Several recommendations have been proposed for detecting land use and land cover change (LULCC) on the environment from, observed climatic records and to modeling to improve its understanding and its impacts on climate. Researchers need to detect LULCCs accurately at appropriate scales within a specified time period to better understand their impacts on climate and provide improved estimates of future climate. The US Climate Reference Network (USCRN) can be helpful in monitoring impacts of LULCC on near-surface atmospheric conditions, including temperature. The USCRN measures temperature, precipitation, solar radiation, and ground or skin temperature. It is recommended that the National Climatic Data Center (NCDC) and other climate monitoring agencies develop plans and seek funds to address any monitoring biases that are identified and for which detailed analyses have not been completed

This Part I presents selected major features of the quasi-stationary (monthly mean) wave field in the troposphere and stratosphere of the Southern Hemisphere. It is confirmed that the quasi-stationary wave with zonal wavenumber 1 (QS-wave 1) is by far the dominant component of the geopotential height field at tropospheric and stratospheric levels. The amplitude of this wave is largest at about 60°S all year round and reaches a maximum during September and October in the upper troposphere and stratosphere. Analysis of the Elliasen-Palm flux vector suggests that at high latitudes the quasi-stationary wave field is primarily forced from lower latitudes, most prominently from the Indian Ocean region during June and October. Orographic and thermal forcing from Antarctic regions seem to also be important sources of wave activity in polar and high latitudes, particularly over southern South America and the Atlantic Ocean. The contribution to the quasi-stationary flow by the transient component of the flow is also analyzed. This analysis suggests that at high latitudes, the low-frequency transients act to strengthen QS-wave 1, while high-frequency transients weaken it. The values found for these contributions suggest that the low-frequency component is dominant.

The authors analyzed an extensive precipitation dataset available for the Mexico City basin that included hourly precipitation in various sectors of the city from 1993 to 2007. Observations indicated that significant changes occurred in the timing and number of intense events (precipitation rate >20 mm h⁻¹) over this time period. Alternative hypotheses that changes in the emission of aerosol pollutants or in the land use can result in the observed variations are tested. The Weather Research and Forecasting Model was used to simulate September precipitation from 2002 to 2011 at the peak of the wet season. Changes were introduced to the microphysical scheme as a proxy for differences in the aerosol population and the droplet activation spectra. Simulations were also performed with the land use of the urban areas set up to represent older and more current conditions. Results indicate that increased pollution (decreased urban area) led to an average precipitation decrease over the mountain areas of 20%–40% (10%–20%) and an increase of 20% (30%) to the east of Mexico City. The timing of intense precipitation shifts from 1900 to 1600 LT for the polluted and decreased urban area cases when compared to a control experiment. These results add valuable information about how precipitation is modified by complex terrain and surface exchange processes in large urban areas under wet conditions.

In this Part II the authors investigate the role that Antarctic elevations, the rest of the world orography, thermal forcing from lower latitudes, and the transient eddy component of the flow play on the generation of the quasi-stationary wave field in the Southern Hemisphere. An approach based on the UCLA GCM is followed. Results from a control simulation with full orography and from experiments without the Antarctic elevations and without the rest of the world orography, suggest that the quasi-stationary wave with zonal wavenumber 1 (QS-wave 1) around Antarctica is primarily generated by mechanisms other than the Antarctic elevations. Comparison of a three-dimensional Eliassen-Palm flux vector in the control simulation, and those where the Antarctic elevation and the rest of the world orography are removed, suggests that wave activity propagates both from the subtropics and from polar latitudes. Although in qualitative agreement with results of Part I, the horizontal and vertical structure of these remote forcings is different in the simulations where a more barotropic wave train is generated from lower and polar latitudes. Antarctica is indeed a source of wave activity but unlike observations it is confined to polar regions at tropospheric levels. Additional evidence of thermal forcing was found in an experiment without orographic elevations and zonal asymmetries south of 45°S. It is found that QS-wave 2 is most affected by the zonal asymmetries in sea ice and SST. The effects of the transient component of the flow were also analyzed. The heat transport by the transient eddies in the absence of Antarctic elevations is greater than in the control simulation consistent with a warming of the polar region. Analysis of the contribution by the low-pass and high-pass transients to QS-wave 1 in the control simulation reveals a very different behavior than in Part I. In the control simulation, the low-pass transients and QS-wave 1 are mostly in opposition of phase. High-frequency transients are uncorrelated with QS-wave 1 in all cases. In the experiments without Antarctic elevations or the rest of the world orography, low-pass transients are in phase with QS-wave 1 over high and polar latitudes. In summary, the results of this study suggest that the generation of QS-wave 1 at high latitudes is predominantly from lower latitudes.