Lapinov, Lapinova, Petrov and Ferrusca (2020) used twelve years of the GEOS-FPIT global assimilation model coupled with radiative-transfer calculations to show that three high Eastern Pamirs sites — Koluch-Kul (~4475 m) and Shorbulak (~4289 m) in Tajikistan and Muztagh-Ata (4526 m) in China — have clear-sky 230 GHz zenith-opacity statistics that rival APEX and fill the January–February high-frequency observing gap of ALMA/APEX, and they proposed these sites as the first points of a future submillimetre array ('PaLMA'). Their evaluation is explicitly preliminary: cloud absorption is excluded because global weather models cannot resolve line-of-sight cloud structure, the statistics are zenith-only and clear-sky-only, instrument-specific performance and interannual variability were deferred to 'forthcoming papers', and no wind, temperature, or topographic data for candidate telescope pads exist. The open problem is the complete, quantified, cloud-inclusive site characterization of the Eastern Pamirs — cloud-affected opacity/transmission distributions, multi-wavelength instrument-specific performance including interannual variability, and ground-level site data — sufficient for an evidence-based siting decision on a submillimetre telescope or array in the Eastern Hemisphere. This core remains open: no published analysis yet quantifies cloud-affected mm/sub-mm transmission or instrument-specific statistics for these sites, and the partial progress that does exist (optical-band cloud statistics at Muztagh-Ata, regional reanalysis site ratings) does not resolve it.

Background

Ground-based astronomy at millimetre (mm) and submillimetre (sub-mm) wavelengths (roughly 1–3 mm and 0.3–1 mm) is limited by absorption and thermal emission from atmospheric water vapour. The key quantity is the precipitable water vapour (PWV, the water-vapour column depth expressed as an equivalent layer of condensed liquid), which together with the dry-air column determines the zenith optical depth τ\tau (the attenuation along the vertical path); observers quote transmission in the atmospheric 'windows' near 1.3 mm (230 GHz) and 0.87 mm (345 GHz). Because PWV falls roughly exponentially with altitude, the premier facilities sit on high, extremely dry plateaus: the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) telescope on the Chajnantor plateau in Chile, and the James Clerk Maxwell Telescope (JCMT) on Mauna Kea, Hawaii. Two structural limitations of this geography motivate the question. First, nearly all stations of the Event Horizon Telescope (EHT, the global very-long-baseline interferometry network that imaged the M87 black hole shadow) lie in the Western Hemisphere, limiting its baseline coverage. Second, January–February is the wet season at Chajnantor, when ALMA and APEX high-frequency observing is largely curtailed; a high, dry Northern-Hemisphere site would fill this seasonal gap.

Soviet expeditions from 1962 and subsequent measurements through the 1980s at the Shorbulak observatory (4350 m) identified the Eastern Pamirs — a high plateau at 4300–4500 m east of the Tajik–Chinese border, ringed by ~7000 m ice-covered ranges that dry the descending air — as exceptionally dry and stable, with winter PWV sometimes as low as 0.27 mm. Lapinov et al. (2020) revisited this with a modern method: they integrated radiative-transfer equations through 12 years (2008–2019) of output from GEOS-FPIT, a NASA Global Modeling and Assimilation Office reanalysis that assimilates ground, air-borne, and satellite data into temperature, pressure, and water-vapour fields on a 4-dimensional grid at 3-hour resolution, for more than 40 sites worldwide. They found that at 230 GHz the Eastern Pamirs candidates outperform APEX in January–February and remain better than LMT and IRAM-30m year-round, while the historically favoured Suffa plateau (Uzbekistan, 2300 m) is unsuitable even at 230 GHz and suffers heavy fractional cloud cover from November to May (Hellemeier et al. 2018). They concluded that the Pamirs sites should be considered as the first points of a future submillimetre antenna array with an angular resolution an order of magnitude better than ALMA, and that an equivalent Eastern-Hemisphere site assessment for Tibet (Ali; Ye et al. 2015, Kuo 2017) shows a wetter summer than the Pamirs.

The 2020 evaluation was, however, deliberately preliminary in exactly the respects its authors flagged. Cloud absorption is absent from the model statistics, because global weather models cannot predict detailed cloud structure along a line of sight, so the impact of fractional cloud cover at the Pamirs sites is unquantified. The analysis is restricted to zenith opacity distributions with a fixed scaling between 129, 230, and 345 GHz, not the performance of any actual telescope. Per-instrument multi-wavelength statistics and interannual variability were promised for 'forthcoming papers'. And a complete site evaluation, in the standard sense of a site-testing campaign (as carried out for optical astronomy at Muztagh-Ata by Xu et al. 2020 and subsequent papers in that series), additionally needs ground-level wind, temperature, and terrain data for specific telescope pads. The unresolved target is therefore a quantified, cloud-inclusive, instrument-specific site characterization of the Eastern Pamirs, sufficient to decide whether a sub-mm observatory or array can be built there — the scientific step that would substantiate (or refute) the broader vision of year-round, high-frequency sub-mm coverage spanning both hemispheres.

Problem Statement

Complete the astronomical site evaluation of the Eastern Pamirs candidate sites — Koluch-Kul (~4475 m) and Shorbulak (~4289 m) in Tajikistan and Muztagh-Ata (4526 m) in China — for ground-based millimetre/submillimetre astronomy, to the standard needed for an evidence-based siting decision on a submillimetre telescope or array (the PaLMA/ESMT concept). Building on the clear-sky, zenith-only GEOS-FPIT statistics of Lapinov et al. (2020), the target is a complete, cloud-inclusive, instrument-specific site characterization, with quantified uncertainty, comprising:

  1. Cloud impact. The effect of fractional cloud cover (liquid- and ice-water content) on mm/sub-mm zenith opacity and transmission at these sites, at the 1.3 mm (230 GHz) and 0.87 mm (345 GHz) windows, thereby replacing the existing clear-sky-only statistics.

  2. Instrument-specific statistics. Performance statistics at multiple wavelengths for at least one concrete telescope or array configuration at one of the three sites, incorporating the opacity statistics — including their interannual variability — receiver sensitivity, observing efficiency and scheduling, and site-specific wind and temperature constraints.

  3. Integrated site assessment. A synthesis of the above with ground-level meteorology and the actual topography of candidate telescope pads, yielding a defensible judgement of whether, and in which seasons, the Eastern Pamirs can support high-frequency submillimetre observations year-round or in defined windows, and of their complementary role in closing the January–February high-frequency gap of ALMA/APEX and strengthening Eastern-Hemisphere coverage for mm VLBI.

Infrastructure development, political agreements, environmental impact, and funding are acknowledged prerequisites for any real observatory, but they are logistical rather than scientific questions and fall outside this problem.

References

  1. Lapinov, A. V., Lapinova, S. A., Petrov, L. Yu., Ferrusca, D. (2020). On the benefits of the Eastern Pamirs for sub-mm astronomy. Proc. SPIE 11453, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy X. DOI: 10.1117/12.2560250; arXiv:2012.04647. https://arxiv.org/abs/2012.04647
  2. Xu, J., Esamdin, A., Hao, J., Bai, J., Yang, J., Zhou, X., Yao, Y., Hou, J., Pu, G., Feng, G., Bai, C., Wei, P., Ma, S., Yisikandee, A., Wang, L., Zhang, X., Ming, L., Ma, L., Liu, J., Cao, Z., Zhao, Y., Feng, L., Shi, J., Chen, H., Pei, C., Jiang, X., Wang, J., Tian, J., Xue, Y., Hu, J., Jiang, Y. (2020). Site-testing at Muztagh-ata site I: Ground Meteorology and Sky Brightness. Research in Astronomy and Astrophysics 20, 86. DOI: 10.1088/1674-4527/20/6/86; arXiv:2003.14079. https://arxiv.org/abs/2003.14079
  3. Xu, J., Feng, G., Pu, G., Wang, L., Cao, Z., Ren, L., Zhang, X., Ma, S., Bai, C., Esamdin, A., Li, J., Tian, Y., Wang, Z., Zhao, Y., Shi, J. (2023). Site-testing at the Muztagh-ata Site. V. Nighttime Cloud Amount during the Last Five Years. Research in Astronomy and Astrophysics 23, 045015. DOI: 10.1088/1674-4527/acc29b; arXiv:2305.03067. https://arxiv.org/abs/2305.03067
  4. Gu, W., Xu, J., Feng, G., Zhang, X., Wang, L., Wang, X., Esamdin, A., Shen, L. (2024). An analysis of the fragmentation of observing time at the Muztagh-ata site. Research in Astronomy and Astrophysics 24, 035003. DOI: 10.1088/1674-4527/ad0a32; arXiv:2311.13935. https://arxiv.org/abs/2311.13935
  5. Zinchenko, I. I., Lapinov, A. V., Vdovin, V. F., Zemlyanukha, P. M., Khabarova, T. A. (2023). Measurements and Evaluations of the Atmospheric Transparency at Short Millimeter Wavelengths at Candidate Sites for Millimeter- and Sub-Millimeter-Wave Telescopes. Applied Sciences 13(21), 11706. DOI: 10.3390/app132111706. https://doi.org/10.3390/app132111706
  6. Bubukin, I. T., Rakut, I. V., Agafonov, M. I., Pankratov, A. L., Lapinov, A. V., Petrov, L. Yu. (2023). On Atmospheric Absorption Values at Millimeter Waves on the Suffa Plateau and Karadag Landfill. Radiophysics and Quantum Electronics 65. DOI: 10.1007/s11141-023-10252-0. https://doi.org/10.1007/s11141-023-10252-0
  7. Shikhovtsev, A. Y., Kovadlo, P. G., Baron, P. (2025). Candidate Sites for Millimeter and Submillimeter Ground-Based Telescopes: Atmospheric Rating for the Eurasian Submillimeter Telescopes Project. Sensors 25(7), 2144. DOI: 10.3390/s25072144. https://doi.org/10.3390/s25072144
  8. Pankratova, E. A., Bubukin, I. T., Rakut, I. V., Agafonov, M. I., Yablokov, A. A., Bubukin, V. I., Pankratov, A. L., Vodzyanovsky, Y. O., Khudchenko, A. V., Tretyakov, I. V. (2026). Analysis of atmospheric absorption data at Khulugaysha site. Atmospheric Environment. DOI: 10.1016/j.atmosenv.2026.122274. https://doi.org/10.1016/j.atmosenv.2026.122274
  9. Duan, R., Khaikin, V., Lebedev, M., Shmagin, V., Yakopov, G., Vdovin, V., Bubnov, G., Zhang, X., Niu, C., Li, D., Zinchenko, I. (2020). Toward Eurasian SubMillimeter Telescopes: the concept of multicolor subTHz MKID-array demo camera MUSICAM and its instrumental testing. arXiv:2008.10154. https://arxiv.org/abs/2008.10154
  10. Ye, Q.-Z., Su, M., Li, H., Zhang, X. (2015). Tibet's Ali: Asia's Atacama? Monthly Notices of the Royal Astronomical Society: Letters. DOI: 10.1093/mnrasl/slv188; arXiv:1512.01099. https://arxiv.org/abs/1512.01099
  11. Kuo, C.-L. (2017). Assessments of Ali, Dome A, and Summit Camp for Mm-wave Observations Using MERRA-2 Reanalysis. The Astrophysical Journal. DOI: 10.3847/1538-4357/aa8b74; arXiv:1707.08400. https://arxiv.org/abs/1707.08400
  12. Suen, J. Y., Fang, M. T., Lubin, P. M. (2014). Global Distribution of Water Vapor and Cloud Cover -- Sites for High Performance THz Applications. IEEE Transactions on Terahertz Science and Technology 4, 86-100. DOI: 10.1109/TTHZ.2013.2294018; arXiv:1312.5918. https://arxiv.org/abs/1312.5918
  13. Gong, Y., Zhong, J., Ren, Y., Zhang, Y., Liu, D., Ao, Y., Yao, Q., Zhang, W., Miao, W., Lin, Z., Duan, W., Liu, D., Zhou, K., Liu, J., Wang, Z., Jin, J., Zhang, K., Wu, F., Li, J., Liu, B., Zhang, X., Luo, Z., Wang, J., Hao, H., Lu, X., Xie, S., Quan, J., Liu, Y., Liang, J., Deng, X., Jiang, J., Li, L., Guo, L., Ji, T., Jiang, P., Zhang, Y., Shu, C., Neupane, S., Mao, R., Shi, S., Li, J. (2026). First Submillimeter Lights from Dome A: Tracing the Carbon Cycle in the Feedback of Massive Stars. Science Advances. DOI: 10.1126/sciadv.aea9433; arXiv:2601.07162. https://arxiv.org/abs/2601.07162
  14. Hellemeier, J. A., Yang, R., Sarazin, M., Hickson, P. (2018). Weather at selected astronomical sites -- an overview of five atmospheric parameters. Monthly Notices of the Royal Astronomical Society 482, 4941. DOI: 10.1093/mnras/sty2982. https://doi.org/10.1093/mnras/sty2982