zone (ITCZ). This seems to be at odds with parameterizations for GW momentum fluxes, where the source is a function of latent heating rates, which are largest in the region of the ITCZ in terms of monthly averages. The authors have examined global distributions of atmospheric latent heating, cloud-top-pressure altitudes, and lower-stratosphere absolute GW momentum fluxes and have found that monthly averages of the lower-stratosphere GW momentum fluxes more closely resemble the monthly mean cloud-top altitudes rather than the monthly mean rates of latent heating. These regions of highest cloud-top altitudes occur when rates of latent heating are largest on the time scale of cloud growth. This, plus previously published studies, suggests that convective sources for stratospheric GW momentum fluxes, being a function of the rate of latent heating, will require either a climate model to correctly model this rate of latent heating or some ad hoc adjustments to account for shortcomings in a climate model’s land–sea differences in convective latent heating.

Why does the Antarctic Peninsula Warm in climate simulations?

The Antarctic Peninsula has warmed significantly since the 1950s. This pronounced and isolated warming trend is collectively captured by 29 twentieth-century climate hindcasts participating in the version 3 Coupled Model Intercomparison Project. To understand the factors driving warming trends in the hindcasts, we examine trends in Peninsula region’s atmospheric heat budget in every simulation. We find that atmospheric latent heat release increases in nearly all hindcasts. These increases are generally anthropogenic in origin, and account for about 60% of the ensemble-mean warming trend in the Peninsula. They are driven primarily by well-understood features of the anthropogenic intensification of global hydrological cycle. As sea surface temperature increases, moisture contained in atmospheric flows increases. When such flows are forced to ascend the Peninsula’s topography, enhanced local latent heat release results. The mechanism driving the warming of the Antarctic Peninsula is therefore clear in the models. Evidence for a similar mechanism operating in the real world is seen in the increasing snow accumulation rates inferred from ice cores drilled in the Peninsula. However, the relative importance of this mechanism and other processes previously identified as potentially causing the observed warming, such as the recent sea ice retreat in the Bellingshausen Sea, is difficult to assess. Thus the relevance of the simulated warming mechanism to the observed warming is unclear, in spite of its robustness in the models.

Radiation entropy flux and entropy production of the Earth system

[1] The study of the Earth's radiation entropy flux at the top of the atmosphere is reviewed with an emphasis on its estimation methods. Existing expressions for calculating radiation entropy flux scattered in different disciplines are surveyed, and their applicabilities are examined. It is found that the Earth's net radiation entropy flux estimated from these various expressions can differ substantially, more than the typical value of the entropy production rate associated with the atmospheric latent heat process. Comparison analysis shows that the commonly used expression of radiation entropy flux as the ratio of radiation energy flux to absolute temperature underestimates the Earth's radiation entropy flux by >30%. Theoretical analysis reveals that the large difference in the Earth's reflected solar radiation entropy flux among the different expressions arises mainly from the difference of the Earth's reflection properties (i.e., Lambertian or specular) assumed in these expressions. For the Earth system with typical shortwave albedo of 0.30 and longwave emissivity between 0.50 and 1.00, the Earth's net radiation entropy flux derived from the most accurate Planck's spectral expression ranges from 1.272 to 1.284 W m−2 K−1, amounting to the overall Earth's entropy production rate from 6.481 × 1014 to 6.547 × 1014 W K−1.

Role of deep soil moisture in modulating climate in the Amazon rainforest

Both local and large‐scale processes affect the Amazon hydrologic cycle. We investigate the impact of deep soils on the atmosphere through local feedbacks. The Simple Biosphere model, version 3 (SiB3), is coupled to a single column model. Historically, land surface schemes parameterize soil moisture stress based on shallow soils and incorrectly capture seasonal cycles in the Amazon. Following observations, SiB3 is updated to allow deep roots to access soil moisture at depth. The new (“Unstressed”) version of SiB3 has a stronger hydrologic cycle, with increased evapotranspiration and moisture export during the dry season. The boundary layer responds through changes in its depth, relative humidity, and turbulent kinetic energy, and these changes feed back to influence wet season onset and intensity. Differences in atmospheric latent heating could affect circulation in a global model. The results have important consequences for modeling the Amazon hydrologic cycle and climate in global climate models.

Global-Scale Energy and Freshwater Balance in Glacial Climate: A Comparison of Three PMIP2 LGM Simulations

Abstract Three coupled atmosphere–ocean general circulation model (AOGCM) simulations of the Last Glacial Maximum (LGM: about 21 000 yr before present), conducted under the protocol of the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2), have been analyzed from a viewpoint of large-scale energy and freshwater balance. Atmospheric latent heat (LH) transport decreases at most latitudes due to reduced water vapor content in the lower troposphere, and dry static energy (DSE) transport in northern midlatitudes increases and changes the intensity contrast between the Pacific and Atlantic regions due to enhanced stationary waves over the North American ice sheets. In low latitudes, even with an intensified Hadley circulation in the Northern Hemisphere (NH), reduced DSE transport by the mean zonal circulation as well as a reduced equatorward LH transport is observed. The oceanic heat transport at NH midlatitudes increases owing to intensified subpolar gyres, and the Atlantic heat transport at low latitudes increases in all models whether or not meridional overturning circulation (MOC) intensifies. As a result, total poleward energy transport at the LGM increases in NH mid- and low latitudes in all models. Oceanic freshwater transport decreases, compensating for the response of the atmospheric water vapor transport. These responses in the atmosphere and ocean make the northern North Atlantic Ocean cold and relatively fresh, and the Southern Ocean relatively warm and saline. This is a common and robust feature in all models. The resultant ocean densities and ocean MOC response, however, show model dependency.

Toward understanding the double Intertropical Convergence Zone pathology in coupled ocean‐atmosphere general circulation models

This paper first analyzes structures of the double Intertropical Convergence Zone (ITCZ) in the central equatorial Pacific simulated by three coupled ocean‐atmosphere general circulation models in terms of sea surface temperatures, surface precipitation, and surface winds. It then describes the projection of the double ITCZ in the equatorial upper ocean. It is shown that the surface wind convergences, associated with the zonally oriented double rainbands on both sides of the equator, also correspond to surface wind curls that are favorable to Ekman pumping immediately poleward of the rainbands. The pumping results in a thermocline ridge south of the equator in the central equatorial Pacific, causing a significant overestimation of the eastward South Equatorial Counter Current that advects warm water eastward. A positive feedback mechanism is then described for the amplification of the double ITCZ in the coupled models from initial biases in stand‐alone atmospheric models through the following chain of interactions: precipitation (atmospheric latent heating), surface wind convergences, surface wind curls, Ekman pumping, South Equatorial Counter Current, and eastward advection of ocean temperature. This pathology provides a possible means to address the longstanding double ITCZ problem in coupled models.

The Response Time of the Temperature of the Equatorial Troposphere to ENSO Heating

Abstract Air temperature anomalies, averaged over the troposphere to 200 mb and around the earth from 10°S to 10°N, lag the similarly averaged El Niño–Southern Oscillation (ENSO) atmospheric latent heating anomalies by about one month. Most of the latent heating is balanced by vertical adiabatic cooling although the zonally averaged imbalance is larger than is typical locally in the Tropics. The excess latent heating heats the atmosphere and generates a temperature anomaly. As the temperature anomaly rises, the atmosphere loses heat until the residual heating is balanced by anomalous cooling. By then the temperature anomaly is typically about 0.4°C. Analysis of the thermodynamic energy equation shows that the ENSO heat loss is highly linearly correlated with the air temperature anomaly averaged over the equatorial troposphere; that is, the adjustment to the residual anomalous heating (or cooling) is Newtonian. Consistent with the observed one-month lag, the Newtonian e-folding time is about 35 days. Similar results apply for latitude bands 5°S–5°N and 15°S–15°N (Newtonian cooling times of 29 and 46 days, respectively). The heat loss is mainly through meridional sensible heat flux rather than radiation. Much of the anomalous cooling is due to the mean meridional flow that diverges more temperature anomaly aloft than it converges near the surface.