The bulk radar reflectivity structures, 85-and 37-GHz brightness temperatures, and lightning characteristics of precipitating systems in tropical Africa, South America, the east Pacific, and west Pacific are documented using data from the Tropical Rainfall Measuring Mission (TRMM) satellite during August, September, and October of 1998. The particular focus is on precipitation features [defined as a contiguous area Ն75 km 2 with either a near-surface reflectivity Ն20 dBZ or an 85-GHz polarization-corrected temperature (PCT) Յ 250 K] with appreciable rainfall, which account for the bulk of the total rainfall and lightning flash density in their respective regions. Systems over the tropical continents typically have greater magnitudes of reflectivity extending to higher altitudes than tropical oceanic systems. This is consistent with the observation of stronger ice scattering signatures (lower 85-and 37-GHz PCT) in the systems over land. However, when normalized by reflectivity heights, tropical continental features consistently have higher 85-GHz PCT than tropical oceanic features. It is inferred that greater supercooled water contents aloft in the tropical continental systems contribute to this brightness temperature difference.Lightning (as detected by the Lightning Imaging Sensor) is much more likely in tropical continental features than tropical oceanic features with similar brightness temperatures or similar reflectivity heights. Vertical profiles of radar reflectivity add additional information to the nonunique lightning-brightness temperature relationships showing that features with lightning tend to have greater magnitudes of reflectivity and smaller decreases of reflectivity with height above the freezing level than systems without detected lightning.Regional comparisons of the lightning, radar, and microwave signatures of precipitating features show that, over the oceans, the west Pacific has the highest frequency of intense precipitation features (by minimum PCT or maximum reflectivity height). Over land, the intense precipitation features occur more frequently in Africa. These observations are consistent with the relative differences in lightning flash density between the land and ocean regions. The quantitative database of land and ocean features presented here provides a substantial observational framework against which cloud and radiative transfer model results can be tested.

Optimized regional climate simulations are conducted using the Polar MM5, a version of the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5), with a 60-km horizontal resolution domain over North America during the Last Glacial Maximum (LGM, 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). The objective is to describe the LGM annual cycle at high spatial resolution with an emphasis on the winter atmospheric circulation. Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO 2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. Polar MM5 produces a substantially different atmospheric response to the LGM boundary conditions than CCM3 and other recent GCM simulations. In particular, from November to April the upper-level flow is split around a blocking anticyclone over the LIS, with a northern branch over the Canadian Arctic and a southern branch impacting southern North America. The split flow pattern is most pronounced in January and transitions into a single, consolidated jet stream that migrates northward over the LIS during summer. Sensitivity experiments indicate that the winter split flow in Polar MM5 is primarily due to mechanical forcing by LIS, although model physics and resolution also contribute to the simulated flow configuration. Polar MM5 LGM results are generally consistent with proxy climate estimates in the western United States, Alaska, and the Canadian Arctic and may help resolve some long-standing discrepancies between proxy data and previous simulations of the LGM climate.

Regional climate simulations are conducted using the Polar fifth-generation Pennsylvania State University (PSU)-NCAR Mesoscale Model (MM5) with a 60-km horizontal resolution domain over North America to explore the summer climate of the Last Glacial Maximum (LGM: 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5.LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO 2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level.The simulated LGM summer climate is characterized by a pronounced low-level thermal gradient along the southern margin of the LIS resulting from the juxtaposition of the cold ice sheet and adjacent warm ice-free land surface. This sharp thermal gradient anchors the midtropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet, some of which produce copious liquid precipitation along and south of the LIS terminus. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow (resembling a contemporary Great Plains low-level jet configuration) in advance of the cyclone is drawn up the ice sheet slope. Composites of wet and dry periods on the LIS southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the summer rain events, it may be possible to reconcile the model depiction of wet conditions on the LIS southern margin during the LGM summer with the widely accepted interpretation of aridity across the Great Plains based on geological proxy evidence.

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